Qualitative Analysis: Detection of elements present in compound

Detection of Carbon and Hydrogen:
Compound is heated with cupric oxide and the produced vapours are passed through lime water or anhydrous CuSO₄.

If lime water turns milky, it shows the presence of CO₂ or carbon.

If anhydrous CuSO₄ turns blue, it shows the presence of H₂O or hydrogen.

Detection of Nitrogen, Sulphur and Halogens — Lassaigne's Test

Compound is heated with sodium in fusion tube (combustion tube) and poured in cold water and heated for some time. It gives Lassaigne's solution or sodium extract.

In Lassaigne's solution, elements present in the compound get converted from covalent to ionic form.

Reactions:

Organic compound + CuO → CO₂ + H₂O + Cu

CO₂ + Ca(OH)₂ → CaCO₃ (Milky) ↓ + H₂O

H₂O + CuSO₄ → CuSO₄·5H₂O (Blue)

Lassaigne's Test:

Na + C + N → NaCN

Na + S → Na₂S

Na + X → NaX

Test of Nitrogen:
Sodium extract is boiled with FeSO₄, which gives sodium hexacyano ferrate (II). Now ferric chloride is added which gives Prussian blue colour, confirming the presence of nitrogen.

FeSO₄ + 6NaCN → Na₄[Fe(CN)₆] + Na₂SO₄

Na₄[Fe(CN)₆] + FeCl₃ → Fe₄[Fe(CN)₆]₃ (Prussian Blue) + 12NaCl

Sulphur is present in the form of Na₂S.

(a) Lead acetate test: The other part of the Lassaigne's extract is acidified with acetic acid and then lead acetate solution is added. Formation of black precipitate confirms the presence of sulphur.

Na₂S + Pb(CH₃COO)₂ → PbS (Black) ↓ + 2CH₃COONa

For testing sulphur, the sodium extract is acidified with acetic acid because lead acetate is soluble and does not interfere with the test. If H₂SO₄ were used, lead acetate itself will react with H₂SO₄ to form white ppt. of lead sulphate which will interfere with the test.

(b) Sodium nitroprusside test: The one portion of the extract, a few drops of sodium nitroprusside are added. The appearance of violet colouration indicates sulphur.

Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS] — violet colouration

Note: If Nitrogen and sulphur both are present in compound they form sodium thiocyanate in sodium.

Na + C + N + S → NaCNS

During test of Nitrogen in place of Prussian blue, we get blood red colour of Fe(CNS)₃.

3NaCNS + FeCl₃ → Fe(SCN)₃ (Blood red) + 3NaCl

or

Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺ (Blood red)

Therefore sodium extract is prepared in presence of excess of sodium which decomposes sodium thiocyanate.

NaCNS + Na (Excess) → Na₂S + NaCN

(iii)Test of halogen: Before the test of halogen sodium extract is boiled with conc. HNO₃, to decompose the Na₂S and NaCN in form of H₂S and HCN.

Na₂S + 2HNO₃ → 2NaNO₃ + H₂S ↑

NaCN + HNO₃ → NaNO₃ + HCN

Now sodium extract is treated with silver nitrate which gives precipitate of AgX.

NaX + AgNO₃ → AgX ↓ + NaNO₃

AgCl — White ppt., soluble in NH₄OH
AgBr — Pale yellow ppt., sparingly soluble in NH₄OH
AgI — Yellow ppt., insoluble in NH₄OH

(iv) Test of phosphorus: Compound is heated with an oxidising agent (Na₂O₂) so the phosphorus in compound is converted into phosphate which on further reaction with ammonium molybdate in presence of HNO₃, gives canary yellow ppt. of ammonium phosphomolybdate and shows the presence of phosphorus.

Compound + Na₂O₂ → Na₃PO₄

Na₃PO₄ + 3HNO₃ → H₃PO₄ + 3NaNO₃

H₃PO₄ + 12(NH₄)₂MoO₄ + 21HNO₃ → (NH₄)₃PO₄·12MoO₃ (canary yellow) + 21NH₄NO₃ + 12H₂O

Ammonium phosphomolybdate