Alkali-metal salts Alkylammonium ions

Tetra-alkylammonium salts have a dramatically different effect from alkali-metal salts on rates of base hydrolysis of the dichromate ion the rate is nearly a hundred times faster in Ca than in equivalent Et4N+ media, at an added salt concentration of 1 mol dm. These effects are similar to those reported earlier for hydrolysis of species such as [VioOag] ", [MesNSOa], and [Fe(5-NO2phen)3]2+, 20 nd must reflect the very different eff ts of these cations on water structure and thus on reactant and transition-state hydration. An analysis of salt effects on the initial and transition states for the solvolysis of t-butyl chloride indicates an approach that should prove fruitful in discussing, for instance, the effects of alkali-metal and tetra-alkylammonium salts on reactivities of transition-metal complexes in aqueous solution. "... 

Nevertheless one might have expected that the kinetic data would reflect clearly the hydration properties of the added salt. This is not usually the case because charge-charge interactions are dominant. Moreover, structural effects often compensate and their influence on rate constants is minimised (p. 247). If, however, the reactants are neutral solutes, it might be anticipated that the kinetic parameters would be markedly sensitive to the particular salt added to the solution (p. 272). This prediction is borne out in practice, striking differences often being observed between the effects of added alkali metal cations and alkylammonium ions as the following examples show. 

Special attention is to be paid to alkali metals. Their reduction potentials are very negative (above -2V vs. SCE) and the electrolyte components must be reducible at very high negative potentials. The tetra-alkylammonium bases or salts are very convenient for such purposes these must be pure. The small variability of the half-wave potential of the reduction of sodium, potassium, rubidium and caesium ions makes impossible their polarographic discrimination. The half-wave potential of lithium is about 200 mV more negative than that of sodium or potassium and therefore it can be determined in the presence of up to a 10-fold excess of sodium or potassium. Some methods for separate determination of sodium and potassium have been described [17]. This procedure is based on the preliminary separation of the potassium by the perchlorate precipitation. 

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