Abstract
The formation of molybdophosphate ions in aqueous solution has been studied by means of an ion-exchange process, using a strongly basic anion exchanger. This was supported by pH titration studies. From the exchange of chloride ions with molybdate and phosphate ions, the number of molybdenum(Vl) and phosphorus(V) atoms per unit charge (R-Value) is calculated. Below pH 6 two stages of condensation occur; one is represented by R(Mo) = 2.0 and R(p) = 0.21 which extends between pH 2-6; and the second is represented by R(Mo) = 4.7 and R(P) = 0.39 and occurs at pH~1. Phase pH 7 separate orthophosphate and molybdate ions coexist* The condensation stages deduced are supported by the observed change in pH of the solution as they come to equilibrium with the resin. The evidence from R-value determination, ion-screening tests and the Mo:P ratio for the exchanging species suggest that the first condensation species is the 9-molybdo-phosphate ion, [PMo9O32]5-. The presence of isopolymolybdate species competing for the resin may account for the slightly enhanced R(Mo) value. At pH 1, the 12-molybdophosphate ions [PMo12O40]3- and [HPMo12O40]2-are proposed as the predominant species. The latter ion exists in strongly acid solution and is stable in solutions of nitric acid up to 10M. Between pH 2-6, a mixture of the 9-molybdo- and the 12-molybdo-phosphates exists the latter predominating with decreasing pH. It is suggested that the 9-molybdophosphate ion is formed by the interaction of phosphate ions and isopolymolybdate ions, possibly the hexamolybdate ion [H5Mo6O22]3- . A structure for the 9-molybdophosphate is suggested which relates it to the 9-molybdoheteropolyanions of the transition metals. Salts containing the 6-molybdoferrate ion have been studied thermally in the solid state and in solution by ion exchange methods. Thus sodium 6-molybdate ferrate was confirmed as a decahydrate which could be dehydrated without decomposition of the heteropolyanion structure. The ammonium salt gave a complex decomposition curve and on the basis of this and the analytical data is assigned the empirical formula (NH4)3 [FeO6. Mo6O15]. 7H2O. Solutions containing the 6-molybdoferrate ion (0.1M in Mo) were found to be stable only in the pH range 2.5-5.0 and solutions of the free acid could not be prepared at this concentration by ion exchange methods. The result of R-value determination and ion screening tests on solutions in this pH range showed that the monomeric ion [FeO6Mo6O15]3- was present. Below pH 2.5 an insoluble iron(lll) molybdate species is formed which was soluble in nitric acid at pH 0.5. In such solutions ion exchange studies showed that an iron(lll) isopolymolybdate cation was present. It is suggested that this may be [FeH2Mo10O32]1+.