Application of the Pitzer Model to Assignment of pH to Phthalate Standard Buffer Solutions

Abstract

Stoichiometric ionization constants of phthalic acid have been determined in potassium chloride solutions at temperatures from 10 to 40 °C, with molalities up to 3 mol⋅kg−1, from potentiometric titrations of potassium hydrogen phthalate. The procedure described by Gans and O’Sulliven (Talanta 51:33–37, 2000) was followed to calibrate the electrochemical cell in terms of hydrogen ion concentration and the SUPERQUAD program was used for the calculations. Pitzer’s ion-interaction equation was applied to this system for the calculation of activity coefficients and a good fit was observed at all temperatures. From these results and from phthalic acid solubilities, Pitzer model coefficients were determined which enabled the evaluation of the activity coefficient of each chemical species, including that of the chloride ion, that are necessary for the establishment of phthalate pH standards at ionic strengths higher than 0.1 mol⋅kg−1(the upper limit of applicability of the Bates-Guggenheim convention).