In the volumetric estimation of calcium in a given solution as calcium oxalate. We convert the calcium oxalate to oxalic acid by dissolving the former in hot (~70 °C) 2 N sulfuric acid solution. This is titrated against $\ce{KMnO4}$.

The reaction would be $\ce{H2SO4 + CaC2O4 -> H2C2O4 + CaSO4}$

The calcium sulfate formed is normally not soluble in water. But here we get a clear solution which we titrate against $\ce{KMnO4}$. Why does the calcium sulfate not precipitate in the solution?

  • 3
    $\begingroup$ It is not "not soluble". It is poorly soluble. $\endgroup$ – Ivan Neretin Mar 11 '16 at 8:14

The overall reaction is really better given as: $$\ce{2H2SO4 + CaC2O4 -> H2C2O4 + Ca^{2+} + 2HSO4^{-}}$$ since at that strong of a pH the predominate sulfuric acid species is $\ce{HSO4^{-}}$.

Better yet the reaction could be written as $$\ce{2H^+ + CaC2O4 ->C[{0.2 N \ H2SO4, 70 °C}]\ H2C2O4 + Ca^{2+}}$$

As was already pointed out in the comments, calcium sulfate has a fair solubility 0.2 g per 100 ml of water at at 20 °C, and the $K_\text{sp} = 4\times10^{-5}$.

The solubility varies with temperature. The solubility of calcium sulfate dihydrate increases from 0.223 g/100 ml at 0 C to about 0.265 g/100 ml at 40 C, then decreases to 0.205 g/100 ml at 100 C.

  • $\begingroup$ I calculated the solubility product of CaSO4 at the required concentration and got as .031 which is higher than the Ksp . The question is would the Ksp increase so much with just a 50 degree temp rise? Added to it is the negative temperature coefficient of CaSO4 which should mean that the solubility decreases with increase in temperature.. $\endgroup$ – Swaroop Chandra Mar 12 '16 at 13:58
  • $\begingroup$ you can look at calcium sulfate solubility as function of temperature here en.wikipedia.org/wiki/Solubility_table Did you account for the fact that most sulfate is $\ce{HSO4^{-}}$ ? $\endgroup$ – MaxW Mar 12 '16 at 15:11
  • $\begingroup$ I would check on that "of course be more soluble in hot water" claim. I have read otherwise. $\endgroup$ – 42- Jul 2 '18 at 23:40

Calcium sulfate is somewhat soluble in acids (CRC Handbook: "s a"). The previous answer accurately states that the overall reaction is really better given as: $\ce{2H2SO4 + CaC2O4 -> H2C2O4 + Ca^2+ + 2HSO4−}$.

If only one sulfuric acid per oxalate were used: $\ce{H2SO4 + CaC2O4 -> H2C2O4 + CaSO4}$. Then insoluble $\ce{CaSO4}$ would replace insoluble $\ce{CaC2O4}$, perhaps coating the oxalate and rendering the analysis inaccurate. Excess acid (and heat) is used to make the mixture go clear, so that the reaction can be seen to be complete.

Similarly, $\ce{CaCO3}$ is insoluble in water, but excess $\ce{CO2}$ gives $\ce{HCO3-}$, and $\ce{Ca(HCO3)2}$ is somewhat soluble - that's why we have stalagtites and stalagmites in some caves (when the $\ce{CO2}$ comes out of solution, the $\ce{CaCO3}$ precipitates).


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