A physico-chemical analysis of co-solvent and electrolyte effects on autoprotolysis equilibria in aqueous acetonitrile solutions

Abstract

ABSTRACTThe autoprotolysis process in aqueous co-solvent mixtures containing 0–60% (v/v) acetonitrile (ACN) was investigated at constant temperature 25.0 ± 0.1°C by using a high-precision and rapid potentiometric technique. During all measurements, the ionic strength was adjusted to 0.25 mol dm−3 with one of the supporting electrolytes KNO3, NaNO3 or KNO3/NaNO3 mixture (1:1) and the autoprotolysis constants (pKap) were calculated from potentiometric data. In the present study, the variation of the pKap values with co-solvent composition was analysed based on non-specific and specific solute–solvent interactions using the Kamlet, Abboud and Taft (KAT) approach. The experimental results obtained show that the general trend of autoprotolysis constants in different ionic media is as follows: KNO3 > NaNO3/NaNO3 > NaNO3, and these variations increase with increasing the amount of co-solvent ACN. The effect of ionic media used on the pKap values is discussed in terms of the weak interactions between the solvent system and the anionic and cationic components of the background electrolytes.