Anodic Wave Polarography of Tungstate Ion

Abstract

It is well known that an anion which forms an insoluble salt with mercurous ion accelerates the anodic dissolution of mercury. Tungstate ion in neutral and slightly acidified potassium nitrate media produces a three-step anodic wave. A study of the characteristic of the anodic waves, the first (I) and the second (II), has revealed the following peculiarities: 1) The limiting current shows the remarkable abrupt rise. A change of 30 mV results for the limiting current to become fully developed.2) The addition of tungstate ion shifts the starting potential of the wave (I) and (II) to less positive value but does not change the limiting currents.3) The temperature coefficient of the height of the wave is relatively small for the wave (I) compared to a diffuMon controlled limiting current.4) The limiting current is proportional to the corrected pressure of mercury, h_corr. and the i_corr.t_d value remains constant at 1.41±0.02 μC for the wave (I) and 6.03±0.07μC for the wave (II). Therefore, by assuming the formation of a mono-molecular film of Hg₂WO₄, the area per molecule is calculated to be 69.6 Ų for the wave (I) and 18.3 Ų for the wave (II) respectively.5) Electrocapillary curves, measured by the droptime method, show that a remarkable change in the interfacial tention occurs in the potential range of the limiting current of the wave (I).
From the results above, it is concluded that the first and the second waves are caused by the oxidation of mercury to produce a thin film of very insoluble mercurous tungstate. Though mono-molecular in thickness, these films are so cohesive that they prevent the oxidation of mercury. Therefore, the second wave shifts to more positive than the first by 140 mV and the third more positive than the second by 120 mV. As the third wave appeares just before the appearance of dissolution wave of mercury, it is difficult to elucidate the electrode reaction but it seems to be not diffusion but kinetically controlled wave.