Polarographic reductions of alkali metal ions have extensively been studied in various non-aqueous solvents ~ . In most cases, the reductions of these ions occur reversibly and the half-wave potentials give important information concerning the solvation of these ions. In some cases, however, the reductions are irreversible: e.g., lithium ions in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) are reduced irreversibly if tetraethylammonium perchlorate (Et4 NCIO4) is used as the supporting electrolyte. According to Gutmann et al.2, lithium ion in 0.1 M Et4 NC104 -dimethylacetamide (DMA) is irreducible up to the potential of the reduction of the supporting electrolyte.
Recently the present authors have studied polarographically the behavior of alkali metal ions in hexamethylphosphoramide (HMPA) a . Because of the extremely basic properties of HMPA a , metal ions in this solvent are solvated more easily than in other aprotic solvents, thus giving interesting polarographic results.
With Et4 NC1Oa as the supporting electrolyte, polarograms as shown in Fig. 1 are obtained: Cesium and rubidium ions give diffusion controlled, reversible waves (E~ A = -2.32 V and -2.35 V vs. Ag/0.1 M AgCIO4 (HMPA) reference electrode, respectively), while sodium and lithium ions are irreducible. The limiting current for potassium ion is smaller than those for cesium and rubidium ions and the test of its dependence on the height of the mercury column shows that the reduction is preceded by a process which seems to be a (partial) desolvation of the solvated potassium ion. The rate of the preceding process, as obtained according to the Koutecky's treatment s , increases with the decrease of the concentration of Et4 NC104 and with the rise of the solution temperature.
During the course of this study, we observed that the reductions of alkali metal ions are influenced significantly by the species of cation of the supporting electrolytes. With tetrabutylammonium perchlorate (Bu4NC104) as the supporting electrolyte, the reduction wave for potassium ion (El A = --2.38 V) becomes reversible and diffusion controlled. Sodium ion, which is irreducible in Et4 NC104, gives a small wave, the limiting current of which is controlled by the rate of a preceding reaction. In Bu4NC104, however, lithium ion is still irreducible up to approximately -3.1 V. If lithium perchlorate (LiC104) is used as the supporting electrolyte, sodium ion gives a diffusion controlled, reversible wave (E~A = -2.47 V) and the reduction of lithium ion begins from about -2.65 V, as shown in Fig. 2.