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When a sample contains $\ce{Al^3+}$ ions (group-III), to test for its presence, before adding NH4Cl/NH3 solution do we need to boil off acidic H2S added earlier to check for the presence of group-ii cations? It can be deduced that $\ce{Al^3+}$ cations would be precipitated and persist as "gelatinous hydroxide" even in the presence of sulfides since Ksp of aluminum hydroxide is far higher than that of aluminium sulfide. So it seems that removal of H2S is not necessary in this case.

But, I know that if the sample to be analysed contains $\ce{Fe^3+}$, the acidic H2S could reduce it to $\ce{Fe^2+}$ ions which would eventually precipitate as FeS even in the presence of OH- barring the detection of $\ce{Fe^3+}$ in the form of Fe(OH)3. is it exactly the reason as to why H2S should be driven off to proceed further? A detailed answer will be appreciated.

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  • $\begingroup$ Aluminum sulfide does not exist. It is totally hydrolyzed into $\ce{Al(OH)3 + H2S}$. So it has no $\ce{K_{sp}}$, despite the information in $4$th line. $\endgroup$
    – Maurice
    Oct 15, 2022 at 18:55
  • $\begingroup$ Can you give the reference for your procedure for qualitative analysis? $\endgroup$
    – Mathew Mahindaratne
    Oct 16, 2022 at 1:18
  • $\begingroup$ Well... I was instructed to do so during my labs. $\endgroup$
    – Sudhagar
    Oct 16, 2022 at 10:28

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