August, 1955
ION-EXCHANGE EQUILIBRIA INVOLVING RUBIDIUM, CESIUM AND THALLOUS IONS
719
ION-EXCHANGE EQUILIBRIA INVOLVING RUBIDIUM, CESIUM AND THALLOUS IONS BY 0. D. BONNER' Ueparlment of Chemistry of the University of South Carolina, Columbia, South Carolina Received February 18, 1966
Equilibrium studies involving rubidium, cesium and thallous ion on Dowex 50 resins of approximately 4, 8 and 16% divinylbenzene content have been made while maintaining a constant ionic strength of approximately 0.1 molar. The characteristic maximum water uptake of the resin in these ionic forms has been measured. These results are correlated with those reported previously for other univalent ions.
Introduction The results of studies of ion-exchange equilibria on Dowex 50 resins of 4, 8 and 16% DVB content involving the common univalent cations with the exception of rubidium, cesium and thallous ion have been reported p r e v i o u ~ l y . ~ -The ~ pyridinium ion, substituted ammonium ions such as tetramethylammonium, hydroxylammonium, etc., and other complex univalent ions have not been included in these studies. The results reported herewith of equilibrium studies involving rubidium, cesium and thallous ion may be correlated with those reported previously to yield a more precise and complete experimental evaluation of certain characteristics of the ion exchange process such as (1) t,he relative affinities of the various common univalent ions for the resin, (2) the relationship between the maximum water uptake of the resin in a particular ionic form and the affinity of that ion for the resin, and (3) the va,riation of the ion-resin affinity with resin loading and with resin divinylbenzene content. It has been recognized by many workers, however, that a quantitative theoretical interpretation of ion exchange equilibria will not be possible until the osmotic properties of concentrated solutions of mixed electrolytes have been more thoroughly investigated. Experimental The methods of equilibration and separation of the aqueous and resin phases have been described in detail.2 The compositions of both phases a t equilibrium were determined by quantitative analysis for each ion in each phase for all of the exchange reactions which were studied. Concentrations of hydrogen ion in the aqueous were determined volumetrically by titration with standard alkali. Rubidium and cesium ion concentrations were determined by weighing directly as the chloride, after evaporation of an accurately measured portion of the solution and drying to constant weight a t 120-130". Thallous ion concentrations were determined volumetrically by titration with standard potassium bromate solution in the presence of hydrochloric acid, methyl orange serving as an indicator. Resin compositions were determined in a similar manner except that an aliquot of the resin, dried to an arbitrary moisture content, was analyzed directly for hydrogen ion content. The remaining iesin having the same moisture content and constituting a known fraction of the whole quantity of equilibrated resin, was then eluted conipletely with ammonium nitrate solution if thallous ion were present or hydrochloric acid if rubidium or cesium ion were present, and the eluent analyzed as before. The determination of the concentrations of cesium and lithium ion in the presence of each other, as was the case for cesium-lithium exchanges, was accomplished (1) These results were developed under a project sponsored by the United States Atomic Energy Commission. ( 2 ) 0.D. Bonner and V. Rhett, THIS JOURNAL, 67, 254 (1953). (3) 0 D. Bonner and W. H. Payne, ibzd., 68, 183 (1954). (4) 0. D. Bonner, ibid., 68, 318 (1954).
by titration of one aliquot of the equilibrium solution containing both cesium and lithium chloride with standard silver nitrate t o determine the total ionic concentration. The evaporation, drying and weighing of the cesium and lithium chloride in another aliquot enabled the calculation of the concentration of each ion. The equilibrium resin sample was completely eluted with hydrochloric acid. The resulting solution was then evaporated to dryness and heated overnight a t 120-130". The residue was then diluted and analyzed in the same manner as the equilibrium solution.
Discussion and Results The experimental data for these exchanges are presented graphically in Figs. 1-4 as a plot of IC, the concentration equilibrium quotient or selectivity coefficient as a function of the resin composition. The thermodynamic equilibrium constants or mass action constants have been calculated from the equation6s6 log K = Jollog k d N
For the purpose of this calculation the standard state for the aqueous solution is taken as the hypothetical one molal solution in which the electrolyte behaves as it does at infinite dilution and the standard state for the resin phase is each "pure" resin with its characteristic maximum water content.4J These mass action constants which are in reality average values of the selectivity coefficients, are listed in Table I. The data representing the maximum water uptake of these resins in these ionic forms and the selectivity of the resins for the ions relative to lithium ion taken as unity are presented in Tables I1 and 111. The data for the other univalent ions are also given for convenience of comparison. It is believed that the maximum error in these data is of the order of 2%. I t is apparent from these data that rubidium and cesium ion are very similar to potassium ion in average affinity for the resins and in water uptake when associated with the resins. Although the ion-resin affinity of these three ions is nearly identical in the 16% DVB resin, the relative selectivities are in the expected order for the lower crosslinked resins. The variation of the selectivity coefficient with resin composition for exchanges between hydrogen ion and the ammonium or alkali metal ions is of unusual interest. This variation becomes more marked as the series progresses from sodium to ( 5 ) E. Hogfeldt, E. Ekedahl and L. G. Sillen, Ada Chem. Scand., 4, 1471 (1950). (15) 0. D. Bonner, W. J. Argersinger and A. W. Davidson, J. A m , Chem. Soc., 74, 1044 (1952). (7) A. W. Davidson and W. J. Argersinger, Ann. N . Y . dcad. Sei., 67, 105 (1953).
0. D. BONNER
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M O L E P E R C E N T RUBIDIUM
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Fig. 1.-Rubidium-hydrogen exchange: A, 16% DVB; B, 8% DVB; C, 4% DVB.
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P E R C E N T THALLOUS R E S I N .
Fig. 3.-Thallous-hydrogen exchange: A, 16% DVB; 8% DVB; C, 4% DVB. 10.0 I-*
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Fig. 2.-Cesium-hydrogen exchange: A, 16% DVB; B, 8% DVB; C, 4% DVB.
cesium ion. Gregor8 and others have advanced a qualitative explanation for this variation based on the swelling properties of the resin. This theory does not explain, however, the reversal in selectivity which occurs when highly Cross-linked resins are SOY0 or more in the sa91tform. Reichenberg,g et al., have suggested that this reversal might be due to the presence of weak acidic groups such as carboxyl in the highly cross-linked resins. The substitution of lithium ion for hydrogen ion in an exchange reaction with one of the heavier alkali metal ions such as cesium OH a highly cross-linked resin ( 8 ) H. P. Gregor, J . Am. Chem. Soc., 73, 643 (1951). (9) D. Reiolienberg, I
