Qualitative Analysis of Zinc Ion
Take the original solution and add NH4OH to make it alkaline. Pass hydrogen sulphide gas (H2S) through the solution, a precipitate is obtained. Formation of white precipitate indicates the presence of Zn2+.
Zn2+ + S2- → ZnS
Confirmatory Test
On dissolving white precipitate of ZnS in dilute HCl, a colourless solution is obtained. Boil off H2S.
ZnS + 2HCl → ZnCl + H2S
On adding small amount of NaOH, a white precipitate is obtained which is soluble in excess of NaOH.
Zn2+ + 2OH– → Zn(OH)2 (Zinc hydroxide, white precipitate)
Zn(OH)2 + 2OH- → [Zn(OH)4]2-
Gray white precipitate is obtained on adding potassium ferrocyanide to solution of ZnS in dilute HCl.
2Zn2+ + K4[Fe(CN)6] → Zn2[Fe(CN)6] + 4K+