Atsuyoshi Saito

A voltammetric study of Keggin-type heteropolymolybdate anions

Abstract The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

Charge dependence of one-electron redox potentials of Keggin-type heteropolyoxometalate anions

Abstract The two-step one-electron reduction processes of Keggin-type heteropolymolybdate and heteropolytungstate anions were investigated in several solvents using cyclic voltammetry. The slope of a plot of standard redox potentials E° against the ionic charge ΔE° was evaluated. It was found by theoretical considerations that ΔE° consists of a constant term independent of the solvent and another term comprising the charge-dependent component of the solvation energy. The difference between ΔE° for water and for 1,2-dichloroethane (1,2-DCE) was in good agreement with the value estimated from the transfer energies across the 1,2-DCE|water interface which were measured by ion transfer voltammetry. The values of ΔE° in various solvents cannot be explained by the Born-type electrostatic solvation energy, suggesting that the short-range interaction is important for the interaction of a Keggin anion with solvent molecules.

A kinetic study of the formation of 12-molybdosilicate and 12-molybdogermanate in aqueous solutions by ion transfer voltammetry with the nitrobenzene-water interface

A novel electrochemical approach was developed for the kinetic study of the formation of heteropolyanions. The method (dual pulse amperometry, DPA) is based on the detections of currents due to the transfers of polyanions at the nitrobenzene-water interface. In this study, DPA was applied to the kinetic study of the formation of two Keggin anions, viz., [SiMo12O40]4− and [GeMo12O40]4−. Prior to the kinetic study, cyclic voltammetric measurements were performed to confirm that the Keggin anion and its lacunary anion ([H3SiMo11O39]5− or [H3GeMO11O39]5−) coexist at equilibrium under certain conditions. In DPA, double voltage pulses of different amplitudes were alternately applied to the interface to follow the concentrations of both the Keggin and the lacunary anions. The concentration-time profiles for the polyanions could be elucidated by the two-step consecutive reactions mechanism. The lacunary anion was then found to be the intermediate of the Keggin anion.

Synthesis and characterization of 15-molybdodiphosphate

Abstract The formation of a 15-molybdodiphosphate anion was ascertained in a 0.05 M (M = mol dm−3) Mo(VI)−0.2 M HCl−4 mM P2O7 4− −40% (vol./vol.) CH3CN system at ambient temperature, and the yellow complex was isolated as a tetrapropylammonium (n-Pr4N+) salt. Once the parent 18-molybdodiphosphate anion, [(P2O7)Mo18O54]4−, was formed in the solution, it transformed spontaneously into the 15-molybdodiphosphate anion, [H6(P2O7)Mo15O48]4−. The transformation reaction was followed voltammetrically at the glassy carbon (GC) electrode. The rate of transformation, which depended on the concentrations of CH3CN and HCl, on the Mo/P mole ratio, and also on the temperature of the solution, was discussed. The complex was characterized by the voltammetric and IR spectroscopic behaviors.

Pulse polarographic study of Mo(VI) in sulfuric acid

Abstract Normal and differential pulse polarographic studies show that, for Mo(VI) concentrations below 0.1 mM, the reduction of Mo(VI) produces a monomeric Mo(V) species. At higher Mo(VI) concentrations, one monomeric and two dimeric Mo(V) species are formed in 0.1–1 M H2SO4. In dc polarography, the reduction wave of Mo(V) to Mo(III) shifts to more negative potentials with an increase in the Mo(VI) concentration. This behavior is accounted for in terms of the increase of the dimeric Mo(V) species at the electrode. The reduction of the monomeric Mo(V) species to Mo(III) catalyzes the reduction of nitrate.

Electrochemical reduction of hexamolybdate(2 – ) ion in acidic aqueous-organic media

Abstract The voltammetric behaviour of hexamolybdate(2−) ion (Mo6O192−) has been investigated in detail. In acidic solutions containing water-miscible organic solvents such as acetone and acetonitrile, Mo6O192− ion is reduced at a glassy carbon (GC) electrode and then decomposes rapidly into Mo(V) and Mo(VI). The reduction stoichiometry in the water + acetone system was confirmed as Mo6O192− + 4 e− + 12 H+ = 2 Mo2vO42+ + Mo2VIO52+ + 6 H2O. Blue mixed-valence isopolymolybdates (the so-called isopoly blues) are formed at the electrode surface by subsequent chemical reactions. Such a reduction mechanism is quite different from that in neat organic media where Mo6O192− undergoes a two-step, one-electron reduction.

Voltammetric determination of sulphate ion through heteropoly blue formation

Abstract The voltammetric determination of sulphate ion is described. A blue molybdosulphate complex was formed at a glassy carbon (GC) electrode surface by short-time (1 min) electrolysis of 70 mM Mo(VI)-1.0 M HCl-60% (v/v) acetone solutions containing sulphate ion at a potential of −0.05 V vs. Ag/AgCl (saturated KCl). The current due to the oxidation of the blue complex is linearly dependent on the sulphate ion concentration in the range 2 × 10 −5 to 1 × 10 −3 M.

Polarographic study of the Co(II)-catalyzed reduction of nitrite

Abstract In the potential region corresponding to the reduction of Co(OH) + to the metal, a catalytic polarographic current is observed in an ammoniacal buffer solution containing nitrite ion. In neutral unbuffered media, the catalytic current appears even on the limiting plateau of aquacobalt(II) reduction wave. The catalytic current is due to the reoxidation of an electrodeposited metallic cobalt to Co(II) by nitrite ion. The condition necessary for the catalytic reaction to proceed is the increase in pH at the electrode surface. It is shown that the hydroxide ion is produced as a result of the catalytic reaction.

Polarographic behavior of Mo(VI) in aqueous sulfuric acid

Abstract The chemical reduction of Mo(VI) to Mo(V) by mercury occurs below 1 M H2SO4, and the polarographic reduction of Mo(VI) is hindered by the presence of an adsorbed layer of Mo(V) on the electrode surface. Above 3 M H2SO4, a kinetic wave is observed at more positive potentials than the diffusion waves for the reduction of Mo(VI) to Mo(V). The kinetic current is controlled by the rate of dissociation of Mo(OH)4(SO4)22− which is not reduced until more negative potentials are reached. Nitrogen oxide, NO, is found to be a reduction product of the catalytic reaction between NO3− and Mo(III).

Voltammetric determination of phosphonate ion as a heteropoly complex

A simple voltammetric method for the determination of phosphonate ion is described. A 12-molybdodiphosphonate complex is formed in a 50mM Mo(VI)-0.5M HCl-70% (v/v) CH(3)CN system containing phosphonate ion. The yellow heteropolyanion undergoes apparent two-step reductions at a glassy carbon electrode. The voltammetric reduction currents depend linearly on the phosphonate concentration in the range 1 x 10(-5)-1 x 10(-3)M.