On the evidence for univalent Mg

Abstract

The mechanism of magnesium dissolution in aqueous solutions and the enhanced reduction of water to form hydrogen observed at a magnesium electrode at increasing anodic potentials has received much recent attention. One explanation for this phenomenon is the assumption that a fraction of dissolving Mg enters the solution as univalent Mg (Mg+), where it is subsequently oxidized by water. A commonly-cited report for the existence of Mg+ is the 1954 paper by Petty et al., in which indirect analytical techniques were used. In this paper, the experiment performed by Petty et al. was reproduced and supplemented with modern in situ Raman spectroscopy. A saturated sulfate solution exposed to a magnesium electrode at open circuit will reduce to form sulfur dioxide, which can serve as a reducing agent. The presence of sulfur dioxide would, therefore, create reducing conditions, which had been attributed to Mg+ by Petty et al. Furthermore, it was found that the reduction of sulfate at a pure magnesium electrode, similar to the reduction of water, is enhanced at increasing anodic potentials. This effect was not observed at a Pt electrode in the same potential range in a saturated sulfate solution. As a result, it must be concluded that the existence of Mg+ was not established by Petty et al.