Q3. How is the ammonium ion detected, and why is it tested on the solid salt rather than in the group scheme?
Heat the salt with NaOH solution: ammonia is given off, which smells, gives white fumes with an HCl rod and a brown precipitate with Nessler’s reagent (Lab Manual, page 69). The group scheme itself adds NH4Cl and NH4OH, so ammonium must be tested for first, on the original salt (reasoned).
Q4. H2S is passed in acid for group II but in ammoniacal solution for group IV. Why?
In acid, the H+ ions hold back the ionisation of H2S (the common ion effect), so the sulphide ion concentration is very low and only the least soluble sulphides, such as CuS and PbS, exceed their solubility products. In ammonia, H+ is removed, the sulphide concentration rises, and the more soluble sulphides, such as ZnS and NiS, precipitate too (reasoned from Lab Manual, page 50).
Q5. A group II precipitate dissolved in nitric acid gives a deep blue solution with excess ammonia. Which ion is present, and how is it confirmed?
Cu2+, as the deep blue [Cu(NH3)4]2+. Acidify with acetic acid and add potassium ferrocyanide: a chocolate-brown precipitate of Cu2[Fe(CN)6] (Lab Manual, pages 73–74).
Q6. Could ammonium sulphate be used instead of ammonium chloride in group III?
No. Its NH4+ would hold back the ionisation of NH4OH just as well, but its sulphate ions would precipitate Ba2+ as white BaSO4, which does not dissolve even in concentrated acids (Lab Manual, page 62). A group V cation could then come down in group III as a white precipitate and be mistaken for Al3+ (reasoned; compare Lab Manual Discussion Question (xxv), page 83).