Dissociation constants of amonium ion (Ka= [H +][NH3]/[NH4+], where [ ] denotes the concentration) in 20 mass % ethanol + water mixtures with different concentrations of sodium chloride were determined by means of a glass electrode. The dissociation constants in pKa units (on molarity and molality bases) increase linearly with increasing ionic strength. Activity coefficients of ammonia in the ethanol + water mixtures were determined by a transpiration method. The observed dissociation constants on the molality scale were compared with those calculated from the Pitzer equations modified on the basis of the principles of corresponding states. The observed values agreed with the calculated ones almost within experimental uncertainties.
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