Effects of Organic Solvents and Salts on the Isomerization Reaction (β→α) of Keggin-type 12-Molybdophosphate Anion

Abstract

Experimental Reagents and chemicalsAll of the chemicals were of analytical grade, and were used without further purification. Mo(VI) and P(V) stock solutions were prepared from Na2MoO4·2H2O and NaH2PO4·2H2O, The isomerization reaction of the β-form of 12-molybdophosphate ([PMo12O40] 3-: PMo12) into the α-form in aqueous-organic mixed media containing various salts was extensively investigated with cyclic voltammetry. It was found that the concentration and type of organic solvent and salt strongly affected the rate of the isomerization reaction. When organic solvents with lower permittivity than water were used as auxiliary solvents, and high concentrations of salts were added to the reaction mixture, the β-form of PMo12 (β-PMo12) became more stable, leading to slow isomerization into the α-form (α-PMo12). Based on the results of this study, it is proposed that the isomerization reaction of β-PMo12 into α-PMo12 is caused by the attack of a proton on a corner-shared oxygen atom. Also, the variation of its reaction rate upon the addition of organic solvents and salts is due to interactions between both the protons and the solvents or anions, and between the cations and the PMo12 anion.

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

“…8) [88,89]. The optical and redox properties of the two isomers differ, in that the b-isomer is reduced at more positive potentials and exhibits more intense IVCT bands [34,55,89].…”

“…8) [88,89]. The optical and redox properties of the two isomers differ, in that the b-isomer is reduced at more positive potentials and exhibits more intense IVCT bands [34,55,89]. This phenomenon is well-known in silicomolybdate chemistry where the two isomers can be isolated in aqueous solution under distinctly different conditions [90e92], but has been overlooked in analytical methods based on phosphomolybdate.…”

“…This phenomenon is well-known in silicomolybdate chemistry where the two isomers can be isolated in aqueous solution under distinctly different conditions [90e92], but has been overlooked in analytical methods based on phosphomolybdate. This is probably because b-MPA is less stable than its silicon counterpart [88], and whilst evidence suggests that the two isomers form simultaneously from different molybdate precursors in fresh 12-MPA solutions [89,91,93], b-MPA isomerises to a-MPA within minutes [89]. However, isomerisation from a / b appears to begin in earnest once the reduction process begins, and voltammetric data for aqueous 12-MPA indicate that nearly 10 min after the first reduction of pure a-MPA, the ratio of a-and b-isomers is nearly 1:1 [42].…”

Colorimetric Determination of Pyrophosphate Anion and Its Application to Adenylation Enzyme Assay

“…The slow reduction of the [(PO4)Mo12O36] 3-anion may be due to isomerization between the α-and β-form of the Keggin structure. 8 It has been found that, instead of AN, a low volatile solvent, propylene carbonate (PC), can be used as the solvent for the reduction process. In PC, [(P2O7)Mo18O54] 4-anion is also reduced to the blue molybdopyrophosphate species, of which the spectrum is similar to that produced in AN.…”

Analytical Methods for the Detection and Purification of ε-Poly-L-lysine for Studying Biopolymer Synthetases, and Bioelectroanalysis Methods for Its Functional Evaluation

“…8 3-anion may be due to isomerization between the α-and β-forms of the Keggin structure. 35 Thus, a 10 mM level ATP would not interfere with the PPi colorimetric assay significantly.…”