Y . MARCUS
1000
Vol. 63
THE ANION EXCHANGE OF METAL COMPLEXES. 111. THE CADMIUM-CHLORIDE SYSTEM'-3 BY Y . MARCUS Contribution ,from the Israel Atotnic Energy Commission Laboratories, Hzkirya, Tel-Aviv, Israel Recewed September 1 I , 1968
The anion-exchange method described in the previous communications was applied to a study of the cadmium-chloride complex system. The distribution of the cadmium between Dowex-1 chloride resin and the solutions was measured by the "batch" method with the aid of the tracer Cd116. Formation of mononuclear complexes up to M cadmium was confirmed. The distribution curves were determined in the range 0.04 to 9.0 M lithium chloride and 0.01 to 9.4 fif hydrochloric acid. These curves coincided up to 0.2 M chloride. Above this concentration the hydrochloric acid curve is lower but both show a maximum around 4 M chloride. Construction of "ideal" distribution curves, valid for constant resinligand activity, made possible the evaluation of the complex system in terms of the species CdCl+, CdC12, CdC13- and C d C l P with HCdCL forming also in hydrochloric acid. The successive formation constants found: log kl* = 1.95, log k2* 0.55, log k3* = -0.15 and log k4* = -0.70, agree with published values. The acid dissociation constant of HCdC13 is about unity.
a
Introduction The cadmium-chloride system is a typical example of a divalent complex system to study by the anion-exchange method2 after the univalent silverchloride ~ y s t e m . ~It already has been studied by a variety of methods, and constants for the successive equilibria have been derived (see Table V). They may be used to check those obtained by the present method. Anion exchange also has been applied to the cadmium chloride system. Leden4 found that the anion-exchange resin Amberlite-400 absorbs cadmium, and rather better from 0.5 M sodium chloride and 0.01 M cadmium chloride than from 0.01 M cadmium chloride alone. He assumed that the species on the resin is CdC13-. Kraus, Kelson and Smith6 reported that in 6 M hydrochloric acid cadmium is so strongly absorbed that it is able to displace the strongly absorbed palladium from Dowex-1 anion exchanger. Jentzsch and Frotschere found that some anionic complex (CdC13-, CdC1d2- or CdCle4-) is absorbed from hydrochloric acid solutions on the anion exchanger Wofatit L-150, and is not eluted till the acid is diluted to 0.05 M . Fomin and co-workers7 published some distribution coefficient data for potassium chloride solutions, using them to calculate ratios of activity coefficieiit functions, employing published complex formation constants for the cadmium chloride system. They did not, however, go the other way round and try to obtain the complex formation constants. They assumed the resin species to be RCdC13, since in the 1-2 M potassium chloride in equilibrium with the resin this happens to be the predominant complex. I n a recent publication, Kraus and Nelsons pre(1) This work is taken from a part of a Ph.D. thesis submitted to the Hebrew University, Jerusalem, Israel, 1955. It was presented in part a t the 132nd Meeting of the American Cheinical Society, New York, September, 1957. (2) Part I, Y. Marcus and C. D. Coryell, Bull. Research Council, Israel, 8 8 , 1 (1959). (3) Part 11, Y.Marcus, ibid., 8 8 , 17 (195D). (4) I. Leden, Suensk K e n . T i d s k r . . 64, 147 (1952). (5) I(. A. Kraris, F. Nelson and G. W. Smith, T H I s JOURNAL, S8, 15 (1954). (6) D. Jentssch and I. Frotscher, 2. anal. Chem., 144, 17 (1955). (7) V. V. Fomin, L. N. Fedorova, V. V. Sinkovskii and M. A. Andreeva, Zhur. Fiz. Khim., 99, 2042 (1955). (8) K. A. Kraus and F. Nelson, "Proc. Int. Conference on Peaceful Uses of Atomic Energy," Vol. 7, p. 113, 1956. This was published after the present work was completed.
sented a curve for the distribution of cadmium between Dowex- l anion-exchange resin and aqueous hydrochloric acid. They msint,ain that cadmium is not eluted even with 0.01 Ai! acid (although zinc is), but dilution to less than 0.001 Ai! is necessary to remove it. It seems from the published data that there is some lack of clarity about the species absorbed, and about the concentration of chloride necessary to remove cadmium from the resin which would imply a distribution coefficient around unity. This suggests that the cadmium-chloride system merits further investigation. The magnitude of the complex formation constants, the availability of activity data for chloride in the resin (Fig. 1 in ref. 2 and Fig. 2 in ref. 3), and finally the availability of the convenient tracer Cd116 made the system attractive to investigate. Experimental Cadmium solutions were prepared by dissolving weighed amounts of cadmium metal foil containing the radioactive isotope Cd"6 in warm dilute hydrochloric acid, with the aid of a few drops nitric acid, precipitating the cadmium as carbonate, dissolving the latter in the stoichiometric amount of 0.002 M hydrochloric acid and boiling to remove the carbon dioxide. Radioactivity was assayed by a dipping G.-M. counter. Cd116 has two isomers, one of 54 hr. half-life, the other 43 days. The specific activity of the former is much larger than that of the latter soon after the irradiation, and as much as possible of this work was done in the lifetime of the former. Care was taken to measure the radioactivity of an aliquot of the solution which had not been contacted with the resin a t the same time as an aliquot which had reached equilibrium with the resin. The distribution coefficient D (liter kg.-l) was calculated using the difference between the two counts, and the results were thus not affected by the decay of the Cd"6. Lithium chloride solutions containing 0.01 M hydrochloric acid were prepared. Impure commercial lithium carbonate was dissolved in hydrochloric acid, about 9 M , and the iron impurity was removed by passing the solution through a Dowex-1 anion-exchange c o l ~ m n . ~Lithium chloride was twice crystallized from the solution. Flame photometric analysis showed only lithium lines in the pure crystalline product LiCl.H20. Chloride was determined by titration with silver nitrate. In lithium chloride solutions the Mohr method was used and in hydrochloric acid a potentiometric method. The anion exchanger used was Dowex-1 chloride, 10% cross-linked, of mesh size 40 to 100. It was air dried and had a capacit of 2.3 meq. per g. Portions o r solution and resin were shaken together for (9) K. A. Kraus and G. E. Moore, J . A m . Chem. S O C . ,72, 5792 (1950).
June, 1959
ANIONEXCHANGE OF METALCOMPLEXES : CADMIUM-CHLORIDE
16 to 20 hr. at 17 3Z 3". Relative amounts of solution and resin were chosen so as to have about half of the cadmium in the resin a t equilibrium. The error of the chloride determination was about ir0.003 M , which affected the accuracy only a t the lowest concentrations. The average statistical error of counting waR irl.5'%, causing an error in the distribution coefficient of ir30J0, except in extreme cases, where it was larger. Possible errors in the relative amounts of resin and solution were within 3~1.5%. The total expected error in the distribution coefficient D is &5% for most cases.
I
I
I
3.0
4
-2
2.0
Results Table I shows the effects of varying the cadmium concentration in the range 10-5 to 10-3 df. The relatively low specific activity of the cadmium did not allow lower concentrations to be tested. This caused rather high loading, but in no case was it higher than 3%. The results show that the distribution is unaffected by changes in the cadmium concentration. This is a strong indication that in this range, and presumably also a t higher concentrations, the complexes are m o n ~ n u c l e a r . ~
Exper8mentol Dirlr~bulian t u n e s
TABLE I THEEFFECTOF CADMIUM CONCENTRATION o s THE ANIONEXCHANGE DISTRIBUTION I N THE CADMIUM-CHLORIDE SYSTEM Appr. equil.
Cd concn., 10-4,M
log of Cd distr. ooeff. log D, I./kg.
0.4 1.4 7.1 0.2 0.7 3.4 0.3 1.6 8.3
2.38 2.42 2.39 2.79 2.78 2.77 2.25 2.27 2.23
1001
HC1 concn., mnql (molarity)
I
I
I
I
3.0
0.144
.144
.
.144 1.08 1.08 1.08 7.00 7.00 7.00
2.0
&
1.o
-
Table I1 shows the distribution of cadmium be- M tween the resin and lithium chloride solutions in the range 0.04 to 9.0 M , while Table I11 shows the ab0.0 sorption from hydrochloric acid solutions in the range 0.01 to 9.4 M.'O The cadmium concentration 0 HCI was about ilf initially, ensuring low loading, and negligible consumption of chloride ions in -1.0 complex formation. Values for the activity function for the chloride ligand, a = V Z H C I ~ ~ ( H CorI ) Ideal Distribution Curves a = n t L i c l y * ( L i c l ) as the case may be, in the hydrochloric acid and lithium chloride solutions, were obtained from Harned and Owen's compilation.'l -2.0 The results appear also in Fig. 1 as curves of log D vs. log a. - 1.0 0.0 1.o 2.0 Discussion log UCl. A detailed derivation and discussion of the mathFig. 2.-The "ideal" anion exchange distribution curves ematical relationship between the distribution coef- (log o ) , using the parameter p = 2 for the predominant ficient D and the ligand activity function a is pre- resin Dcomplex R2CdC14, plotted against the chloride activity (10) Kraus and Nelson8 presented a curve for the distribution of cadmium for the same nominal conditions as in the piesent investigation. These authors probably refer their data t o "anhydrone dry" resin, not to "air dry" resin as in the present work. This would account for the constant factor of 2.5 found between their values for the distribution coefficient and those found here in the range 0.01 to 3 M hydrochloric acid. This does not affect the slopes, and hence the calculated solution complexity constants. The divergencies beyond the above range could not be explained. (11) H . S. Harned and 8 . B. Owen, "Physical Chemistry of Electrolyte Solutions," 2nd Ed,, Reinhold Publ. Corp., New York, N . Y . , 1850.
function (log a) for hydrochloric acid and lithium chloride solutions. Dots (LiCl) and open circles (HC1) were calculated from the experimental data using eq. 7, solid curves are calculated with the constants found (Table V).
sented elsewhere,2 and only the main equations are given here. Consider the complex cadmium species to be formed from the uncharged complex CdClz
CdCL'--I
+ iC1-
(1)
1002
VOl. 63 ANIONEXCHANQE LiCl concn., WZLiCi,
M
0.040
.loo
.200 .400 .700 .goo 1.40 2.70
OF
TABLE I1 CADMIUM FROM LITHIUM CHLORIDE SOLUTIONS
Log of c1 activity function log a
Log of Cd distribution coefficient log D (L’kg.)
-1.42 -1.22 -0.82
1.52 2.21 2.45 2.72 2.94 3.06 3.13 3.43
-
.49
.21 .13 .10 .50
ANIONEXCHANQE OF HC1 concn. maci, Ai
LOR of CI activity fiinction
0.010 ,020 .030 .060
-2.0 -1.7 -1.6 -1.34 -1.15 -1.00 -0.03 - .73 - .53 .40 .05
-0.3 f0 . 2 0 . 8 zk 0.1 1.02 1.85 2.15 2.40 2.38 2.49 2.55 2.65 2.75
.loo
.I44 .170 .270 .400 ,510 1.080
-
Log of c1 aativity function log a
Log of Cd distribution coefficient log D (IJkg.)
3.55 4.30 5.00 6.00 7.00 8.00 8.50 9.00
0.75 1.00 1.20 1.45 1.70 1.05 2.05 2.17
3.57 3.50 3.60 3.51 3.46 3.42 3.32 3.34
TABLE I11 HYDROCHLORIC ACIDSOLUTIONS
CADMIUM FROM Log of Cd distribution coefficient log D (IJkg.)
log a
mLici. 1M
LiCl concn.,
HC1 concn. maci , AI
Log of c1 activity function log a
Log of Cd distribution Coefficient log D (I./kg.
2.00 2.70 3.30 3.90 4.70 5.40 6.60 7.00 8.30 9.40
0.30 .55 .73 .90 1.11 1.30 1.65 1.74 2.03 2.25
2.88 2.90 2.81 2.77 2.67 2.63 2.48 2.25 2.15 1.82
Do = log D - p rFa = log Kr‘ - log ZBi’* a+ (7) Lithium Chloride Solutions.-Chemical considerations predict that the limiting complex in solution would be C d C P . The predominant resin complex would then most likely be R2CdC14,so Emi’ = ao’ zpi’* a-i (2) where ao’ is the thermodynamic activity of the neu- that p = 2. Taking values of ,Fa from Fig. 1, ref. tral species CdC12, and Pa’” are complexity parame- 2, a log DO vs, log a’curve was constructed, and is ters involving the thermodynamic equilibrium con- shown in Fig. 2. The average charge number i stants and activity coefficient functions, which are and the average ligand number rs. were obtained from the slopes (see eqs. 31, 43 and 44c of ref. 2) assumed independent of a (see discussion in 2). Let the charge of the cadmium in the resin be d log DO/d log a = i = 2 - ii (8) designated by i = - p , which may be t.he average charge or the charge of a predominant resin species. I n the range of experimental data the slope ranges The concentration of cadmium in the resin will be from +1.5 to exactly -2.00 (from 4.5 M lithium chloride onwards). This integral limiting slope is given by an expression analogous to ( 2 ) considered as justification of selecting p = 2, since r m b = ?ao’ (3) other reasonable values of p would give fractional Using the same standard state in both phases, it limiting slopes, which would be difficult to interis easy to see that ra‘o = a’o, and thus the distribu- pret. Rjerrum’s “half-integral Z, or method and tion coefficient D,which is the ratio of the quantities SillBn’s “curve fitting” methodI2 were used to obgiven in eq. 3 and 2 will be given in logarithmic form tain the successive formation parameters ki’* or as k,”, and the over-all complexity parameters Pi‘* or P,*13 for the complexes CdClzi-i(i = 2 , i- 1and - 2 ) log D = log (rm’p/Zm’i) = or CdCln2-n (n = 1, 2, 3 and 4) which are shown in log p@‘*-p + p log ra - log E@
