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 pKa 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 (3) values were deduced: H(HP)O3–, 6.09(2); H2(HP)O3, 7.23(4) (pKa = 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.
2007. 124-32 p.