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I have a reference that says some L.Thompson obtained sulfuric acid of
the highest degree of purity by treating calcium sulphate with oxalic acid,
and evaporating the filtrate. And if one used Oxalic acid with Calcium nitrate, could one get nitric acid in solution? I know that HNO3 won't oxidise oxalic, so calcium oxalate would possibly precipitate, and then what remains in solution ? Finally, reacting AgNO3 with HCl one gets HNO3 (soln). But HNO3 is stronger than HCl, how can this be ? The third question is easiest, so I'll answer it first. The reaction AgNO3(aq) + HCl(aq) ® HNO3(aq) + AgCl(s) is not an acid-base reaction but rather is possible because of solubilities: to wit, silver chloride is insoluble in water. Any combination which will produce silver chloride will tend to occur, especially if it can leave something soluble (like a solution of H+ and NO3-) behind.My first suspicion is that the first and second reactions happen by similar mechanisms. So I checked the water solubilities of calcium oxalate ("practically insoluble") and calcium sulfate (0.2 g/100 g) in the Merck Index. What this means is that the calcium ion has a much stronger affinity for oxalate than for sulfate, and so the sulfate ions (along with the H+ from the oxalic acid) can be removed by washing with water. All that then remains is to evaporate the water, and you will have sulfuric acid "of the highest degree of purity." Presumably the same trick would work using calcium nitrate ("very soluble in water") and oxalic acid.
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