Abstract
The pH value of a equimolal phosphate buffer solution has been accurately measured using a pH cell composed of a hydrogen and silver-silver chloride electrode. Four pairs of hydrogen and Ag-AgCl electrodes were prepared by an electrolytic method, and measurements of the electromotive force of the pH cell were repeated for each pair assembled in the cell. The pH cell was filled with buffer solutions KH2PO4, Na2HPO4 mixed with KCl at three different concentrations. The pH values of the buffer solution were calculated from the measured emf values in accordance with Bates-Guggenheim convention. In addition, the standard electrode potential(E°) of the Ag-AgCl electrode was precisely determined. The pH values obtained are 6.946±0.006, 6.864±0.003 and 6.838± 0.003, and the E° values also determined are 234.54± 0.14mV, 222.76±0.14mV and 204.64±0.13mV, at 5, 25 and 50°C respectively. These pH values agree with those of Bates et al. within 0.005, but the E° value shows a relatively large difference of 0.42mV from that of Bates et al. Consequently, it may be said that for the precise measurement of pH the E° value of each electrode must be measured to reduce the systematic error.
