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Why must zinc sulfate solution be highly purified before electrolysis?

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A labeled diagram of the periodic table entry for zinc, showing its atomic number 30, symbol Zn, atomic weight 65.38, electron configuration, name, hexagonal crystal structure, solid state at room temperature, and classification as a transition metal.
zinc

In the electrolysis of zinc sulfate solution, extreme purification is essential because the process relies on hydrogen overvoltage to deposit zinc onto the cathode.

Theoretically, the voltage required to deposit zinc is higher than that needed to decompose water, which would normally result in the production of hydrogen gas instead of zinc. However, using a zinc cathode creates an overvoltage that prevents this hydrogen generation, allowing zinc to deposit successfully.1

The presence of certain impurities, even at very low concentrations, drastically lowers this overvoltage. If the electrolyte is not highly purified, hydrogen will be produced at the cathode rather than zinc, effectively halting the deposition process. To prevent this, the solution undergoes a two-stage purification process:1
* Iron removal: Eliminating iron as jarosite, goethite, or hematite.1
* Cementation: Using zinc dust to remove metallic impurities such as copper, nickel, cadmium, cobalt, and germanium.1

This rigorous purification ensures the final zinc product achieves a purity of 99.99 percent or higher.1