Re-Evaluation of Stoichiometric Dissociation Constants from Electrochemical Cell Data for Propionic and n-Butyric Acids at (0 to 60) °C and for Some Other Aliphatic Carboxylic Acids at (18 or 25) °C in Aqueous Sodium Chloride Solutions

Abstract

Equations were determined for the calculation of stoichiometric (molality scale) dissociation constants Km of propionic and n-butyric acids in buffer solutions from their thermodynamic dissociation constants Ka and molalities of components in solution at temperatures from (0 to 60) °C. These equations are based on single-ion activity coefficients from Hückel-type equations. The required thermodynamic dissociation constants and propionate and n-butyrate parameters were determined from Harned cell data of Harned and Ehlers (1933) and Harned and Sutherland (1934). The parameters agreed closely with parameters for acetate species determined recently. The required interaction parameters between hydrogen ions and chloride ions were taken from recent HCl data reassessment, and between hydrogen ions and sodium ions and between sodium ions and chloride ions were taken from the recent acetate work. The new equations were used to calculate pH values of propionate buffers with equal molalities of propionic acid and sodium propionate of (0.01 or 0.1) mol kg-1 at temperatures from (0 to 60) °C. Values of p(mH), giving molality of protons directly, calculated from these equations are tabulated, and p(mH) values for buffer solutions with NaCl as major component and propionate species as minor components are also given. The Hückel equations were also applied to a re-evaluation of Ka for formic acid and activity parameters for formate species at 25 °C from Harned cell data of Prue and Read (1966) and the parameter values for species from acetic, formic, propionic (propanoic), n-butyric (n-butanoic), 2-methylpropanoic, n-pentanoic, 2,2-dimethylpropanoic, n-hexanoic, 4-methylpentanoic, and n-heptanoic acids at 18 °C from quinhydrone electrode data of Larsson and Adell (1931).