Abstract
The stable ionic species formed as the pH value of molybdate solutions is lowered have been examined by means of the exchange reactions they undergo with strongly basic anion exchangers. The system is found to be a simple one consisting of the normal molybdate (MoO42-) above pH 7 and two condensed ions on the acid side of neutrality. Ratios of molybdenum atoms to ionic charge (R values) of 2.0 and 5.0 have been established for these two condensed ions in order of their formation as the acidity is increased. The results of pH titrations support these conclusions. The system is apparently a continuous one, the ion of R value 2.0 initially co-existing with simple molybdate but attaining predominance between pH 5 and 6. The R value 5.0 ion is formed and exists, together with the first condensed species, in increasing proportion as the pH of the solution is dropped below 5.5. At molybdenum concentrations greater than 0.05 g. atom/1 and pH ca 1, R value 5 is shown to represent the ion of maximum condensation which under these conditions is virtually the sole molybdate species present. Using exchangers of various porosity which were specially manufactured the two condensed ions have been separated by an ionic sieve mechanism. By comparison of these experimental results with those of similar tests employing the 12-molybdophosphate anion as a reference, the R value 2.0 ion is shown to be considerably smaller than the species of R value 5.0 and that the latter approximates to the hetoropolymolybdate in size. From stereooheinical considerations, and by comparison of the metal to charge ratios of the ions suggested in the literature with those found experimentally, the formulae Mo 40132- and H2Mo100322 proposed for the two condensed species.