Ion Exchange Separation of Microgram Quantities of Osmium from Large Amounts of Base Metals.

Abstract

measuring the dissociation constants of the S-methyl or Se-methyl derivatives (Table 111) since in these methyl derivatives participation of the zwvjtter ion forms is excluded (b). From the values of Kog as well as from K,, and K,, from use of Equa,tions 1 to 3 the other K, values are readily obtained. With this information (Table IV:l it now becomes possible to evaluate the effect of structural changes on the dissociation phenomena in detail. esaniination of the dissociation scheme shown above leads to the following predictions. The values of pKaD, expected to be sensitive to the electronegativity of X, should decrease considerably from 0 to Se. Similarly, values of pK,, which correspond t'o the loss of a proton from i;hp group-XH in the cationic species, !should be lower than those of pK,, by virtue of the presence of the positive charge, but should change in the same manner as pK,,.Ais can be seen from Table IV values cf pK,, drop 4.51 units from oxine to sele;.oxine in the same order as observed in the acid strengths of H20, H,S, and H2Se. The magnitude of these values seem to be reasonable since that for osine is very close to the pK, of a-naphthol (9.85) and l.hat for thiooxine is close to the pK, of thiophenol (6.5). I t is of interest to note the similarity of the value of pK,, of oxine with that (6.8) of the 'V-methyl analog of the cationic oxine species (12). The relationship between pK,, and pK,, can be expressed as: pK,, = 1.5 pKaD -8.4. The relationship shows two consequences of the presence of the positive charge. First, its presence causes a general lowering of the pK,, and second, this lowering varies linearly with X, being greatest with selenoxine. This might be due to the greater sensitivity of the more highly polarizable selenium to the presence of the neighboring positively charged proton.The effect of changing X on the values of pKaB and pKac would not be expected to be appreciable since the X atom is not directly involved in the dissociation processes nor is there any appreciable conjugative interaction between the X and S atoms. This is the case in the pK,, values observed and, were it not for oxine, in the pK,, values as well. The deviation of the value of pK,B for oxine may reflect the stabilization of the neutral species through hydrogen bonding.K , would be eypected to increase with increasing strength of -XH and with increasing dielectric constant of the solvent. Both selenoxine and thiooxine exist almost entirely in their zwitter ion forms in water. In 50% dioxane-water (dielectric constant = 32) selenoxine is still present entirely as the zwitter ion form but thiooxine, with its significantly lower K 1 even in water, is probably largely in its neutral form in 50% aqueous dioxane. For osine, even in aqueous solution, the concentration of the awitter ion form is negligible.