o-phthalic acid as a standard substance for calibrations of hydrogen ion concentrations with a glass electrode

o-Phthalic acid is proposed as a standard substance for buffer solutions of known hydrogen ion concentration (I ? 0.2 M KCl, pH⁺] = 3.0–5.4, 25°C). Its crystallinity, purity and slightly wide buffer range afford advantages over acetic acid. Empirical relationships between measured pH (pH_m) and calculated H⁺] were derived for sequences of buffer solutions at several ionic strengths: pH_m - MpH⁺] + C. These calibration lines were parallel and of unit slope as required by theory. A table of pH⁺] values for o-phthalic acid buffer solutions at I = 0.1 M (KCl) is presented and the method of calculation of pH⁺] values for a buffer series generated by additions of potassium hydroxide is outlined.