Qualitative Analysis - Test for Halogens
Description:
The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.
Na + X → NaX
NaX + AgNO3 → AgX (X − Cl, Br, I)
AgF is soluble in water and does not precipitate out.
A white precipitate soluble in ammonium hydroxide (NH4OH) shows the presence of chlorine.
Cl− + AgNO3 → AgCl ↓ (white ppt) + NO3−
AgCl + 2NH4OH → [Ag(NH3)2]Cl (soluble complex) + 2H2O
Yellowish precipitate sparingly soluble in NH4OH shows the presence of bromine.
Br− + AgNO3 → AgBr ↓ (yellow ppt) + NO3−
Yellow precipitate insoluble in NH4OH shows the presence of iodine.
I− + AgNO3 → AgI ↓ (yellow ppt) + NO3−
In the presence of nitrogen and Sulphur, the sodium fusion extract, from Lassaigne’s test is first boiled with conc.HNO3 to decompose cyanide or sulphide of sodium.
These ions otherwise, interfere with the silver nitrate test.
NaCN + HNO3 → NaNO3 + HCN ↑
Na2S + 2HNO3 → 2NaNO3 + H2S ↑
Summary
| Test | Observation | Inference |
|---|---|---|
Sodium fusion extract + FeSO4 FeCl3 + HCl | i) A prussian blue colour is formed. ii) Blood red colouration is observed. | i) Nitrogen is confirmed. ii) The presence of both nitrogen and Sulphur are confirmed. |
i) Sodium fusion extract + sodium nitroprusside ii) Sodium fusion extract + CH3COOH + Pb(CH3COO)2 | i) A violet colour is observed. ii) A black precipitate is formed. | Presence of Sulphur is confirmed. |
Sodium fusion extract + HNO3 + AgNO3 | i) A white precipitate soluble in NH4OH is formed. ii) A yellow precipitate slightly soluble in NH4OH is formed. iii) A yellow precipitate insoluble in NH4OH is formed. | The presence of halogens is confirmed. i) Chlorine ii) Bromine iii) Iodine |