When conc. H2SO4 was treated with K4[Fe(CN)6], CO gas was evolved. By mistake, somebody used dilute — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Group III cations (Fe3+, Al3+, Cr3+) are precipitated as hydroxides using the weak base NH4OH in the presence of NH4Cl. Step 2: The addition of NH4Cl provides a high concentration of NH4+ ions, which shifts the dissociation equilibrium of NH4OH to the left (NH4OH <=> NH4+ + OH-) via the common ion effect. Step 3: This decreases the concentration of OH- ions so that only the very low solubility product Group III hydroxides precipitate, preventing the co-precipitation of more soluble Group IV and V hydroxides. Thus, option (c) is correct.