Dee., 1957
ADSORPTION OF ZN(II)FROM BROMIDE
t e r n ~ , ~ *diffusion ,~S in the resin phase is rate determining, the chemical adsorption processes involved being, probably, quite fast like the majority of simple ionic metatheses. Exchange processes (runs 30,31 and 32) are somewhat slower than adsorption processes at comparable concentrations (run 13) and quite a bit slower for the more massive zinc(I1) chloro-complexes than for simple anions. Although the strong Lewis base and complexing agent, pyridine, markedly depresses the adsorption on anion-exchange resins of zinc(I1) from chloride media, once adsorbed the zinc(I1) species do not (29) G. E. Boyd, A. W. Adainson and L. S. Myers, Jr., ibid., 69, 2836 (1947).
1661
readily desorb into pyridine (runs 21 and 22). They do not desorb a t all into ethyl acetate (run 19), acetone (run 20) or ethanol (runs 26, 27 and 29), especially when the weight per cent. of ethanol exceeds 60. These effects evidence important solvation phenomena and will be treated further in a subsequent paper of this series. Acknowledgment.-The authors wish to express their gratitude for the valuable assistance of Professors C. D. Coryell and J. w. Irvine, Jr. of M.I.T. and R. M. Diamond of Cornell University and for facilities and funds supplied by the United States Atomic Energy Commission through the Division of Sponsored Research of the Massachusetts Institute of Technology.
THE ADSORPTION OF ZINC(I1) ON ANION-EXCHANGE RESINS. 111. THE ADSORPTION FROM BROMIDE, FLUORIDE, CYANIDE, OXALATE, ACETATE, NITRATE, PHOSPHATE, SULFATE, AND ALKALINE MEDIA; THE PERCHLORATE EFFECT, AND THE ADSORPTION FROM MIXED SOLVENT MEDIA BY It. A. HORNE,'R. H. HOLMAND M. D. MEYERS Contribution of the Department of Chemistry and Laboratory for Nuclear Science of the Massachusetts Institute of Technology Cambridge, Massachusetts Received J u l y $9,1967
Zinc(I1) adsorbs on Dowex 1-X8 from bromide, fluoride and oxalate media but not from acetate, nitrate, sulfate and alkaline media. The adsorption curve from HBr at 24.8 f 0.2" has been interpreted to yield the following constants for the formation of ZnBr+, ZnBrz, ZnBrs- and ZnBrr-: 1.0,0.7,0.5 and 0.3. The presence of perchlorate ion markedly depresses the adsorption of zinc(I1) on Dowex 1 chloride from HC1 solution. I n mixed solvent media of dielectric constant lower than that of water the differences in the adsorption of zinc(I1) from HC1 and LiCl solution observed in the case of aqueous solutions tend to diminish., the adsorption peak is shifted to lower chloride concentrations, and the value of the adsorption coefficient at the peak is increased: these observations are explained in terms of the decreased dielectric constant of the medium resulting in changes in the extent of ionic association.
Introduction The adsorption of metals from chloride media on anion-exchange resins has been exhaustively studied for separative purposes,2 but the adsorption from other anionic complexing media has received comparatively little attention. Burstad, Kembler, Forrest and Wells3 have studied the elution Of adsorbed Zn(CN)d- under varying conditions and Herber and Irvine4 have investigated the anionexchange resin adsorption of zinc(I1) from, HBr. The purpose Of the present study was to extend research the anion-exchange resin adsorption of zinc(I1) to non-chloride media and alkaline systems, to investigate the roles of solvation and medium dielectric constant, and to test the secondary cation effect hypotheses that were outlined earlier.6 Adsorption experiments in mixed media are of further interest because, while the effect of (1) Radio Corporation of America, Needham Heights, Mass. (2) K. A. Kraus and F. Nelaon, Collected Papers of the Geneva Conference on the Peaceful Uses of Atomic Energy, Paper UN-837 (1955). (3) F. H. Burstad, P. J. Kembler, N. F. Forrest and R. A. Wells, Ind. EnQ. Chem., 46, 1648 (1953). (4) R. H. Herber and J. W. Irvine, Jr., J . A m . Chem. Soc., 76, 987 (1954). (5) R. -4.Horne, THISJOURNAL,61, 1651 (1057).
mixed and non-aqueous solvent systems on cation exchange and the hydration of cations in Polystyrene resins has been the effect on anion exchange, especially of complex anions, has received little Experimental The preparation of the tracer 245 day zinc~6and the determination of the distribution coefficient, D (vol. of soln. in ml.)(radioactivity in resin) D = (wt. of resin in g.)(radioactivity in soln.) (1) from batch runs a t 24.8 f 0.2" are described in the first paper of the present series.6 In the alkaline and perchlorate runs the resin (Dowex I-X8) was in the chloride form,. in the other runs in the form of the anion of the added solution electrolyte, being converted to these forms from the chloride form by conversion to the hydroxide form followed by treatment with the appropriate acid.
Results and Conclusions No adsorption of zinc(I1) on Dowex 1 was obup to 8.0 in acid, served from 2.5 &! in HzS04, 5.0 &! HF and 13 M "01. Resin discoloration indicating resin decomposition occurs at high concentrations of the latter. AI-
''
(6) R. W. Gable and H. A. Strobel, ibid., 60, 513 (1956). (7) E. Glueckauf and G. P. Kitt, Proc. Roy. SOC. (London), A228,
322 (1955).
.
1662
R. A. HORNE,R. H. HOLMAND M. D. MEYERS I
Vol .61
alogously, was Leden12 able to find anionic cadmium(I1) sulfate complexes. No adsorption 56 of zinc(I1) from nitrate solutions was anticipated. The coefficient of the adsorption of zinc(I1) from &Po4 solution increases rapidly in the 1 to 3 M region, log D reaching a value of about 1.25, and then remains fairly constant up to high concentrations. The variations of log D with the concentration of NaCN and oxalic acid are shown in Figs. 1 and 2. Both curves show high adsorption at low concentrations of complexing ligand as might be 30 expected from the considerable stability of zinc(I1) 0 05 1.0 I5 20 25 30 35 40 cyanide and oxalate c ~ r n p l e x e s ,(Zn(CN)4-)/ ~~~~~ Total Added Concentrotion of NoCN,MFig. 1.-The adsorption on Dowex 1 cyanide a t 25' of tracer (Zn++)(CN-)( = 10l6 and (Zn(CzO4)~')/(Zn++)radiozinc( 11) from sodium cyanide solutions. (c,04-)2 = 107. The unusually strong adsorption of zinc(I1) from cyanide media indicates that the equilibrium constants for reactions of the type 3.5L 5.5
I
11
30L
2R+ 2RCN
-----
5
Y
l
;;;,
I
I
I
IO
05
0
1
= R*Zn(CN)4
(2)
Oi .
10-
I
+ Zn(CN)4'
2
3 4 5 Bromide Concentrolion,M.
6
7
8
Fig. 3.-The adsorption on Dowex 1 bromide at 25' of tracer radioainc(I1) from hydrobromic acid and lithium bromide solutions.
though ZnCH3COO* (Kf = 0.38) and ZnFf (IG = 5 ) have been reported18*9 evidently zinc(I1) does not form anionic complexes with these ligands under present experimental conditions since no adsorption is observed. Edwardslo quotes a large and calculates a small yet appreciable formation constant for anionic zinc(I1) sulfate complexes; Purser and Stokes," however, found no evidence for anionic zinc(I1) sulfate complexes, nor, an(8) 9. Bardhan and S. Aditya, J . Indian Chem. Soc.. 38, 105 (1955). (9) A. D. Paul, Univ. of California Radiation Lab. Report 2926 (1955). (10) J. 0. Edwards, J . Am. Chem. SOC.,7 6 , 1540 (1954). (11) E. P. Purser and R. H. Stokes,ibid., 73,6650 (1951).
+ Zn(CN)r-
= RpXn(CN)a
+ 2CN-
(3)
are possibly very large while the large and near constant adsorption in the 2 to 4 M NaCN region may be due to the weakly acidic character of HCN, an inference supported by the marked acid dependence of the elution of adsorbed zinc(I1) cyanide ~pecies.~ The oxalic acid curve also shows some trailing, and in addition it appears to have no maximum, the zinc(I1) adsorbing strongly on the oxalate form of the resin even in the absence of added oxalic acid in the external solution. While this may be due t o residual free oxalic acid in the resin, it also suggests the possibility of preparing highly selective cationexchange resins by the adsorption of polyfunctional strongly complexing anions on an anion exchange resin. Unfortunately, tentative studies of the adsorption of nickel(II), which does not adsorb on Dowex 1 from chloride solution but which is known to form strong oxalate c o m p l e x e ~ , ~from ~ ~ ' ~HC1 media onto the oxalate form of the resin were ambivalent. The variation of the coefficient of adsorption of tracer quantities of zinc(I1) on Dowex 1-X8 bromide from solutions of varying HBr and LiBr concentration is shown in Fig. 3 together with HCl and LiCl curves for comparison. The ionic strength was allowed to varv. Analyzing the HBr data kith a plot of log D versus log mr"-(nGt shown), - the bromideion activity evaluated from where U B ~ is HBr activity coefficients given by Biermann and Yamasakils and International Critical Tables,17by the method of Coryell and Marcus1*and Irving and Rossottils one obtains the estimates of the successive formation constants of the zinc(I1) bromo(12) I. Leden, Svensk Kem. Tidekr., 64, 145 (1952). (13) J. Bjerrum, "Metal Ammine Formation in Aqueous Solution," P. Haase and Son, Copenhagen, 1944, p. 64. (14) R. H. Herber and J. Irvine, Jr., J . Am. Chem. Soc., 78, 905 (1956).
W.
(15) J. E. Barney, W. J. Argersinger, Jr.. and C. A. Reynolds, ibid.. 73,3785 (1951). (16) W. J. Bierrnann and R. S. Yamasaki, ibid., 17, 241 (1956). (17) E. W. Washburn, Ed., "International Critical Tables of Numerical Data, Physics, Chemistry, and Technology," Vol. VII, McGraw-Hill Book Co., Inc., New York, N. Y., 1930, p. 234. (18) C. D. CoryeU and Y. Marcus, Bull. Res. Council Israel, 4 , 7 (1954). (19)
H.Irving and H. 9. Roseotti. J. Chem. SOC..3397 (1953).
!
I
I
1
1
, i
I 6
1 I
ADSORPTIONOF ZN(II)FROM BROMIDE
Dec., 1957
complexes summarized in Table I. Above 1.0 Zn++
I
1663
I
1
I
i
+ Br = ZnBr+,
K1 = (ZnBr+)/(Zn++)(Br-) (4)
+ Br- = ZnBrs, Kz = (ZnBrz)/( ZnBr +)(Br -) ZnBr2 + Br- = ZnBrs-, K s = (ZnBrr-)(ZnBrz)(Br-) ZnBrs- + Br- = ZnBr4',
ZnBr+
(5) (6)
ICr = (ZnBr,-)/(ZnBr,-)(Br-)
(7)
M HBr an invasion correction of 2 log
I
I
b
t
(aBr-)S
0.5 I 1u' ' 10.' O 0-2 L 10-1 a 10-0 l was subtracted from log D,it being assumed that the activity coefficient of bromide ion in the resin Totol Concentration of H C I Q . M 4 is nearly equal to that in solution, an approxiFig. 4.-The effect of perchlorate ion on the adsorption of mation which has been found to be satisfactory tracer radiozinc(I1) on Dowex 1 from chloride solutions a t in the case of HC1.20 The plot of invasion corrected 25' and a hydrogen ion concentration of 3.0 M. a limiting slope of -2 inlog D versus log U B ~ has dicating2 t.hat, since the charge of the complex is -2 and of the complexing ligand -1, the average number of complexed ligands, E, is 4 in this concentration region.
TABLE I THE SUCCESSIVE FORMATION CONBTANTS OF COMPLEXES OF ZINC(II) AT 25' (a)
(b)
THE
BROMIDE (0)
KI (1.0) 0.25 20 K2 0.7 .. .. K8 .5 .. K4 .3 .. a Present work. * L. G. Sillen and B. Lilje vist, Svensk Kem. Tidskr., 56, 85 (1944). CE. Ferrell, J.%. Ridgion and H. L.Riley, J . Chem. Soc., 1121 (1936).
..
0 LiCl
..
The adsorption of tracer zinc(I1) on Dowex 1-X8 chloride at 24.8 0.2' from aqueous solutions 0.25 to 6.0 M in NH40H, 0.25 to 12.5 M NaOH and 0.25 to 4.0 M LiOH was not reproducible, presumably because of resin instability under these strongly alkaline conditions. It may be said, however, that the adsorption is small, Le., D is less than 10, and that the adsorption is consistently greater from NH40Hthan from NaOH solutions of comparable concentration. Evidently, then, ZnO a and hydroxyl zinc(I1) complexes do not adsorb strongly. The adsorption coefficients from solutions 1.0 M in ",OH and 1.0 to 5.0 M in NH4C1 and solutions 0.10 M in LiOH and 2.0 to 6.0 M in LiCl are also less than 10, indicating that neutral or anionic chlorohydroxyamino-complexes are not formed in appreciable amounts or if formed are but weakly adsorbed. The presence of perchlorate ion markedly depresses both the solvent extraction21-2aand anionexchange resin absorption24 of metals from HC1 media. Clearly, HC104,or perchlorate salts cannot be used to maintain constant ionic strength in experiments of these types because of this complication. Figure 4 shows the adsorption of zinc(I1)
A HCI 0 KCI
GSCl
*
(20) K. A. Kraus and G. Moore, J. Am. Chem. Boc., 75, 1467 (1953).
(21) R. H. Herber, E. Rudzitis and J. W. Irvine, Jr., M.I.T.Lab. Nua. Sci., Ann. Progress Report, May 31. 1955, p. 16. (22) R. J. Dietz, L. C. Boger, G. 8. Golden and J. W. Irvine, Jr., ref. 21, ibid., May 31, 1954, p. 18. (23) C. H. Brubaker, Jr., M.I.T. Lab. Nuc. Sci.. Progress Report, Nov. 30, 1955, p. 7. (24) R. A. Horne, ref. 23, Feb. 29, 1956, p. 14; Ann. Progress Report, June 1, 1957 to May 31, 1956, p. 25.
0
2
I
3
5
4
Chloride Concentration .M.
Fig. 5.-'The adsorption of tracer radiozinc(I1) on Dowex 1 a t 25' from hydrochloric acid, lithium chloride, potassium chloride and cesium chloride solutions of 0.24 mole fraction ethanol and 0.76 water compared with the functions
+ 50X X 1 + 20/X + 1/X2 + 1 + 20/X + 1/X* 1.8 X 1 0 ' + 70X + 1 + 2O/x 4- 1/x2 D=70X + 1 + 20/X + l/Xa D = -1.8 X IO6 + 500X4 + 1 + 20/X + l/Xs 500Xz+ 1 + 20/X + l / X p D = -1.8 X lo4
50X
where X =: ( M + ) = (Cl-),
as a function of the added perchloric acid concentration in HC104-HC1 media 3.0 M in hydrogen ion, while in 'Table I1 are listed the results in the presence of various acids, including perchloric, of TABLE I1 THE ABSORPTION OF TRACERZINC(TI) ON DOWEX1-X8 FROM 2.0 M HYDROCHLORIC ACIDAT 25' I N THE PRESENCE OF VARIOUS ANIONS Resin form
Chloride Phosphate Acetate Sulfate Nitrate Chloride
Concn. of added acid
log D
Ka
1.0 M HCI (3.0 m total) 0.33 M HaP04 1.0 M CH,COOH 0.5 M HzSOI 1.O M HNOI 1.0 M HCIO,
3.25 3.15 3.04 3.04 2.38 0.70
l.O(Cl-) 0.25(HzP01-) 0.17(CHa000-) 4.1(HS04-) 3.8(NOa-) 35( Clod-)
R. A. HORNE, R. H. HOLMAND M. D. MEYERS
1664
Dowex 2.2'3 If the principal desorption process is of type (8) then it is a t least qualitatively clear,
3'51F-
R2ZnCL
I
in 0.31 Mole Fraction Methanol
1.5
in020Mofe Fraction Acetone
1.0
0
I
Vol. 61
2 3 4 Chloride Concentratlon ,M.
5
6
Fig. 6.-The adsor tion of tracer radiozinc(I1) on Dowex 1 a t 25' from hydrocgloric acid and lithium chloride, methanol-water and acetone-water solutions.
+ 2A-
= 2RA
+ ZnC14'
(8)
with the exception of H2S04, why these various anions do or do not significantly depress the adsorption of anionic zinc(II) species. From a comparison of the concentrations of chloride and perchlorate ion required to yield a given decrease in D (vide the Dowex 1-HC1 curves in Figs. 3 and 4) the latter appears to be about one hundred times more effective in producing depression. This effect is greater than anticipated from the selectivity differences in Table 11. In addition to its greater selectivity, perhaps the resin dehydrating properties of perchlorate ion27 contribute to its desorptive effectiveness if solvation equilibria ?re to be maintained in the resin phase with its diminished water content. Two explanations of the "lithium chloride effect" have been consideredb: ion pair formation in the resin phase only and ion pair formation in both the internal and external solution. While the former is probably more important when the solution phase has a high dielectric constant, as the dlelectric constant of the external solution is lowered and becomes comparable to that of the resin phase the latter explanation might be expected to become dominant. On the basis of the hypothesis of ion-pair formation in both external and internal solution the following expressions have been derived6 D =
0
I 2 3 Chloride Goncentro t ion , M.
4
Fig. 7.-The adsorption of tracer radiozinc(I1) on Dowex 1 at 25" from hydrochloric acid and lithium chloride solutions 0.40 mole fraction ethanol and 0.60 water.
1
Kc = (M2ZnClJ/(M+)'(ZnC&-) Ka' = K2(R+)Z = (R2ZnCL)/(ZnCL') Ka = (ZnCL-)/(ZnCl,-)(Cl-)
(11) (12) (13) (14)
Kb = (ZnC14")/(znC12)(C1-)'
(15)
and
Kb and K , will decrease and Kz, K 3 and Kh will in-
crease with decreasing dielectric constant. Figure 5 shows the adsorption curves of tracer zinc(I1) on Dowex 1-X8 chloride from LiC1, HCI, KC1 and CsC1-water-ethanol solutions, the mole fraction I of the latter being, ignoring the dissolved salts, ea' 0 10 20 30 40 50 60 70 80 90 0.24 (which corresponds to about 44 weight 5% Welphl Percent of Ethanol Fig. %-The adsorption of tracer radiozinc(I1) on Dowex 1 ethanol and a dielectric constantz8 of 52, again ignoring the dissolved electrolyte). Functions formed at 25" from ethanol-water mixtures 0.5 M in chloride ion. from equations 9 and 11 With constants estimated zinc(I1) adsorption measurements from 2.0 M HC1. by trial and error to give the best fit (broken lines The last column in Table I1 gives the selectivity in Fig. 5 ) are now, as expected, in much better constants, K,, of Dowex 1 for the anions indicated (26) C. D. Coryell and D. H. Freeman, private aommunication, in parenthesesz6; the perchlorate value is for 1956. I
I
I
I
d
(26)
(IQiil),
R. M. Wheaton snd W.
C. Bwman, Ind, Bnu. Chem., 48, 1088
(27) (28)
G.E. Boyd and B. A. Soldano, 2. Elektrochem., ST, 182 (1963).
a. Akerlof, J . A m . Chem. S o h 154, 4126 (1863).
. NOTES
Dec., 1957
1665
agreement with the observed ethanol-water data than in the case of the aqueous data reported previously.6 Divergencies between theory and observation, however, are still apparent at higher concentrations of HC1 and particularly LiC1. Evidently ion-pair formation in the resin phase is still somewhat favored. I n 0.31 mole fraction methanol (Fig. 6) these divergencies diminish, and finally when the mole fraction of ethanol is increased to 0.40 vanish as anticipated (Fig. 7), although difficulties now appear in the region of the adsorption peak. The adsorption curves in 0.20 mole fraction acetone solutions (Fig. 6) are similar to but fall somewhat below the corresponding ethanol curves, suggesting, inasmuch as the dielectric constants of these two media are very similar, specific solvation effects. Figure 8 shows the variation of the adsorption of tracer zinc(I1) on Dowex 1-X8 chloride at 24.8 A 0.2" with the per cent. by weight of ethanol in ethanol-water solution. Log D increases as the dielectric constant decreases up to about 35 weight yo ethanol, presumably due to the increase of Kz',remains fairly constant in the region 35 to 65 weight % ethanol, and then at 65 weight yoethanol, the same concentration at which desorption of adsorbed zinc(I1) species into ethanol-water solution ceases,29 abruptly increases. This increase is accompanied by an inversion of the LiCl and HC1 selectivities, the greater adsorption now being from HC1 solution. I n the resin phase where competition for water of solvation is keenest, ion dehydration, resulting in an inversion of the order of effective ionic radii and greatly enhanced ionpair formation, is occurring. Such abrupt transitions in the water content of the solvation spheres of cations in ethanol-water mixtures have also been observed in the case of iron(III),80although
at lower water concentrations. Adsorbed zinc(I1) complex anions do not desorb into ethanolic media above 65 weight Yobecause ethanolic solvation, unlike hydration, is insufficiently strong to disrupt the ionic associations in the resin phase. The present results and conclusions are similar to those of Gable and Strobe16 who explained the enhanced selectivity of cation exchange in methanol in terms of ion solvation and ion-pair formation. Solvation is also known t o play an important role in solvent extraction where, commonly, the acid of the metal halide complex extracts with a definite number of waters of h y d r a t i ~ n . ~ ~ , ~ ~ Conclusion On the basis of the results reported in this and the previous papers of the present seriessJ9 we conclude that the adsorption of metals on anion exchange resins from complexing media is an equivalent exchange phenomenon involving anionic complexes of the metal, similar to cation adsorption on cation-exchange resins or simple anions on anionexchange resins, complicated by (a) the as yet poorly understood physical chemistry of concentrated solutions of electrolytes, (b) the dependence of the concentrations. of absorbing species on the concentration of complexing ligand and (c) ionic association in the resin phase, and possibly in the external solution phase as well, of the anionic metal complexes with the cation of the supporting electrolyte. Acknowledgment.-The authors wish to express their gratitude for the valuable assistance of Professors C. D. Coryell and J. w. Irvine, Jr., o f ' M.I.T. and R. N. Diamond of Cornel1 University and for facilities and funds supplied by the United States Atomic Energy Commission through the Division of Sponsored Research of the Massachusetts Institute of Technology.
(29) R. A. Horne, R. H. Holm and M. D. Meyers, THISJOURNAL, 61,1655 (1957). (30) R. A. Horne, Ph.D. Thesis, Columbia University, 1955.
(31) H.Irving and F. J. C. Rossotti, J . Chem. SOC.,1938 (1955). (32) P.C.Yates, R. Laran, R. E. Williama and J. E. Moore, J . A m . Chem. Soc.. 71, 2212 (1953).
NOTES THE PHOTOCHEMlCAL OXIDATION OF ZINC AND CADMIUM SULFIDE BY M. CLAREMARKHAM, JOANBARRY,MARIONIAVA AND JANET HADDAD Department of Chemistry, Saint Joseph CoUeoe, West Hartford, Conn. Received July 10, 1967
I n the past few years considerable attention has been focused on the effects of ultraviolet light on photoconducting oxides and sulfides.' Many data have accumulated to show that zinc oxide is a photocatalyst for the formation of hydrogen peroxide from molecular oxygen, but that there must be oxidizable organic substances present if the hydrogen peroxide is to accumulate to any ap'
(1) M. C. Markham, J . Chsm. ad., 89, 640 (1966).
preciable e ~ t e n t . ~ The J net reaction seems to be the oxidation of the organic material and the reduction of molecular oxygen, part of which is found as hydrogen peroxide, and the remainder in the oxidized organic product^.^ Recently two articles have appeared in THIS JOURNALwith the conclusion that some photoactive metallic sulfides are catalysts for the formation of hydrogen peroxide from oxygen and water.6p6 Our finding of considerable amounts of (2) T. R. Rubin, J. G . Calvert, G . T. Rankin and W. MacNevin, J . A m . Chem. SOC.,71, 2850 (1953). (3) M. C. Markham and K. J. Laidler, THISJOURNAL, 67, 363 (1953). (4) C. B. Vail, J. P. Holmquint and L. White, Jr.. J . Am. Cham. ~ o c . ,76, ea4 (1954).
