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Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant K m of formic acid in dilute aqueous NaCl and KCl solutions at 298.15 K from the thermodynamic dissociation constant K a of this acid and from the ionic strength I m of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study, and the equations for K m were based on the single-ion activity coefficient equations of the Hü ckel type. The data measured by potentiometric titrations in a glass electrode cell were used in the estimation of the parameters for the Hü ckel equations of formate ions. The Hü ckel parameters obtained in these estimations agree well with those published previously for acetate ions, and therefore, the acetate parameters are also recommended for formate ions. By means of the calculation method based on the Hü ckel equations, K m can be obtained almost within experimental error at least up to I m of about 2 mol kg -1 for formic acid in NaCl and KCl solutions.

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