中性リン酸塩pH標準液のpHの精密測定  [in Japanese] Precise measurement of pH value of equimolal phosphate buffer solution.  [in Japanese]

電解法で製作した銀-塩化銀電極と水素電極を使用し,Bates-Guggenheimの規約に従って中性リン酸塩pH標準液のpHを測定した.又,pH測定に使用した銀-塩化銀電極の標準電極電位を測定した,中性リン酸塩pH標準液のpHは5,25及び50℃の温度でそれぞれ6.946±0.006,6.864±0.003及び6.838±0.003,銀-塩化銀電極の標準電極電位は同じ温度で234.54±0.14mV,222.76±0.14mV及び204.64±0.13mVであった.pHは文献に見られる値と±0.005以内で一致したが,銀-塩化銀電極の標準電極電位の値は最大で0.42mVの偏りを示した.このことから,pHの精密測定には銀-塩化銀電極の標準電極電位を直接測定することが測定の偏りを小さくするために必要であると結論できる.

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 KH_2PO_4, Na_2HPO_4 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±0.14 mV, 222.76±0.14 mV and 204.64±0.13 mV, at 5,25 and 50℃ respectively. These pH values agree with those of Bates et al. within 0.005, but the E° value shows a relatively large difference of 0.42 mV 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.