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An equilibrium analysis of the aqueous H+- MoO4(2-)-(HP)O3(2-) and H+-MoO4(2-)-(HP)O3(2-)-HPO4(2-) systems
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
2007 (English)In: Dalton Transactions, 124-32 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2007. 124-32 p.
URN: urn:nbn:se:umu:diva-12370DOI: doi:10.1039/b613214fOAI: diva2:152041
Available from: 2007-06-04 Created: 2007-06-04 Last updated: 2011-01-11Bibliographically approved

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