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I am a pool maintenance technician. In the industry, we pour bags of Vitamin C (ascorbic acid) into the pool to remove rust stains from the fiberglass surface. I am trying to determine the chemical equation behind this phenomenon as it works really well but was unable to find anything online. Any help would be appreciated.

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Conceptually, it's something like $$\ce{\text{ascorbic acid} + 2Fe^3+ -> \text{dehydroascorbic acid} + 2Fe^2+ + 2H+}$$

I say "conceptually" because the ferric iron will be some form of oxide rather than a free ion, and the ferrous iron might also be an oxide, depending on pH. The ferrous ion also can be chelated by ascorbic acid. The low pH of the ascorbic acid solution could also slightly solubilize some of the ferric iron without it needing to be reduced to ferrous.

But regardless of the details, the result is that the ferric iron that gives the orange stain color is solubilized in some form and washes away.

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