Abstract
A.C. Impedance method has been applied to reactions between aquous solution and metal in the field of corrosion science. There has been, however, only a little application of this method to the reactions, like molten salt-metal electrode reactions, at elevated temperatures because of difficulty in the experiments.
In the present study, impedance measurements of sodium silicate-platinum electrode reaction have been made by A.C. impedance method in the frequency range from 5 Hz to 100 kHz. The electrolyte resistance of the slag has been considered together with the double layer capacitance, charge-transfer overvoltage at the interface, reaction overvoltage and diffusion overvoltage in the slag. The overpotential of platinum in the slag was kept at values ranging from −0.6 to +0.6 V against platinum crucible which was a counter electrode. The experiments were carried out at 1200 to 1500 K under an argon atmosphere. The frequency response of complex impedance of the electrode reaction consisted of a semicircle at high frequencies and a straight line with a slope of 45° at low frequencies.
The conductivity of the melts increased slightly with increasing Na2O concentration and temperature. The reacting species have not been clarified, but the charge-transfer resistance for both of anodic and cathodic reactions obtained from the radius of the semicircle decreased markedly with increasing temperature. The double layer capacitance increased with increasing temperature and depended strongly on the electrode potential. The mass transfer process, therefore, would behave as a rate-control step at higher temperatures.
