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Recently I conducted an experiment to investigate the effect of HCL concentration on the solubility of hydroxyapatite [Ca5(PO4)3(OH)]. This was done by taking a sample of the solution after hydroxyapatite was left to dissolve in HCL, and conducting an EDTA titration using this solution to find how many calcium ions were present. I have already established that hydroxyapatite is more soluble in solutions of HCL at higher concentrations (lower pH). However, I was wondering what the exact scientific theory behind this reaction was.

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The $Ksp$ expression for hydroxyapatite is: $\ce{ Ca5(PO4)3(OH)_{(s)} -> 5Ca^{2+}_{(aq)} + 3PO4^{3-}_{(aq)} + OH-_{(aq)}}$.

When $\ce{HCl_{(aq)}}$ is added, it reacts with the dissolved $\ce{OH-}$ to form water, thereby decreasing the $\ce{OH-}$ concentration. Le Chatlier's principle predicts that more hydroxyapatite will dissolve (shift to the right).

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