Qualitative Analysis of Cupric Ion
On passing hydrogen sulphide, gas through acidified original solution obtained indicates the presence of Cu2+ ions.
M2+ + S2– → MS
If no precipitate is obtained even on dilution of test portion, in that case group II is absent. To obtain a granular precipitate pass H2S for 2 to 3 minutes through hot solution.
Confirmatory Test
On heating black precipitate of CuS with dilute HNO3, a blue colour solution is obtained.
3CuS + 8 HNO3 → 3 Cu(NO3)2 (Copper nitrate, blue colour) + 3S + 2NO ↑ + 4H2O
On adding ammonium hydroxide, precipitate is obtained which is soluble in excess reagent to produce deep blue solution.
Cu2+ + 2OH- → Cu(OH)2 (Copper hydroxide, blue precipitate)
Cu(OH)2 + 4 NH3 → [Cu(NH3)4]SO4 (Tetraammine cupric sulphate, deep blue colour)
On adding acetic acid followed by potassium ferrocyanide to blue solution, a chocolate brown precipitate is obtained.
2Cu2+ + K4[Fe(CN)6] → Cu2[Fe(CN)6] + 4K–