Voltammetry of Dissolved Water and the Role of the Hydrated Electron in Fused Nitrates

Abstract

In the early ’sixties, Swofford and Laitinen [1,2] first reported that small amounts of water dissolved in molten alkali nitrate solvents yielded steady-state current-voltage curves whose sigmoid shape resembled classical polarograms which exhibited a well-defined limiting current domain. This phenomenon has since been referred to as the so-called “water wave” [3] and reinvestigated by several authors at mercury, platinum and gold indicator electrodes [4–6]. Using Levich’s rotated platinum disk electrode (RDE), Peleg [5] has successfully made use of the “water wave” for the quantitative determination of moisture in nitrate melts. In a preliminary investigation, T.E. Geckle in our laboratories [7] has substantiated that the relevant limiting currents at the RDE were indeed proportional to the partial pressure of water in the supernate. However, the same investigation revealed that neither hydrogen nor oxygen were produced during the electrolysis. Results of a comprehensive study of the water wave phenomenon are presented and discussed in this paper, including voltammetry at the RDE and controlled potential coulometry, complemented by analysis of reaction products via appropriate chemical methods and mass spectrometry. A mechanism is postulated, invoking involvement of a hydrated electron species, which accounts for the paradoxical experimental findings previously reported in the literature.