Applicability of 0.05 mol kg-1 potassium hydrogen phthalate as reference value pH standard in water-organic solvent mixtures

The applicability of 0.05 mol kg(-1) potassium hydrogen phthalate (KHPh(S), S= the solvent under study) as reference value pH standard (RVS) was studied in water-organic solvent mixtures. Phthalic acid in various aqueous organic solvent mixtures was titrated potentiometrically with Bu,NOH (MeOH), using a pH-ISFET and a glass electrode as pH-sensors, and the buffer capacity of the HPh- solution was roughly estimated from the difference between the first and second half-neutralization potentials. The influence of potassium ion to the buffer capacity was also studied, titrating KHPh and Et4NHPh with CF3SO3H and Bu4NOH (MeOH) and comparing the titration curves. The KHPh solution loses its buffer capacity if the solvent mixture has an aprotic property. For aqueous mixtures of protophilic aprotic solvents (e.g. DMSO and DMF), it is unsuitable as RVS if the water content is less than ca. 40 (v/v)%. The necessity of selecting subsidiary pH standard(s) is suggested for the solvent systems in which KHPh is inapplicable. (C) 1998 Elsevier Science B.V. All rights reserved.