Electrochemical measurements of isopolyoxomolybdates: 1. pH dependent behavior of sodium molybdate

MoO₄ ^2- condenses on increasing acidity in aqueous solution and the extent of condensation depends on the level of pH. Frequency response analysis in the range 1000 Hz to 35 mHz and cyclic voltammetry were used to characterize the admittance, impedance, double layer capacitance, and semiconduction behavior of different protonated mono-, hepta- and octa- isopolyoxomolybdate species in solution. It was found that the dominant species was MoO₄ ^2- in the pH range 7-12 and the protonated species were of Mo₇O₂₄ ^6- and of Mo8O₂₆ ^4- in the pH range 3-5 and below pH 2 respectively. This observation is consistent with the potentiometric titration data. Frequency as well as potential dependent color oscillations between blue and brown or gold at the mercury electrode were also observed. The results promise a new way of investigating polyoxometalates in solution.