In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :
- decrease concentration of -OH ions
- increase concentration of Cl- ions
- prevent interference by phosphate ions
- increase concentration of NH4+ ions
Answer
(1) decrease concentration of -OH ions
Solution
NH4Cl NH4+ + Cl- (strong electrolyte)
NH4OH NH4+ + OH- (weak base)
∵ Common ion NH4+ suppresses ionisation of NH4OH ⇒ [OH-] decreases
⇒ only group III hydroxides (Fe(OH)3, Al(OH)3, Cr(OH)3; very low Ksp) precipitate; Zn2+, Mn2+, Mg2+ hydroxides do not.
Answer: (1)