use of fixion 50x8 cation exchange plates for heterogeneous isotope exchange in radiochemical...

Journal of Radioanalytical Chemistry Vol. 51, No. 2 H979) 197-203

R a d i o a n a l y t i c a l M e t h o d s

U S E O F F I X I O N 5 0 X 8 C A T I O N E X C H A N G E P L A T E S

F O R H E T E R O G E N E O U S I S O T O P E E X C H A N G E

I N R A D I O C H E M I C A L A N A L Y S I S

G. N. BILIMOVICH, V. V. ATRASHKEVICH, N. N. NEKRASOVA

E L Vernadsky Institute of Geochemistry and Analytical Chemistry, USSR Academy of Sciences, Moscow (USSR)

(Received January 3, 1979)

A method for the simultaneous isolation of a number of elements (In, Y, Zn, Co, Cd) from their mixture has been developed. The method is based on heterogeneous isotope exchange between the radionuclide of the element to be determined in the solution and a cation exchange plate saturated with the inactive form of the same element. The investiga- tions were carried out with "Fixion 50X8" plates (Hungary) measuring 1 cm 2 . 5 -6 ml of the test solution was used. In order to properly choose the optimum saturation conditions of the plates with the element to be determined the kinetics of sorption and isotope exchange and the pH were studied. It can be excepted that in the future this procedure will be widely used in the radioanalytical isolation and determination of many elements by isotope exchange and activation analysis. The automation of this radioanalytical procedure for serial analysis will become possible owing to the simplicity and elegance of the experimental technique.

Introduction

The isotopic ecxhange method is based on chemical reactions in which a

labelled and a nonlabelled compound o f the element to be determined exchange

stable and radioactive atoms of this element. For analytical purposes most con-

yenient are the reactions o f isotopic exchange occurring in a heterogeneous sys-

tem 1-3 as in this case the problems o f phase separation and sampling are very sim-

ply solved. The principle of the isotopic exchange method in application to micro-

amount determinations has much in common with the principle o f substoichio-

merry. *'~ Namely, in one phase there is an unknown amount of the element und-

er determinat ion labelled by its radioactive isotope and in the other phase - a pre-

eiLqy known amount of the non-radioactive form of the element under determina-

tion. After some strictly fixed t ime o f phase contact, isotopic exchange comes to

Jm oquih~orium and a redistribution o f the radioactive form of the element under

I. lCadimml. Chem. M (1979} 197

G. N. BILIMOVICH et al.: USE OF FIXION 50X8

determination occurs in accordance with the amount of carrier in both phases. As the amount of the element being determined is known in one of the phases, the share of isotope exchange for this phase is the analytical signal of the unknown concentration of this element in the other phase. As a phase containing a fixed amount of the element under determination, we propose to use in the given method "Fixion 50X8" plates made in Hungary.

The method

The cation exchange plate "Fixion 50X8" represents a very thin layer of the Dowex 50X8 cationite evenfly attached to a polymeric substrate. The substrate possesses a high resistance to the action of various acids and organic sol- vents. The cationite layer is so uniform that even very small strips of "Fixion 50X8", being equal in area (~ 1 cm2), contain almost ideally equal amounts of resin on the substrate. The resin is sufficiently firmly retained on the substrate even when dipped into an aqueous solution under intensive stirring of the latter. This simplifies strongly the experiments with plates, as no phase separation and sampling operations are needed. After isotopic exchange has been carded out, the plate itself is directly a target for radioactivity measurement. Owing to the simpli. city of the method, it is possible to carry out large series of experiments under absolutely identical conditions, which influences positively the reproducibility of results and permits to automate the process of analysis. The above qualities of the "Fixion 50X8" plates have in many respects determined the choice of the given system for carrying out isotopic exchange reactions.

The method of determination consists in this case in the following. The "Fixion 50X8" plate with a very small cationite amount (in our experiments 10 mg) is placed into a concentrated solution of the element to be determined for conver- sion of the cationite to the non-radioactive form of this element. Then the plate is placed into the solution under analysis, containing an unknown amount of the target element labelled by its radioactive isotope. After isotopic exchange has been fmished, the plate is withdrawn from the solution, its radioactivity measured and the degree of ion exchange calculated; this serves as the concentration of the element under determination in the solution

Results and discussion

In order to correctly choose the conditions for the experiments, we have studied, on the example of I, Y, Zn, Cd and Co, the influence of pH upon the sorption of elements from the solution on the plate, as well as the kinetics of sorption and isotope exchange of these elements.

198 J. Radioanal. Chem. 51 (1979)


~ 0 . 6

0.4

O~ I I I 1 I It 3 4 S w 0 1 2 3 4 5 w

pN

�9 Cd

0 1 2 3 4 5 - - pH pH pH

Fig. 1. Sorptton curves o f the elements studied on a plate, depending on the pH of the solution. A e - radioaetiviW on the plate; Ao - total radioactivity in the system

The influence of pH

To study the influence of pH upon the sorption of the indicated elements, the plates were converted to the H-form, were saturated in solutions of various HC1 concentrations and then placed into solutions of pH equal to the pH of saturation of the plates and containing equal (10-3M) amounts of the element being studied. The influence of pH upon the sorption of the elements chosen was studied using a contact time of 2 hours between plates and solutions. As it will be shown below, this time suffices for reaching equilibrium of the sorption process.

The percent sorption by the plate, depending on the pH of solution, is given in Fig. 1. As is seen from the figure, yttrium sorption depends weakly on the pH in the pH interval used. The sorption of the other elements strongly increases in the pH interval from 1 to 2 and does not depend or only very slightly depends

on the pH at pH ~ 2.5. From Fig. 1 it is seen that the maximum sorption of the elements studied is attained in the region of pH = 3. This value was later on ad- mitted as the optimal one.

Kinetics of sorption and isotope exchange

The kinetics of sorption and isotope exchange were studied by us at the optimal pH equal to 3. In order to have the possibility to consider the isotope exchange reaction in its pure form (not connected with the sorption or desorption of the element studied), kinetic curves of sorption were taken in order to establish the time necessary for attaining sorption equilibrium. The concentration of the element studied was 10-3M.

In order to obtain kinetic curves of isotope exchange, two parallel series of solutions with the same concentrations of the cation studied were taken. In one of the series the solutions cor~tained the radioactive isotope of the element under

J. Raclioanal. Chem. 51 (1979) 199


2 "~ 0,16

O.t,

s

f ;

In

I I I ~ 50 100 r-~"-

Time7 rr~n

l L A ~

Tirne~ mir~ Time ~rnin

F Co

so lOO 15o~ Time)rain

o2

oy 1 Cd

I I 1|05 SO IO0 Time~rnin

Fig. 2. Kinetic curves of sorption (1) and isotope exchange (2)

determination. In all solutions "Fixion 50X8" plates of equal areas were dipped

and kept there for 2 hours in contact with the solutions under intensive mixing. As is seen from the results of sorption kinetics shown in Fig. 2 (curves 1), this

time is quite sufficient for the sorption reaction to come to equilibrium. After this all plates were taken out from the solutions and those of them which had sorbed the non-radioactive form of the element, were placed into radioactive solutions. The kinetic curves of isotope exchange between the radioactive and non-radioactive forms of all elements on "Fixion 50X8" plates are given in Fig. 2 (curves 2).

All curves in Fig. 2 were drawn by least squares calculations on a computer according to a non-linear program. The analytical form of these dependences has been established by solving the differential equations for sorption and exchange rates for these processes. The mathematical treatment of the results allows to cal- culate the conventional rate constants of sorption and desorption as well as the exchange rate constants for each dement studied. All constants may be considered equal within the experimental error. This points to the identity of sorption-desorption processes for triply and doubly charged cations with processes of isotope and ion exchange. Besides, it was possible to determine (by calculation) the time of equilibration for sorption and isotope exchange. It must be noted that the time of equilibration of the elements studied practically does not depend on the concentration of the element in the solution owing to the intensive stirring of the solutions in all experiments and is of the order of 2 hours. For equilibration of the isotope exchange reaction two hours also suffice, but only if the amount of the cation being exchanged on the plate is at least three times higher than its amount in the solution. In experiments on the determination of the element in the solution according to the share of isotope exchange, this ratio was always greater than 3; therefore, the period of two hours was chosen as optimal for carrying out isotope exchange.

200 1. Radioanal. Chem. 51 (1979]

G. N, BILIMOVICH et el.: USE OF FIX/ON 50X8

t 0

~{" 0.40'6'-~

O.Z In

-Ig Cln

t

- Ig Cv -Ig CZn -Ig Coo -ig Cce

Fig. 3. Dependenco of the share of isotope exchange on the concentration of the analyzed element in the solution

Determination of the element according to the share of isotope exchange

The dependence of the share of isotope exchange on the concentration of the element to be determined is given in Fig. 3. Apparently, the dependence of the share of isotope exchange on the concentration of the element in the solution at concentrations greater than 10-4M has a linear character and is observed for all the elements studied. At a concentration of the element lower than 10-4M, the ratio between the amount of the element being determined on the plate and its amount in the solution strongly increases and at concentrations of the order of 10-SM, it approaches the value of 100. As a result of this, isotope exchange becomes insensitive to slight concentration changes of the analyzed element in the solution with respect to its concentration on the plate. As is seen from Fig. 3, for reproducible results, the element concentrations in the solution should be greater than 10-~M.

In order to use the system proposed in the radiochemical variant of neutron activation analysis, we have studied the influence of interfering elements and the possibility of sorption suppression of their radioactive isotopes in carrying out reactions of isotope exchange of the dement to be determined. The results of these experiments are given in Fig. 4. Each graph in Fig. 4 represents the dependence of the sorption of the interfering element on the plate converted into the exchangeable cation form on the negative logarithm Of the concentration of the latter in the solution. In the upper line of the figure, the cation form on the plate is indicated and in the graphs themselves the name of the interfering element is shown. The concentration of the interfering element is in each of the given systems equal to 10-SM. As is seen from Fig. 4, in systems with equally charged cations the interfering dement is noticeably sorbed by the plate. However, as is seen from the

J.. Radioanal. Chem..51 (1979) 201


In R vn tar znR ~ C~ I / I

l

- - o / I I I ~ o . . . X - - . . l ~ 6 c - - I _ - - L ~ 6 o l l ~ ~ 40 ~ S 4 3 2 ~ ~ 4 3 l ? ( 4 3 Z S 4 ) 2~

0l I ~ l 01 I " ~ e e l ~0 I I_hl ac I I ~ - 5 4 3 2 5 4 3 2 4 3 2 ; 4 3 2 - - k

1 o zo \ S. oH_~_ t ~ ~o,.._~ 5 ~'--~~"

- S 8 50 40

S - c d 4 4

oH _-J, -t9 Cln -19 Cv -t9 CLo -19 CZn -19 CCo

Fig. 4. Suppression of the sorption of interfering elements on a plate converted to the form of the exchangeable cation, from the exchangeable cation solution

corresponding graphs, the sorption has a regular character and this fact may be successfully used for determining the concentration of the interfering element. If the charge of the interfering element is less than the charge of the cation partici- pating in isotope exchange, the sorption of the interfering cation is practically completely suppressed ff the concentration of the cation exchanged in the solution is ~10-3M. This is illustrated also by Fig. 5, where the suppression of sodium sorption from a zinc solution on a Zn-form plate is shown. When the charge of the interfering cation is higher than that of the cation being exchanged, no suppression of the interfering cation occurs and at least this process bears no regular character. It follows for example that In, Y and La may be determined by isotope exchange in the system proposed at least in the presence of both doubly and singly charged interfering cations without preliminary separation of the latter. It is also obvious that the presence of foreign singly charged cations will not impede the determination of doubly charged cations.

202 J. Radioanal. Chem. 51 (1979)


0.06

0.04

0.02

L

4 3 -I(3 Czn

J 2 r

Fig. 5. Suppression of Na sorption from zinc solution on a plate converted to the Zn-form

In conclusion we note that the suggested system does not solve all the difficm. ties connected with the simultaneous determination of elements in multicomponent objects. However, in our opinion, taking into account the possibilities of varying the compositions of solutions with the purpose of masking and eliminating the interfering influence of foreign elements, as well as the simplicity and elegance of the method, it is possible with the aid of this system to effectively solve a certain class of analytical problems.

References

1. M. CSAJKA, Anal. China. Aeta, 68 (1974) 31. 2. 1~. BANYAI, O. GIMESI, A. FARKAS, J. Radioanal. Chem., 16 (1973) 173. 3. M. HEURTEBISE, W. J. ROSS, Anal. Chem., 43 (1971) 1438. 4. I. STARY, K. KRATZER, A. ZEMAN, J. RadioanaL Chem., 5 (1970) 69. 5. G. N. BILIMOVICH, I. STARY, J. RadioanaL Chem., 28 (1975) 69. 6. K. KUDO, N. SUZUKI, J. Radioanal. Chem., 26 (1975) 327.

Z Raclioanal. Chem. 51 (1979) 203

use of fixion 50x8 cation exchange plates for heterogeneous isotope exchange in radiochemical...

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