Polarographic Investigation of Alkali Metal Ions in Hexamethylphosphoramide

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The polarographic behavior of alkali metal ions in hexamethylphosphoramide (HMPA) has been found to be greatly influenced by the cations of the supporting electrolytes. With 0.05 M Et4NClO4 as the supporting electrolyte, cesium and rubidium ions give diffusion-controlled, reversible waves at−2.32 and −2.35 V, respectively, vs. the Ag/0.1 M AgClO4 electrode, but sodium and lithium ions are not reduced until the reduction of the supporting electrolyte (−3.1 V). The potassium ion gives a wave at −2.37 V with a limiting current controlled by the rate of the preceding process. In 0.05 M Bu4NClO4 solutions, the reduction wave of the potassium ion is diffusion-controlled, but that of the sodium ion is kinetic and is very small. The lithium ion is not reduced until −3.1 V. In 0.05 M LiClO4, even the sodium ion is reduced reversibly with E1⁄2 at −2.47 V, while the reduction of the lithium ion starts from −2.65 V. In mixed supporting electrolytes, the presence of the tetraethylammonium ion is the determining factor of the electrode processes. From the measurements of the electrocapillary curves and of the conductivities, the effect has been attributed to the change in the double-layer properties with the cations of the supporting electrolytes.
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