Alkane-lnsoluble Trialkylammonium Double Salts Involving the Dodecamolybdophosphate Anion. II. Effect of Amine Structure on Third-Phase Formation

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Abstract
In this paper, a rational basis is presented for the control of molybdenum “third phase” formation in tertiary amine extraction through appropriate selection of amine structure. The studies have been founded on the observation that third-phase formation in the amine-extraction of molybdenum is mainly caused by the formation of heteropolymolybdates, especially the dodecamolybdophosphate anion PMo12O40 3− (DMP). Eight trialkylamine structures with straight and methyl -branched, 8-to 13-carbon chains have been examined with regard to the type of third phase formed and the maximum molarity of the dissolved salt (R3NH)3[DMP± that can be tolerated in the solvent before precipitation occurs (i.e., the DMP “tolerance”, a type of solubility measurement). In the experiments, a standard solvent consisting of 0.05 M tertiary amine (primarily as the trialkylammonium sulfate salt) and 0.15 M n-dodecanol in n-nonane was used. For all of the amines tested, third-phase formation first involved co-extraction of Mo(VI) and phosphate from 0.1 M H2S04 giving the dissolved salt (R3NH)3[DMP± over extended equilibration times (days to weeks). With no NaCl added to the aqueous phase, liquid third phases were obtained except in the case of triisooc-tylamine (TIOA), which gave solid (TIOAH)3[DMP±. In the systems containing added chloride, third-phase formation involved combination of (R3NH)3[DMP± with