Anion exchange of metal complexes—V

I. inorg, nucl. Chem., 197 I, Vol. 33, pp. 137 to 145.

ANION

EXCHANGE

Pergamon Press.

OF

Printed in Great Britain

METAL

COMPLEXES-V*

THE C H R O M I U M - T H I O C Y A N A T E

SYSTEM

J. T. M A S O N , Jr.t and C. J. C U M M I S K E Y D e p a r t m e n t of Chemistry, St. M a r y ' s University, San Antonio, T e x a s 78228

(First received 29 August 1969; in revised form 13 May 1970) A b s t r a c t - T h e m e a s u r e m e n t s of the distributions of C r ( l l l ) between D o w e x 1 × 8. a strongly basic anion exchanger, and solutions of sodium, lithium, potassium, and a m m o n i u m thiocyanate have been determined by the batch m e t h o d employing 51mCr as a tracer. D u e to the tendency of 51mCr to adsorb on the surfaces of the e q u i p m e n t used in the investigation, carrier chromium, as c h r o m i u m nitrate, was added. Distribution i s o t h e r m s were determined for thiocyanate solutions between - 0 . 0 5 and 4-1 M. Increasing adsorbability above 1.0 M was in the order NH4 + > N a ÷ > K + > Li +. Taking resin invasion into account, corrected distribution curves were made from which overall complexity parameters related to the neutral complex were evaluated as: k + 3 - 9-8 × 10-z; k+*2 = 1.4× 10-2; k* a = 0.78; k* 1 = 10-2; k* 2 = 17.0; and k* 3 = 35.6. A 200-fold stabilization of the neutral species over the a q u e o u s phase is effected by resin sorption. INTRODUCTION

VERY little work has been performed on chromium systems using anion exchange techniques; however, a basic theoretical approach to metal-ligand systems was presented by Marcus and Coryell[1]. Their treatment for a complex series referred to the neutral species, MLv, fits the chromium-thiocyanate system very well since the ratio of the central metal charge to the ligand charge, v, is an integer. The present work involved the measurement of the distribution of Cr(III) between Dowex 1 × 8, a strongly basic anion exchanger, in thiocyanate form and solutions of sodium, potassium, ammonium, and lithium thiocyanate employing 51mCr as a tracer. Using resin invasion data, corrected distribution coefficients were determined. Complexity parameters, L*, were evaluated from the above data by means of a curve fitting method[2] involving successive and sweeping approximations. EXPERIMENTAL Materials T h e radioactive 5J"Cr was obtained from N e w England N u c l e a r Corporation as the chloride in hydrochloric acid solution. D u e to the short half-life, 27 days, a stock solution was prepared and maintained, by s u b s e q u e n t activity additions, such that its count rate was relatively constant throughout the investigation. T h e resin used was D o w e x 1, Baker Analyzed Reagent, 5 0 - 1 0 0 m e s h , with 8% divinyl b e n z e n e content for crosslinkage. T h e chloride form of the resin was converted to the thiocyanate form by eluting with 5 M sodium hydroxide followed by 20% sodium thiocyanate. T h e resin was air dried at room temperature and stored over " a n h y d r o n e " in a dessicator. All chemicals * Part 1V; J. F. N e u m a n n , J. R. P a x s o n and C. J. C u m m i s k e y , J. inorg, nucl. Chem. 30, 2243 (1968). t F r o m the thesis submitted in partial fulfillment for the degree Master of Science at St. Mary's University. 1. Y. M a r c u s and C. D. Coryell, Bull. Res. Coun. Israel A8, 1 (1959). 2. L. G. Sill6n, Acta chem. scand. 10, 186 (1956). 137

138

J . T . MASON, Jr. and C. J. C U M M I S K E Y

used were of reagent grade with the exception of lithium thiocyanate which was the purified grade obtained from City Chemical Corporation, New York,

Equipment Equilibrations were carried out in 50-ml Erlenmeyer flasks with polyethylene stoppers. Radioactivity of the gamma-emitting 51mCr was assayed by means of a well-type counting system employing a thallium activated sodium iodide crystal. Method The batch technique used in this investigation was described by Kraus et al. [3]. The distribution coefficients were evaluated as: D = (radioactivity in resin)(volume of solution) (radioactivity in solution)(weight of resin) where the activity in the resin is indirectly determined as the difference between the total activity equilibrated with the resin and the activity remaining in the solution after equilibration. 1 gm samples of resin were shaken with 20 ml of thiocyanate salt solution, 1"0 ml of 51mCr stock solution, and 1.0 ml of 2.7 × 10-a M chromium nitrate carrier stock solution. On the basis of the data presented in Table l it was concluded that 16 hr was adequate time for attaining the desired equilibria. Table 1. The determination of equilibration time in the chromium-thiocyanate system at 20°C Time (hr)

log D

6 16 20 24

2"28 2"34 2'43 2"30

RESULTS

It was necessary to add carrier chromium since the tracer, 51mCr, was found to adsorb on the surface of the containers used in equilibrating. Results of the tests run to determine the effects of the addition of the chromium carrier on increasing the distribution Coefficients (reducing tracer losses) and improving precision are listed in Table 2. Carrier chromium nitrate solutions were added ranging from 6 × 10-5 to 6 x 10-1 M. At concentrations above this, the results tend to confirm the reports of others [4], i.e. that chromium is effectively nonadsorbable from solutions of thiocyanate. The 2-7 × 10 -3 M solution was chosen as the carrier since it gave good precision and yet represented a maximum resin loading of only 0.3 per cent. According to the theoretical approach of Marcus and Coryell [ 1], the distribution for a tracer metal M 3+, such as Cr a+, between the aqueous and resin phases is 3. K. A. Kraus, H. O. Phillips and F. Nelson, Radioisotopes in the Physical Sciences and Industry Vol° I ! I. Proceedings of the I AEA Conference, Copenhagen, Sept. 1960, Vienna (1962). 4. J. B. Turner, R. H. Philp and R. A. Day, Analytica chim. Acta 26, 94 (1962).

Anion exchange of metal c o m p l e x e s - V

139

Table 2. Tests with chromium nitrate carrier using 1.5M sodium thiocyanate Conc. Cr(NO3)3 in sample

Av. logD

% deviation

0 2-7 x 10-~ 2.7x 10-5 2.7 x 10 4

0.97* 1.12" 1.13" 1.34" 1.45"t 1.35" 1.37~ 0.46{

± 12.8 +-8.9 +-5.5 +-4.8 +- 12.0 +-6.0 +-7-4 +-26.5

2.7 x 10-3 2.7 x 10-2

*Based on all eight samples run. Based on first eight samples plus additional four. :~Based on all four samples run.

given by: log D = log K ' + p r F ~ -- log ~k~A -~

(1)

and a corrected distribution coefficient can be defined as: log D O= log D - - p r F a -----log K ' -- log X k i A -i

(2)

where ,4 is defined as the activity of the ligand in the aqueous phase expressed as the activity of the supporting electrolyte, re_y+; K' is the system parameter which is independent of A ; --p is the effective charge of the anionic metal complex in the resin phase; rFa, a function of A, is the correction for resin invasion by the nonexchange electrolyte; ki is the over-all complexity parameter referred to the neutral species; and i is the charge on the anionic metal complex in the aqueous phase. The plot of the observed distribution, log D, of Cr 3+ vs. concentrations of the various thiocyanate salt solutions employed is given in Fig. 1. The same data as a function of ligand activities are shown in Fig. 2. Values of the mean activity coefficients needed to determine activities were obtained from the following sources: for sodium thiocyanate-Robinson and Stokes[5] for concentrations up to 4.0 molal, and Miller and Sheridan[6] for higher concentrations; for potassium thioc y a n a t e - R o b i n s o n and Stokes [5]. The activity coefficients £or ammonium thiocyanate were based upon a comparison of the mean activity coefficients of ammonium and potassium chlorides with approximations of the ammonium coefficients from the potassium thiocyanate coefficients [7]. An assumption must be made as to the effective charge of the anionic metal complex in the resin phase, i.e. p = 3. Since the resin as well as the aqueous phase will tend to maintain overall charge neutrality we have assumed, as have Marcus and Coryell[1], that the species added to the resin is effectively the neutral 5. R.A. Robinson and R. H. Stokes, Electrolyte Solutions. Butterworth, London (1959). 6. M. L. Miller and C. L. Sheridan, J. phys. Chem. 60, 184 (1956). 7. G. Alexandrides, A Study of the Cadmium-Thiocyanate System by Means of Dowex 1 × 8 Anion Exchange Resin. M. S. Thesis in Chemistry, St. Mary's University, San Antonio, Texas (1964).

140

J . T . MASON, Jr. and C. J. C U M M I S K E Y

,o t

'P

.

A

5.5

30 2.5

2-0

1.5

I-0

OI,s

I1.0

1!5

210

21,5

3!0

31.5

4!0

41.5

MscN Fig. 1. Distribution coefficients (log D) of chromium vs. thiocyanate molarity (MscN) for: © - K S C N ; A - N a S C N ; O - N H 4 S C N ; A - LiSCN.

complex. This reaction may be represented by the following equation:

Rz(SCN)3 + Cr(SCN)3 = R3Cr(SCN)e. We further assume that the high effective thiocyanate concentration in the resin will favor the formation of the ultimate chromium complex showing a characteristic coordination number of 6. Values for the invasion correction data, rFa, taken from previous work in this laboratory [8], were used to construct corrected distribution curves; these are shown in Fig. 3. Following the practice of one of the standard collections of data in this field [9], we will let the stepwise constants, which express the stability of the various equilibria involved in this sytem, be denoted by the symbol Kn. Thus, MLn_I + L = MLn defines Kn = (MLn)/{(ML,~_O(L)}, where M is the metal ion and L is the ligand. The overall (or cumulative) constants are expressed by the symbol fin. Thus, M 3+ + nL = MLn defines fin = (MLn)/{ (M) (L)~}. The overall complexity parameter referred to the neutral species, denoted by kt, is defined as follows: if ML~ = ML~_~ + iL defines a thermodynamic equilibrium constant

then

k~ = {~4MLv_ I . ALi} /~MLv, = { (ML~_ ~)(L) ~/(MLv) }. T,TL'Yv-~ = k, {~/i~/Ll~/v-1 } = { (MLv-i)T+_t. A i } / A v

= ~*T-+t

8. G. Zachariades, W. R. Herrera and C. Cummiskey, J. inorg, nucl. Chem. 28, 1707 (1966). 9. J. Bjerrum, G. Schwarzenbach and k G. Sill6n, Stability Constants. Chemical Society Special Publication No. 17, London (1964).

Anion exchange of metal complexes- V

141

3'5

3.0

C~

2-5

20

Z~

15

j

I-O

I

-,.2

i

-~o

I

-o.8

-o16

-o'-,

-o'.2

o!o

o'.2

, i

o,~

I

o.6

log AsCN

Fig. 2. Distribution coefficients (log D) of chromium vs. effective ligand activities (log ASCN)for: O-- KSCN ; A - NaSCN ; • - N H4SCN. Where ")/+t ~

l+i

,~l--i

"Y+.(MLv_i, iL) • I + _ ( C , L )

H ~ for i > / 0 and A = (L)T~(C,L) while A~, = for i ~< 0 or Y+_i---- "Y+_(tC,MLv_i) • "Y-+(C,L) (MLv)')/-,-(MLv). NOW since the m a x i m u m a m o u n t o f C r 3+ e m p l o y e d is 1 ml of 2 - 7 / 1 0 - 3 M per 22 ml batch, the m a x i m u m c o n c e n t r a t i o n o f a n y Cr 3+ species w o u l d be 1.2 × 10-4 M. In addition, with the value o f log D being b e t w e e n 1.0 and 4.0 m o s t o f the C r 3+ ends up on the resin so that 3'±(M%) ~ 1. T h e value o f Y+_w,L) for K S C N solutions e m p l o y e d varies f r o m 0.73 to 0.53 so that the r e p o r t e d k* values can be adjusted to ki values as follows: k+3 = k+3/Y±(C,L) * 3 = (0"0980)(0"73) 3 = 2"52 × lO -1 for log k+3 = - 0 . 6 0 , etc. as given in T a b l e 3. T h e s e ki values and the ft, and K , values w h i c h o t h e r m e t h o d s o f determining the stabilities o f these species determine are related as follows: k+3 = f13 -1 = ( K 1 K 2 K 3 ) -~ k+2 = fl~/33-1 = (K2K3) -~ k+l =

f12f13 - 1 =

K.C 1

k-1 =

•4•3

K4

-1 :

k-2 = fl5fla -] : K 4 K ~ k-3

=

[36fla -1

=

K4KsK6.

142

J . T . MASON, Jr. and C. J. C U M M I S K E Y 4.5

4.0

3.5

3.0

o, _o

2-5

2.0

1.5

i.o -I.4

-I.2

-I.0

-0.8

-0-6

-0.4

-0.2

0.0

0,2

0.4

0.6

log .4SCN

Fig. 3. Distribution coefficients corrected for invasion (log D °) vs. effective ligand activities (log AscN) for: C ) - KSCN; ~ - N a S C N ; • - NH4SCN.

The results shown in Table 3 apply these relationships to previously published data[9]; the results of this present study are given in the last column for the sake of comparison. It is noted that differences in experimental conditions, such as temperature and aqueous media limit the value of such a comparison. The overall complexity parameters referred to the neutral species, k*, and the system parameter, log K ' , were determined by fitting a curve to the experimentally determined log D o vs. logA points. This was done in several steps. First, each k* was determined in the region in which it predominated assuming that the others were effectively zero. This was done for each k* in order from i = + 3 to i = - 3. The next approximation included the k* being determined and the one immediately before and after it. Values were improved by trial and error. The experimental data used was that gathered for potassium thiocyanate. Such very little data is available from other sources and the values are often so divergent that it is difficult to draw any conclusions. In reducing all values to those determined in this study, i.e. the complexity parameters referred to the neutral species, it would seem that the ion-exchange studies seem to show as a pattern a greater stability to these species than other methods. The calculated values of log K ', the system parameter, and complexity parameters, k*, when inserted into Equation (2), using a program developed for an IBM 1620, yielded a set of back-calculated distribution coefficients or log D o values. Figure 4 demonstrates the fit between this back-calculated data, the solid line, and the original experimental points, the open circles. By subtracting 3 IogrA ° from the values of log K ' [10], the equilibrium con10. Y. Marcus, J. phys. Chem. 63, 1000 (1959).

143

Anion exchange of metal c o m p l e x e s - V Table 3. Equilibrium parameters for the Cr(I I I ) - S C N system Chem. anal., Chem. anal., Method conductivity conductivity Chem. anal. Chem. anal. Chem. anal. Log of k+:, k+e k+~ k , k_., k :~ B1 B2 B:~ B4

B;, B, Kt K2 K3 K4 Ks K,~ Ref.

- 4.42 - 1-90 -0.66 0.29 0.20 -0.19

- 5.80 - 2.7 - 1.0 0-3 - 0.40 2.00

2.52 3.76 4.42 4-71 4.62 4.23

3.10 4.80 5.80 6.10 5.40 3-80

2.52 1-24 0.66 0.29 -- 0-09 --0.39 *t

3.1 1.7 1"0 0-3 - 0"7 -- 1'6 *-t~§

Spectrophotometry Ani. ex.

- 0.60 - 1.54 0.08 0.79 0.76 0.75 1-87 2.98

1.79 2-79

1.72 2.32

3,04

-- 0.94 0.68 0.60 1.39 1-36 1.35

1.87 1-11

1.79 1-0

1.72 0-60

3.04

,k

,k

,t

¶

--0.93 1.62 -- 0-08 0.79 -- 0.03 -0.01 Present work

*N. Bjerrum, Z. anorg, allg. Chem. 119, 179 (1921). t N . Bjerrum, K. danske Vidensk. Selsk. Skr. 12, No. 4 (1915). SN. Bjerrum, Naturwissensehaften 5, 125 (1926). §N. Bjerrum, Ergebn. Exakt. Naturwiss 5, 125 (1926). IlK. G. Poulsen, J. Bjerrum and 1. Poulsen,Acta chem. scand. 8,921 (1954). ¶C. Postmus and E. L. King, J. phys. Chem. 59, 1208 ( 1955).

stant in the resin phase, ~k*s, for the reaction: Cr(SCN)3 + Rz(SCN)3 = R3Cr(SCN)6 m a y be calculated. Previously[11], it was shown that log rA ° = 0-41 for K S C N Thus, log rk_*3 = log K ' -- 3 log r A ° = 5"07-- (3 × 0"41) = 3~84. This value can be used for comparison with the value for the corresponding reaction in the solution phase: Cr(SCN),~ + 3 S C N - --- Cr(SCN)6311. W. R. H e n e r a , A Study o f the Invasion o f Dowex 1 x 8 by Potassium Thiocyanate and SulJi~ric AcidSolutions. M. S. Thesis in Chemistry, St. Mary's University, San Antonio, Texas (1965).

144

J . T . MASON, Jr. and C. J. C U M M I S K E Y

4,2

5.8

3"4

_o

3'01

2.6 ¸

2.2

_1.10

I -o-8

, I -o.6

-o'.4

-o.2I

o'.o

o'.2

log ,4SCN Fig. 4. Distribution coefficients corrected for invasion (log D °) vs. effective ligand activities (logAscN) for KSCN solutions: O - E x p e r i m e n t a l values; c u r v e - C a l c u l a t e d values.

where log k*3 = 1.55. A stabilization of 200-fold of the complex results from its interaction with the resin. DISCUSSION

The normal effect of resin invasion on the graph of log D vs. ligand activity is to inhibit the decline of the log D values after reaching some peak. As seen in Fig. 1, the KSCN curve is not much inhibited in its decline. Thus it appears that there is not much effect of resin invasion observed. The region of interest is for values of log AscN greater than -- 0.5 or in the region where the anionic chromium complexes are predominant in the aqueous phase. A more normal pattern is seen for sodium thiocyanate and ammonium thiocyanate solutions. This observation may be explained by the secondary cation effect. The so-called "Secondary Cation Effect" was treated at some length by H o m e [12] for the Zinc-CI-Dowex 1 × 8 system. He interpreted the results in terms of ionic association of the secondary cation with anionic complexes. A similar explanation will be advanced here. At low salt concentrations, the secondary cation causes no difference; however, at higher concentrations, the association of the secondary cation with anionic chromium thiocyanate complexes increases. Although the tendency toward resin invasion is strong, its effect is not seen in the C r - K S C N system since the associated complexes are too large to readily enter the resin and the concentration of Cr(SCN)3 is minimized. Desorption of the resin due to the ionic association of the secondary cation with the anionic chromium complexes is also possible, giving rise to even less chromium on the resin. 12. R. A. Horne, J.phys. Chem. 61, 1651 (1956).

Anion exchange of metal complexes - V

145

It should be recalled that for compounds of the secondary cations which involve large anion groups, such as the chlorates, the perchlorates, and the periodates[13], the solubilities of both the sodium and the ammonium salts are greater than those of the potassium salts. Similar occurrences in this system would tend to strengthen the above argument. Marcus interpreted a similar effect [14] in the Fe(III)-HCI system as being due to the association of the secondary cation with an iron complex to form a nonsorbable ion-pair in the aqueous phase. Mizumachi also attributed these effects [14] in the Co(II)-HC1 system to the formation of a complex in the aqueous phase which was not sorbable by the resin phase. The correction for resin invasion brings the three curves closer together; however, the effect on the graph of potassium thiocyanate is to accentuate its anomalous behavior. A slope o f - 12 is reached at high thiocyanate concentrations which requires a very improbable coordination number in terms of complex formation. Fortunately, for the calculations of the complex stability parameters presented here, it was not necessary to use the high thiocyanate concentration end of the curve where invasion effects are large and in this case apparently anomalous. It is possible that similar results will be seen for the Cr-LiSCN system; however, no conclusions can be drawn until activity coefficients are made available. Taking into account the results shown in Fig. 1, it may be said that the association of the secondary cation with anionic chromium thiocyanate complexes, resuiting from interionic interaction, if it exists, shows the following increasing order of stability: Li + > K + > N a + > N H 4 +. Acknowledgement-The authors wish to thank the Robert A. Welch Foundation for Grant U-087 which supported this work. 13. International Critical Tables Vol. IV. McGraw-Hill, New York (1928). 14. Y. Marcus, Metal Chloride Complexes Studied by Ion Exchange and Solvent Extraction Method. Israel Atomic Energy Commission, Yavne, Soreq Nuclear Research Center (Dec. 1966).