Since calcium oxalate is 3000 times less soluble than calcium sulfate, would it be possible to concentrate sulfuric acid from let's say 50% solution using following steps? :

  1. adding calcium carbonate to cold dilute H2SO4
  2. filtering off resulting CaSO4
  3. mixing CaSO4 with oxalic acid in 1:1 molar ratio
  4. adding just enough water to submerge the solids
  5. vaccum filtering off the now concentrated (hopefully) H2SO4

My reasoning behind this is that even though at any given time theres only a small amount of CaSO4 dissolved, the reaction producing sulfuric acid would eventually go to completion with high yield due to it's the massive kinetic favourability. The only problem I see is that the amount of water needed would limit the final concentration. Would adding merely enough water to wet all the solid still work, improving the final concentration?


Calcium oxalate solubility depends on acidity, as the weak oxalic acid is protonated in highly acidic sulphuric acid.

Additionally, oxalic acid would be dehydrated if sulphuric acid was concentrated enough.

If we for convenience mark oxalate anion $\ce{(COO)2^2-}$ as $\ce{Ox^2-}$, then

$$\ce{CaOx <<=> Ca^2+ + Ox^2-}$$

$$\ce{Ox^2- + H+ <=>> HOx^-}$$

$$\ce{ HOx^- + H+ <=>> H2Ox}$$

$$\ce{ H2Ox ->[H2SO4] H2O + CO ^ + CO2 ^}$$

Therefore oxalic acid has no chance to create concentrated sulphuric acid.


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