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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2$\begingroup$ Why do vinegar and other acids remove rust?; Acetic(vinegar) and citric acid as the rust remover $\endgroup$– andselisk ♦Commented Jun 12, 2022 at 19:34
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1$\begingroup$ @andselisk - I don't think this is just an acid reaction here. Ascorbic acid can also reduce Fe(III) to Fe(II), which is more soluble and can wash away. $\endgroup$– AndrewCommented Jun 12, 2022 at 21:04
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$\begingroup$ And ascorbic acid also forms stable complexes with iron: amazon.com/Adapter-Desktop-ALINKVITE-Adapters-Wireless/dp/…, pubmed.ncbi.nlm.nih.gov/6514543 . This chelation of iron can be used to enhance its absorption in the body, pubmed.ncbi.nlm.nih.gov/15743017 $\endgroup$– DrMoishe PippikCommented Jun 13, 2022 at 1:21
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$\begingroup$ @Andrew I agree 100 %, that's why I dropped my dupe-close-vote and just left some links to related posts. $\endgroup$– andselisk ♦Commented Jun 14, 2022 at 5:42
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1 Answer
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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.
