Calculation of stoichiometric dissociation constants of monoprotic carboxylic acids in dilute aqueous sodium or potassium chloride solutions and p[m(H+)] values for acetate and formate buffers at 25°C

Equations are given for calculation of the stoichiometric (molality scale) dissociation constants, Km, of weak acids in dilute aqueous electrolyte solutions at 298.15 K from the thermodynamic dissociation constant, Ka, of the acid and the ionic strength, Im, of the solution. The equations for Km were based on the single-ion activity coefficient equations of the Hückel type. The equations were tested with the conductivity data for formic, acetic, propionic, n-butyric, lactic, chloroacetic, α-crotonic and cyanoacetic acids, and with data measured by Harned cells for formic, acetic, propionic, n-butyric and glycolic acids. These data were taken from the literature. According to these tests, Km can be obtained by the Hückel method within experimental error at least up to Im of about 0.1 mol kg−1. On the basis of the equations for Km, it is suggested p(mH) values {p(mH)=−lg[m(H+)/(mol kg−1)] where m refers to the molality} for buffer solutions containing acetic or formic acid. A new calibration method is suggested for glass electrode cells, and this method is based on the p(mH) values instead of pH values (pH=−lg[a(H+)] where a refers to the activity).