Analysis of Category –I : Acid Radicals
Small amount of salt/mixture is treated with dilute H2SO4 or dilute HCI. Observation is made first in cold and then after the warming the mixture. Effervescence or evolution of odorous gases indicates the presence of acid radicals of this group.
Experiment/Observation | Inference |
1. Carbonate (CO32–) : | |
(i) A colorless, odourless gas is evolved with brisk effervescence. | It may be CO2 from CO32– or HCO3– |
(ii) On passing the evolved gas through lime water, it turns milky | Gas is CO2 and lime water turns milky due to formation of CaCO3. |
(iii) On passing the gas through milky lime water in excess, solution becomes colourless. | CO32– or HCO3– is confirm in the mixture and in presence of excess CO2, CaCO3 is converted into Ca(HCO3)2 (water soluble) |
Reactions: | |
(i) Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2 ↑ | |
(ii) 2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2 ↑ | |
(iii) Ca(OH)2 + CO2 → CaCO3↓ + H2O Calcium carbonate (white) | |
(iv) CaCO3 + H2O + CO2 → Ca(HCO3)2
Calcium bicarbonate (water soluble) |
|
2. Sulphide (S2-): | |
(i) A colourless gas with smell of rotten eggs. | May be H2S from sulphide. Black volour is due to PbS and gas is H2S thus S2– (sulphide) confirmed. |
(ii) Gas turns lead acetate paper black | |
(iii) On using cadmium acetate paper instead of lead acetate paper turns yellow. | Yellow substance is CdS and S2– is confirmed. |
Reactions: | |
(i) Na2S + H2SO4 → Na2SO4 + H2S ↑ | Smell of rotten eggs is due to H2S. |
(ii) Pb(CH3COO)2 + H2S → PbS ↓ + 2CH3COOH Black lead Sulphide | Black colour is due to PbS. |
(iii) Cd(CH3COO)2 + H2S → CdS ↓ + 2CH3COOH Yellow cadmium sulphide | Yellow colour is due to CdS. |
3. Sulphide (SO32–) : | |
(i) A colourless gas with pungent suffocating smell like burning sulphur. | May be SO2 from sulphite |
(ii) On burning acidified potassium dichromate paper in contact of gas or on adding some crystals of K2Cr2O7 in test tube, green colour is observed. | Gas is SO2 and sulphite (SO32–). is confirmed. |
(iii) On passing the evolved gas in dil. KMnO4 solution excess, the solution is decolorized. | SO2 gas and (SO32–). radicals are confirmed. |
Reactions: | |
(i) Na2SO3 + H2SO4 → Na2SO4 + SO2 ↑ + H2O | Pungent smell is of sulphur dioxide (SO2). |
(ii) K2Cr2O7 + H2SO4 + 3SO2 → Cr2(SO4)3 + K2SO4 + H2O Green | Green colour is of chromic sulphate [Cr2(SO4)3]. |
(iii) 2KMnO4 + 2H2O + 3SO2 → K2SO4 + 3MnSO4 + H2SO4 colourless | Manganese sulphate is colourless |
4. Acetate (CH3COO–): | |
(i) A Colourless gas with smell of vinegar is evolved. | May be vapour of CH3COOH from acetate. |
(ii) Pungent smelling gas is evolved when salt/mixture is rubbed on palm with oxalic acid. | CH3COO– is confirmed. |
(iii) Neutral FeCl3 is added in water extract of mixture and red colour is obtained | CH3COO– is confirmed and red colour is of ferric acetate. |
(iv) Fruity smell is observed when mixture is heated with concentrate H2SO4 and iso-amyl alcohol. | Smell is due to esters and CH3COO– |
Reactions: | |
(i) 2CH3COONa + H2SO4 → Na2SO4 + 2CH3COOH | Smell is of vapour of acetic acid. |
(ii) 2CH3COONa + H2C2O4 → Na2C2O4 + 2CH3COOH | Oxalic acid decomposes acetates. |
(iii) 2CH3COONa + H2SO4 → (CH3COO)5Fe + 3NaCl | Ferric acetate gives red colour. |
(iv) 2CH3COONa + H2SO4 → Na2SO4 + 2CH3COOH | |
(v) CH3COOH + C2H5OH → CH3COOC2H5 + H2O
Ester |
Ester, smell like fruits. |
5. Nitrite (NO2–) : | |
(i) A light brown coloured gas with pungent smell. | May be (NO2 + NO) from nitrite (NO2–). |
(ii) On passing the gases through fresh aqueous solution of FeSO4, dark brown colour is observed. | (NO2–) is confirmed and dark brown colour is of ferrous nitroso sulphate. |
(iii) On bringing acidified KI/starch paper in contact of gases, paper turns blue. | (NO2–) is confirmed and blue colour is due to I2 starch complex. |
Reactions: | |
(i) 2NaNO2 + H2SO4 → Na2SO4 + 2HNO2
3HNO2 → HNO3 + 2NO + H2O 2NO + O2 → 2NO2 |
Brown colour and pungent smell is due to (NO2 + NO). |
(ii) FeSO4 + NO → [Fe(NO)]SO4
Dark brown |
Dark brown colour is due to dark brown ferrous nitroso-sulphate. |
(iii) 2NaNO2 + 2KI + 2H2SO4 → Na2SO4 + K2SO4 + I2 + 2NO + 2H2O
I2 + Starch → Blue complex |
NOTE:
1. Some basic carbonate like those of Bi, Ba, Pb etc. evolve CO2 gradually with H2SO4. Here dilute HCl must be used.
2. SO2 also turns lime water milky. Into Solution SO32– or S2–, a few crystals must be added in the mixture and then for CO32– must be performed.
3. PbCO3 initially reacts vigorously with HCl or H2SO4. But reaction slows down due to formation of protective covering of PbCl2 or PbSO4 on the surface of salt.
4. Some sulphide (e.g., PbS, VdS, NiS, SnS, Sb2S3 etc,) are not decomposed by dilute HCl and conc. HCl is used here. HgS is insoluble even in conc. HCl for HgS, aqua-regia is used.
In case of these sulphide Zn/HCl or Sn/HCl may be also be used. H2SO4 may also be used with Zn or Sn.
Zn + H2SO4 → ZnSO4 + 2[H]
HgS + 2[H] → Hg + H2S
5. Before testing CH3COO– by ferric acetate, the solution should not contain radicals which combines with Fe3+. Thus, radicals like CO32–, SO32–, PO42–, I– etc, must be removed by adding AgNO3 before performing ferric chloride test.
6. Like SO2, H2S also turns potassium dichromate paper green. So before testing SO32– S2– must be tested and if present it should be removed.
7. If a mixture containing (NO2–) also contains I–, then violet fumes will be evolved on addition of dil H2SO4.