An equilibrium analysis of the aqueous H+–MoO42−–(HP)O32−and H+–MoO42−–(HP)O32−–HPO42−systems
| Autor: | Ingegärd Andersson, Masato Hashimoto, Lage Pettersson |
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| Rok vydání: | 2007 |
| Předmět: | |
| Zdroj: | Dalton Trans.. :124-132 |
| ISSN: | 1477-9234 1477-9226 |
| Popis: | 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 logβ (3σ) 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. |
| Databáze: | OpenAIRE |
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