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I'm trying to dissolve small pyrite (FeS2) grains. It's supposed to be done in nitric acid, but my lab doesn't have any. It does have a lot of 37% HCl and various salt oxidizers, such as KNO3, KMnO4, or NH4NO3.

I've tried dissolving the pyrite in full-strength HCl with no success. I have read that the dissolution comes from the oxidizing effect of the nitric acid's conjugate base, and not necessarily the H+. So if I dissolve some of the aforementioned salts into HCl, would it be able to then dissolve the pyrite?

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    $\begingroup$ Properties of the solution containing particular concentrations of hydrated ions H+, K+, Cl- and NO3- do not depend on origin of these ions. What is essential here is low activity of water and high activity of H+ and NO3^-. When based on HCl, there may be too much of water. $\endgroup$
    – Poutnik
    Commented Aug 16, 2022 at 9:15
  • $\begingroup$ Dissolving $\ce{KNO3}$ in concentrated $\ce{HCl}$ will produce some $\ce{HNO3}$. But as soon as it is produced, $\ce{HNO3}$ will react with $\ce{HCl}$ in excess to produce "aqua regia", according to the equation : $$\ce{HNO3 + 3 HCl -> Cl2 + NOCl + 2 H2O}$$ And $\ce{Cl2}$ will attack pyrite $\endgroup$
    – Maurice
    Commented Aug 16, 2022 at 19:13

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Per this 1991 study "Effect of Additives on Pyrite Oxidation" by By G. W. Reimers and D. F. Franke, apparently the presence of chloride reduces pyrite oxidation with air/oxygen and water.

The cited products of this air oxidation includes soluble ferrous sulfate and also sulfur dioxide. The latter forms bisulfite with water, and with transition metal tap water (with increased ferrous content) upon vigorous aeration, the sulfite radical is introduced (from Fenton hydroxyl radicals acting on bisulfite). This sulfite radical can apparently rapidly react with oxygen creating a long chain of reactive species (see, for example, this ScienceDirect account and even more recent work by Rivas discussed here).

So, my recommendation, do not use nitrate or chloride source, just vigorous aeration (like with an air pump) of tap water containing the pyrite should suffice especially with optional strong sunlight.

However, if you want to add some acid along with the aeration actually consider a combination of ascorbic acid and citric acid per the 2009 work of Vidrio et al.,"Generation of Hydroxyl Radicals from Dissolved Transition Metals in Surrogate Lung Fluid Solutions" (where an available but impure source would be just common lemon juice) that would act mainly as an initial driver of hydroxyl radicals to further induce sulfite radicals, etc.

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