Anodic Wave Polarography of Oxalate Ion

Abstract

The polarographic behavior of oxalate ion, which forms an insoluble salt with mercurous ion has been studied in neutral and acidified potassium nitrate and acetate buffer solutions. In neutral and slightly acidified solution, oxalate ion produces a three-step anodic wave. The first (I) and the second (II) are attributed to the phenomenon that a mono-molecular film of mercurous oxalate are formed by the oxidation of mercury. Peculiarities of the anodic wave are as follows: 1) The limiting current rises with the remarkable abruptness. Changes of 6 mV for the wave (I) and 7 mV for the wave (II) are resulted for the limiting currents to become fully developed.2) The addition of oxalate ion shifts the starting potential of the waves (I) and (II) to less positive value but does not change the limiting currents.3) The temperature coefficients of the height of the waves (I) and (II) are relatively small.4) The limiting current is proportional to the corrected pressure of mercury, and the i_corr. t_d, value remains constant at 1.36 ± 0.03 μC for the wave (I) and 3.21 ± 0.07 μC for the wave (II). Therefore, by assuming the formation of a monomolecular film of Hg₂(COO)₂, the area per molecule is calcu- lated to be 68.6 Ų for the wave (II) and 28.8 Ų for the (II) respectively.5) Electrocapillary curves 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 they are both absorption waves. As the third wave appears just before the appearance of dissolution wave of mercury, it is difficult to elucidate the electrode reaction.
The anodic wave of oxalate ion is remarkably affected by the addition of small amounts of halogen ions. The first wave shifts to more negative potential and a new maximum appears just after the second wave. The height of the maximum is proportional to the halogen ion concentration. Therefore, the method is found to be suitable for the determination of halogen ion at the concentration range of 10^-5 mol/l.