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The behavior of the amalgamated zinc electrode in unsaturated and supersaturated zincate solutions has been studied by measuring equilibrium potential. Excess zinc oxide can be dissolved in the KOH solutions to form supersaturated zincate solutions. The electrode potential was measured as a function of zincate concentration in the zincate solutions containing constant free hydroxyl ion. It has been found that the equilibrium potential of the amalgamated zinc electrode first increased and then decreased with the increase of zincate concentration from unsaturated solutions to supersaturated solutions. The influence of zincate concentration and its activity coefficient on the equilibrium potential of the amalgamated zinc electrode were discussed.It is well known that the zincate content in the alkaline solution in which the zinc electrode is anodized can be substantially higher than that in a solution in equilibrium with solid ZnO. Supersaturated zincate also exists in the cooled solutions obtained from zinc oxide and hot aqueous potassium hydroxide. These solutions have considerable stability and may be kept unchanged for a period of time. Further study on the behavior of the zinc electrode would be expected to reveal any difference in nature between unsaturated and supersaturated zincate solutions.The equilibrium potential of the zinc electrode in alkaline zincate solutions has been studied extensively. 1-5 Many authors studied the equilibrium potential of the zinc electrode in unsaturated and nearsaturated zincate solutions. Non-Nernstian behavior was observed at high zincate concentration. It was thought that water was a reactant in the electrode reaction and the zincate ion was hydrated to some extent. 2 It was also explained as being due to the dependence of the hydroxyl and zincate ion activities on concentration in strong potassium hydroxide solutions. 3 The junction potential between the electrolyte in the reference electrode and the electrolyte under test was also a major contributor to this non-Nernstian behavior. 4 Another suggestion was that the zincate ion was somewhat acidic and a neutralization reaction took place between the zincate and the potassium hydroxide as the potassium hydroxide concentration increased. 6 A driving force for this neutralization was the scarcity of free water molecules needed for the hydration of the ions.The above-mentioned suggestions that interpret the curvature for the equilibrium potential of the zinc electrode vs. Log a OH Ϫ and Log a Zincate are based on a zinc electrode in unsaturated or saturated alkaline zincate solutions. In actual situations, zincate solutions are often supersaturated during discharge of the zinc electrode in zincbased batteries.Zinc oxide is very soluble in the KOH electrolyte ͑see Fig. 1͒. 7-9 Moreover, the saturation solubility of zinc oxide increases with the increase of KOH concentration. Under the saturation solubility, zinc oxide is dissolved to form the unsaturated zincate, and above the saturation solubility, zinc oxide is also dissolved ...