What material is used as the positive electrode in a zinc-mercury cell?

In a zinc-mercury cell, mercuric oxide is used as the positive electrode. This is because mercuric oxide serves as a cathode material in electrochemical cells, allowing it to facilitate the reduction reactions necessary for the cell's operation. The chemical reactions in the cell involve zinc as the anode, which oxidizes, and mercuric oxide at the cathode, which reduces, thus enabling the flow of electrical current.

Given the other materials listed, zinc oxide is typically a compound resulting from the oxidation of zinc, which does not function as the cathode in this context. Lead sulfate is primarily associated with lead-acid batteries and does not participate as a positive electrode in a zinc-mercury cell. Copper sulfate is commonly used in copper electroplating and other applications but does not serve as an electrode in this specific type of cell.