What is the way to dissolve oxalates? I have tried to clean the seashells with lemon (or citric, as I was corrected) acid and then I washed them with soap (with pH above 10). There is a white coating on the shells now, which disappears when is wet and appears back when is dry. I suspect I accidentially covered the shells with some oxalate. Please help.
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$\begingroup$ It might just be too late, but I don't see where oxalate might come from if you used citric acid (I guess that what you mean by "lemon acid"). Anyway, I think the white coloring is the remaining fine conchiolin network(pores) remained after the $\ce{CaCO3}$ has been dissolved with an acid. Maybe try to polish the surface in a less visible spot, say, with a Dremel's polishing nozzle, and see whether it'll become clear again when dry. In addition, providing photos before and after the treatment may help. $\endgroup$– andselisk ♦Aug 3, 2017 at 21:45
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$\begingroup$ @andselisk I am very thankful for your comment! I thought oxalate appeared after reaction of citric acid with the soap. I read some info again and understood I was wrong about oxalate. So that white substance is to be the calcium citrate, right? If yes, is there any non-mechanical way to remove it? $\endgroup$– AStenAug 4, 2017 at 8:27
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$\begingroup$ I tried to summarize what I think might've happen in an answer, also check out related question: Citric acid chelates calcium carbonate more than ethanoic acid, why does this result in a faster rate of reaction? $\endgroup$– andselisk ♦Aug 4, 2017 at 10:24
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1 Answer
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Seashells' outer layer (periostracum) mainly consists of conchiolin and polymorphic modifications of $\ce{CaCO3}$ (aragonite, calcite). To sum it up, I see two possible sources of the white coating after treating the seashells with citric acid:
Formation of calcium citrate, which has limited water solubility ($\pu{0.9 g/L}$ at $\pu{20^\circ C}$):
$$\ce{3CaCO3 + 2C6H8O7 -> Ca3(C6H5O7)2 v + 3CO2 ^ + 3H2O}$$ You can try to use acetic acid instead of citric acid in order to produce calcium acetate, which is very soluble in water ($\pu{347 g/L}$ at $\pu{20^\circ C}$). I don't think you can easily dissolve calcium citrate with acetic acid because calcium is complexated by citrate, and this is a very stable formation. The problem here is that the prolonged exposure to acidic medium can further damage the outer layer as more calcite will be washed away.Conchiolin porous network remained after $\ce{CaCO3}$ has been eliminated with an acid might also appear white when dry and then, when the pores are filled with water, return to it's original appearance. This is rather a speculative theory, but if this is the case, you will need to perform mechanical surface polishing in order to get rid of the porous layer. This can potentially damage the seashells and should be done cautiously.
answered Aug 4, 2017 at 10:23
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$\begingroup$ Thank you for the answer! I will try both and will leave a feedback after. $\endgroup$– AStenAug 4, 2017 at 13:11