Wet Test Analysis for Acid Radicals: Examination of Volatile Group | Chemistry by Burhan Ishaque

 

(Group I = CO₃^-2, HCO₃^-, NO₂^-, S^-2, SO₃^-2, S₂O₃^-2).

a.       Threat about 1 gram of the given mixture with dilute H₂SO₄  and observe what happens in the cold and on heating.

b.       Add K₂Cr₂O₇ solution to about 1 gram of the mixture and then dilute H₂SO₄ and test the gas evolved with lime water.

c.       Prepare aqueous solution for confirmatory test.

Sr. No.	Observations	Interferences & Confirmations
1.	Effervescence in the cold. The gas evolved in the presence of K2Cr2O7 turns lime water milky. CO2 is confirmed.	Carbonate or bicarbonate (CO3-2 or HCO3-) Confirmation: 1.       Dissolve the mixture in cold water and filter. Heat and filtrate, evolution of CO2 proves HCO3-. If no gas is evolved on heating then CO3- is confirmed. 2.       Filtrate + MgSO4 → white ppt. in cold CO3-2. 3.       Filtrate + MgSO4 → white ppt. in cold HCO3-. 4.       Filtrate + MgSO4 → no ppt. even on boiling insoluble CO3-2
2.	Brown fumes of NO2 are given out; KI + starch paper is turned blue & K2Cr2O7 paper green.	Nitrite (NO2-) Confirmation: 1.       O.S + dilute CH3COOH + FeSO4 solution → blackish-brown colour. 2.       O.S + NH4Cl → effervescence. 3.       Aq. Solution + KI +H2SO4 → brown solution of Iodine. 4.       O.S + NH4Cl solid + CoCl2 + CH3COOH glacial in excess & shake → thick yellow ppt. 5.       Aq. Solution + Diphenylamine in H2SO4 → Blue Color
3.	A colorless gas with rotten egg smell. Lead acetate paper is turned lack. (gas is H2S)	Sulphide (S-2). Confirmation: 1.       O.S + Sodium Nitroprusside → violet color. 2.       O.S + dilute HNO3 + AgNO3 → Black ppt. 3.       O.S + dilute HCl + SbCl3 → Orange red ppt.
4.	A colorless gas with a pungent odor like burning Sulphur evolves, which turns K2Cr2O7 paper green and Litmus paper red. (Gas is SO2).	Sulphite (SO3-2). Confirmation: 1.       O.S + BaCl2 → white ppt. which is soluble in dilute HCl. 2.       O.S + AgNO3 → White ppt. which is soluble in dil. HNO3. 3.       Aq. Solution + KMnO4 acidified with dilute H2SO4 → Colour is discharged.
5.	A colourless gas with pungent odor, turns K2Cr2O7 paper green and a light-yellow ppt. is formed on the cooler parts of the test tube. (Gas is SO2 & Light-yellow ppt. is that of sulphur).	Thiosulphate (S2O3-2). Confirmation: 1.       O.S + (CH3COOH)2Pb → white ppt. which blackens on heating. 2.       Aq. Solution +FeCl3 → violet color which fades away quickly. 3.       Aq. Solution + AgNO3 → white ppt. immediately changing to yellow, orange, brown and finally black. 4.       Aq. Solution + Iodine solution → color is discharged.