Abstract
Isopolymolybdate ions in aqueous solution have been studied by means of an ion-exchange process, using mainly a liquid ion-exchange, tri-n-octylamine hydrochloride (TOAH.C1) in chloroform. From the exchange of chloride ions with the molybdate ions, the number of molybdenum(VI) atoms per unit charge (R value) is calculated. It has been found that there are two stages of condensation in molybdate solutions of pH below 5.0; one is represented by R = 2.0, which extends approximately from pH 4.8 to 3.5; and the other is represented by R = 2.5, and extends from pH 1.9 to 1.1. The ion of R = 2.0 was also isolated as a solid from the organic layer by precipitation with kerosene. The nature of these ions was further investigated by carrying out vapour pressure lowering measurements and infrared spectroscopy of the amine layer obtained after the extraction of isopolymolybdate ions from aqueous solution. Vapour pressure lowering measurements have given a measure of the degree of aggregation of molybdenum(VI), and also the charge on an isopolymolybdate species. From the information obtained it was concluded that the stage of condensation represented by R = 2.0, consists mainly of the ions and H3Mo6O213- and H5Mo6O223- whilst the stage represented by R = 2.50, consists mainly of Mo10O324- ions. The structure of the postulated ions have been derived by deducing stability in terms of formal charge on the oxygen atoms and by a consideration of known structure Prom the available information the following scheme for the formation of isopolymolybdate ions in solution has been deduced.