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I want to know the gases produced(primarily nitrogen oxides) in leaching of Galena in nitric acid and the effect of concentration of nitric acid on the gases evolved.

I found papers about the efficiency of leaching in different concentrations of nitric acid but none mentioned which gases are produced.

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    $\begingroup$ The site expects that you write explicit compact summary of your prior effort to answer the question, based on your knowledge and on searching for existing related info or answers. It helps preventing others to tell you what you already know or what you could easily find yourself. $\endgroup$
    – Poutnik
    Sep 3, 2022 at 13:33
  • $\begingroup$ Some information about a similar dissolution : pyrite $\ce{FeS2}$ in acidic media. Strangely enough, the reaction pyrite + acids is not yet well known. Whatever the proposed equation, the temperature, the concentration the nature of the acid ($\ce{HCl, HNO3, HClO4}$), and the dimension of the grains, the ratio Sulfur/Iron is NEVER equal to 2 at the end of the reaction. It changes from report to report, but it is usually about 1.5. Some sulfur is always lost somewhere. Of course, Sulfur may be produced in elementary form $\ce{S8}$, in a gaseous form $\ce{SO2}$ or in various polyoxygenated ions. $\endgroup$
    – Maurice
    Sep 3, 2022 at 21:20
  • $\begingroup$ $62$ publications have been written about the problem of pyrite dissolution in acidic media. and analyzed, then compared by M. Descostes et al., in a $10$ pages report : Pyrite Dissolution in Acidic Media, Geochimica Cosmochimnica Acta, Vol. 68. No. 22, p. 4559 - 4569, 2004. Nineteen equations have been compared. None is satisfactory. Descostes has also analyzed the system by numerous methods (spectroscopy, X-ray, solution chemistry, ion chromatography, etc.) In vain. Sulfur is lacking in all explanations. $\endgroup$
    – Maurice
    Sep 3, 2022 at 21:21
  • $\begingroup$ Nitric oxide: chemistry.stackexchange.com/questions/51543/… $\endgroup$ Sep 4, 2022 at 3:06

2 Answers 2

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The answer is hydrogen sulfide with dilute nitric acid, otherwise little to none using the concentrated acid.

Here is a source: "Leaching of Lead Sulfide with Nitric Acid", quoting:

Galena, lead sulfide, is leached with nitric acid in order to prepare higher value-added products such as lead nitrate, lead carbonate, lead basic carbonate, etc. Leaching behavior has varied with the concentration of nitric acid. At the lower concentration of nitric acid, galena is dissolved into lead nitrate and hydrogen sulfide. However, at the higher concentration of nitric acid, galena is not dissolved but it is changed into lead sulfate as precipitate. The leaching efficiency is strongly dependent upon temperature and nitric acid concentration. The optimal conditions are found to be 40°C and 4–8 N nitric acid where about 95% of lead can be dissolved from lead sulfide.

I hope this helps.

For those students interested in the advanced underlying chemistry surrounding the action of hydrogen sulfide on nitric acid and associated nitric oxides, see this source: "Potential Biological Chemistry of Hydrogen Sulfide (H2S) with the Nitrogen Oxides".

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Adding to the previous answer (with a fume hood recommendation) is that $\ce{NO2}$ and $\ce{SO2}$ can also evolve using the concentrated acid at higher temperatures.

It is really interesting that $\ce{H2S}$ (gas) can be produced with dilute $\ce{HNO3}$ conditions because hydrogen carries a very slightly stronger positive charge than $\ce{Pb^2+}$, $\ce{NO3-}$ serve as counter ions, and the oxidation rate of sulfur is very low.

With more concentrated nitric acid at higher temperatures, the oxidation of sulfur rate becomes higher than the evolution of $\ce{H2S}$ gas as:

$\ce{HNO3 -> NO2 (g) + H2O}$
$\ce{ [S] atoms -> $\ce{SO2}$ (g) -> SO4^2-}$

resulting in $\ce{PbSO4}$ salt. The loss of $\ce{H2S}$ and $\ce{SO2}$ during the reaction explains the less than 2:1 ratio of $\ce{S}$ to $\ce{Pb}$ after the galena leaching reaction is complete.

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    $\begingroup$ Robert, I would amend your reply as your 'evolution' link leads to a white paper noting sulfur and nitric acid reaction. However, you answer largely remains correct as per my advanced source reference, to quote: "Electrochemical evidence indicates −SH acts as a weaker two electron reducing agent (forming So)", where here the −SH ion is created from H2S/H2O. However, the direct action of HNO3 with −SH ion appears to be NO which can even further react with H2S, $\endgroup$
    – AJKOER
    Sep 4, 2022 at 12:39
  • $\begingroup$ @AJKOER noted. The "tipping" point where higher conc. HNO3/heat increases rate of oxidation of H2S is of interest to me, as there are similarities to silver recovery reactions I have done. $\endgroup$ Sep 4, 2022 at 15:51

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