A study of alkali metal ion hydration by polarographic half-wave potential shifts

Abstract

Differential pulse polarograms of 0.001<em>M</em> solution of alkali ions are obtained with specially prepared, bridged, saturated calomel electrode. The differential pulse peaks are corrected for pulse amplitude and bridge potentials. The half-wave potentials are measured over the complete range from pure water to pure dimethylformamide (DMF). Shifts in polarographic half-wave potentials of the alkali ions in water-(DMF) mixtures are interpreted to be, primarily, the result of direct binding between the solvent molecules and the cations, forming solvent cationic complexes that exist in dynamic equilibrium with the precursors. Equations are derived to show that the half-wave potential shift of a cation in the mixed solvent system is related not only to the concentrations ot the component solvents but also to the solvation numbers, as well as the complex formation constants. Using the derived equations, the hydration numbers of Na⁺, K⁺, Rb⁺, and Cs⁺ are determined to be 4.0 &plusmn; 0.2, 3.4 &plusmn; 1.3, 4.3 &plusmn; 1.3 and 3.9 &plusmn; 1.3 respectively. The <em>pK</em>a values for the formation of the hydrated species of these ions are found to be - 4.9 &plusmn; 0.1, -4.7 &plusmn; 0.1, - 4.3 &plusmn; 0.1 and - 3.4 &plusmn; 0.2 respectively.