Analysis of Complex-formation Reaction in Molybdenum Blue Method by ESI-MS

• Published in: BUNSEKI KAGAKU Vol. 61; no. 12; pp. 1049 - 1054
• Main Authors: TAKAHASHI, Mariko, TANAKA, Miho
• Format: Journal Article
• Language: Japanese; English
• Published: Tokyo The Japan Society for Analytical Chemistry 2012; Japan Science and Technology Agency
• Subjects: Ascorbic acid; complexation; Derivatives; electro spray ionization mass spectrometry; Molybdenum; molybdenum blue method; molybdo antimonyl phosphate; molybdophosphate; Phosphates; Potassium; Precursors; Reduction
• ISSN: 0525-1931
• DOI: 10.2116/bunsekikagaku.61.1049

JIS K 0102 defines Molybdenum Blue Method as a spectrophotometric determination of phosphate. Molybdo antimonyl phosphate is formed by the reaction of phosphoric and molybdic acids and potassium antimonyl tartrate, which is then reduced by ascorbic acid. The absorbance of the reduced form of molybdo antimonyl phosphate has been known to be higher than those of other complexes. However, the reaction system of the Molybdenum Blue method and the structure of molybdo antimonyl phosphate have not been elucidated so far. To investigate the reaction system and structure, phosphoric and molybdic acids and their derivatives were monitored using electrospray ionization mass spectrometry (ESI-MS) at each reaction step in the procedure. In earlier reports, [PSb2Mo10O40]3− (m/z 625) was considered to exist as molybdo antimonyl phosphate, but in our studies with ESI-MS this complex was not observed. In a sample solution, a peak at m/z 1033 was identified as [PSb2Mo(VI)7Mo(V)5O40]2−; this complex was formed by the addition of both ascorbic acid and potassium antimonyl tartrate. Also, reports suggest [PSb2Mo10O40]3− (m/z 625) to be first formed and reduced by ascorbic acid. According to our observation by ESI-MS, the formation of molybdophosphate was completed and followed by reduction with ascorbic acid. After reduction, antimony would be attached to the reduced molybdophosphate complex. The complex [PSb2Mo12O40] is considered neither to be formed in gas-phase nor to be derived from precursors in the solution, but exists as a stable complex. We have thus illustrated that ESI-MS is a useful tool that can be applied to study reactions and metal complexes in solution.