Effect of anion basicity in influencing the acidity function of concentrated aqueous solutions of oxyacids

Abstract

The measurement of Hammett acidity of anhydrous acids and their concentrated aqueous solutions by amine indicators is viewed in terms of the transference of a proton to the —NH₂ group from a solvating cluster provided by the medium. It is argued that a measure of the difficulty with which the proton can be transferred should be given by the optical (Lewis) basicity of the deprotonated solvating cluster. Optical basicities for clusters of various sizes are calculated for the systems H₃PO₄+ H₂O, H₂SO₄+ H₂O and HClO₄+ H₂O, and the values are found to increase with increasing water content of the solvating cluster in line with the decreasing acidity of the medium. When they are plotted against the H₀ values of the corresponding acid solutions, straight line relationships are observed which are especially good when based upon two molecule and three molecule solvating clusters. These relationships indicate the dependence of H₀ not only upon the water content but also the decreasing basicity in going from phosphate to sulphate to perchlorate. Changes in H₀ with change of indicator and the prediction of H₀, e.g. for oleum and orthosilicic acid, are also discussed.