Ion association and its reaction mechanism (Review)

Abstract

Studies on ion association and its reaction mechanism were reviewed from the view-point of analytical chemistry. Ion association has been extensively used for the separation and analysis of ionic substances by coupling with other methods, such as solvent extraction, precipitation, adsorption on hydrophobic adsorbents, titration, reversed-phase/HPLC, flow injection analysis and capillary electrophoresis. First, a historical background concerning ion association in analytical chemistry, electrolyte solutions and water structures around ions is reviewed; secondly, theoretical aspects of ion association are summarized; thirdly, analysis methods for ion-association equilibrium in aqueous solutions are reviewed; last, ion-association mechanisms in aqueous solutions and in liquid-liquid distribution are considered. From the present reviews, classifying ions into three groups is proposed : they are water-structure former (WSF), water-structure breaker (WSB) and hydrophobic water-structure promoter (HSP). The ion associability of each ion with its pairing ion is considered. In analytical chemistry, important ion associates are electrostatic interaction-induced ion associates and hydrophobic water structure-induced ion associates.