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The efficient separation of platinum group metalsusing centrifugal partition chromatography.
Item Type text; Dissertation-Reproduction (electronic)
Authors Surakitbanharn, Yosyong.
Publisher The University of Arizona.
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The efficient separation of platinum group metals using centrifugal partition chromatography
Surakitbanharn, Yosyong, Ph.D.
The University of Arizona, 1992
300 N. Zccb Rd. Ann Arbor, MI 48106
THE EFFICIENT SEPARATION OF PLATINUM GROUP METALS USING
CENTRIFUGAL PARTITION CHROMATOGRAPHY
by
Yosyong Surakitbanharn
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF CHEMISTRY
In the Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 992
THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that we have
read the dissertation prepared by Yosyong Suraldtbanharn
entitled The Efficient Separation of Platinum Group Hetals using
Centrifugal Pcrtition Chromatography
and recommend that it be accepted as fulfilling the dissertation
requirement for the Degree of Doctor of Philosophy
I I I '1: tJ L
Date i/. /. /1/~//4
Date
1/ /7 /?.:l-Date
/ (il/v/J q'Z,. Date
Date
Final approval and acceptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.
2
I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.
'(/9A':,-it,
Dissertation Director Date(
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
:v. ~' r +l /' SIGNED: _-=::L-+-=--_~ __ MJ_:lI_,_i..i.._j\"_'t-f.\._Y-,'v;.;..''\-,fJ/-,\=-'''I,,-,/V;....-.. __
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4
ACKNOWLEDGEMENTS
I wish to express my gratitude to my mother, Mrs. Kingkarn Surakitbanharn. Without her love, encouragement and support I would have never been successful throughout my academic career. Sincerest thanks and appreciation are given to my wonderful wife and son, Puongpen and Chittayong (Jao) Surakitbanharn, for their patience and encouragement throughout my graduate work.
The gratitude is tremendous to Professor Henry Freiser for his guidance and support throughout this project. His patience and inspiration have been greatly appreciated. Without him, this dissertation could have never been completed.
Special thanks are given to Dr. Subramaniam Muralidharan for his ideas, discussion and assistance throughout this project. Many thanks are given to Dr. Roger Sperline for his suggestion and assistance from his computer expertise for data acquisition and manipulation.
Last but not least, this dissertation is dedicated in memory of my father, Mr. Yiamyong Surakitbanharn, who had always encouraged me to be a scientist since I was young.
5
TABLE OF CONTENTS
LIST OF FIGURES ..................................... , 7
LIST OF TABLES ...................................... 11
ABSTRACT .......................................... 13
I. INTRODUCTION ..................................... 15
II. STATEMENT OF PROBLEM ............................ , 22
III. EXPERIMENTAL ................................... , 24
A. Apparatus .................................... , 24
B. Reagents ..................................... 32
C. Procedures .................................... 33
1. Equilibrium Extraction . . . . . . . . . . . . . . . . . . . . . . . .. 33
2. CPC Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33
3. Kinetic Studies using Stopped Flow ................. 34
IV. RESULTS AND DISCUSSION ........................... , 38
A. Equilibrium Study ............................... , 38
1. Extraction Equilibria of Coordination Complexes of PGM by Solvent Extraction and CPC ..................... , 38
o Palladium(II) ............................. 38
o Platinum(Il) ............................. , 44
2. Extraction equilibrium of ion pairs of PGM at high acid concentration .............................. , 50
o Palladium (II) and Platinum (II) ................. , 50
o Platinum (IV) and Iridium(IV) .................. , 60
6
B. Chromatographic Separation of PGM . . . . . . . . . . . . . . . . . . .. 68
1. Separation of Pd from Pt, Ir and Rh by TOPO . . . . . . . . .. 68
2. Separation of Pd, Pt, Ir and Rh by Protonated TOPO . . . . .. 72
3. Separation of Pd(I1) , Pt(II) from Ir(III) and Rh(III) by Stepwise Chloride Gradient Technique ............... 75
C. Kinetic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 78
1. Preliminary Indication of Role of Chemical Kinetics in CPC Efficiency. ................................ 78
2. Dissociation of PdCI2(TOPO)2 Complex . . . . . . . . . . . . . .. 87
3. Dissociation of PtC14.(TOPO.H)2 Complex . . . . . . . . . . . .. 92
4. Dissociation of PdCI4(TOPO.H)2, PtCI4(TOPO.Hb IrCI6(TOPO.H)2 and PtC16(TOPO.H)2 Ion Pairs ......... 97
V. CONCLUSIONS .................................... 106
VI. APPENDIX A Derivation of The Kinetic Expressions for The Dissociation and The Formation of PdCI2(TOPO)2' ............. 109
VII. APPENDIX B Derivation of The Kinetic Expressions for The Dissociation and The Formation of PdC14(TOPO.H)2' ........... 111
VIII. LITERATURE CITED ............................... 113
7
LIST OF FIGURES
FIGURE 1. Motion of mobile phase passing through stationary phase held by centrifugal force (G). . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17
FIGURE 2. A flow sheet of CPC system. a) ascending mode (mobile phase flow from J2 to JI), b) descending mode (mobile phase flow from J1 to J2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18
FIGURE 3. The UV-VIS spectrum of 1Q4 M chIaro complex of Pd(II). .... 25
FIGURE 4. The UV-VIS spectrum of 10-4 M chloro complex of Pt(II). . . . .. 26
FIGURE 5. The UV-VIS spectrum of 1Q4 M chIaro complex of Pt(IV). 27
FIGURE 6. The UV-VIS spectrum of 1Q4 M chiaro complex of Ir(III). 28
FIGURE 7. The UV-VIS spectrum of 10-4 M chIaro complex of Ir(IV). 29
FIGURE 8. The UV-VIS spectrum of lQ4 M chiaro complex of Rh(III). 30
FIGURE 9. The UV -VIS spectrum of 1 Q4 M 3-picoline. . . . . . . . . . . . . .. 31
FIGURE 10. A block diagram of the stopped flow apparatus. ........... 35
FIGURE 11. The spectral change due to the dissociation of TOPO-Pd(II) complex in 0.5 % Triton X-100 in water. [Pd(II)] = 10-3 M and [Tapa] = 10-3 M. a) initial spectrum at 10-2 M HCl and b) final spectrum at 10-2 M HCI and 0.5 M NaCl. ............... 36
FIGURE 12. The spectral change due to the dissociation of TOPO-Pd(II) ion pair in 0.5% Triton X-IOO in water. [Pd(II)] = 104 M and [Tapa] = 5x104 M. a) initial spectrum at 1.0 M HCI and b) final spectrum at 10-2 M HCl. ...................... 37
FIGURE 13. The Pd(II)-chloro species as a function of chloride concentration, a) Pd, b)PdCI+, c) PdCI2, d) PdCI3- and e) PdCI4
2-. ••••••••• 41
FIGURE 14. Dependence of log D as a function of log [Tapa] at a) 10-3 M [Cn from solvent extraction, b) 10-3 M [CI"] from CPC, c) 0.1 M [Cn from solvent extraction and d) 0.1 M [CI"] from CPC. . .................................... 42
8
FIGURE 15. Dependence of log D of each Pd species as a function of log [Cn, a) PdCI2, b) PdCI3- and c) PdCIl-. ................ 43
FIGURE 16. Dependence of CPC chromatograms on chloride concentrations for Pt(II)-TOPO complex at 104 M Pt(II), 10-3 M HCI, 0.50 M TOPO and flow rate of 2.00 mL/min. ................. 46
FIGURE 17. Dependence of CPC chromatograms on TOPO concentrations for Pt(II)-TOPO complex at 104 M Pt(II), 10-3 M Hel, and flow rate of 2.00 mL/min. .............................. 47
FIGURE 18. The Pt(II)-chloro species as a function of chloride concentration, a) PtCI2, b)PtCI3- and c) PtCll-. ..................... 48
FIGURE 19. Dependence of CPC chromatograms on TOPO concentrations for Pt(II)-TOPO ion pair at 10-4 M Pt(II), 0.10 M HCI, 0.50 M NaCI and flow rate of 4.00 mLimin. . . . . . . . . . . . . . . . . . . . . .. 53
FIGURE 20. Dependence of CPC chromatograms on acid concentrations for Pt(II)-TOPO ion pair at 104 M Pt(II), 0.20 M TOPO, 0.50 M chloride and flow rate of 4.00 mLimin. ................ 54
FIGURE 21. CPC separation of Pt(II) and Pd(II) as their ion pairs as a function of chloride concentrations at lQ4 M Pt(II) , 10-3 M Pd(II) , 0.50 M TOPO, 0.10 M HCI and flow rate of 4.00 mL/min. . .................................. 55
FIGURE 22. Dependence of CPC chromatograms on chloride concentrations for Pd(II)-TOPO ion pair at 10-3 M Pd(II), 0.10 M HCI, 0.50 M TOPO and flow rate of 4.00 mLimin. ................. 58
FIGURE 23. Dependence of CPC chromatograms on acid concentrations for Pd(II)-TOPO ion pair at 10-3 M Pd(II), 0.50 M TOPO, 0.60 M chloride and flow rate of 4.00 mL/min. ................ 59
FIGURE 24. Dependence of CPC chromatograms on TOPO concentrations for Pt(IV)-TOPO ion pair at 10-4 M Pt(IV), 0.10 M HCI, 0.50 M NaCI and flow rate of 4.00 mLimin. .................. 63
FIGURE 25. Dependence of CPC chromatograms on acid concentrations for Pt(IV)-TOPO ion pair at 104 M Pt(IV), 0.20 M TOPO, 0.50 M chloride and flow rate of 4.00 mL/min. ................ 64
FIGURE 26. Dependence of CPC chromatograms on TOPO concentrations for Ir(IV)-TOPO ion pair at 10-2 M Ir(IV) , 0.04 M HCI, 0.50 M
9
NaCI and flow rate of 4.00 mL/min. .................. 65
FIGURE 27. Dependence of CPC chromatograms on acid concentrations for Ir(IV)-TOPO ion pair at 10-2 M Ir(IV), 0.50 M TOPO, 0.50 M chloride and flow rate of 4.00 mL/min. ................ 66
FIGURE 28. Chromatogram of the separation of Pd(II) from Pt(II) at 0.10 M [CI-] , 0.50 M [TOPO] , 10-3 M HCI and flow rate of 0.54 mL/min., a) dead volume peak, b) 10-3 M Pt and c) 4XlO-3 M Pd ........................................ 69
FIGURE 29. The computer simulated chromatogram of the separation ofPd(ll) and Pt(II) at N = 360. a) DPt = 0.11 and b) Dpd = 1.40. ..... 70
FIGURE 30. The chromatogram of the separation of Pd(II) from Pt(II), Ir(III) and Rh(III) at 0.08 M [Cn, 0.5 M [TOPO], 10-3 M HCI, and flow rate of 0.63 mLlmin, a) 5XlO-4 M Ir(III) and Rh(III), b) IQ-4 M Pt(II) and c) 10-3 Pd(II) ................... " 71
FIGURE 31. The chromatogram of the ion pair separation of Pt(II) and Pd(II) at 0.50 M TOPO, 0.10 M HCI, 0.50 M NaCI and flow rate of 4.00 mL/min. a) dead volume peak, b) 10-3 M Pd(II) and c) 10-4 M Pt(II). . ............................ " 73
FIGURE 32. The chromatogram of the ion pair separation of Ir(IV) and Rh (III) concentrations at 0.50 M TOPO, 0.02 M HCI, 0.50 M NaCI and flow rate of 4.00 mLlmin. a) 10-3 M Ir(IV) and b) 10-3 M Rh(III). .................................... 74
FIGURE 33. The CPC separation of Pd(II), Pt(II) from Ir(III) and Rh(III) by stepwise chloride gradient technique at 0.50 M TOPO, 10-3 M HCI and flow rate of 2.00 mL/min. Peaks elute at a) 10-3 M Ir(III) and 10-3 M Rh(III) (10-3 M HCI), b) 10-4 M Pt(II) (0.08 M NaCI) and c) 10-4 M Pd(II) (0.50 M NaCl). .............. 76
FIGURE 34. The CPC separation of Pt(II), Pd(II) from Ir(III) and Rh(III) by proton and chloride gradients at 0.50 M TOPO and flow rate of 4.00 mLlmin. Peaks elute at a) 10-3 M Ir(III) and 10-3 M Rh(III) (0.10 M HCI), b) 10-3 M Pd(II) (0.10 M HCI and 0.80 M NaCI) and c) 10-4 M Pt(II) (10-3 M HCI and 0.80 M NaCI). ........ 77
FIGURE 35. The comparison of the chromatograms at identical retention volumes (132 mL) of a) Pd(II) (0.30 M TOPO, 0.1 M NaCI and 10-3 M HC1), b) 3-picoline (PH = 6.10) and c) the dead volume
10
peale ...................................... 79
FIGURE 36. The Van Deemter type plot for a) Pd(II)-TOPO system (0.10 M TOPO, 10-2 M NaCI and 10-3 M HC1) and b) 10-3 M 3-picoline in succinic acid buffer of pH = 6.10. ................. 80
FIGURE 37. Dependence of CPC chromatograms on chloride concentrations for Pd(II)-TOPO complex at 10-3 M Pd(II), 10-3 M HCI, 0.50 M TOPO and flow rate of 2.00 mLlmin. ................. 85
FIGURE 38. Dependence of CPC chromatograms on TOPO concentrations for Pd(II)-TOPO complex at 10-3 M Pd(II), 10-3 M HCI, 0.05 M NaCI and flow rate of 2.00 mL/min. .................. 86
FIGURE 39. The correlation between CETPs and half lives at a) 0.20-0.50 M TOPO and b) 0.05-0.30 M NaCl. .................... 91
11
LIST OF TABLES
TABLE 1. Equilibrium data for extraction of Pd(II) by TOPO at low acid concentration. ................................. 40
TABLE 2. Equilibrium data for extraction of Pt(II) by TOPO at low acid concentration. ................................. 45
TABLE 3. Equilibrium data for extraction of Pd(II) by protonated TOPO. ... 51
TABLE 4. Equilibrium data for extraction of Pt(II) by protonated TOPO. . . .. 52
TABLE 5. Distribution ratios for Pd(II) extraction as TOPO complex and ion pair. ....................................... 57
TABLE 6. Equilibrium data for the extraction of Pt(IV) by protonated TOPO ....................................... 61
TABLE 7. Equilibrium data for the extraction of Ir(IV) by protonated TOPO ...................................... , 62
TABLE 8. CETP values of 3-picoline at the viscosities of TOPO solutions in heptane at a flow rate of 2.00 mL/min and the distribution ratio of 0.90 ........................................ 82
TABLE 9. CETP and D values for Pd(II) at different TOPO and chloride concentrations at low acid concentration (2.00 mLlmin). ....... 83
TABLE 10. Variation of ~5 as a function of Triton X-IOO, Chloride and TOPO concentrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., 88
TABLE 11. CETP and D values for Pt(II) at different TOPO and chloride concentrations at low acid concentration (2.00 mLlmin). ....... 93
TABLE 12. CETP and D values for Pt(II) at different TOPO and chloride concentrations at high acid concentration (4.00 mLlmin). . . . . . .. 98
TABLE 13. Variation of k"bs as a function of [HCI] and [TOPO] in 0.5% Triton X-100 ....................................... 99
TABLE 14. CETP and D values for Pt(IV) at different TOPO and chloride concentrations at high acid concentration (4.00 mLlmin). . . . . .. 102
12
TABLE 15. CETP and D values for Ir(IV) at different Tapa and chloride concentrations at high acid concentration (4.00 mL/min). . . . . .. 103
TABLE 16. Extraction equilibrium constants and formation and dissociation rate constants for TOPO ion pairs of Platinum Group Metals (pGM).. 104
13
ABSTRACT
Centrifugal Partition Chromatography (CPC) is a multistage liquid-liquid
countercurrent distribution technique which utilizes rotating teflon cartridges to hold a
liquid phase stationary while the other liquid phase is pumped at a constant flow rate. It
has been demonstrated to be a valuable technique for the base line separations of families
of metal ions such as the platinum group metals (pGM) - Pt, Pd, Rh and Ir. The
separations of these metals as their anionic chloro complexes were achieved using the
heptane-water phase pair with a stable and relatively inexpensive extractant
trioctylphosphine oxide (TOPO) functioning as a ligand in its neutral form and as a cation
in its protonated form.
A striking feature of the chromatograms of the complexes and ion pairs were their
much poorer efficiencies compared to the efficiency of an organic analyte like 3-picoline
under identical distribution ratios. The inefficiencies of the PGM separations were also
a function of the concentrations of the aqueous and organic phase components. These
inefficiencies could be attributed to slow kinetics of the back extraction of the complexes
and ion pairs and could be used to derive the mechanisms of these slow chemical kinetic
steps. A correlation was established for the Pd(II) system between the CPC inefficiencies
and the half lives of the slow reactions measured independently by stopped flow in
micelles. This correlation was utilized to derive the rate constants for the back extraction
of the TOPO complexes and ion pairs of Pt and Ir. The mechanisms of the extraction
reactions were derived using the principle of microscopic reversibility based on the
mechanisms of the back extraction reactions. This was then used to obtain estimates for
14
the rate constants for the extraction reactions as well. The PGM were thus separated and
their equilibrium and kinetics (extraction and back extraction) completely characterized
using CPC. This is a significant development with CPC because such complete
equilibrium and kinetic characterizations are hard to achieve with conventional liquid
chromatographic techniques.
15
I. INTRODUCTION
The separation of platinum group metals (pGM) , while widely investigated,
continues to pose challenges (1,2). Ion pair extractions of the chloro anions are
reasonably rapid but not so selective. Extraction as chelates while being more selective
often suffer from very slow formation/dissociation kinetics. A way to improve the
separation of PGM is multistage liquid-liquid separation techniques using ligand systems
that have reasonably rapid kinetics and ion pair extraction.
The separation of platinum group metals by multistage methods such as,
extraction chromatography has been previously attempted (3). This process, in which
an organophosphorus reagent was loaded on a silica solid support, suffered from, being
tedious, poor efficiencies and resolution. Further, the coated extractant leached from the
solid support. The resolution of these problems have been addressed by the use of the
multistage liquid-liquid countercurrent distribution method, Centrifugal Partition
Chromatography (CPC).
CPC has been applied for the separation of organic compounds, biological
materials and natural products (5,6). It is only recently that this technique has been
applied for the separations of metal ions, like lanthanides (7,8). The efficient separation
by CPC of adjacent lanthanides including the separation of both light and heavy of these
elements in a single run was successfully demonstrated using the extractant Cyanex 272TM
(bis(2,4,4-trimethylpentyl)phosphinicacid) in the heptane stationary phase and an aqueous
mobile phase at the appropriate pH (9).
CPC is based on the principle of countercurrent distribution (10) or liquid-liquid
16
multistage extraction. Two immiscible phases, an organic phase such as heptane
containing an extractant and an aqueous phase at the appropriate pH constitute the phase
pair used. One phase is held stationary in rotating cartridges by the action of centrifugal
force and the other phase is pumped through the stationary phase, achieving multistage
solvent extraction as shown in Figure 1. Compared to normal phase and reverse phase
in other conventional LC, CPC can be operated in either ascending or descending modes
depending upon the densities of the stationary and mobile phases. Ascending mode must
be carried out when the mobile phase is less dense than the stationary phase and the
descending mode is used for the reverse situation (Figure 2).
The multistage extraction or the countercurrent distribution of each solute can be
represented as a binomial expansion equation (11). The fraction, Fa, of a given analyte
with a distribution ratio, D, in the organic phase where the ratio of the volume of the
organic phase to the volume of the aqueous phase is R. is given in the Equation 1.
DR + 1 v
(1)
In a countercurrent distribution involving N stages,
(2)
where Fw is the fraction of the analyte, and Cm, its concentration in the aqueous phase.
The concentration of the analyte in the mth stage of the separation is given by
C = m N! FN-mFm
m!(N-m)! a w (3)
CPC is different from droplet counter current chromatography (DCCC) where the
stationary phase is held by gravitational force. CPC, unlike DCCC is not hampered by
17
~t)fl Mobile Phase
D Stationary Phase
FIGURE 1. Motion of mobile phase passing through stationary phase held by centrifugal force (G).
Eo- CENTRIFUGAL FORCE -
Vi V2
P
mWill WS SP MP R
~
SP: stationary phase MP: mobile phase WS: wash solvent SL: sample loop VI: sample injection valve V2: elution mode selection valve P: constant-flow metering pump C: partition cartridges M: UV-VIS monitor R: recorder
FIGURE 2. A flow sheet of CPC system. a) ascending mode (mobile phase flow from J2 to 11), b) descending mode (mobile phase flow from 11 to 12). .....
00
19
long separation times and restricted choice of immiscible solvents. An analyte mixture
is injected into the CPC and the "chromatogram" of the separated components is obtained
using a UV-VIS detector. The chromatogram can be analyzed by the typical
chromatographic equations as shown in equation 4-8.
The distribution ratio, D, is determined from the following equation:
v = V + DV r m • (4)
where Vr = retention volume of sample, V m = mobile phase volume or dead volume and
V. = stationary phase volume. This D value is the same as determined from single stage
extraction in equation 5:
(5)
where Co = concentration of solute in organic phase, and Caq = concentration of solute
in aqueous phase.
The efficiency of separations in CPC is determined by the number of theoretical
plates (N). A quantity similar to the reduced plate height in conventional
chromatography can be defined for CPC. This quantity, the channel equivalent of a
theoretical plate (CETP) is obtained by dividing the total number of channels (2400) by
N. The CETP is a measure of the inefficiency of the column in contrast to N which is
a measure of the efficiency of the column. The N of the column is calculated by the ratio
of the elution volume, Vr> and the band width, W, as in equation 6.
20
(6)
The degree of separation of two peaks is defined as resolution, ~, determined by
the following expression:
(7)
where d is the difference in the Vr of the two separated metals. The larger of the
resolution the better of the separation. The resolution can be expressed in terms of the
D values of the components as follows:
where D = (D, +
D R. = (a-I) ~ . 4a V l''1----,V~
D m +-
Vs D2)/2 and a is the selectivity (a =
(8)
traditional LC is very large, typically 20-30 while in CPC can be adjusted to be as small
as 0.25. The same resolution, therefore can be accomplished at a much lower efficiency
(smaller N) in CPC compared to LC.
The separations of metal ions by CPC involves the formation and dissociation of
extractable complexes using suitable ligands or ion pairs using the appropriate counter
ions. The ligands and ion pair extractants are usually dissolved in the stationary organic
phase. The column efficiency in these separations is also a function of the distribution
ratio of a given metal species, unlike the case of the organic compounds. Similar low
efficiencies and the dependence of these efficiencies on the distribution ratios of the
21
extracted metal species have also been observed in the separation of metals by derivatized
solid supports (12). Generally, in chromatography the column efficiency is constant for
a given set of operating conditions, exhibiting no dependence on the distribution ratios
of the species being separated. This puzzling observation in metals separations in
conventional LC and CPC has been addressed systematically to discern the underlying
factors. CPC being a liquid-liquid separation technique has the advantage over
conventional LC in term of elucidating the influence of factors other than mass transport
and diffusion on chromatographic efficiencies.
This study describes the efforts to determine whether, and in what manner,
chemical factors, in contrast to simple solvation and desolvation as well as mass transfer
factors, are responsible for the differences in CPC efficiencies for metals and organic
separations. CPC studies coupled with solution kinetic studies using stopped flow have
allowed the elucidation of the influence of chemical kinetics on CPC column efficiencies
and allowed the correlation of them to the half-lives of the chemical reaction responsible
for the lowered chromatographic efficiencies.
22
II. STATEMENT OF PROBLEM
The separation of PGM continues to pose challenges and is currently achieved by
batch extraction methods under harsh conditions such as high acid and chloride
concentrations. Attempts to use conventional LC techniques such as coated silica supports
have been only partially successful and have suffered from poor column efficiencies.
Centrifugal Partition Chromatography (CPC) is a relatively new liquid-liquid
countercurrent distribution technique which could be useful for the separation of PGM.
The objectives of this study were (1) develop CPC separation methodologies for
the separation of PGM as their anionic chloro complexes, (2) characterize the species
extracted by examination of the dependence of the distIibution ratios on the
concentrations of the aqueous and mobile phase components and the extraction
equilibrium constants, and (3) elucidate the factors responsible for the observed
inefficiencies by examination of the efficiencies of the CPC chromatograms as a function
of the concentrations of components in the aqueous and mobile phase. The last aspect is
a fundamental and important problem encountered in conventional extraction
chromatography as well. The hypothesis that the inefficiencies in the multistage liquid
liquid separations of metals is caused by their slow complexation and dissociation kinetics
was proposed. A final and major objective of this work was to test this hypothesis using
CPC separations and independent stopped flow kinetic studies. A significant part of this
last objective was the examination of a relationship between the half-lives of the slow
chemical kinetic steps and the chromatographic inefficiencies. Such a correlation could
be a powerful tool in obtaining information on metal complexation and dissociation
23
kinetics of significance in practical separation procedures and seeking ways to improve
separation efficiencies by accelerating slow kinetic steps that are identified. Thus, the
general objectives of the work were to investigate the practical and fundamental aspects
of the separation of PGM by CPC.
24
III. EXPERIMENTAL
A. Apparatus
A Sanki, Co., Japan assembly consisting of a Model SPL centrifuge containing
6 analytical/semi-preparative cartridges each having 400 channels (2400 total channels),
a Model CPC FCU-V loop injector and a model LBP-V pump were used for CPC
experiments. The internal volume of these six cartridges is 125 mL. A block diagram of
the CPC instrument system is given in Figure 2.
A UV-VIS spectrophotometric detector (Model 770, Schoeffel Instrument Co.)
with a 0.1 mL cell volume and 8 mm path length was used. It was set at 238 nm and 255
nm for PGM and 3-picoline experiments respectively. The chloro complex spectra of
these PGM, Pd(II), Pt(II), Pt(IV), Ir(III), Ir(IV) , Rh(III) and 3-picoline are shown in
Figure 3-9. Chromatographic data was acquired every 10 seconds using an IBM/PC
interfaced to the detector and a DASH-8 A-D board and Labtech Acquire program
(Metrabyte Co., USA).
An ICP-AES (Model 6500, Perkin-Elmer Co., USA) was used to characterize the
extraction equilibria of PGM by single stage extraction experiments. A cross flow
nebulizer was used through out the studies except at the high salt content (> 0.1 M NaCl)
where the micro Babington nebulizer was utilized (J 3).
A HI-TECH Scientific Stopped Flow SHU spectrophotometer was used for the
kinetic study of the formation and dissociation of both Pd(II)-TOPO complex and its ion
pair. Data acquisition and treatment were accomplished with the associated Hi-Tech
25
.---------------------------------~g
-I000o4 I000o4
Q ---"0 ..... --~ "0 /l.t - 4-t e 0
8 S >< Q) -0.. \r)~ 8
0
" u
e ffi 0 -.c: ......l u
~~ ::E 'b
~ ...... 4-t 0
8 2 "-'
8 [ ~
Vl
CI) ..... > I
> :::> Q)
.c:
8 Eo-<
..-i oC'l M
'tIJNVffiIOSHV ga :::> 0 ..... IJ."
3.,------------------------------------------------------------~
ril 2 U
~ § o en
~ 1
Pt(II)
o I I I I ............... I Iii I I I I I I I I I I I 200 300 400 500 600 700
WAVELENGTH (nm)
FIGURE 4. The UV-VIS spectrum of lQ4 M chloro complex of Pt(II). IV 0\
3
~ 2
~ o CI)
~ 1
o I :;: 200 3CO
Pt(IV)
400 500 600 WAVELENGTH (nm)
FIGURE 5. The UV-VIS spectrum of 10-4 M chioro complex of Pt(IV).
700
N -....]
28
r---------------------------------~~
..-~ ~ -~ --- -1-0 ---~ 1-0 -..- .....
s 0 ><
SS <U -a V)r5 e 8
d 8 ~ 0 :a .....:l u
~~ ~
'b ~ ......
4-< 0
e 2 .....
8 ~ en Cf)
CI) -:> I
:> ::J
<U .c
8 E-<
..-l ON \0
t[JNVffiIOSHV § ::J t:J -u..
29
~--------------------------------~~
>- --1-004 > --1-4 -1-004 '-' .... -..- '-EI 0
~
gS co C5..
ln~ E 0
d u 0
Z .... 0
~ ::a ....l u
~~ ::E 'h
~ ...... '-0
E 2 ..... ~ til
CI) -> I
> ::> co .c
8 E-<
.-4 0<:'1 ['-
tlJNVffilOSHY ga ::> 0 -~
2 Rh(III)
E3
~ @ 1
~
01 I I i=-=r iii Iii iii iii I i I 200 300 400 500 600 700
WAVELENGTH (nm)
FIGURE 8. The UV-VIS spectrum of lQ4 M chloro complex of Rh(III). w o
~--------------------------------~g
C1) 1:1 ..... 8 ..... ~
I
M
----------------------~
3JNVffiIOSHV
I-
l
I-
o 1-0
M
I-
115 .5 "8 .s.
o M
:::E b -t.o-. o E 2
i r.n en ...... > I
> ::; Q)
..r:: E-
31
32
software.
All pH measurements were made with an Fisher Accumet pH meter (model 925)
which was calibrated daily with standard buffer solutions.
The viscosities of heptane solutions of TOPO were measured using an Ostwald
viscometer.
B. Reagents
Trioctylphosphine oxide, TOPO, (Kindly supplied by American Cyanamid Co.)
was recrystallized from acetone (M.P. = 53-54°C). Metal-free n-heptane (Baker
Analyzed, HPLC Grade) and water solutions were equilibrated overnight before use.
Pd(II), Pt(Il), Pt(IV), Ir(IIl), Ir(IV) and Rh(III) stock solutions of 10-2 M were prepared
by dissolving a weighed quantity of palladium(II) chloride (59.9% Pd, Alfa Products),
sodium tetrachloro platinite(II) (44.3% Pt, Johnson Mathey, Inc.), sodium
hexachloroplatinate(IV) (34.0% Pt, Johnson Mathey, Inc.), sodium hexachloroiridite(III)
(32.0% Ir, Alfa Product), sodium hexachloroiridate(IV) (Na2IrCI6.6H20, Johnson
Mathey, Inc.) and Rhodium(III) chloride trihydrate (39.0% Rh, Aldrich Chemical
Company, Inc.) in 0.1 M HCI solution. 3-Picoline (Eastman Kodak Co.) was purified
by distillation before use. The stock solution of 10-2 M 3-picoline was prepared in water.
Succinic acid buffer of pH = 6.10 (3.88XlO-3 M succinic acid and 6.94XlO-3 M NaOH)
was prepared using Perrin's method (14). Deionized-distilled water was used throughout
this study.
33
C. Procedures
1. Equilibrium Extraction
Single stage solvent extractions were carried out by shaking equal volumes (10
mL) of heptane containing TOPO with an aqueous phase containing 4XlO-4 M PGM in
a glass vial with a box type Eberbach shaker. The heptane-aqueous mixtures were shaken
for one hour to ensure complete equilibrium. The aqueous Pd, Pt, Ir and Rh
concentrations were determined using the ICP-AES. The most sensitive wavelengths of
Pd (340.458 nm), Pt (214.423 nm), Ir (244.268 nm) and Rh (233.477) were used to
measure the emission intensities with the detection limit of 1.47 ppm, 1.0 ppm, 0.91 ppm
and 1.49 ppm respectively. Standard solutions of PGM (10, 20, 30, and 40 ppm) were
prepared daily for calibration in the same matrix as sample to eliminate all matrix effects.
2. CPC Preparation
The CPC cartridges, injection loop, valve box and pump were always washed and
loaded with methanol. Methanol was removed from the CPC apparatus before use by
pumping water in the ascending mode through the system. The stationary phase is loaded
by replacing the water with heptane containing TOPO (0.1-0.5 M) in the ascending
mode. The total internal volume VI (124-125) was determined from the amount of water
displaced by heptane. The mobile phase of aqueous solution at appropriate pH and
chloride concentration (using NaCl) was pumped through the stationary phase using
descending mode. Equilibration of the two phases at 800 rpm resulted in 20 mL of
stationary phase (Vs) and 105 mL of mobile phase (VnJ as indicated by the amount of the
34
stationary phase displaced. One mL ofPGM stock solution (10-3 M) was injected into the
CPC for each run using the loop injector. Flow rates between 0.5 and 4.0 mL/min.
were used in these experiments. A flow rate of 4.00 mL/min was typically used for
experiments involving higher [HCl] and [Cn to minimize corrosion of stainless steel
parts of the CPC apparatus. The mobile phase was replaced by water after each run to
wash out the HCl and chloride.
The experimental configuration for 3-picoline experiment was identical to that for
palladium except that the detection wavelength was set at 255 nm. The mobile phase was
adjusted to pH 6.10 using succinic acid buffer before injecting 1 mL of 10-3 M 3-
picoline.
3. Kinetic Studies using Stopped Flow Apparatus
Pd(II) and TOPO as well as chloride (NaCl) and acid (HCI) were dissolved in
aqueous solutions of various concentrations of surfactant (0.5-4% Triton X-lOO, Aldrich
Chemical Company, Inc.). Figure 10 shows a block diagram of the stopped flow with
two syringes, where one side was loaded with TOPO-Pd complex and the other with
NaC!. The dissociation reaction occurred in the mixing chamber when the contents of the
two syringes were mixed by a pneumatic piston. The kinetics was monitored where the
change of spectra were observed at 420 nm for TOPO-Pd(II) complex (Figure 11) and
at 248 nm for protonated TOPO-Pd(II) ion pair (Figure 12). All measurements were
performed at 25°C by thermostating the syringes.
TUBE LENGTH 102 mm
VOLUME 180 pi
REAGENT DRIVE SYRINGES
DRIVE RAM
STOPPING BLOCK
8: EVENT MARKER D STOPPING SYRINGE
TUBE LENGTH 240 mm
VOLUME 424 ... 1
FIGURE 10. A block diagram of the stopped flow apparatus.
lIGHT(ATH 10 mm
MIXER
LIGHT PATH 2 mm
" OBSERV A TBON CELL
CELL VOLUME 40 ... 1
DEAD VOLUME (MIXER) 6 ... 1
w VI
0.4 (.
Pd(II)-TOPO complex
0.3
~ ~ a
ga 0.2 0 b CI)
t:Q <t:
0.1
... -....... _0'" -"'"
.. "" ...... ",.
,./" .. \ .' ...
' , . , . \ . , . , ~/ \,
/ .... ... "" '. .. -.~.... "'-~
~~o ~.~ 350 400 450 500 550 600 650 700
WAVELENGTH(nm)
FIGURE 11. The spectral change due to the dissociation of TOPO-Pd(II) complex in 0.5% Triton X-IOO in water. [Pd(II)] = 10-3 M and [TOPO] = 10-3 M. a) initial spectrum at 10-2 M HCl and b) final spectrum at 10-2 M HCI and 0.5 M NaCI.
W 0'1
4.----------------------------------------------------.
B ~
3
j:Q 2 ~ o CI)
~ 1
Pd(II)-TOPO Ion Pair
b
a
.. ...... _.
o I =-=-r " ·"_M"
200 250 300 350 400 450
WAVELENGTH(nm) 500
FIGURE 12. The spectral change due to the dissociation of TOPO-Pd(II) ion pair in 0.5% Triton X-IOO in water. [Pd(II)] = 10-4 M and [TOPO] = 5xlO-4 M. a) initial spectrum at 1.0 M HCI and b) final spectrum at 10'2 M HCI.
w ....J
38
IV. RESULTS AND DISCUSSION
A. Equilibrium Study
1. Extraction Equilibria of Coordination Complexes of PGM by Solvent
Extraction and CPC.
o Palladium (II)
The extraction equilibria of Pd(II) species were characterized by both solvent
extraction and CPC as a function of chloride and TOPO concentrations. The chloro-
palladium species extracted is a function of chloride concentration (Figure 13) (15). The
extraction equilibria were inferred from the dependence of log Dpd on log [CI"] and log
[TOPO], namely from the slopes of these plots (16). These equilibria are:
PdCI;- + 2TOPO ..... PdCI2(TOPO)2 + 2Cl-
PdCI; + 2TOPO ..... PdClz{TOPO)2 + Cl-
PdCl2 + 2TOPO ~ PdCI2(TOPO)2
(9)
(10)
(11)
The extraction equilibrium constants, Km for each Pd species can be calculated by using
the equilibrium expressions:
LogKex
•4 = LogDpd + 2Log[CI-] - 2Log[TOPO] - Loga4 (12)
LogKex ,3 = LogDl'd + Log[CI-] - 2Log[TOPO] - Loga3 (13)
LogKex ,2 = LogDI'd - 2Log[TOPO] - Loga2 (14)
where a4' a3, and a2 are the fraction of each Pd species, PdCl/, PdCI3-, and PdCI2 •
respectively in aqueous phase as shown in Figure 13.
Plots of Log Dl'd vs Log [TOPO] at 10-3 M and 0.1 M [Cn yielded a slopes of
39
1.89 and 1.88, respectively (Figure 14). This indicates that each Pd species is bound to
2 TOPO. The plot of Log Dpd - Log an of each Pd species as a function of Log [Cn is
illustrated in Figure 15. Slopes of -1.97, -0.97, and 0.003 were obtained for PdCLt,
PdCI3-, and PdC12 species respectively. These indicate the number of chloride ions
released from PdCLt, PdCI3-, and PdCl2 species to be 2, 1, and 0 respectively (Equations
9-11).
The extraction constants, K ex.4 , Ked and K ex.2• can be calculated from equations
12, l3 and 14 to be 0.14, 2.75 M-I, and 794.3 M-2 respectively (Table 1). This ~x.2
value is smaller by about 5 orders of magnitude compared to the Kex.2 value for the
extraction of PdCl2 by R2S, (2)as expected for a harder ligand.
The comparison of distribution ratios (D) determined by single stage batch solvent
extraction and by CPC are also shown in Figure 14. The data from two methods are
well in agreement within experimental error.
40
TABLE 1. Equilibrium data for extraction of Pd(II) by Tapa at low acid concentration.
A. Dependence on [Tapa]; Pd(II) = 4x1Q4 M, [HCI] = 10-3 M
[TOPO],M LogD Log ~x.2 Log Kex.3 Log Kex.4 (a2=0.01l) (a3=0.330) (a4=0.658)
0.50 0.32 2.86 0.40 -0.8
0.40 0.16 2.89 0.43 -0.86
0.30 -0.03 2.95 0.49 -0.80
0.20 -0.39 2.94 0.47 -0.82
0.14 -0.71 2.93 0.48 -0.81
B. Dependence on [CI-]; Pd(II) = 4x104 M, [Tapa] = 0.5 M, [HCI] = 10-3 M.
[Cl-],M Log D Log ~x.2 Log Kex.3 Log Kex.4
0.10 0.32 2.86 0.40 -0.89
0.14 0.08 2.87 0.41 -0.88
0.20 -0.19 2.87 0.41 -0.88
0.30 -0.52 2.86 0.40 -0.89
0.40 -0.72 2.89 0.43 -0.86
0.50 -0.92 2.88 0.42 -0.87
1 ___ :::::::==='1
O.B
0.6
~
0.4
0.2
o I - i -,=:= r====== :=y---<: ,..... =-r-- i =-===.j -7 -6 -5 -4- -3 -2 -1 0 1
lDg [Cl-]
FIGURE 13. The Pd(II)-chloro species as a function of chloride concentrations. a) Pd. b)PdCI+, c) PdCI2• d) PdCI3- and e) PdClt.
~ ......
Q b.O o
.....:l
2~1-------------------------------------------------------'
1
c
-11 I I I I I
-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 Log [TOPO]
FIGURE 14. Dependence of log D as a function of log [TOPO] at a) 10-3 M [en from solvent extraction, b) 10-3 M [en from epe, c) 0.1 M [en from solvent extraction and d) 0.1 M [en from epe.
,J:>. tv
4~1--------------------------------------------------------~
C 2 ~ bD 3
Q
.90
a o c:J ~ I::] Cl 0
b
..................................................................... ~ ............................................................................. _ .............•. _ ........... .
-21 I I I I 1 -1.2 -1 -0.8 -0.6 -0.4 -0.2
Log [CI-]
FIGURE 15. Dependence of log D of each Pd species as a function of log [CI"], a) PdCI2,
b) PdCI3' and c) PdCI/. .J::a. Vol
44
o Platinum (II)
The extraction equilibria of Pt(II) by complexation with TOPO are difficult to
characterize completely by the variation of the concentrations of both TOPO and chloride
like the Pd(U) case (17). This is due to the low extractibility of Pt(II) compared to Pd(II)
by complexation with TOPO. The stoichiometry and equilibrium constants have been
determined only by the variation of the concentration of TOPO. It is evident from Table
2B and Figure 16. (slope oflog D vs log [Cn = -0.51±0.01), that in the [Cn range of
0.005-0.10 M, the extraction of Pt(II) occurs by the complexation of a mixture of PtCI2,
PtCI3- and ptCIl. Table 2A and Figure 17 show that the plot of 10gD vs 10g[TOPO] has
a slope of 2.08+ 18 at 0.001 M RCI. The likely species are PtCl2 and PtCI3- as evident
from Figure 18 which shows the fraction of each platinum species, ex, as a function of
chloride concentration. Thus the extracted complex, irrespective of the chloroplatinite
species present in the aqueous phase is PtCliTOPOb which is similar to the extraction
of Pd(II) (17). The extraction equilibria for the different chloroplatinite species are given
in equation 15-17.
PtCIJ- + 2TOPO ~ PtCI2(TOPO)2 + 2CI- (15)
PtCI; + 2TOPO ~ PtCI2(TOPO)2 + CI- (16)
PtCl2 + 2TOPO ~ PtCI2(TOPO)2 (17)
The extraction constants, Kex•4 , Kex•3 and Kex•2, can be determined from Equations 18-20,
for PtCI/, PtCI3- and PtCl2 at different [en, Table IIB and the fraction of each species,
ex calculated from their formation constants (18) (Figure 18) to be 0.018±0.0006,
0.047±0.02 M-1 and 48±6 M-2 respectively.
45
TABLE 2. Equilibrium data for extraction of Pt(II) by TOPO at low acid concentration.
A. Dependence on [TOPO]; Pt(II) = 1Q4 M, [HCI] = 10-3 M
[TOPO],M
0.50
0.40
0.30
0.20
LogD
0.361
0.146
-0.187
-0.455
B. Dependence on [CI-]; Pt(II) = 10-4 M, [TOPO] = 0.5 M, [HCI] = 10-3 M.
[CI-],M Log D (CPC)
0.005 0.146
0.01 0.000
0.05 -0.346
0.10 -0.455
0.10 -0.420 (ICP)
0.08
UJ ~ 0.06
~ o ~ 0.04
0.02
Or 7 I 7 7 I
o 100 200 300 400
RETENTION VOLUME (mL)
FIGURE 16. Dependence of CPC chromatograms on chloride concentrations for Pt(II)-TOPO complex at 1<r1 M Pt(II), 10.3 M HCI, 0.50 M TOPO and flow rate of 2.00 mLlmin.
~ 0\
0.08
Ul ~ 0.06
~ o ~ 0.04
0.02
o-r 7 7 7 7 I
o 100 200 300 400
RETENTION VOLUME (mL)
,L. 0.30 4" ~ §
~
FIGURE 17. Dependence of CPC chromatograms on TOPO concentrations for Pt(II)-TOPO complex at 10-4 M Pt(II), 10-3 M HCl, and flow rate of 2.00 mLlmin.
.J:>. -..J
1,----------------------------------------------====--,
a c
0.8
0.6
o 0.4
0.2
f::::::== ::::::::;:= --- -- I o , , i i
-5 -4 -3 -2 -1 0 1
Log [CC]
FIGURE 18. The Pt(II)-chloro species as a function of chloride concentration, a) PtCh, b)PtCI3-
and c) PtCil-. ~ 00
49
LogKex,4 = LogDPt + 2Log[CI-] - 2LogrrOPO] - Logcx4 (18)
LogKex,3 = LogDPt + Log[CI-] - 2LogrrOPO] - Logcx3 (19)
LogKex,2 = LogDPt - 2LogrrOPO] -Logcx2 (20)
It is evident that the extractibility of Pd(II) species is much less compared to the
Pd(II) species. As will be discussed in the kinetic section, this is due to the slower
formation and more rapid dissociation kinetics of PtCI2(TOPO)2 compared to
PdCI2(TOPO)2. The difference in the Kcx,4 values can be exploited to obtain an efficient
separation of Pt(II) and Pd(II) from Rh(III) and Ir(III) . This was achieved by using
stepwise chloride gradient elution, Figure 31.
50
2. Extraction equilibrium of ion pairs of PGM at high acid concentration.
o Palladium (II) and Platinum (II)
It has been found that TOPO can be protonated at higher HCI concentration, and
the protonated TOPO can extract both PdClt and PtClt. These experiments were
carried out at high acid concentration « 1.0 M HCI) where the C¥4 value is close to unity
and PdCIl- and PtCIl- are the predominant species. The extraction equilibria of Pd(lI)
and Pt(lI) were confirmed by both solvent extraction and CPC as a function of TOPO,
acid, and chloride concentrations (Figure 19-21 and Table 3-4).
MCI;- + 2TOPO + 2H+ ... MCliTOPO'H)2 (21)
The equilibrium constant, K jp ,2, can be calculated from the following equation:
LogKjp
,2 = LogDM - 2Log[TOPO] - 2Log[H+] - Logc¥4 (22)
When C¥4 is very close to unity at very high chloride concentration, K jp,2 can be expressed
as follows:
LogK jp,2 = LogDM - 2Log[TOPO] - 2Log[H+] (23)
The dependence of log D for Pd(lI) on log [TOPO] and log [H+] yielded the
slopes of :2.01 and 1.95 respectively (Table 3). The ion pair extraction of Pt(lI) also
showed a linear relationship of log D vs log [TO PO] and log [H+] with a slopes of
2.21 ±O.03 and 1.82±O.08 respectively (Table 4). These indicate that the extracted Pd
and Pt species are associated with 2 TOPO and 2 protons. Dpd and DPt were also found
to be independent of [Cn (Table 3 and 4). The extracted species are mononuclear and
have M(II):Cl ratio of 1:4. The Kjp ,2 values for Pd(II) and Pt(II) determined in this
manner were 93.3 M-4 and 1961 ± 138 M-4 respectively.
TABLE 3. Equilibrium data for extraction of Pd(II) by protonated Tapa.
A. Dependence on [TOPO]; Pd(II) = 4x1Q4 M, [HCI] = 1 M, Q'4 = 0.995
[Tapa], M Log D (ICP) Log K jp,2
0.30 0.94 1.98
0.20 0.58 1.97
0.14 0.29 1.99
0.10 -0.05 1.95
0.08 -0.28 1.91
B. Dependence on [HCI]; Pd(I1) = 4xlO-4 M, [TOPO] = 0.1 M
[HCI], M Log D (ICP) Log K jp,2
2.50 0.66 1.86
2.00 0.47 1.88
1.50 0.23 1.87
1.25 0.06 1.86
1.00 -0.11 1.89
C: Dependence on [Cn; Pd(II) = 4xlO-4 M, [Tapa] = 0.1 M, [HCl] = 1 M.
[Cn, M
2.00
1.50
1.25
1.00
Log D (ICP)
-0.05
-0.07
-0.06
-0.10
51
52
TABLE 4. Equilibrium data for extraction of Pt(II) by protonated TOPO.
A. Dependence on [TOPO]; Pt(II) = 104 M, [HCI] =0.1 M, [NaCl] =0.5 M, IY4 =0.952
[TOPO], M Log D (CPC) Log K jp,2
0.20 -0.10 3.30
0.30 0.30 3.34
0.40 0.58 3.38
0.50 0.78 3.38
B. Dependence on [HCI]; Pt(I1) = 104 M, [TOPO] = 0.20 M, [NaCl] = 0.5 M
[HCI], M Log D (CPC) Log K jp,2
0.10 -0.11 3.29
0.15 -0.18 3.22
0.20 0.42 3.21
0.25 0.62 3.22
C: Dependence on [Cn; Pt(II) = 10-4 M, [TOPO] = 0.5 M, [HCl] = 0.1 M.
[Cn, M
0.20
0.60
0.80
Log D (CPC)
0.89
0.89
0.89
0.04
UJ ~ 0.03
§
~ 00021 ~H t-JU "- r03°i~~ ~
/ Il---' " '- ;L 0.20 0.01
07' 7 7 7
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 19. Dependence of CPC chromatograms on TOPO concentrations for Pt(lQ-TOPO ion pair at 104 M Pt(II), 0.10 M HCl, 0.50 M NaCI and flow rate of 4.00 mL/min.
Ul W
0.04
U.l II IW~b'
\ c 7 7 0.25
~ 0.03
~ 0 ~ 0.02l Al\ 1\ I \ / ~
cY ~
/ III I /
0.01
DryV' 7 7 7 '
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 20. Dependence of CPC chromatograms on acid concentrations for Pt(II)-TOPO ion pair at 10-4 M Pt(II}, 0.20 M TOPO, 0.50 M chloride and flow rate of 4.00 mLlmin.
VI ~
0.08
Jb' '\ Z k_ ""= 7 0.30
Ul U 0.06 Z <C ~ ~
7 0.60 ~ -.
0
~11 '\ ' v
~
~ 0.04 I ~
/ III I '\ /
0.02
07" 7 7 7 7'
o 100 200 300 400
RETENTION VOLUME (mL)
FIGURE 21. CPC separation of Pt(II) and Pd(II) as their ion pairs as a function of chloride concentrations at 104 M Pt(II), 10-3 M Pd(II), 0.50 M Tapa, 0.10 M HC} and flow rate of 4.00 mLlmin.
VI VI
56
It also has been found that the PdCI3- can be extracted as its ion pair by TOPO.H+
at high acid concentrations. It is also evident from Pd chromatograms in Figure 21 that
the Pd(II) peak shifts with the chloride concentration while the Pt(II) peak is stationary.
As it has been shown for Pd(II) at 1.0 M HCI before ion pair extraction ofPdCll should
be independent of the concentration of chloride (l7). This indicates that at 0.1 M HCI
concentrations employed in Figure 21, Pt(II) is extracted purely as the ion pair
PtCliTOPO.Hh while the extraction behavior of Pd(II) is more complex. The slope of
the plot of log D vs log [Cn for Pt(II) is zero (Table 4 and Figure 21) while for Pd(II)
the slope is -1.02+0.03 (Figure 22). The major Pd(II) species under the experimental
conditions employed is PdCI4- with PdCI3- being present in minor quantities as indicated
by their a values, Table 5. The slope of negative one with respect to [Cn and the D
value for Pd cannot be explained by the extraction of Pd(II) as either PdCI2(TOPO)2 or
PdCI4(TOPO.H)2. Clearly the D value for Pd determined from the chromatograms is
much larger than the sum of the D values due to PdCI2(TOPO)2 and PdCI4(TOPO.H)2
which can be calculated from the extraction equilibrium constants of these species that
we have previously reported (16). The plot of log (Dob• - Dept" - Dip) vs log [Cn and log
[H+] yielded slopes of -1 and + 1 respectively (Table 5 and Figures 22 and 23). A
limited number of chromatograms as a function of the concentration of TOPO also
yielded a slope of + 1 for the plot with respect to log [TOPO]. These observations could
be rationalized by the extraction of PdCI3(TOPO.H), equation 24.
(24)
Thus the major species extracted in the case of Pd(II) is PdCI3(TOPO.H). The extraction
57
TABLE 5. Distribution ratios for Pd(II) extraction as TOPO complex and ion pair.
A. Dependence on [Cn; [HCI] = 0.1 M, [TOPO] = 0.50 M.
[CI-],M 01./ 0I.3b Dobs Dcplx
c Dip2d Dipl C
0.2 0.797 0.200 5.69 0.87 0.19 4.64
0.3 0.855 0.143 3.73 0.38 0.20 3.14
0.4 0.888 0.111 2.71 0.22 0.21 2.29
0.6 0.922 0.077 1.77 0.10 0.21 1.46
0.8 0.941 0.059 1.42 0.054 0.22 1.15
Slope of log Dipl VS log [Cn = -1.01 ±0.006
B. Dependence on [H+]; [Cn = 0.6 M, [TOPO] = 0.50 M, <l'4 = 0.922.
[H+],M Dobs Dcp1x c DiP2d Diplc
0.05 0.90 0.096 0.054 0.65
0.10 1.77 0.096 0.215 1.46
0.15 2.84 0.096 0.484 2.26
0.20 4.04 0.096 0.860 3.08
0.25 5.33 0.096 1.344 3.89
Slope of log D vs log [H+] = 1.02±0.02
a fraction of PdCIl-b fraction of PdC13-
c distribution ratio due to PdC12(TOPOh d distribution ratio due to PdCI4(TOPO.Hh C distribution ratio due to PdCI3(TOPO.H)
0.04
/ /
Jt I ;7 '\ ~ 70.20
L.Ll ~ 0.03
~ 0 ~ 0.02l / I \ A / \ / ~
~ ~
/ I \ v / 0.01
04 > > > '
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 22. Dependence of CPC chromatograms on chloride concentrations for Pd(II)-TOPO ion pair at 10-3 M Pd(II), 0.10 M HC}, 0.50 M TOPO and flow rate of 4.00 mUmin.
lJ\ 00
0.04
)lvi \
~ ~ / r- 0.25 UJ ~ 0.03
~ 0 ~ 0.02 l / I \ II / \ / ~
~ ~C ~
" " 0.01
o -j£---~-~-== 7 o 7
7---~
100 200 300
RETENTION VOLUME (mL)
FIGURE 23. Dependence of epe chromatograms on acid concentrations for Pd(II)-TOPO ion pair at 10-3 M Pd(II), 0.50 M TOPO, 0.60 M chloride and flow rate of 4.00 mLimin.
Ut \0
60
equilibrium constant Kip,l for the species according to equation 24 is given by
LogKip,l = LogDpd - Log[TOPO] - Log[H+] + Log[CI-] + loga4 (25)
The Kip,l value determined from the variation of Cl- and H+ was 18,25±0.42 M-1•
o Platinum(lV) and Iridium(IV)
The PGM in the +4 oxidation state such as Pt(IV) and Ir(IV) can also be extracted
as an ion pair by protonated TOPO. We have demonstrated here that TOPO.H+ can be
used to separate platinum group metals by employing HCl concentrations L 0.10 M.
High acid concentration such as the one employed in single stage extraction are
unsuitable for CPC experiments due to the corrosion problem. An example of such a
separation is shown in Figure 21, for Pd(II) and Pt(II).
The extraction of Pt(IV) (Figure 24-25) and Ir(IV) (Figure 26-27) proceed
exclusively by the formation of MCI6(TOPO.H)2 ion pairs, Table 6 and 7. These are
evident from slopes of two for log D vs log [TOPO] (Pt(IV) = 2.35 ±0.05 and Ir(IV)
= 2.09±0.20) and log [HCI] (Pt(IV) = 1.92±0.1O and Ir(lV) = 2.01 ±0.12 ) and
slopes of zero for log D vs log [Cn from both Pt(lV) and Ir(IV) system. The extraction
equilibrium and the corresponding equilibrium constant can be written as in equations 26
and 27.
MCI~- + 2TOPO + 2H+ .... MCI6(TOPO'H)2
where M = Pt(IV) and Ir(IV)
LogKip,2 = LogDM - 2Log[TOPO] - 2Log[H +]
The Kip,2 values were Pt(lV) = 1576±146 M-t and Ir(lV) = 8035±700 M-t.
(26)
(27)
TABLE 6. Equilibrium data for the extraction of Pt(lV) by protonated Tapa.
A. Dependence on [Tapa]; Pt(IV) = 10-4 M, [HCI] = 0.1 M, [NaCl] = 0.5 M
[Tapa], M Log D (cpe) Log Kir.2
0.20
0.30
0.40
0.50
-0.22
0.18
0.50
0.71
3.18
3.22
3.30
3.31
B. Dependence on [HCI]; Pt(IV) = 10-4 M, [Tapa] = 0.2 M, [NaCI] = 0.5 M.
[HCI], M Log D (CPC) Log KiP•2
0.10
0.15
0.20
0.25
-0.22
0.07
0.32
0.54
3.18
3.12
3.12
3.15
C: Dependence on [Cn; Pt(II) = 10-4 M, [Tapa] = 0.5 M, [HCI] = 0.03 M
[Cn, M Log D (CPC)
0.23
0.33
0.43
0.53
0.01
0.00
-0.01
-0.01
61
TABLE 7. Equilibrium data for the extraction of Ir(IV) by protonated Tapa.
A. Dependence on [Tapa]; Ir(IV) = 10.2 M, [HCI] = 0.04 M, [NaCl] = 0.5 M
[Tapa], M Log D (CPC) Log Kjp,2
0.20
0.30
0.40
0.50
-0.30
-0.05
0.28
0.52
3.89
3.80
3.88
3.92
B. Dependence on [HCI]; Ir(IV) = 10.2 M, [Tapa] = 0.5 M, [NaCI] = 0.5 M.
[HCI], M Log D (CPC) Log Kjp,2
0.01
0.02
0.03
0.04
-0.70
0.00
0.27
0.53
3.90
4.00
3.92
3.93
C: Dependence on [CI']; Ir(IV) = 10.2 M, [Tapa] = 0.5 M, [HCI] = 0.03 M
[Cn, M Log D (CPC)
0.23
0.33
0.43
0.53
0.25
0.24
0.25
0.25
62
0.40
UJ U 0.30
~ § 0
~\ I \ / ~ ~ 0.201
fJ~ (::0
0.10
0-1' > > > '
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 24. Dependence of CPC chromatograms on TOPO concentrations for Pt(IV)-TOPO ion pair at 10-4 M Pt(IV), 0.10 M HCI, 0.50 M NaCI and flow rate of 4.00 mLimin.
0\ W
0.40
~ AL=JOd \
70.25 "'= Ul ~ 0.30
~ 0
/ ~ tI) 0.20
/1\ I \ ~ ~
~ 0.10
o -r 7 ' / 7
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 25. Dependence of CPC chromatograms on acid concentrations for Pt(lV)-TOPO ion pair at 10-4 M Pt(lV), 0.20 M TOPO, 0.50 M chloride and flow rate of 4.00 mLimin.
~
0.20
[.IJ
U 0.15
~ ~ o CI) 0.10
~
0.05
... " 0.50
\" / 0.20
~ O~
oq; ~
o T 7 7 7 o 100 200 300
RETENTION VOLUME (mL)
FIGURE 26. Dependence of CPC chromatograms on TOPO concentrations for Ir(IV)-TOPO ion pair at 10-2 M Ir(IV), 0.04 M HCI, 0.50 M NaCI and flow rate of 4.00 mLimin.
~
0.20
~ dt==i ' ""
7 7 4.00
UJ U 0.15
~
~ 00101 ~nrL 73000~ ~,
~ 0.05-1 / I I /
~(J
o Y 7 7 7
o 100 200 300
RETENTION VOLUME (mL)
FIGURE 27. Dependence of CPC chromatograms on acid concentrations for Ir(lV)-TOPO ion pair at 10.2 M Ir(lV) , 0.50 M TOPO, 0.50 M chloride and flow rate of 4.00 mLlmin.
0'1 0'1
67
The variation of Kip,2 values between the different metals is difficult to interpret.
One may expect that the Kip,2 values for Pd(I!) and Pt(II) will be about the same and
smaller than those for Pt(IV) and Ir(IV) based on the size of these anions. It is interesting
that Ir(lV) has the largest Kip,2 value. A separate set of studies on the separation of PGM
by ion pair formation with a quaternary ammonium cation indicates similar Kip,2 value
for the Pt(lI) and Pd(II) with the values for Pt(IV) and Ir(IV) being larger (19). Even in
this case Ir(IV) has the largest ~p,2 value, being bigger by an order of magnitude than
the value for Pt(IV) and by two orders of magnitude than the values for Pd(II) and Pt(II).
Thus the difference in the values of ~p,2 for Pd(II) and Pt(II) with TOPO.H"~ is
interesting but the reasons for this are not clear.
68
B. Chromatographic Separation of PGM
1. Separation of Pd from Pt, Ir and Rh by TOPO.
CPC experiments were conducted at 0.50 M TOPO, 0.10 M NaCI and 10-3 M HCI.
The distribution ratio of Pt as a complex is very small under these conditions, the Pt peak
in the chromatogram always appeared at a position close to the dead volume peak. The
separation of Pd from Pt in Figure 28 shows base line separation of Pt and Pd with a
resolution of 1.54 or < 0.3 % peak overlap. The retention volumes (R) of Pt and Pd
are 110 mL and 135 mL respectively. The equation for countercurrent distribution
together with the D values of Pt and Pd from the experiment in Figure 28 (DPt =0.11,
Dpd=1.4 and N=360) can be used to simulate the chromatogram.
A computer-simulated chromatogram (written in QPRO 3.0 using equation 3) shows
the retention volume of both Pt and Pd corresponding to the experiment, 111 mL and 134
mL respectively (Figure 29). The experimental chromatogram, Figure 30, shows the
base line separation of Pd from the other platinum group metals (Pt, Ir, and Rh). It may
be seen that partial resolution of Pt(II) from Ir(III) and Rh(III) was obtained under these
conditions. The complete separation of the four platinum group metals was not possible,
however, using TOPO.
Q04'I----------------------------------------------~
a b
UJ
0.03
1 ( III c U
~ ~ 0.02 0 rn
~ 0.01
o I I I I I I I I I I 90 100 110 120 130 140 150 160 170 180
RETENTION VOLUME (mL)
FIGURE 28. Chromatogram of the separation of Pd(II) from Pt(II) at 0.10 M [Cn, 0.50 M [TOPO], 10-3 M HCl and flow rate of 0.54 mLimin., a) dead volume peak, b) 10-3 M Pt and c) 4XIO-3 M Pd.
0'1 \0
0.08~ a
A -.. ::! 0.06
11 b co:!
I ---z 0 E=:
~ 0.04
U.l u Z o 0.02 U
0 1 I \. d ?- I Iii i
100 110 120 130 140 150 160 RETENTION VOLUME (mL)
FIGURE 29. The computer simulated chromatogram of the separation of Pd(II) and Pt(II) at N = 360. a) DPt = 0.11 and b) Dpd = 1.40.
c3
0.09
I a 0.08
0007
1 ~ 0.06
~ I b c
~ 0.05
~ o 0.04 U'1
~ 0.03
0.02
0.01
0 0 50 100 150 200 250 300 350
RETENTION VOLUME (mL)
FIGURE 30. The chromatogram of the separation of Pd(II) from Pt(Il), Ir(lII) and Rh(III) at 0.08 M [Cn, 0.5 M [TOPO], 10-3 M HCl, and flow rate of 0.87 mLlmin., a) 5XI04 M Ir(III) and Rh(III), b) 104 M Pt(II) and c) 10-3 Pd(II).
-l -
72
2. Separation of Pd, Pt, Ir and Rh by Protonated TOPO.
The Kip,2 values for Pt(II) (1961 M-2) yields higher D values compared to the Pd(II)
species, PdCI3- and PdCll- CKip,l = 18.25+0.42 M-l and Kip,2 = 93.3 M-2 respectively)
and hence is eluted after Pd(II) in Figure 31. The separation of Ir and Rh cannot be
achieved in their +3 oxidation states but the oxidation of Ir to the +4 oxidation state
enables the base line separation of these two metals through ion pair extraction of Ir(IV)
as in Figure 32. In Figure 31 (0.1 M HCl and 0.5 M NaCl), the base line separations
of Pt from Pd and Pd from Ir and Rh were obtained with the resolutions of 1.05 and
1.26 respectively. Under this condition of CPC operation, Pd(II) is extracted as a
combination of two ion pair species, PdCI3(TOPO.H) and PdCliTOPO.Hb as discussed
in the previous section (Table 5)_
The conditions of the chromatogram in Figure 21 are such that Pd(II) is extracted
mainly as PdCI3(TOPO.H) and to a minor extent as PdCI2(TOPOh and PdCI4(TOPO.H)2,
while PtCll-is only extracted as PdC4(TOPO.H)2' Interestingly, the ion pair peak is very
symmetrical similar to the 3-picoline peak rather than co-ordination peak as in Figure 33.
This indicates that the rate of back extraction of ion pairs is faster than that of the
complexes. This will be discussed later in the kinetic section.
0.07""T'"1-------------------------.
a 0.06
~ 0.05
~ b 0.04
o CJ)
~ 0.03
0.02
I .k .• ~ I 0.01 I I I I I I
o 50 100 150 200 250 300 350 400 RETENTION VOLUME (mL)
FIGURE 31. The chromatogram of the ion pair separation of Pt(I1) and Pd(I1) at 0.50 M TOPO, 0.10 M HCI, 0.50 M NaCI and flow rate of 4.00 mUmin. a) dead volume peak, b) 10-3 M Pd(II) and c) 10-4 M Pt(I1).
'I W
0.3
fj ~ 0.2
~ o U')
~ 0.1
00
b
a
50 100 150 200 250 300 RETENTION VOLUME (mL)
FIGURE 32. The chromatogram of the ion pair separation of Ir(IV} and Rh (III) at 0.50 M TOPO, 0.02 M HCl, 0.50 M NaCl and flow rate of 4.00 mUmin. a} 10-3 M Ir(IV} and b) 10-3 M Rh(III}.
~
75
3. Separation of Pd(II) , Pt(II) from Ir(III) and Rh(III) by Stepwise Chloride
Gradient Technique.
The separation of Pd(II) from Pt(II), Ir(III) and Rh(III) using CPC was investigated
at 0.1 M [Cn (17). As may be seen from Figure 28, the efficiency of Pd(II) separation
is rather low. This efficiency can be improved with either increasing [Cn or decreasing
[TOPO]. It is much more convenient and practical to vary [Cn in the mobile phase in
order to obtain better efficiency.
As mentioned in the experimental section, a stepwise chloride gradient (lO-3, 0.08
and 0.50 M) was applied to separate these PGM. The base line separation of Pd(U) and
Pt(II) from Ir(In) and Rh(Ill) was achieved with a very high efficiency as shown in
Figure 33. This is very useful for practical separations because it can reduce both the
volume of sample eluted from the system and the operation time as well.
The separation of PGM by gradient elution can also be carried out under ion pair
extraction as well. Figure 34 shows the separation of PGM where Pt(II) elutes after
Pd(II) and Ir(Ill) and Rh(III) are washed out at the dead volume.
n14
c n12
0.1 b ~ U ~noo ~ @ n~ a
~ nM
nm I-----~, / U l 0
0 50 100 150 200 250 300 RETENTION VOLUME (mL)
FIGURE 33. The CPC separation of Pd(II) , Pt(II) from Ir(III) and Rh(lII) by stepwise chloride gradient technique at 0.50 M TOPO, 10-3 M HCl and flow rate of 2.00 mLlmin. Peaks elute at a) 10-3 M Ir(III) and 10-3 M Rh(III) (10-3 M HCl), b) 10-4 M Pt(II) (0.08 M NaCI) and c) 10-4 M Pd(ll) (0.50 M NaCl).
-J 0\
0.12..,...·--------------------.,
0.1
~ 0.08 U
~ § 0.06 o CI)
~ 0.04
0.02
o 50
b
a c
100 150 200 250 300
RETENTION VOLUME (mL)
FIGURE 34. The CPC separation of Pt(II), Pd(II) from Ir(III) and Rh(III) by proto~ and chloride gradients at 0.50 M TOPO and flow rate of 4.00 mUmin. Peaks elute at a) 10-3 M Ir(III) and 10-3 M Rh(III) (0.10 M HCI), b) 10-3 M Pd(II) (0.10 M HCI and 0.80 M NaCl) and c) lQ4 M Pt(II) (10-3 M HCI and 0.80 M NaCI).
-....l -....l
78
C. Kinetic Studies
1. Preliminary Indication of Role of Chemical Kinetics in CPC Efficiency.
The extraction of Pd(II) by TOPO using batch solvent extraction and CPC has been
completely characterized in terms of the nature of the Pd(II) species extracted and the
dependence of the distribution ratio of this species on the concentrations of TOPO and
Cl- using batch solvent extraction and CPC (17). It was shown that Pd(II) was extracted
as the PdCliTOPO)2 complex and that the distribution ratio increased with [TOPO] and
decreased with [Cn.
Channel equivalent of a theoretical plate (CETP), which is the equivalent of
reduced plate height, is defined to characterize the separation efficiency of CPC. The
dependence of CETP on flow rates for Pd(II) and 3-picoline were compared in the
heptane:water system. The peak widths for palladium and 3-picoline at the same retention
volume and flow rate of 2.0 mL/min are compared in Figure 35, which shows a
significantly lower efficiency for Pd(II) compared to 3-picoline. It is evident from Figure
36 that the CETP for Pd is higher than that for 3-picoline and that it exhibited a greater
sensitivity to flow rate. The Pd(II) and 3-picoline separations became less efficient
(higher CETP) with increasing flow rate, analogous to conventional liquid
chromatography.
Chromatographic efficiency is typically limited by vanous types of diffusion
processes which are operating for both 3-picoline and Pd(II). Simple diffusion alone can
not account for the significantly lower efficiency for Pd(II), however, even when the
effect of TOPO on increasing the viscosity, 7}, of heptane solution is taken into account.
0.5
0.4
~ U 0.3 Z -< CO 0:: 0 U)
CO -<
0.2
0.1
o o 50
I
fI I
(I,
c a
V b
I I I
100 150 200 250 300 350 400 RETENTION VOLUME (mL)"
FIGURE 35. The comparison of the chromatograms at identical retention volumes (132 mL) of Pd(I1) and 3-picoline. a) Pd(II) (0.30 M TOPO, 0.1 M NaCI and 10.3 M HCI), b) 3-picoline (PH = 6.10) and c) the dead volume peak.
-...J \0
25~1-----------------------------------------------------'
20
15 a
~ U
D
10 D
D
5 b
O~I----~.-----.------.-----.----~------r-----'------.----~
o 0.5 1 1.5 2 2.5 3 3.5 4 4.5 FLOW RATE (mL/min)
FIGURE 36. The Van Deemter type plot for a) Pd(I1)-TOPO system (0.10 M TOPO, 10-2 M NaCI and 10-3 M HCI) and b) 10-3 M 3-picoline in succinic acid buffer of pH = 6.10.
00 o
81
The CETP values of 3-picoline at the viscosities of Tapa solutions in heptane were
calculated from the CETP value in pure heptane (Table 8) assuming a linear relationship
between CETP and TJO.5, given the narrow range of viscosities, as indicated by the Knox
equation (21). As seen from Tables 8 and 9, the viscosity of Tapa solution in heptane
and the CETP values for 3-picoline and Pd(II) increase with the concentration of Tapa.
The CETP values for Pd remain distinctly higher indicating that viscosity alone does not
account for the increase in CETP for Pd with increasing Tapa concentration. It is also
evident from this table that the CETP values for Pd(II) are dependent on its distribution
ratio. The CETP values are larger at higher distribution ratios indicating poorer column
efficiencies at higher D values. The additional peak broadening in the case of Pd(II)
compared to 3-picoline under identical conditions and at the higher distribution ratios
of Pd(II) must arise from chemical kinetic factors. Approximately 50% of the 3-picoline
molecules are protonated at pH = 6.1 (pKa = 6) at which its CPC chromatograms were
obtained. The deprotonation reaction will not cause CPC band broadening in the case of
3-picoline as it is very rapid and indeed is termed diffusion controlled. In the manner
of the Van Deemter and Knox equations in which the reduced plate height is seen as a
sum of contributions from various factors we can express the observed CETP for Pd
(CETP ob.) as a sum of contributions from diffusion (CETPdif) and chemical kinetics
(CETPcJ, equation 28.
CETP obs = CETP dif + CETP CK (28)
We can isolate the contribution due to the chemical kinetics to the observed CETP
for Pd(II) by subtracting the CETP values for 3-picoline at the appropriate TOPO
82
TABLE 8. CETP values of 3-picoline at the viscosities of TOPO solutions in heptane at a flow rate of 2.00 mLlmin and the distribution ratio of 0.90.
[TOPO],M 17,cP CEPTA
0.00 0.386 3.2
0.20 0.485 3.6
0.30 0.556 3.9
0.40 0.643 4.2
0.50 0.738 4.4
a Calculated values except in pure heptane.
83
TABLE 9. CETP and D values for Pd(II) at different Tapa and chloride concentrations at low acid concentration (2.00 mL/min).
A. Dependence on [Tapa]; [NaCI] = 0.10 M, [HCI] = 10-3 M
[TOPO],M CETP D
0.20 10 0.4 2.2
0.30 17 0.9 3.3
0.40 25 1.4 4.4
0.50 33 2.8 5.5
B. Dependence on [CI-]; [Tapa] = 0.50 M, [HCI] = 10-3 M
[Cl"],M CETP D tl12 ,sB
0.05 68 8.3 11.0
0.10 33 2.8 5.5
0.20 13 0.6 2.7
0.30 11 0.4 1.1
a Calculated from equation 6.
84
concentrations. We will show that the CETPCK is related to the half-life for the kinetics
of back extraction of the PdCI2(TOPO)2 complex and the D value of Pd(II).
A further examination of the involvement of chemical kinetic factors in the CPC
efficiency for Pd(II) was obtained by studying the dependence of CETP on the
concentration of chloride in the aqueous phase and the concentration of TOPO in the
heptane stationary phase. It is evident from equation 29 that the distribution ratio of
Pd(II) should depend on the chloride and TOPO concentrations, with D being directly
proportional to [TOPO]2 and inversely proportional to [Cn2. It is not obvious, however,
that CETP should depend on these concentrations as well. It was found that increasing
[Cn and decreasing [TOPO] provided better CPC efficiencies for Pd(II) (Figures 37 and
38). As may be seen, CETP is directly proportional to [TOP 0] and inversely
proportional to [Cn, Table 9. These observations are consistent with the hypothesis that
the kinetics of the back extraction reaction, equation 29, is the major factor in affecting
the CPC efficiency for Pd(II).
PdCI2(TOPO)2 + 2Cl- .... PdCI~- + 2TOPO (29)
0.08
Ul U 0.06
~ ga 0 V) 0.04
~ J \ III '---- / ~ 0.15 ~
~ I 1',1 .
0.02
o Y 7 7 7 7
o 100 200 300 400
RETENTION VOLUME (mL)
FIGURE 37. Dependence of CPC chromatograms on chloride concentrations for Pd(Iij-TOPO complex at 10-3 M Pd(lij, 10-3 M HCI, 0.50 M TOPO and flow rate of 2.00 mUmin.
00 Ul
0.08 1\ " 1 \ . I \ / -------- /
0.50
/I 1\ / I \~ I \ / UJ U 0.06
~ ~ JH VV "--- / ~ § o,o4l 7 0
.30 j>"t <::::0
~ \---IV ,L O?O 0.02
oy 7 7 7 7 I
o tOO 200 300 400
RETENTION VOLUME (mL)
FIGURE 38. Dependence of CPC chromatograms on TOPO concentrations for Pd(II)-TOPO complex at 10-3 M Pd(II), 10.3 M HCI, 0.05 M NaCI and flow rate of 2.00 mLimin.
00 0\
87
2. Dissociation of PdC12(TOPO)2 Complex.
Since the CPC results indicated the involvement of slow chemical kinetics, a
systematic study of the kinetics of the process of extraction and back extraction of Pd(II)
seemed in order. We learned from preliminary experiments of extraction and back
extraction of Pd(II) by TOPO in heptane-water system using the microporous teflon phase
separator (MTPS) (21), that these kinetics were too fast to be measured by this system
which can measure half-lives 50 s and longer.
In order to understand the influence of the kinetics of formation and dissociation
of PdCI2(TOPO)2 on CETP, we resorted to stopped-flow analysis and examined the
kinetics in the presence of micelles, which we have demonstrated earlier, provide
excellent models for extraction systems (22). The micellar system containing up to 4%
Triton X-IOO, a neutral surfactant, to solubilize TOPO was chosen to compare the
formation and dissociation rates ofPdCI2(TOPO)2' The formation ofPdC12(TOPO)2 under
a variety of conditions was too fast even for the stopped flow apparatus (limit of tl/2 =
0.4 ms). The dissociation kinetics of PdCI2(TOPO)2 could be followed by stopped flow.
The PdCI2(TOPO)2 complex was formed in the Triton X-lOa system using 10-3 M
Pd(II). The dissociation of this complex under pseudo first order conditions in [Cn, was
monitored as a function of [Triton X-lOa], [Cn and [TOPO]. The observed pseudo first
order rate constant was independent of the Triton X-lOa concentration (Table 10),
indicating that the dissociation of PdCI2(TOPO)2 occurred essentially in the bulk aqueous
phase. This also indicated that no appreciable dissociation occurred in the micellar phase
and that mass transfer of PdCI2(TOPOh between the bulk aqueous and the bulk micellar
88
TABLE 10. Variation of ~s as a function of Triton X-100, chloride and Tapa concentrations.
A. Dependence on [Triton X-100]; [NaCI] =0.1 M, [Tapa] = 10-4 M, [Pd(II)] = 10-3 M.
[Triton X-l00],M
0.011 15.99
0.016 17.05
0.032 17.97
0.064 16.49
B. Dependence on [CI-]; [Triton X-100] =0.016 M, [TOPO] = 10-4 M, [Pd(II)] = 10-3 M.
[CI-], M kobs S·l ,
0.10 17.97
0.20 32.18
0.30 46.59
0.40 61.36
0.50 80.64
0.60 96.54
C. Dependence on [TOPO]; [Triton X-100]=0.016M, [NaCI] =0.5 M, [Pd(II)] = 10-3 M.
[TOPO]XlO4, M
5
10
20
30
75.00
66.72
56_50
47.52
89
phases was rapid.
The value of kobs increased with increasing [Cn at constant [TOP 0] and
decreased with increasing [TOPO] at constant [Cn. These observations can be
rationalized based on the mechanism in equations (30)-(32) for the decomposition of
K PdCI2(TOPO)2 ....
fast PdCI2(TOPO) + TOPO (30)
k_2
PdCI2(TOPO) + Cl- - PdCl3" + TOPO (31)
slow
(32)
The rapid preequilibrium step, equation 30 and the rate limiting step, equation
31, yield the following expression, equation 33, for k,Obs under pseudo first order
condition in [Cl-].
k_2K[Cl-]
K + [TOPO]
Equation 33 can be rewritten as:
[CI-] _ 1 [TOPO] - - + ..::...........-:--....:.
kob• k_2 Kk_2
(33)
(34)
The plot of [CI-]/kob• as a function of [TOPO] yielded a straight line. The k_2 and
K values derived from the intercept and slope of this plot are 168±8 M-ts- t and
0.004±0.0002 M, respectively. Equation 33 indicates two limiting cases, K> > [TOPO]
and K < < [TOPO] (Equations 35 and 36). Equation 35 is encountered in stopped flow
90
experiments at lQ4 M TOPO where koos is linear in [Cn, Table 10.
k_2[Cl-]
k_2K[Cl-]
[TOPO]
(35)
(36)
The half life of the dissociation reaction from equation 36 (t1/2 = In2/k.,b.) is given by
equation 37:
t = In2[TOPO] (37) 1/2 k_
2K[Cl-]
These observed rate constants yield a k..2 value of 158+4 M-Is-I which is in
excellent agreement with the value from equation 34. Equation 37 indicates that tl/2 Q!
[TOPO]/[Cl-]. It is evident from Table 9 and Figures 37 and 38 that CETP has a similar
relationship to [TOPO] and [Cn.
Plotting the difference in the CETP for Pd and 3-picoline to eliminate the
contribution from viscosity changes (equation 28), at constant [Cn as a function of tl/2
yielded a straight line with slope 6.4±0.2 (Table 9A and Figure 39a). A similar plot at
constant [TOPO] yielded a straight line with slope 5.9±0.3 (Table 9B and Figure 39b).
The CETP determined chromatographically, is thus a measure of the half-life for the
Idnetics of back extraction of PdCI2(TOPO)2' It is also evident that the dependence of
CETP on [TOPO] and [Cn is different from the dependence of the 0 value of Pd(II) on
these concentrations (equation 2). Interestingly, we find that CETP is directly
proportional to DO,S, Table 9. Such a relation is not obvious from the basic
chromatographic equations used to calculate the D value from retention volume and the
efficiency from the retention volume and the peak width. The relationship between
70~1---------------------------------------------------'
60
50
~ 40 U
30
20
10 10 2 3 4 5 678 9 10 11
HALF LIFE (s)
FIGURE 39. The correlation between CETPs and half lives at a) 0.20-0.50 M TOPO and b) 0.05-0.30 M NaCI.
\0 -
92
CETP and tIn and CETP and D have become evident through the independent elucidation
of the mechanism of the kinetics of back extraction of PdCI2(TOPO)2 using the stopped
flow technique.
This analysis of chromatographic efficiency in separations involving the
distribution of metal complexes clearly reveals the influence of the kinetics of complex
formation and dissociation on the chromatographic efficiencies and affords a semi
quantitative description of the relevant kinetic parameters.
3. Dissociation of PtCI4(TOPO.Hh Complex.
The CETP of Pt(II) under condition where it is extracted as PtCliTOPO)2
increased with the concentration of TOPO and decreased with the concentration of
chloride, Table 11 and Figures 16 and 17, similar to the case of Pd(II) (20). This, by
analogy to the Pd(II) system, is due to the slow kinetics of dissociation of the
PtCI2(TOPO)2 complex. Thus the pseudo first order rate constant for the dissociation of
PtC12(fOPO)2 is described by equation 36.
An independent verification of this by stopped-flow is hampered by the
interference of Triton X-IOO and TOPO spectra in the UV where PtCI}" and
PtC12(TOPO)2 absorb. We can, however, derive kinetic information on the dissociation
of PtCI2(TOPO)2 from the relationship between CETP and t1/2 that we have established
(20). The t1/2 values for PtCI2(TOPO)2 calculated from this relationship is related to the
concentration of TOPO at constant concentration of chloride, Equation 38 and to the
concentration of chloride at constant concentration of TOPO, Equation 39.
93
TABLE 11. CETP and D values for Pt(II) at different Tapa and chloride concentrations at low acid concentration (2.00 mL/min).
A. Dependence on [Tapa]; [HCI] = 10-3 M, [Pt] = 10-4.
[Tapa], M
0.20
0.30
0040
0.50
D (CPC)
0.35
0.65
104
2.3
CETP
2.5
10.4
19.2
30.0
B. Dependence on [CI-]; [HCI] = 10-3 M, [Tapa] = 0.50 M.
[Cn, M D (CPC) CETP
0.005 1.4 20.8
0.010 1.0 11.9
0.050 0.45 4.8
0.100 0.35 1.7
1.05
2.24
3.57
5.20
tin,s
3.81
2.46
1.55
0.93
94
tll2 ln2 + ln2 . [TOPO] =
k_2[CI-] Kk_2[CI-] (38)
tl/2 = {102 + 102[TOPO] }. _1_ k_2 Kk_2 [CI-]
(39)
The k_2K, k_2 and K values obtained from equations 38 and 39 are 50.24±2.2 S-1
M) respectively, with the values in parenthesis being those for PdCliTOPO)2 (20).
It is evident that the major difference between the Pd(II) and Pt(II) complexes in
their dissociation kinetics lies in the preequilibrium step, which involves the dissociation
of a TOPO molecule (20). This equilibrium constant is about two orders of magnitude
larger for PtCI2(TOPO)2 compared to PdC12(TOPOh indicating that the Pt complex
dissociates more readily than the Pd complex. This is also indicated by the ~x.4 values
for the extraction of PdCIl- and PtCll-. It is evident from the CPC results, that the
condition in equation 36 exists in these experiments. While the experimental conditions
employed provide the mechanistic and kinetic informations on the dissociation of
MCI2(TOPO)2 complexes, we can derive information on the formation reaction as well
using the principle of microscopic reversibility. The mechanism of the formation of
MCI2(TOPO)2 from MCI/ [M = Pt(II), Pd(II)] is as follows:
MCli-K_4
..... MC1; + Cl-fast
Is MC1; + TOPO ..... MCI2(TOPO) + Cl-
slow
Kl MCI2(TOPO) + TOPO ..... MCI2(TOPO)2
fast
This yields kra, the pseudo first order formation constant.
k:bS = IsK_4[TOPO]
K_4 + [Cl-]
95
(40)
(41)
(42)
(43)
krOOa like ~ OOa has two limiting cases depending on the concentration of chloride. At low
[Cn when K-4 > > [Cn
kt" = Is[TOPO] (44)
At high [Cn when [Cn > > K-4
= IsK_4[TOPO] (45)
[Cl-]
The latter condition is encountered in the CPC experiments. When extraction equilibrium
is attained we have
k~b"[MCI2(TOPO)2] = krObs[MCl;-]
Substituting (45) and (33) for krobs and ~ obs we find
IsK_4 [MCI2(TOPO)2][CI-]2 =
k_2K [MCl;-] [TOPO]2
= K K2
ex L KDC
(46)
(47)
where KL and KDC are the distribution constant for TOPO and MCI2(TOPO}z complex
96
between heptane and water and ~x is the extraction equilibrium constant. If we assume
that KL and KDC are about the same, then the only unknown quantity k2 in equation 47
can be calculated. Thus the k2 value, using KL = lOS (24) and K-4 values for PdCI/- and
PtC14-2 from literature (18) is 2.52XI06 M-1s-1 and 3.00X106 M-1s-1 for PdCMTOPO)2 and
PdC12(TOPO)2 respectively. Surprisingly, TOPO complexes PdCI/- and ptCIl with
similar rate constants. Thus, the differences in the ~x values of PdCIl and ptCIl and
the aqueous stability constants for PdCI2(TOPO)2 and PdCI2(TOPO)2 stem from the
dissociation reactions of the respective MCI2(TOPO)2 complexes, with the Pt(lI) complex
dissociating more readily than the Pd(II) complex. Further as mentioned above, the
kinetics of dissociation of these complexes says that the major difference in the
dissociation of these two complexes lies in the fast preequilibrium step involving the
dissociation of TOPO molecule. This dissociation equilibrium constant is about two
orders of magnitude larger for PtCI2(TOPO)2 compared to PdCI2(TOPO)2. The K4 for
ptCIl is about 10 times the value for PdCIl. It is easily seen from these values and
equation 47 that this should lead to ~x.4 for Pt(II) that is 10 times smaller than that for
Pd(II).
97
4. Dissociation of PdCI4(TOPO.H)2, PtCI4(TOPO.Hb IrCI6(TOPO.Hh and
PtCI6(TOPO.H)2 Ion Pairs.
The CETP of the ion pair complex of Pt(II) in CPC showed significant change,
increasing with either TOPO or acid concentration as in Table 12 and Figures 24 and 25.
This again indicates that the back extraction reaction is a major factor affecting the CPC
efficiency. Only the dissociation of PdC4(TOPO.H)2 could be studied by stopped flow.
The spectral change for PtCI4(TOPO.H)2 due to its dissociation could not be followed
spectrophotometrically due to interference from the spectra of TOPO and Triton X-lOa.
The value of kobo was constant when the product of [TOPO] and [HCI] was very
small and decreased with increasing [TOPO] and [H+], Table 13. These observations can
be rationalized based on the mechanism in equations (48)-(53):
PdCI4(TOPO-H)2 ... 2TOPO + 2H+ + PdCI;
K PdCI4(TOPO-H)2 .... TOPO·H+ + PdCI4(TOPO·Hr
fast
k
kK
TOPO·H+ ..... TOPO + H+ fast
kK' K' + [TOPO][H+]
where Kl is the dissociation constant of [TOPO.H+] and K' = KKt •
(48)
(49)
(50)
(51)
(52)
(53)
The Ie value was determined (when the product of [TOPO] and [H+] is negligible
98
TABLE 12. CETP and D values for Pt(II) at different TOPO and chloride concentrations at high acid concentration (4.00 mL/min).
A. Dependence on rrOPO]; [HCI] = 0.10 M, [NaCI] = 0.5 M.
rroPOl, M D (CPC) CETP t1l2 , s
0.20 0.80 3.5 1.20
0.30 2.00 9.6 2.12
0.40 3.84 17.0 3.24
0.50 6.04 25.0 4.45
Slope of log D vs log rrOPO] = 2.21 ±0.03
B. Dependence on [HCI]; [NaCI] = 0.5 M, rrOPO] = 0.20 M.
[HCI], M D (CPC) CETP t1/2 , s
0.10 0.78 3.4 1.18
0.15 1.51 6.7 1.68
0.20 2.60 10.2 2.21
0.25 4.17 14.0 2.78
Slope of log D vs log [HCI] = 1.82±0.08
99
TABLE 13. Variation of JC>"8 as a function of [HCl] and [Tapa] in 0.5% Triton X-100.
A. Dependence on low [HCl]; 0.60 M NaCl, 2 x 10-4 M Tapa and 10-4 M Pd(II).
[HCl] x 1()3, M
2.00 38.00 38.00
5.00 37.55 37.55
10.00 38.02 38.02
20.00 40.23 40.23
B. Dependence on [Tapa]; 0.2 M HCl, 0.5 M NaCI and 10-4 M Pd(II).
[Tapa] x 103,M
1.00 1.47 0.6803
1.50 0.75 1.3333
2.00 0049 2.0408
2.50 0.34 2.9412
C. Dependence on high [HCl]; 0.7 M [Cn, 5 X 10-4 M Tapa and 10-4 M Pd(II).
[HCl], M kobs M-ts- t , lIkobS, S
0.10 12.14 0.0823
0.20 5.25 0.1905
0.30 3.18 0.3145
0040 2.10 004762
0.50 1.23 0.6452
100
compared to K' in equation 53) to be 38.45 M-Is-I (Table 13). The K' values was
determined from the reciprocal of equation 53 under the condition of either constant
[TOPO] or [H+], Table 13B and 13C.
The reciprocal of equation 53 is:
1 _ 1 [TOPO] [H +] - - + ~---,,:-::...-~
kobs k kK' (54)
The average K' from both these determination is 6.45 x 10"6 M2.
The half life of this dissociation reaction from equation 57 (t1/2 = In2/kobS) is given
by the equation 55:
(55)
Equation 55 indicates that the half life for the dissociation of the ion pair
PdCI4(TOPO.Hh is directly proportional to [TOPO] and [HCI] like the CETP values in
the CPC chromatograms. The inefficiency of the CPC chromatograms is thus caused by
the slow dissociation of the ion pair. Hence, we can expect a linear correlation between
CETP and tl/2 for the ion pairs of PdCll, ptCIl, ptCll and IrClt with protonated
TOPO.
We can determine the tl/2 values for the dissociation reactions of the TOPO_H+
ion pairs of PtClt, ptCll and IrCll using the CETP values for these species and the
CETP-tl12 correlation that we have established (19). The CETP values in Tables 12, 14
and 15 thus provide tl12 values as a function of [TOPO] and [H+]. The k and K' value
can be calculated using equation 54 as in Table 16. The overall rate constant for the
dissociation of the ion pairs under the conditions of CPC experiment is kK'. This value
101
is about the same for ptCIl, PtClt and IrClt and about 30 times the value for PdClt.
The rate constant for the rate limiting step, k (equation 50), and the equilibrium constant,
K', however do not exhibit any systematic variation. The K' value indicates that the
chloride assisted dissociation of the ion pair is least favored in the case of PdCI/- and
most favored in the case of IrClt while the subsequent dissociation of the mono TOPO
ion pair species is the exact reverse, i.e., most favored for the PdCl/- and least favored
for IrClt. The reasons for these differences are not clear.
We can use the principle of microscopic reversibility and the ~p,2 values to
determine the rate constant for the rate limiting step of the ion-pair formation reaction.
The mechanism for the ion pair formation reaction is given in 56 and 57.
(56) slow
The relationship between the rate constants for the ion pair formation and dissociation
reactions, and the extraction equilibrium constant (K ip,2), and the distribution ratio of
TOPO.HCI, KL(ip), and the distribution ratio of the extracted ion-pair, PdCI4(TOPO.Hb
KD(ip), is given in the equation 58.
(58)
K can be determined from K' and K\ provided (K' = KK\).
102
TABLE 14. CETP and D values for Pt(IV) at different Tapa and chloride concentrations at high acid concentration (4.00 mL/min).
A. Dependence on [Tapa]; [HCI] = 0.10 M, [NaCI] = 0.5 M.
[Tapa], M D (CPC) CETP t1/2 , s
0.20 0.60 5.5 1.50
0.30 1.51 15.9 3.07
0040 3.20 24.0 4.3
0.50 5.16 35.8 6.07
Slope of log D vs log [Tapa] = 2.35 +0.05
B. Dependence on [HCI]; [NaCI] = 0.5 M, [Tapa] = 0.20 M.
[HCI], M D (CPC) CETP t1/2 , s
0.10 0.60 5.5 1.5
0.15 1.18 10.0 2.18
0.20 2.10 14.9 2.92
0.25 3.5 20.5 3.77
Slope of log D vs log [HCI] = 1.92±0.1O
103
TABLE 15. CETP and D values for Ir(IV) at different TOPO and chloride concentrations at high acid concentration (4.00 mL/min).
A. Dependence on [rOPO]; [HCl] = 0.04 M, [NaCI] = 0.5 M.
[rOPO], M D (CPC) CETP tl/2, s
0.20 0.5 1.6 0.89
0.30 0.9 2.1 1.09
0.40 1.92 4.1 1.31
0.50 3.36 6.0 1.58
Slope of log D vs log [TOPO] = 2.09±0.20
B. Dependence on [HCl]; [NaCl] = 0.5 M, [rOPO] = 0.50 M.
[HCl], M D (CPC) CETP tl/2, S
0.01 0.20 1.46 0.89
0.02 1.00 2.77 1.09
0.03 1.85 4.26 1.31
0.04 3.37 6.00 1.57
Slope of log D vs log [HCI] = 2.01 ±0.12
104
TABLE 16. Extraction equilibrium constants and formation and dissociation rate constants for TOPO ion pairs of Platinum Group Metals (pGM).
Metal Ion kK' x 1Q4 k K' K jp,2 k M-ls-l (,
Pd(II) 2.48 38.45 6.5 x 10-6 93.3 2.3 x 105
Pt(II) 133.0 6.99 1.9x 10-3 1795 4.9x108
Pt(IV) 97.0 10.22 9.5 x 10-4 1528 4.0x108
Ir(IV) 152.0 1.07 1.4 x 10-2 8395 3.4 x 109
105
Our observations and those of Petkovic (25) on the protonation of TOPO indicate
that its KI is in the range of 0.01-0.10. The lower limit of 10-2 is used to estimate the Kr
value. Thus the estimation of Kr is a lower limit and will be higher if the true KI is lower
than the assumed value of 10-2. Thus, the kr value for PdCI4(TOPO.H)2 is about 106 M
IS-I, Table 16. Similarly the kr for the ion pairs of Pt(lI), Pt(lV) and Ir(lV) can be
calculated as well, Table 16. The kr values for the ion pairs of these metals is 2-3 orders
of magnitude larger than that for Pd(II). The kr value for the Ir(IV) ion pair is the
highest, approaching the diffusion controlled value of 1010 M-Is-1•
The above kinetic analysis based on CPC inefficiencies indicates that the
TOPO.H+ ion pair of PdCI/- is formed much more slowly than those of PtCI{, ptCIl
and IrCIl. The PdCI/- ion pair also decomposes much more slowly compared to the
other ion pairs. The net effect of the rate constants for the ion pair formation and
dissociation reactions is that the extraction equilibrium constant for PdCIl- is 1-2 orders
of magnitude smaller than the values for PtCIl, ptCIl and IrCll. The rate constants
for the formation and dissociation of these ion pairs are probably indicative of the fact
that these are not ion pairs by the conventional definition but the protonated TOPO may
be wealdy coordinated to the metal.
106
V. CONCLUSIONS
The objectives outlined in the statement of the problem have been accomplished.
The important results and conclusions of the studies are:
1. An inexpensive and stable extractant, trioctylphosphine oxide (TOPO) has been
identified for the separation of PGM. This compound functions as a ligand in
its neutral form and as a cation in its protonated form for the separations of
PGM as their anionic chloro complexes.
2. TOPO as a ligand can provide base line separation of Pt(II) and Pd(II) from
Ir(III) and Rh(III) with the extracted species being MCI2(TOPOb where M =
Pd(II) and Pt(II).
3. Protonated TOPO can provide base line separations of Pd(II), Pt(II) and Ir(IV)
from Ir(III) and Rh(III). The species extracted are MCL4(TOPO.H)2 where M
= Pd(II), Pt(II), Pt(IV)CI2 and Ir(IV)CI2.
4. The use of a chloride gradient in the mobile phase affords efficient separation
of Pd(II) and Pt(II) from Rh(III) and Ir(III). Similarly a proton gradient is
useful for the efficient separation of Pt(II) and Pd(II) from Rh(III) and Ir(III).
The proton gradient also enables the separation of the oxidation states of Pt,
namely Pt(II) and Pt(IV), the former being extracted as its complex and the
latter as its ion pair. Protonated TOPO can also separate Ir(III) and Ir(IV) with
the latter being extracted as ion pair while the former is not extracted.
5. A striking feature in the chromatograms ofPGM, whether it is due to complex
formation or ion pair formation with TOPO, is their poor efficiency. These
107
efficiencies are much poorer (at identical D values) compared to the efficiency
for an organic analyte like 3-picoline, whose efficiency is limited only by mass
transport and diffusion. Thus, the lower efficiencies of PGM are a result from
factors in addition to mass transport and diffusion, and is probably due to slow
chemical ldnetics.
6. The hypothesis that slow chemical kinetics limits CPC efficiencies has been
tested by studying the variation in the CPC efficiencies of PGM as function of
the concentrations of the components in the aqueous and organic phases. These
studies clearly indicated dependence of CETP on the concentration of species
like chloride and acid in the aqueous phase and TOPO in the organic phase.
It could be inferred from these studies that the slow kinetics of back extraction
(dissociation) of the MCI2(TOPOh complexes and the MCI4(TOPO.Hh ion
pairs were responsible for the poor efficiencies. The results for the ion pairs
are interesting because ion pair formation and dissociation reaction are
generally in the diffusion controlled regime. It is possible that MCliTOPO.H)2
are not true ion pairs.
7. The fact that the back extraction of MCI2(TOPO)2 and MCI4(TOPO.H)2 are
indeed slow was independently confirmed by stopped flow kinetic experiments
in micelles. These studies indicated that chemical reactions with half lives as
short as 1 ms can adversely affect the CPC efficiencies.
8. A linear correlation was found to exist between the CPC CETP' s and the t1/2' s
measured by stopped flow. The mechanisms of the slow kinetic step can be
108
derived from the dependence of CETP on chemical parameters and the t1/2 for
this reaction can be obtained from this plot. This correlation between CETP
and tll2 is a very significant finding which makes CPC an invaluable tool not
only for the separation of metals but also for the examination of the kinetics
of their metal complex and ion pair formation and dissociation reactions.
9. All systems examined indicated that back extraction kinetics was slow which
adversely affected their CPC efficiencies. While the CPC experiments yielded
the back extraction mechanism and rate constants directly, the mechanism of
the complex and ion pair formation reactions could be derived using the
principle of microscopic reversibility and the ~x and Kip values. The CPC
separations achieved here have also been useful to completely characterize the
extraction equilibria and the extraction and back extraction kinetics. This is a
significant development in liquid chromatography where such complete
characterizations are very difficult to accomplish.
109
APPENDIX A
Derivation of The Kinetic Expressions for The Dissociation and The Formation of PdCI2(TOPO)2·
A. Complex Dissociation
K PdCI2(TOPO)2 ~ PdC12(TOPO) + TOPO
fast
PdC12(TOPO) + CI- - PdCI; + TOPO
Rate =
slow
PdCI3- + CI- - PdC142-
fast
d[PdCI;-]
dt
(1)
(2)
(3)
(4)
[pdCI2(TOPO)2]t = [PdCI2(TOPO)2]r + [PdCI2(TOPO)]r (5)
K = [pdC12(TOPO)] [TOPO] (6) [PdC12(TOPO)2]
[PdCI (TOPO)] = [pdCI2(TOPO)] [TOPO] (7) 2 2 K
[pdCI (TOPO) ] = [pdCI2(TOPO)] [TOPO] + [PdCl (TOPO)] (8) 2 2t K ~
[PdCl,(fOPO),l, = [PdCl,(TOPO)l{ K • ~OP01} (9)
[pdCI (TOPO)] = K[PdCI2(TOPO)2]t (10) 2 K + [TOPO]
d[PdCI;] k_2K[pdCI2(TOPO)2]t[CI-] =
dt K + [TOPO]
k_2K[Cl-]
K + [TOPO]
(11)
(12)
110
B. Complex Formation
PdCli- K_4 PdCI; + CI- (13) .....
k_2 (14) PdCI; + TOPO - PdCliTOPO) + Cl-
slow
PdCliTOPO) + TOPO .....
fast PdC~(TOPO)2 (15)
Rate -d[PdCl;]
Is[PdCI;][TOPO] (16) = = dt
[PdCli-]t = [PdCI;] f + [pdClt] f (17)
= [PdCI;] f + [PdCI;][CI-] (18) K_4
[PdCl;] { 1 + [i~:]} (19) =
-d[PdCl;] { k,K ... [fOPO] } [PdCli-] (20) =
dt K_4 + [CI-]
k;ms = IsK-4[TOPO]
(21) K-4 + [Cl-]
ifK4 < < [Cn
lobs IsK_4[TOPO] (22) (f = [CI-]
111
APPENDIX B
Derivation of The Kjnetic Expressions for The Dissociation and The Formation of PdCliTOPO.H)2·
A. Ion Pair Dissociation
PdCI4(TOPO·H)2 ~ 2TOPO + 2H+ + PdCI;-
PdCI4(TOPO·H)2 + CI-
Rate = dt
K ~ TOPO·HCI + PdCI4(TOPO·Hr
fast k
(1)
(2)
(3)
(4)
[TOPO·HCI][PdCI4(TOPO·Hr]r (6) = + [PdCI4(TOPO·Hrl r K[CI-]
= {[TOPO.HCI] + I} [PdCI (TOPO·Hr] (7) K[CI-] 4 r
[PdCI4(TOPO.Hf]r = { K[CI-] } [pdCI
4(TOPO·H)2]t (8)
K [CI-] + [TOPO·HCI]
= { kK[CI-] } [pdCI (TOPO.H) ] K[CI-] + [TOPO.HCI] 4 2 t
(9)
TOPO·HCI Kl TOPO + H+ + CI- (10) ~
substituting [TOPO.HCI] = [TOPO][H+][Cn/KI in equation 9:
d[PdCl;-] = { kKK1[CI-] } PdCI OPO.H) ] dt KK1[CI-] + [TOPO][H+][Cl-] [4(T 2 t
(11)
if K' = KKI> equation II can be rewritten as following:
d[PdCI;-] = { kK' } [PdCI (TOPO oR). ]
dt K' + [TOPO][H+] 4 2 t (12)
kK'
B. Ion Pair Formation
- PdCI/fOPO·Hr slow
PdC14(TOPO·Hf + TOPO·H+ f:t PdCI4(TOPO·H)2
-d[PdC]2-] dt 4 = kf[PdC]~-][TOPO·H+]
substituting [TOPO.H+] = [TOPO][H+]/K1
krebs = krfTOPO][H+]
KI
112
(13)
(14)
(15)
(16)
(17)
113
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