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Why does zinc sulfate solution need to be purified before electrolysis?

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sphalerite — zinc processing Sphalerite from Baxter Springs, Kansas.

In the process of zinc processing, the purification of the zinc sulfate solution is essential for successful electrolysis.

Theoretically, the voltage required to deposit zinc from a sulfate solution is higher than that needed to decompose water, which would normally result in the production of hydrogen at the cathode instead of zinc. However, when using a zinc cathode, a phenomenon known as "overvoltage" prevents hydrogen generation, allowing zinc to deposit.

This hydrogen overvoltage is highly dependent on the purity of the electrolyte. The presence of even trace amounts of certain impurities can drastically lower the overvoltage, causing hydrogen to form and effectively interfering with the deposition of zinc. Consequently, extreme purification is a critical necessity to ensure the process functions correctly and to achieve a final product purity of 99.99 percent or more.1