J . Chem. SOC., Faraday Trans. I , 1985, 81, 687-693

Preferential Solvation of Ions in Mixed Solvents Part 4.-Preferential Solvation of Cu+ in Acetone + Acetonitrile

and N,N-Dimethylacetamide + Acetonitrile Mixtures Using Conductance Measurements

BY DIP SINGH GILL*

Department of Chemistry, Panjab University, Chandigarh- 1600 14, India

AND NEENA KUMARI AND MOHINDER SINGH CHAUHAN

Department of Chemistry, Himachal Pradesh University, Shimla-1 7 1005, India

Received 4th June, 1984

The molar conductances of Bu,NBPh,, Bu4NC10, and CuC10, - 4AN have been measured in the concentration range (0.3-90) x lo-, mol dm-3 in acetone (Ac), acetonitrile (AN) and N,N-dimethylacetamide (DMA) and in Ac + AN and DMA + AN mixtures containing 12.0, 33.2, 58.2, 73.4 and 93.2 mol % AN at 25 "C. The conductance data in all cases have been analysed by the 1957 Fuoss-Onsager conductance equation to obtain A. and K A values for the electrolytes. The A. values have been split into the limiting ion conductances (A:) using a model recently proposed by Gill and Cheema. Good applicability of this model to some organic solvents has also been demonstrated. The present model gives a better splitting than that obtained by use of Gill's equation:

The non-linear variation of the true solvated radii (ri) of Cu+ with solvent composition in the binary mixtures shows the strong preferential solvation of Cu+ by AN in Ac + AN mixtures over the whole solvent composition range, by AN in DMA-rich region and by DMA in the AN-rich region of the DMA + AN mixtures.

The study of the solvation behaviour of Cus in mixed solvents1-* and the utility of such a study in the hydrometallurgical purification'. 3 9 6$ 9-11 of copper have been the subject of much interest during the last few years. However, there are few studies in the literature where copper(1) salts have been extensively investigated in binary mixtures of two dipolar aprotic solvents.' Since solvation studies of Cu+ in mixed solvents have practical utility in discovering new methods for the purification of copper, more detailed investigations using copper(1) salts are needed. In the present work the molar conductances of Bu,NBPh,, Bu,NClO, and CuClO;4AN in acetone + acetonitrile (Ac +AN) and N,N-dimethylacetamide + acetonitrile (DMA +AN) mixtures over the entire solvent composition range have been measured. The results are analysed and show a strong preferential solvation of Cu+ in these solvent mixtures.

EXPERIMENTAL AN (Merck, 99% pure), Ac and DMA (both B.D.H. AnalaR, 99.5% pure) were purified by

the methods already reported.2- 12-14 Ac +AN and DMA +AN mixtures of appropriate composition were prepared by weight and the dielectric constant (e0), viscosity (q,) and

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688 SOLVATION OF IONS IN MIXED SOLVENTS

Table 1. Dielectric constant ( E ~ ) , viscosity (q,) and density ( d ) for Ac + AN and DMA + AN mixtures at 25 "C

Ac + ANa DMA + AN*

solvent mol% AN E, q,/cP d/g cm-3 EO qo/cP d/g cmP3

1 0.0 20.60' 0.304" 0.7851 37.8 0.919 0.9365 2 12.0 22.36 0.316 0.7845 37.7 0.861 0.9261 3 33.2 25.18 0.323 0.7835 37.4 0.729 0.9019 4 58.2 28.76 0.330 0.7820 37.0 0.575 0.8659 5 73.4 31.17 0.335 0.7804 36.7 0.483 0.8407 6 93.2 34.62 0.339 0.7779 36.2 0.381 0.7985 7 100.0 36.00" 0.341' 0.7767' 36.0" 0.341 0.7767"

a E~ and d values for Acf AN mixtures have been taken from F. Mato and F. Fernandez- Polanco, Ann. Quim., 1974, 70, 76; the qo values have been measured in the present study. * e,, qo and d values of DMA +AN mixtures in this table are our measured values. ' Ref. (14).

density (d ) of the DMA+AN solvent mixtures and the viscosity and density of Ac+AN mixtures were measured by methods described ea~1ier.l~ The E,, q, and d values for various Ac +AN and DMA + AN mixtures are reported in table 1 . The electrolytes used were prepared by methods reported previously.**

Conductances were measured at 25.00+0.01 "C at 1000 Hz with a calibrated digital conductivity meter (model NDC-732, Naina Electronics, Chandigarh). Details of the conduc- tance cell and the experimental procedure for making the conductance measurements have been given 1 4 9 l5

An Ubbelohde suspended-bulb level viscometer with a flow time of 780 s for water at 25 "C was used for the viscosity measurements of the pure solvents and the solvent mixtures. The method ofcalibration of the viscometer and the procedure for making the viscosity measurements have already been given." l6

The overall accuracy of the conductance and viscosity measurements was estimated as &0.1%.

RESULTS AND DISCUSSION

The molar conductances of Bu,NBPh,, Bu,NClO, and CuC10, - 4AN have been measured in the concentration range (0.3-90) x lo-, mol dm-3 in Ac, AN and DMA and Ac+AN and DMA+AN mixtures containing 12.0, 33.2, 58.2, 73.4 and 93.2 mol % AN. The conductance data in all cases have been analysed by the 1957 Fuoss-Onsager conductance equation,l7~ l8 the details of which are given in our previous papers.l49 l5 The derived values of A,,, K A and the ion-size parameter (ti), along with their standard deviations and the standard deviations of the individual points oA, are reported in tables 2 and 3. Values of the dielectric constant ( E J and the viscosity (qo) for Ac, AN and DMA and Ac+AN and DMA+AN mixtures for the analysis of conductance data were taken from table 1 . Conductance data (values of A as a function of concentration) for all the systems studied are given in Supplementary Publication no. Sup 56129.*

To obtain an indication of the precision of the present conductance measurements, our A,, values for Bu,NBPh, (120.0), Bu,NClO, (165.4) and CuClO, (167.8) (in cm2 mol-1 Sz-l) in AN given in table 2 can be compared with the corresponding values (1 19.65, 165.06 and 168.38 cm2 mol-l 0-l) reported by Coetzee and

* See Notice to Authors, J. Chem. SOC., Faraday Trans. I, 1985,81(1).

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Tab

le 2

. A,

(cm

2 mol

-' a-

l), K

A (

mol

dm

-3),

H (A

) and

oA

valu

es f

or s

ome

elec

troly

tes i

n A

c + AN

mix

ture

s at

25

"C d

eriv

ed fr

om th

e 19

57 F

uoss

-Ons

ager

con

duct

ance

equ

atio

n

elec

troly

te

Bu,

NB

Ph,

Bu,

NC

10,

CU

C10

, so

lven

t (m

olx

AN

) A

0

KA

ii

cr,

A0

KA

H

a, A0

K

A

H O

A

0.0

130.

0f0.

2 40

f8

4.8k

0.2

0.10

18

2.5f

0.3

80+

8 4.

5k0.

2 0.

24

198.

6f0.

3 50

f8

4.3k

0.3

0.26

u

33.2

12

6.5f

0.1

29f5

4.

7k0.

2 0.

08

173.

2f0.

2 48

f7

4.4f

0.2

0.16

17

7.9f

0.2

33f6

4.

2f0.

2 0.

16

3 58

.2

124.

1f0.

1 27

f4

4.3f

0.1

0.08

17

1.2f

0.1

29+

5 4.

2f0.

1 0.

10

173.

6f0.

3 30

f4

4.5k

0.2

0.21

r

73.4

12

2.1f

0.1

21f5

4.

5k0.

1 0.

07

169.

4f0.

2 29

k6

4.4f

0.1

0.14

17

1.0f

0.2

30f6

4.

3f0.

2 0.

16

z 93

.2

120.

8f0.

2 16

&3

4.6f

0.2

0.10

16

6.6k

0.2

25+

4 4.

0f0.

2 0.

15

168.

7f0.

1 24

55

4.0f

0.1

0.09

7;1

10

0.0

120.

0k0.

2 53

13

4.5f

0.2

0.18

16

5.4f

0.3

10+

4 4.

4f0.

2 0.

22

167.

820.

2 8

+4

4.

5k0.

3 0.

20

e E

12.0

12

8.0f

0.1

34f4

4.

6f0.

2 0.

09

175.

4f0.

2 55

f7

4.4f

0.2

0.17

18

4.1 f

0.2

36f7

4.

2f0.

2 0.

18

[" 5 ?2

1957

Fuo

ss-O

nsag

er c

ondu

ctan

ce e

quat

ion

F U ["

Tab

le 3

. A,

(cm

2 mol

-1 W

l),

KA

(mol

drn

p3),

H (A

) and

0,

valu

es fo

r var

ious

ele

ctro

lyte

s in

DM

A+

AN

mix

ture

s at

25

"C d

eriv

ed fr

om th

e

g el

ectro

lyte

Bu,

NB

Ph,

Bu,

NC

10,

CU

C10

, so

lven

t H

OA

A0

K

A

H C

A

A0

KA

H

t7A

(mo

lx A

N)

A0

KA

0.0

44.3

0k0.

06 -

4.2k

0.1

0.05

65

.80f

0.08

9

f4

3.7f

0.1

0.07

70

.40f

0.08

-

4.0f

0.1

0.08

12

.0

47.7

0f0.

05

-

4.3f

0.1

0.05

68

.50f

0.08

9

+5

3.

8f0.

2 0.

08

71.4

0f0.

08

-

3.8k

0.1

0.08

33

.2

56.4

0f0.

04

-

3.9f

O.l

0.04

79

.40f

0.08

8

f5

3.9k

0.1

0.07

81

.60f

0.08

-

4.2k

0.2

0.07

58

.3

71.2

0f0.

08

-

4.4f

0.2

0.06

99

.70k

O.l

9+

3

3.7f

0.2

0.09

97

.70k

0.1

-

4.0f

0.1

0.10

73

.4

85.3

0f0.

1 -

4.0k

0.2

0.08

11

8.6f

0.1

9+

4

3.8k

0.2

0.10

11

6.3f

0.2

-

4.3f

0.2

0.20

3.

9f0.

2 0.

20

Q\

100.

0 12

0.0f

0.2

5f3

4.

5k0.

2 0.

18

165.

4f0.

3 1

0f4

4.

4f0.

2 0.

22

167.

8k0.

2 8

k4

4.

5f0.

2 0.

20

\b

93.2

10

7.9f

0.1

-

4.3f

0.2

0.10

15

8.0f

0.1

8+4

3.9f

0.2

0.10

15

5.7k

0.2

-

00

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690 SOLVATION OF IONS IN MIXED SOLVENTS

Cunningham19 and Yeager and Kratochvil.20 Similarly, our A, value for Bu,NClO, (182.5 cm2 mol-1 C2-l) in Ac (table 2) is in good agreement with the value of 182.7 cm2 mol-1 R-l reported by Gill et aZ.15

LIMITING ION CONDUCTANCES

In order to obtain a better understanding of ion-solvent interactions and hence of ion solvation, accurate data on the properties of individual ions of the electrolytes are needed because cation-solvent and anion-solvent interactions are different.21 For many properties of electrolytes it is a complicated problem to divide the total parameter of a given salt into those of the individual ions. The limiting molar conductance of an electrolyte can be easily split into those of the individual ions by means of transference-number data and Kohlrausch’s law of independent ion conductances using the relation

A: = A, t:.

Unfortunately no transference-number data are available for Ac + AN and DMA+AN mixtures in the literature, so that limiting ion conductances in these solvent mixtures could not be calculated by the direct method. In the absence of transference-number data in mixed solvents an indirect method has usually been followed for the evaluation of single-ion conductance.6’

Krumga1z22 has recently reviewed the indirect methods used for the separation of A, values of the electrolytes into the contribution from individual ions. In another recent paper Krumgalz and F l e i ~ h e r ~ ~ have discussed Gill’s approach to the evaluation of single limiting ion conductances in organic solvents and have tested the validity of this equation for the prediction of limiting ion conductances of Et,N+, Pr,N+, Bu,N+ and Ph,B- ions. They have found that Gill’s eq~at ion:~ ,

where 2 and ri are the charge and crystallographic radius for the respective ions, F is Faraday’s constant, N is Avogadro’s number, yo is the solvent viscosity, E , is the static dielectric constant of the medium and r , is a parameter equal to 0.85 A for non-associated solvents and 1.13 A for associated or hydrogen-bonded solvents, predicts A: values for all these ions in n-butyl alcohol, AN and nitromethane (NM) in good agreement with the corresponding values obtained using precise transference- number data. The results of these authors also showed that the deviations between the A: values predicted by eqn (2) and the corresponding experimental values obtained in DMF, DMA and ethanol (with the exception of Bu,N+ in ethanol) were not large and remained within +0.59 and -3.62% [see table 1 of ref. (23)]. Large deviations (> 5 % ) between these two sets of A: values were recorded only in a few cases and especially in case of formamide. The invalidity of eqn (2) for formamide and N-methylformamide solutions was first mentioned by Gill,25 indicating that the equation was applicable for solvents of dielectric constant between 17 and 77.

In recent studies in mixed solvents, Gill et aZ.’* found that eqn (2) predicts A: values for Et,N+, Pr,N+, Bu,N+ and Ph,B- ions which deviate between 1 and 4% from the corresponding values obtained using transference-number measurements. In order to resolve this difficulty, Gill and Cheema’ proposed a method based on Gill’s equation2, whereby limiting ion conductances in pure organic solvents and in binary mixtures of two dipolar aprotic solvents can be computed more precisely than by the use of

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D. S. GILL, N. KUMARI A N D M. S. CHAUHAN 69 I

Table 4. Comparison of the A: values for Bu,N+ calculated using eqn (3) and (4) with the corresponding experimental values in organic solventsa

solvent

methanol acetonitrile

32.6 1.13 36.0 0.85 36.0 0.85 37.1 0.85

75.61b 120.0" 1 19.63b 50.70d

39.59 62.70 62.45 26.48

39.14 61.70 61.70 26.68

- 1.15 - 1.62 - 1.23 + 0.75 N,N-dimethyl-

formamide N,N-dime thyl- acetamide

dimethyl sulphoxide

1,1,3,3-tetra- methylurea

ni tromethane

37.78 0.85 44.30" 23.13 22.90 - 1.00

46.6 1.13 21.21e 11.12 10.93 - 1.74

29.19f + 1.29 23.45 1.13 15.27 15.47

35.94 0.85 67.1lIb 35.08 34.06 - 2.99

a and A,"(exptl) values in this table are taken from ref. (23). Obtained from the sum of Ref. (7).

D. E. Arrington and E. Griswold, J . Phys. Chem., 1970,74,123. f Obtained by adding A&4N+ from ref. (21) to &h4B- reported by B. J. Barker and J. A. Caruso, J. Am. Chem. Soc., 1971, 93, 1341.

and &h4B- values reported in ref. (23). Present values from table 2.

eqn (2) and almost with the same accuracy as can be done with precise transport-number data. The equations used can be written as

and A i u 4 N + + Agh4B- = AO Bu4NBPh4* (4)

The good applicability of this model has been demonstrated in table 4 for some organic solvents in which precise A; values for Bu,NBPh, are available. The results show that the experimental values for Bu,N+ and the corresponding 1; values calculated by the use of eqn (3) and (4) are in good agreement with each other in all cases. In conclusion, the use of the present model in the evaluation of single limiting ion conductances in pure and mixed non-aqueous solvents should be preferred over Gill's approach [i.e. eqn (211 in future investigations.

Using eqn (3) and (4), A; values for Bu,N+ and Ph,B- in Ac, AN and DMA and Ac+AN and DMA+AN mixtures have been calculated from the A. values of Bu,NBPh, from tables 2 and 3. From these values and the A. values of all other electrolytes in tables 2 and 3 the limiting ion conductances for various ions have been computed using Kohlrausch's law of independent ion conductances. Single-ion conductances thus obtained are reported in table 5 .

Using Gill's m0dification~~7 26 of Stokes' law:

and the 1; values for various ions from table 5, ri values for various ions in Ac, AN and DMA and Ac+AN and DMA+AN mixtures have been calculated and are also

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692 SOLVATION OF IONS IN MIXED SOLVENTS

Table 5. 1; (cm2 mol-l i 2 - l ) and rl (A) for various ions in Ac + A N and DMA + A N mixtures at 25 "C

solvent (mol % AN)

0.0 12.0 33.2 58.2 73.4 93.2 100.0

ion 1: ri 1; ri 12; ri 1; ri 1; ri 1; rt 1; ri ~~ ~ ~ _ _ _ ~ -

Bu,N+ Ph,B-

Cu+ c10,

Bu,N+ Ph,B-

c u + ClO,

Ac + A N mixtures 67.8 5.0 66.7 5.0 66.0 5.0 64.7 5.0 63.7 5.0 63.1 5.0 62.7 5.1 62.2 5.4 61.3 5.3 60.5 5.3 59.4 5.3 58.4 5.4 57.7 5.4 57.3 5.4

114.7 3.5 108.7 3.5 107.2 3.5 106.5 3.5 105.7 3.5 103.5 3.5 102.7 3.6 83.9 4.3 75.4 4.5 70.7 4.7 67.1 4.8 65.3 4.9 65.2 4.9 65.1 4.9

DMA + AN mixtures 23.1 5.1 24.9 5.1 29.5 5.1 37.0 5.1 44.5 5.1 56.1 5.1 62.7 5.1 21.2 5.4 22.8 5.4 26.9 5.4 34.2 5.4 40.8 5.4 51.8 5.4 57.3 5.4 42.7 3.4 43.6 3.4 49.9 3.5 62.7 3.5 74.1 3.5 101.9 3.5 102.7 3.5 27.7 4.4 27.8 4.7 31.7 4.8 35.0 5.3 42.2 5.3 53.8 5.2 65.1 4.9

reported in table 5. In eqn ( 3 ) and ( 5 ) the E, values for the calculation of A: and ri were taken from table 1 , and the value of ry for all solvent systems was taken as 0.85 26

In binary solvent mixtures, if the ri value of an ion shows a linear dependence on solvent composition there is no preferential solvation, and if it shows a non-linear dependence there is preferential solvation for that ion. A perusal of the ri values in table 5 shows that these values for Bu,N+ and Ph,B- in Ac+AN and DMA+AN mixtures remain constant and equal to the corresponding crystallographic radii (5.00 A for Bu,N+ and 5.35 A for Ph,B-).l9? 27 Similarly, the ri value for C10, remains constant with the change of solvent composition in Ac+AN and DMA+AN mixtures. The anions are poorly solvated in dipolar aprotic and the change of solvent composition in dipolar aprotic solvent mixtures should bring about no change in the solvation behaviour of the anion^.^?^^ The variation of the value of ri for Cu+ with solvent composition shows interesting behaviour : it increases non- linearly from pure Ac to ca. 73 mol % AN in Ac+AN mixtures and then becomes constant and equal to the value in pure AN. This shows that the solvation of Cu+ increases from pure Ac upto ca. 73 mol % AN; beyond this solvent composition, when the solvation sphere of this cation becomes complete, no further increase in solvation takes place. Studies by Parker and coworkers1 show that Cu+ forms [Cu(AN),J+, [Cu(AN),]+ and [Cu(AN),]+ complexes with AN. When the AN content in the mixture is relatively small, the predominant complex is [Cu(AN),J+, and in the AN-rich region the [Cu(AN),]+ complex is generally formed. Thus the increase in solvation of Cu+ with the increase in the AN concentration in Ac+AN mixtures continues until the [Cu(AN),]+ complex changes into the [Cu(AN),]+ complex through [Cu(AN),]+. When [Cu(AN),]+ complex formation is complete at ca. 73 mol % AN, no further increase in solvation, and hence in Ti, takes place. These results show a strong preferential solvation of Cu+ by AN in Ac+AN mixtures over the entire solvent composition range. Similar results for the preferential solvation of Cu+ have been reported by Gill and Nording6 in H,O+AN mixtures.

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D. S. GILL, N. KUMARI AND M. S. CHAUHAN 693

In DMA+AN mixtures, an increase in the concentration of AN in the mixture increases the ri value of Cu+ from that in pure DMA; also, an increase in the DMA concentration in AN increases the ri value of Cu+ from that in pure AN. In other words, the value of ri for Cu+ passes through a maximum. This shows the preferential solvation of Cu+ by AN in the DMA-rich region and by DMA in the AN-rich region of DMA +AN mixtures. These results are similar to those found by Gill and Cheema’ in DMF + AN mixtures using conductance and viscosity measurements. Both DMF and DMA have almost the same Gutmann donor so they are expected to possess similar solvation behaviour towards d10 cations such as Ag+ and‘Cu+. Studies of copper(1) salts in DMF + DMA mixtures are in progress and will be reported in a later paper in this series.

We are grateful to the University Grants Commission, New Delhi for financial assistance under the research scheme no. F. 12-18/81-SRIII. M. S. C. thanks C.S.I.R., New Delhi for the award of a Senior Research Fellowship.

I. D. Macleod, D. M. Muir, A. J. Parker and P. Singh, Aust. J. Chem., 1977, 30, 1423. D. S. Gill and J. S. Cheema, Electrochim. Acta, 1982, 27, 1267. P. Singh, I. D. Macleod and A. J. Parker, J. Solution Chem., 1982, 11, 495. J. I. Kim, A. Cecal, H. J. Born and E. A. Gomaa, Z. Phys. Chem. (Neue Folge), 1978, 110, 209. V. V. Giridhar and C. Kalidas, J. Solution Chem., 1982, 11, 539. D. S. Gill and R. Nording, 2. Phys. Chem. (Neue Folge), 1983, 136, 117. D. S. Gill and J. S. Cheema, 2. Phys. Chem. (Neue Folge), 1983, 134, 205. D. S. Gill and M. B. Sekhri, Z. Phys. Chem. (Neue Folge), 1983, 137, 221. D. S. Gill and R. Srivastava, J. Chem. Soc., Faraday Trans. 1, 1982, 78, 1533.

lo D. S. Gill and R. Srivastava, Indian J. Chem., 1983, 22A, 140. I1 D. S. Gill and R. Srivastava, Indian J. Chem., 1983, 22A, 479.

D. S. Gill and J. S. Cheema, Electrochim. Acta, 1982, 27, 755. l 3 D. S. Gill and H. Schneider, Indian J. Chem., 1980, 19A, 313. l4 D. S. Gill, J. Solution Chem., 1979, 8, 691. l5 D. S. Gill, A. N. Sharma and H. Schneider, J. Chem. Soc., Faraday Trans. 1, 1982, 78, 465. I6 D. S. Gill and A. N. Sharma, J. Chem. Soc., Faraday Trans. I , 1982,78,475.

l8 R. M. Fuoss and F. Accascina, Electrolytic Conductance (Interscience, New York, 1959). Is J. F. Coetzee and G. P. Cunningham, J. Am. Chem. SOC., 1965, 87, 2529. 2o H. L. Yeager and B. Kratochvil, J. Phys. Chem., 1969,73, 1963.

22 B. S. Krumgalz, J. Chem. SOC., Frrraday Trans. 1 , 1983, 79, 571. 23 B. S. Krumgalz and Z. Fleisher, J. Chem. SOC., Faraday Trans. I , 1985, 81, 241. 24 D. S. Gill, J. Chem. Soc., Faraday Trans. 1 , 1981, 77, 751. 25 D. S. Gill, Electrochim. Acta, 1979, 24, 701. 26 D. S. Gill, Electrochim. Acta, 1977, 22, 491. 27 D. S. Gill, M. S. Chauhan and M. B. Sekhri, J. Chem. SOC., Faraday Trans. 1 , 1982, 78, 3461. 28 A. J. Parker, Q. Rev. Chem. SOC., 1962, 14, 163. 2s V. Gutmann, Coordination Chemistry in Non-aqueous Solutions (Springer, Vienna, 1968).

R. M. Fuoss and L. Onsager, J. Phys. Chem., 1957, 61, 668.

C. H. Springer, J. F. Coetzee and R. L. Kay, J. Phys. Chem., 1969, 73, 471.

(PAPER 4/91 1)

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d on

01

Janu

ary

1985

. Dow

nloa

ded

by U

nive

rsity

of

Wes

tern

Ont

ario

on

26/1

0/20

14 0

5:41

:35.

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