Limit test for Heavy Metals
( Heavy Metal = Cu, Fe, Cd, Bi, Pb )
Principle The test depends on the precipitation of heavy metals as sulfides by sodium sulfide solution in presence of ammonium hydroxide. No color should be produced under the conditions of the test. In compounds containing lead as impurity lead will interfere with the test , so we had to prepare two solutions of the same substances under test to one of which add KCN solution and then add Na2S . The two nesslers must be equal in color if heavy metals are absent but in presence of heavy metal impurity , the one does not contain KCN will be darker in color.
2 CuSO4 + 4 KCN ———→ 2 Cu(CN)2
6 KCN
2 Cu(CN)2 ——→ 2 CuCN + NC-CN ——→ 2 K3Cu(CN)4
Limit test for heavy metals in absence of lead :
Principle Precipitation of heavy metal as sulfides by Na2S in presence of NH4OH using one nessler tube & no KCN.
- If dark color produced so heavy metal present.
- If no dark color produced so heavy metal absent.
Procedure :
In one Nesseler tube
5 ml sample + 1 ml NH4OH + 3 drops Na2S + mix thoroughly
- If Brown color produced so heavy metal present.
- If no brown color produced so heavy metal absent.
Limit test for heavy metals in presence of lead
Principle Precipitation of heavy metal as sulfides by Na2S in presence of NH4OH using two nesseler tubes & KCN in one of them.
Tube A
Tube B
Contain KCN
( Form soluble complex with heavy metals but Pb free ) Color due to Pb only
No KCN
( Pb & heavy metals are free )
Color due to Pb & heavy metals
- If B = A ( Matched ) so heavy metal is absent as color in both due to Pb only.
- If B > A ( B darker than A ) so heavy metal is present.