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I am aware that arsenic pentasulfide is soluble in hot concentrated nitric acid giving arsenic acid ($\ce{H3AsO4}$) + nitric oxide + sulfur, but am not sure whether it is soluble in 50% nitric acid or in hot dilute nitric acid as well. I know that the precipitate of arsenic trisulfide is soluble in 50% nitric acid or in hot dilute nitric acid, so I am wondering whether the same is true for arsenic pentasulfide.

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  • $\begingroup$ Is this arsenic pentasulfide, As2S5? The subscript on S does not show up on my device, but you do contrast it from As2S3. $\endgroup$ Commented May 15 at 12:23
  • $\begingroup$ Yes, the subscript that doesn't seem to be showing up is 5, it's arsenic pentasulphide. $\endgroup$
    – ABRTPDR
    Commented May 15 at 13:50
  • $\begingroup$ Arsenic pentasulphide is insoluble in hydrochloric acid and also in dilute nitric acid but it is attacked by more concentrated nitric acid with evolution of nitric oxide. With acid of 20 to 40 per cent concentration, reaction is observed on boiling or vigorous agitation (source) $\endgroup$ Commented May 18 at 5:07

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Arsenic pentasulfide is insoluble in aqueous solutions in acidic medium. To dissolve it, basic medium is required. According to Vogel's Qualitative Inorganic Analysis:

Arsenic pentasulphide, like the trisulphide, is readily soluble in alkali hydroxides or ammonia (a), ammonium sulphide (b), ammonium polysulphide (c), sodium or ammonium carbonate (d):
$\ce{As2S5 + 6OH^{-} -> AsS4^{3-} + AsO3S^{3-} + 3H2O}$
$\ce{As2S5 + 3S^{2-} -> 2AsS4^{3-}}$
$\ce{As2S5 + 6S2^{2-} -> 2AsS4^{3-} + 3S3^{2-}}$
$\ce{As2S5 + 3CO3^{2-} -> AsS4^{3-} + AsO3S^{3-} + 3CO2}$

Upon acidifying these solutions with hydrochloric acid, arsenic pentasulphide is reprecipitated:
$\ce{2AsS4^{3-} + 6H+ -> As2S5\downarrow + 3H2S\uparrow}$

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