An equilibrium analysis of the aqueous H+–MoO42−–(HP)O32− and H+–MoO42−–(HP)O32−–HPO42− systems

Abstract

The speciation in the phosphitomolybdate system, H⁺–MoO₄²⁻–(HP)O₃²⁻, has been determined from combined potentiometric and ³¹P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5 < −log[H⁺] < 6.2, 40.0 ≤ [Mo]_tot ≤ 240.0 mM, 10.0 ≤ [P]_tot ≤ 40.0 mM and 1 ≤ [Mo]_tot/[P]_tot ≤ 10. ³¹P NMR data were collected in the same ranges with the exception that −log[H⁺] was decreased to 0.8. The pK_a values of phosphite were determined by means of potentiometric (1.5 < −log[H⁺] < 8.6) and ³¹P NMR chemical shift data (0.0 < −log[H⁺] < 12.0). The following log β (3σ) values were deduced: H(HP)O₃⁻, 6.09(2); H₂(HP)O₃, 7.23(4) (pK_a = 1.14). In the phosphitomolybdate system, the following complexes were found along with their formation constants ((HP) denotes phosphite): Mo₅(HP)₂⁴⁻, 65.31(4); Mo₆(HP)₂⁵⁻, 71.94(5); Mo₅(HP)⁴⁻, 51.25(8); Mo₅(HP)³⁻, 54.9(3); Mo₆(HP)²⁻, 68.99(9); Mo₇(HP)⁵⁻, 71.25(3); Mo₇(HP)⁴⁻, 74.2(2). Because hydrolysis and oxidation of phosphite to phosphate were observed by NMR, with a mixed-hetero species of low [Mo]_tot/[P]_tot ratio being detected, the four-component H⁺–MoO₄²⁻–(HP)O₃²⁻–HPO₄²⁻ system was also studied in the same way over the range 0.8 < −log[H⁺] < 6.2 and [Mo]_tot/[P]_tot fixed at 1.5. The following mixed heteropolyanion species have been found (P denotes the phosphate group): Mo₅(HP)P⁵⁻, 62.27(6); Mo₅(HP)P⁴⁻, 66.76(4). The equilibrium speciation in the two systems are illustrated by distribution diagrams. In addition, the aqueous structures are proposed for all the species formed in the systems.