Anion exchange of metal complexes—II the cadmium-thiocyanate system

J. inorg, nucl. Chem., 1966, Vol. 28, pp. 2025 to 2032. Pergamon Press Ltd. Printed in Northern Ireland

A N I O N E X C H A N G E OF M E T A L COMPLEXES~II m THE CADMIUM-THIOCYANATE

SYSTEM

G. ALEXANDRIDES(2) a n d C. CUMMISKEY Department of Chemistry, St. Mary's University, San Antonio, Texas

(Received 28 September 1965; in revised form 20 December 1965) Abstract--The distributions of tracer ttsmCd between Dowex-l, strong base anion exchanger, and solutions of lithium thiocyanate, sodium thiocyanate, potassium thiocyanate and ammonium thiocyanate have been measured. Mononuclear complexes were verified in potassium thiocyanate up to 10-5 M in tracer. Distribution isotherms were determined for thiocyanate solutions between ~10 -2 to ~ 6 M. Above 0.2 M increasing adsorbability was in the order: Na + > Li + > K + > NH4 + with a maximum at H1 M in each case. These distributions, when invasion was taken into account, gave corrected distribution curves from which the overall complexity parameters related to the neutral complex were evaluated by curve fitting as: k2+ = 4'0 × 10-3; kt+ = 0"22; k l _ - 0.1; ka_ = 3"25. Resin sorption effects a 500-fold stabilization of the neutral species over the aqueous phase. THE CADMIUM-halide system has been studied with a n i o n exchangers b y several investigators (3-6) a n d m o s t recently by MARCUS a n d ELIEZER. (7) T h e t h i o c y a n a t e ion is a c o m m o n c o m p l e x i n g agent whose equilibria have n o t been extensively studied by this technique. A basic theoretical a p p r o a c h to these systems has been presented by MARCUS a n d CORYELL.(8) The t r e a t m e n t for a c o m p l e x series referred to the neutral species, M L v, fits the system very well since nil, the ratio o f central m e t a l charge to ligand charge, is an integer. This p e r m i t s a further test o f the c o n c e p t o f an effective l i g a n d activity A = mETe- T h e present w o r k concerns the distribution, D, o f 115n'Cd between Dowex-1 strongly basic a n i o n exchanger in the t h i o c y a n a t e f o r m a n d solutions o f l i t h i u m t h i o c y a n a t e , s o d i u m thiocyanate, p o t a s s i u m t h i o c y a n a t e , a n d a m m o n i u m t h i o c y a n a t e . The possibility o f p o l y n u c l e a t i o n has been investigated. U s i n g the resin invasion d a t a for these solutions, rFa values, f r o m a previous study m corrected distributions, D °, are determined. Curve fitting tg) for a p l o t o f log D ° vs. log A is e m p l o y e d to evaluate a p p r o p r i a t e c o m p l e x i t y p a r a m e t e r s , ki. EXPERIMENTAL

Materials

~tS~Cd (43d) was obtained from Oak Ridge National Laboratory in nitric acid solution, 2.16 mg/ml and 0.925 ± 10% mc/ml. It was diluted for use to 10-6 M, so that resin loading was a small ttl G. ZACHARIADES,W. R. HERRERA,and C. CUMrVnSKEY. J. inorg, nucl. Chem. 28, 1707 (1966). c2~From the thesis submitted in partial fulfillment for the degree Master of Science at St. Mary's University. ia~ y. MARCUS,J. phys. Chem. 63, 1000 (1959). 141 V. V. FOMIN, L. N. FEDOROVA, V. V. SINKOVSKIIand M. A. ANDREEVA, Zh. j~z. Khim. 29, 2042 (1955). c~ I. LEDEN, Svensk kern. Tidskr. 64, 145 (1952). ~"~ K. A. KRAUS and F. NELSON, Int. Conf. Peaceful Uses Atom. Energy 7, 121, 1955. ~7~ y . MARCUS and I. ELIEZER, J. inorg, nucl. Chem. 25, 867, (1963). ~8 y . MARCUS and C. D. COmCELL, Bull. Res. Coun. Israel A8, I (1959). ~9~ L. G. StoLEN, Acta chem. scand. 10, 186 (1956). 2025

2026

G. ALEXANDRIDESand C. CUM~SKEY

fraction of 1 per cent. The resin was Dowex 1 × 8, Baker Analysed Reagent, 50--100 mesh. It was converted to the thiocyanate form by eluting with 21. of 2 0 ~ sodium thiocyanate solution per 100 g of resin at a flow rate of 10 ml/min. The resin was air dried at room temperature until free flowing and then stored over anhydrone in a dessicator. All chemicals were reagent grade with the exception of lithium thiocyanate, purified, obtained from City Chemical Corp., New York, N.Y. Equipment

Equilibrations were carried out in polyethylene stoppered erlenmeyer flasks and solution activities were determined in a glass jacketed GM tube whose effective volume was ~ 5 ml. Methods

The "batch" equilibration method as described by KRAUS et al. (1°) was used. Sixteen hours equilibration time was adequate for attaining equilibrium. The distributions were evaluated as: D = (radioactivity in resin) (vol. of soln.) (radioactivity in soln.) (wt. of resin) ' the radioactivity in the resin being the difference in solution count rate before and after equilibration. One gram samples of resin were used with salt solutions of 40 ml total volume. RESULTS T a b l e 1 shows the effects of varying the c a d m i u m c o n c e n t r a t i o n i n the range 10- ° to 10- 5 M. T h e results show that the d i s t r i b u t i o n is n o t affected by changes in the c a d m i u m c o n c e n t r a t i o n . This strongly suggests that at least i n the range studied, p o l y n u c l e a t i o n does n o t occur. TABLE 1 . - - T H E EFFECT OF CADMIUM CONCENTRATION ON THE ANION-EXCHANGE DISTRIBUTION IN THE CADMIUM--THIOCYANATE SYSTEM

Cd ~+ Concentration × 10-6 M

log D (eq/kg)

Molarity KSCN

1 5 10

3"514 3"513 3-500

0"3762 0"3762 0"3762

1 5 10

3'380 3.394 3"343

1'7585 1'7585 1.7585

I 5 10

2.663 2.676 2"671

5.5766 5.5766 5"5766

A c c o r d i n g to MARCUS a n d CORYELL(8) the d i s t r i b u t i o n for a tracer metal M 2+, such as Cd 2+, c a n be expressed as: log D = log K ' - - p~F~ - - log Zkt A - i

(1)

a n d a corrected d i s t r i b u t i o n coefficient D °, can be defined as: log D ° ---- log D - - prFa = log K ' - - log E k i A - i

(2)

where A is defined as the activity o f the ligand in the a q u e o u s phase expressed as activity o f the s u p p o r t i n g electrolyte = m~:7:~ ; K ' is a c o n s t a n t i n d e p e n d e n t o f A ; (x0) K. A. KRAUS, H. O. PHILLIPSand F. NELSON,Radioisotopes in the Physical Sciences and Industry, Vol. III., Vienna (1962).

Anion exchange of metal complexes--II

2027

--p is the effective charge of the anionic metal complex in the resin phase; i is the charge of the metal complex in the aqueous phase; k~ is the over-all complexity parameter referred to the neutral species; and ,Fa is a function which corrects for the invasion of the resin by the non-exchange electrolyte. The observed distributions of I

I

I

i

1

3.5

3.0 O

\,

2.5

\

I

I

I

i

I

0

2

4-

6

8

MSCN FIG. 1.--Distribution coefficients (log D) of cadmium vs. thiocyanate molarity (M) for: • LiSCN; A NaSCN; C) KSCN; • NH4SCN; 0 Common point.

radiocadmium in thiocyanate solutions are given in Fig. 1. The distribution of radiocadmium as a function of ligand activities are shown in Fig. 2. Values for the activity coefficients of potassium thiocyanate, required to calculate A, were obtained from HARNED and OWEN.m) Values for sodium thiocyanate activity coefficients were obtained from ROBINSONand STOICES~12)except for high concentrations, where MILLER a n d SHERIDAN(18) were used. For ammonium thiocyanate, no source of mean activity coefficients was available. However, the values were estimated by comparison of the mean activity coefficients of ammonium and potassium chlorides followed by appropriate approximation of the values of ammonium thiocyanate from those of potassium ,11) H. S. HARNEDand B. B. OWEN, The Physical Chemistry of Electrolytic Solutions, 2nd Ed., Reinhold, New York (1950). ,az, R. A. ROBINSONand R. H. STOKES,Electrolyte Solutions, Butterworth, London (1955). ~a3, M. L. MILLER and C. L. SHERIDAN,J. phys. Chem. 60, 184 (1956).

2028

G. ALEXANDRIDESand C. CUMMISKEY I

" I

I

1

I

I

I

3.5

3.0

o~ 2 . 5 2.0

1.5

I

-2.0

[

I

-I.5

-I.0

I

-0.5

I

I

I

0.0

0.5

1.0

log ASC N Fro. 2.--Distribution coefficients (log D) of cadmium vs. effective ligand activities (log As0~r) for: A NaSCN; © K S C N ; • N H , SCN.

thiocyanate. Values of the invasion correction function, rF~ were taken from previous work in this laboratory. ~I~ Since the Cd z+ is a d 1° ion it shows a characteristic co-ordination number of four with the arrangement of the ligands being tetrahedral, as sp3 hybridization would suggest. However, a maximum co-ordination number of six, using sp3d~ hybridization, is not unreasonable but has been reached only for certain ligands, t14~ The plot of log D ° vs. log A can interpret the experimental data in terms of such species provided an assumption is made as to the effective charge of the anionic metal complex in the resin phase, i.e.p in Equation (2). Since the resin as well as the aqueous phase will tend to maintain overall charge neutrality we have assumed, as have others/s~ that the species added to the resin is effectively the neutral complex. This reaction may be represented as follows: R2(SCN)2 q- Cd(SCN) 2~ R2 Cd(SCN)4 Other species held in the resin would result in charge inbalance as well as unusual co-ordination numbers leading to a secondary attraction for thiocyanate anions if a positive species is attracted by the resin or to the liberation of excess thiocyanate if a higher complex is attracted. Such effects might be represented thus: R2(SCN) 2 q- CdSCN + q- SCN- ~ R 2 Cd(SCN)3 + -k SCN- ~ R 2 Cd(SCN)4 or

R2(SCN)2 -t- Cd(SCN)z-~- R2 Cd(SCN), q- SCNHence the interpretation presented in Fig. 3 is on the basis o f p = 2. t141 M. M. JONES, Elementary Coordination Chemistry, p. 119, Prentice-Hall, N.J. (1964).

Anion exchange of metal complexes~II

2029

Several ways of expressing the stability of the various equilibria involved in systems such as this one with Cd(II)-SCN- have been employed. Following the practice of one of the standard collections of data in this field 115)the symbol K is used to refer to the step-wise constants. Thus: ML,~_i + L ~ - M E , defines K~ = [ML,,]/{[ML~_I][L]} etc. Then the overall (or cumulative) constants are expressed by the symbol fi,~. Thus: M 2+ + n L ~ - M L , defines ft, = [ML,]/{[M][L]n}. Using the techniques of the ion exchange method reference to the neutral complex is desirable by means of an overall complexity parameter symbolized by k~. Thus: ML~+-MLi, l i ÷ i L defines a thermodynamic equilibrium constant B i = (A51L~_?~A L O / A 3 i L , which can be shown to be ts) related to the overall complexity parameter referred to the neutral species by the expression: k i = B~/7~. Likewise: k~ = {[ML~I_i][L]~}/[ML~] × ( y L i / y v (1)) where the 7 refer to the activity coefficients of the appropriate species. To the extent that these 7 and/or their ratios approach unity the relationships between these several methods of representation have been worked out and are as follows: /¢+2 -

1 1 fi2 ' - KIK2

k - 2 - - fI2 - - K'~K4

k+l

fit

k_ a = ¢~5 _

fi3

k_ 1 . . . . .

1

x, Ks

fi4

E - C3XK5

k_4 --

fi6

-- KaK4KsK 6

Thus using the data available in Reference (15) we have compiled the data presented in Table 2. The last column of the Table lists the source of the information as quoted in Reference (15). The results of this present study are included in the last line. They were initially evaluated as k i ' s by a method of curve fitting(9) using the corrected distributions for potassium thiocyanate in Fig. 3. The system parameter log K' as defined in Equation (2) was determined from this same graph and found to be 4.00 ~ 0.05. Figure 3 shows as well the agreement of the D°'s obtained experimentally with those obtained by employing the calculated k i. Open circles stand for experimental points in potassium thiocyanate solutions and the curve has been calculated by inserting the k i and K' into Equation (2). By subtracting 2 log rA ° from the value of log K' the equilibrium constant, rk 2, for the reaction: Cd(SCN)2 + Rz(SCN)2 ~- R2Cd(SCN) 4 may be calculated (a thus, log ~ k 2 = log K' --2 log rA ° =- 4 -- (2 × 0.41) = 3.18 This value can be used for comparison with the value for the corresponding reaction in the solution phase: Cd(SCN)2 + 2 S C N - ~ - Cd(SCN)42-, where log k 2 = 0.51. It can be seen that a 500-fold stabilization of the complex results from its interaction with the resin. The curve for the NaSCN data is merely the best fit of the experimental points. 115) j. BJERRUM, G. SCI-IWARZENBACH and L. G. SILLEN, Stability Constants, Chem. Soc. Spec. Public., No. 17, L o n d o n (1964).

1"04 1.75

1"90 2"24

1.36

1-08

1.40

1.78

1.74

pol.

M Hg

pol.

pol.

pol.

pol.

ix

2"38

2'40

1.40

2"51 1.78

1 - 8 8 1'93

1"62 0'96

2.09

2.36

0.78

1 . 9 8 2.58

1.39

M Hg

log of fit fl~

fl~

Method

1-39

/(1 0.59

/(2 0.60

log of K3

2.91

3"80

2"38

1-64

2"48

0"73

1 " 7 4 0"66

1"78 0-73

1 " 4 0 0.48

1 " 0 8 0"54

1.36

1"90 0"34

--1"0

--0'73

0"05

--0.66

0-29

0"08

1"78 1 " 0 4 0 " 7 1 --0"97

f14

1.51

2"02

0.45

0"68

0'10

1.00

K4

--2"40

--2"51

--1-88

--1-62

--2.09

--2-24

--1"75

--1-98

k+~

--0'68

--0.73

--0"48

--0.54

--0.73

--0"34

--0"71

--0.58

--1"0

--0"73

0.05

--0"66

0"29

0"08

--0"97

0.60

0"51

1"29

0-5

0"02

0"39

0.03

k_2

Reference

Present work

H-S. HSIUNG, Thesis, Univ. Cincinnati, Univ. Microfilms 60-6444, Diss. Abs. 21, 3629.

P. SENISEand E. F. DE ALMEIDANEVES,J. Am. chem. Soc. 83, 4146 (1961).

YA. I. TUR VAN and N. BONDARENKO, Zh. neorg. Khim. 4, 1070 (1959).

H-G. TSlANG and K-H. Hso, Acta. Chim. Sin. 23, 196 (1957).

A. M. GOLUB and O. G. BILYK, Zh. neorff. Khim. 2, 2723 (1957).

D. N. HUME, D. D. DEFORD and G. C. B. CAVE, ,Jr. Am. chem. Soc. 73, 5323 (1951).

I. LEDEN, Z. phys. Chem. 188, 160 (1941)

Cd(II)-SCN SYSTEM

log of k+x k_l

TABLE 2 . - - E Q U m m R I A PARAMETERS FOR THE

r~

~r

ga gx,

X

>

8

Anion exchange of metal complexes--II

2031

DISCUSSION The so-called "Secondary Cation Effect" was treated at some length by HORNEC16) for the Zn-C1-Dowex 1 × 8 system. He did not, however, employ the concepts of resin invasion and ligand activity that follow from MARCUS' treatment. ~8) MARCUS and ELmZER(71 employed these concepts in the halide systems in a rather limited way and without reference to data from direct invasion. In our case, when observed log D's are plotted against concentration, whether as M or log A, (Figs. 1 or 2), the values for salts of different secondary cations start out together and separate at ~-O-1 M.

[

;

[

I

I

I

4.0

3.5 O

c'~

o, 5,0 o

2.5

2.0

i

-2.0

:

-I.5

I

-I.0

L

-0.5

i

]

I

0.0

0.5

1.0

log Asc N

FzG. 3.---Corrected distribution coefficientsfor invasion (log D °) based on the species in the resinphase being Cd(SCN)4~- vs. effectiveligand activities (log Asc~) for: /x NaSCN; © KSCN. When A is used instead of M they seem not to overlap thereafter. In order to employ the correction of log D for resin invasion some assumption must be made as to the effective charge of the anionic metal complex in the resin phase, p. Figure 3 has been made with the assumption t h a t p = 2 as previously explained. The fact that the curves now no longer agree at low log A is a matter of some concern. This may be due to the fact that invasion in this region is small and thus its measurement is least precise. Or it may be that the different salt systems have different activity coefficients for the various complex species for which the theory does not correct. At any rate the slopes are unchanged so that the predominant species in the aqueous phase goes progressively from Cd 2+ to Cd(SCN) + to Cd(SCN)2 at the maximum logA ~ --0"6. After this the negative species are detected: Cd(SCN)3- and Cd(SCN)4 -2. The evidence, ~1~)R. A. HORNE,d. phys. Chem. 61, 1651 (1957).

2032

G. AJ.~XANDRtDES and C. CUMMIS~:~V

especially in the case of NaSCN, indicated that even higher complexes may be formed but insufficient experimental data are available to evaluate any parameters. The curve in Fig. 2 for NH4SCN would indicate this more strongly but its activity coefficients have only been estimated.

Acknowledgement--The authors wish to thank the Robert A. Welch Foundation for Grant U-087 which supported this work. We are grateful to Mr. JOHNR. PAXSONfor discussions concerning the calculations involved.