Alkane-insoluble trialkylammonium double salts involving the dodecamolybdophosphate anion. I. Model studies using trioctylamine in dodecanol-modified nonane

Third-phase formation in the extraction of Mo(VI) from dilute H₂SO₄ containing phosphate by 0.05 M tri-n-octylamine (TOA) in nonane modified with n-dodecanol can be attributed to molybdate-phosphate condensation reactions leading to the appearance of dodecamolybdophosphate (DMP) anion, PMo₁₂O₄₀³⁻, as the dissolved amine salt (TOAH)₃ [DMP]. The condensation reaction was briefly examined showing that after small amounts of Mo(VI) are initially extracted, at least partly in the form of a molybdophosphate intermediate, the DMP anion forms in a slow, quantitative reaction over a period of several weeks. What little ability the solvent has to dissolve the condensation product (TOAH)₃ [DMP] appears to involve a solubilizing mixed-salt interaction with other amine salts in the system, raising the solvent tolerance for (TOAH)₃ [DMP] well above the value for the modified nonane alone (< 10⁻⁷M). Even so, DMP-containing third phases form at (TOAH)₃ [DMP] concentrations less than 1.3 × 10⁻⁴M, the exact tolerance varying within a limited range as a function of (1) the proportion of the amine sulfate which is converted to the chloride form via anion exchange with aqueous chloride and (2) the dodecanol : TOA mole ratio at constant (0.05 M) TOA molarity. Two types of third phases form, both having mixed-salt compositions. When the system contains sufficient chloride, the solid double-salt compound (TOAH)₃ [DMP] · 3(TOAH)Cl precipitates at the interface. When the system is deprived of chloride, a dense, liquid third phase forms consisting of the amine DMP, sulfate, and bisulfate salts plus nonane and possibly water. Technological implications are discussed in terms of hydrometallurgical applications of amine extraction. The double salt (TOAH)₃ [DMP] · 3(TOAH)Cl may be important in understanding compound formation between two alkylammonium salts containing different anions.