Question

When a sample contains Al3+ 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 Al3+ 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 Fe3+, the acidic H2S could reduce it to Fe2+ions which would eventually precipitate as FeS even in the presence of OH- barring the detection of Fe3+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.

Answer 1

The removal of H2S before testing for Group III cations like Al3+ is important when Fe3+ might be present, as H2S can reduce Fe3+ to Fe2+ leading to its precipitation as FeS and interfering with accurate identification.

Step-by-step explanation:

When analyzing for the presence of cations in a qualitative analysis, the sample often undergoes sequential treatment with various reagents to separate and identify different groups of metal ions. In this context, testing for the presence of Group III cations (such as Al3+ ions), involves the addition of ammonia or sodium hydroxide to precipitate these ions as their respective hydroxides.

However, when previously added substances like H2S might interfere with the test, it is typically advisable to remove them. Although Al3+ will precipitate as Al(OH)3, which is relatively unaffected by the presence of sulfide due to the higher solubility product (Ksp) of aluminum hydroxide compared to aluminum sulfide, the situation is different if Fe3+ is present. The acidic H2S can reduce Fe3+ to Fe2+, which then could precipitate as FeS, therefore obscuring the detection of Fe3+ as Fe(OH)3.

In such cases, it is necessary to boil off H2S to prevent this reduction and subsequent misidentification. Hence, the removal of H2S is a crucial step in the analytical procedure used to maintain the accuracy of cation identification, particularly when certain cations like Fe3+ are suspected to be in the sample.