Group I Radicals which are detected by dilute H2SO4 or dilute HCl, by liberating a gas (i) Carbonate (ii) Sulphite (iii) Sulphide (iv) Nitrite (v) Acetate Group II Radicals which are detected by concentrated H2SO4 (i) Chloride (ii) Bromide (iii) Iodide (iv) Nitrate (v) Oxalate Group III Radicals which do not give any characteristic gas with dilute and concentrated H2SO4 (i) Sulphate (ii) Phosphate (iii) Borate (iv) Fluoride Identification of Acid Radicals Group : I Group acidic radicals : CO32–, SO32–, S–2, NO2– Group reagent : dil. HCl or dil H2SO4 Procedure : Take some quantity of substance and add dil. HCl or dil. H2SO4. Observation : (i) Brisk efferrescence in cold with evolution of colourless and odourless gas – Carbonate ion (CO32–) (ii) A colourless gas with suffocating odour having smell of burning sulphur – Sulphite ion (SO32–) (iii) A colourless gas with smell of rotten eggs – Sulphide ion (S–2) (iv) A light brown gas – Nitrite ion (NO2–) Group : II Group acidic radicals : Cl–, Br–, I–, NO3–, C2O42– Group reagent : Conc. H2SO4 Procedure : Take salt and add conc. H2SO4 and warm gently. Observation : (i)Colourless gas with pungent smell which fumes in air. Addition of pinch of MnO2 in the solution gives pale green gas. – Chloride ion (Cl–) (ii) Reddish brown fumes which intensify on addition of MnO2. Bromide ion (Br–) (iii) Violet pungent fumes evolved which may condense as black spots on the cooler part of test tube. These violet fumes intensity on addition of MnO2 – Iodine ion (I–) (iv) Light brown fumes of NO2 which intensify on adding copper turning – Nitrate ion (NO3–) (v) Colourless, odourless gas which burns with blue flame at mouth of tube – Oxalate ion (C2O42–) Confirmatory Test 1. For Chloride ion (Cl–) : If salt contains chloride ion than it evolves HCl gas on reaction with conc. H2SO4. (i) The HCl gas so evolved when reacted with NH4OH gives fumes of ammonium chloride. (ii) HCl gas also gives curdy white precipitate of silver chloride with silver nitrate solution. (iii) When chloride salt is directly heated with conc. H2SO4 in presence of K2Cr2O7, deep red vapours of chromyl chloride are evolved. This test is known as chromyl chloride test. When vapours of chromyl chloride are passed through NaOH solution, solution becomes yellow due to the formation of sodium chromate, which on reaction with CH3COOH and Pb(CH3COO)2 gives a yellow precipitate of lead chromate. Note : Test is not given by HgCl2, SnCl2, AgCl, PbCl2. 2. For bromide ion (Br–) : (i) The bromide salt which is under analysis gives pale yellow precipitate of AgBr on reaction with NH4OH. If NH4OH is taken in excess then the soluble complex of silver is formed. (ii) Bromide salt when treated with dil. H2SO4, CHCl3 (or CCl4) and chlorine water, chlorine is able to displace bromine due to which Br2 gas is liberated. This Br2 gas dissolves in CHCl3 or CCl4 forming brown layer. 3. For Iodide ion (I–) : (i) The salt if with iodide ion when treated with conc. H2SO4 evolves iodine vapour which on reaction with starch produce blue colour. (ii) The iodide salt solution gives yellow precipitate of AgI with AgNO3 solution which is insoluble in NH4OH. (iii) The iodide salt is treated with chlorine water, chlorine replaces iodine from salt. The iodine thus evolved dissolves in CHCl3 or CCl4 layer giving violet colour. 4. For Nitrate ion (NO3–) : (i) Brown ring test : If salt under analysis contains nitrate ion, to confirm it, aqueous solution of the salt is treated with freshly prepared solution of ferrous sulphate and conc. H2SO4, a brown ring complex is formed. (ii) Salt of nitrate when reacted with Zn or Al in presence of conc. NaOH evolves NH3 gas which can be detected by its characteristic odour. Group :III Group acidic radicals : SO42–, BO33–, PO43– and F– Group reagents (i) BaCl2 – (For SO42–) (ii) C2H5OH and conc. H2SO4 – (For BO33–) (iii) Conc. HNO3 and ammonium molybdate – (For PO43–) (iv) Sand and conc. H2SO4 – (For F–) Procedure & Observation : Take salt and add above reagents one by one to observe the result. (i) For Sulphate ion : Take salt and add BaCl2, if salt containing SO42– then precipitate of Barium sulphate (BaSO4) is obtained which is insoluble in conc. HNO3. Note : If sulphate salt is of silver or lead then on reaction with BaCl2 it will give AgCl or PbCl2. So to avoid this BaNO3 can be used for instead of BaCl2 to analyse SO42– ion .
