The hydrolysis of some tripositive ions

A review of experimental methods for investigating hydrolysis equilibria by emf measurements is given. Liquid‐junction potentials and the activity factors of the reacting species can be estimated by a suitable combination of cells. An emf method is mentioned for the accurate determination of the analytical hydrogen ion concentration of a solution containing metal ions of appreciable acidity constant. Of the tripositive ions investigated only TI(III) forms exclusively mononuclear ions, TIOH2+, and TI(OH)2+ (pka1 = 1.14, pka2 = 1.49). The main products of hydrolysis of In(III) are ions of the formula In[(OH)2In]n3+n (for the first step pK = 5.21, for all following steps pK = 4.70). At low [In(III)]total, InOH2+ and In(OH)2+ become appreciable (pka1 = 4.4, pka2 = 3.9). Similar results were found by Kilpatrick and Pokras for Sc(III) and by Hedström for Fe(III), the main hydrolysis products being Sc[(OH)2Sc]n3+n and Fe[(OH)2Fe]4+; at low metal ion concentration. ScOH2+ and FeOH2+, Fe(OH)2+ are predominant species. Al(III) seems to be an exception; Brosset's emf studies and other available data are consistent with the assumption that Al6(OH)153+ is formed in acid solutions.