Identification Of Acid Radicals (Anions)

The identification of the acid radicals is first done on the basis of preliminary tests. Dry heating test is one of the preliminary tests performed earlier which may give some important information about the acid radical present. The other preliminary tests are based upon the fact that:
1. CO32-,  S2-, SO32-and NO2–  react with dil. H2S04 to give out CO2, H2S, SO2 and NO2 gas respectively which can be identified by certain tests.
2. Cl, Br, I, NO3, C2O42- and CH3COO react with cone. H2S04 but not with dil. H2S04 to produce characteristic gases.
3. S042- and P043- react neither with dil. H2S04 nor with cone. H2S04. These are therefore, identified by individual tests.
Thus, the acid radicals may be identified by performing the following tests in the order given below :
(i) Dil. H2S04 test. Treat a pinch of the salt with dil. H2S04 and identify the gas evolved.
(ii) Cone. H2S04 test. If no action takes place with dil. H2S04, warm a pinch of the salt with cone. H2S04 and identify the gas evolved.
(iii) Independent Group. (SO42- and PO43-). If the salt does not react with dil. H2S04 as well as with cone. H2S04, test for SO42- and PO43- by performing their individual tests.
Let us now discuss these tests in detail one by one.

Dilute Sulphuric Acid Test

Take a small quantity of the salt in a test-tube and add 1-2 ml of dilute sulphuric acid. Observe whether some gas is evolved or not. If some gas is evolved, identify the gas and draw inferences from Table 9.7.

Table 9.7. Dilute Sulphuric Acid Test

Observations Inference
Gas Evolved Possible Radical
1. Colourless, odourless gas with brisk effervescence, turns lime water milky. CO2

 

CO32-
2. Colourless gas, smell like that of rotten eggs, turns ‘ lead acetate paper black. H2S

 

S2-
3. Colourless gas, smell like that of burning sulphur, turns acidified potassium dichromate paper green. SO2

 

SO32-

 

4. Reddish brown gas, pungent smell, turns ferrous sulphate solution black. NO2

 

NO2
5. No gas is evolved. CO32-, S2-, SO32-, NO2 absent

Note:
1. Do not treat the salt with a large quantity of dilute acid.
2. Do not heat the salt with dilute acid.

Chemical Reactions Involved in Dil. H2SO4 Test

Dilute H2S04  (or dilute HCl) decomposes carbonates, sulphides sulphites and nitrites in cold to and liberates different gases. These gases on identification indicate the nature of the acid radical present in the salt.
1. Carbonates. On treating the solid carbonate, CO2 is given off in the cold with brisk effervescence.qualitative-analysis-identification-acid-radicals-anions-12. Sulphides. Sulphides when treated with dil. H2S04 give H2S gas.qualitative-analysis-identification-acid-radicals-anions-23. Sulphites. Sulphites when treated with dil. H2S04 give SO2 gas.qualitative-analysis-identification-acid-radicals-anions-3

4. Nitrites. On treating the solid nitrite with dil. H2S04, nitric oxide (NO) gas is evolved which readily gives dense brown fumes of NO2 with oxygen of the air.

qualitative-analysis-identification-acid-radicals-anions-4

Potassium Permanganate Test

To a pinch of salt in test tube add about 2 ml of dilute sulphuric acid. Boil off any gas evolved, add little more of dilute acid and then potassium permanganate solution d’ropwise. Note the changes as given in Table 9.8. This test helps in detection of Cl, Br, I, C2O42-, and Fe2+ radicals.

Table 9.8. Potassium Permanganate Test

Observations Inference
1. Potassium permanganate decolourised

without the evolution of any gas.

Presence of Fe2+  salts.

 

2. Potassium permanganate decolourised :

(a) In cold

(i) With the evolution of chlorine.

(ii) With the evolution of bromine.

(iii) With the evolution of iodine.

(b) On warming

(i) With the evolution of carbon dioxide

 

 

Cl

Br

I

 

C2O42-

3. KMnO4 not decolourised. Absence of Cl, Br, I, C2O42- and Fe2+

Note:
Do not perform this test if the salt reacts with dilute sulphuric acid because sulphides, sulphites and nitrites also decolourise KMn04 solution.

Chemical Reactions Involved
qualitative-analysis-identification-acid-radicals-anions-5qualitative-analysis-identification-acid-radicals-anions-6

Concentrated Sulphuric Acid Test

This test is performed by treating small quantity of the salt with cone, sulphuric acid (2-3 ml) in a test tube. Identify the gas evolved in cold and then on heating. Draw inferences from Table 9.9.
Table 9.9. Cone. Sulphuric Acid Test

Observations Inference
Gas Evolved Possible Radical
1. Colourless gas with pungent smell. The gas gives white fumes with aqueous ammonia (NH4OH) and white ppt. with AgNO3 solution. HCl

 

Cl

 

2. Reddish brown vapours with pungent smell, turns starch paper yellow. It does not turn FeSO4 solution black. Br2 Br

 

3. Deep violet vapours with pungent smell, turns starch paper blue. A sublimate is formed on the sides of the tube. I2 vapours I

 

4. Reddish brown gas with pungent smell, turns FeSO4 solution black. NO2

 

NO2
5. Colourless gas which turns lime water milky and a gas which burns with blue flame. CO2 and CO C2O42-
6. No gas/vapours evolved. Cl, Br, I, CH3COO, C2O42-  absent

Note :
1. If some gas is evolved with dilute sulphuric acid, then there is no need for performing cone, sulphuric acid test.
2. Do not boil the salt with cone, sulphuric acid. On boiling, the acid may decompose to give  SO2 gas.
3. Nitrates give vapours of nitric acid (colourless) when heated with cone, sulphuric acid. When a paper pellet or copper chips is added, dense brown fumes evolve. Paper pellet acts as a reducing agent and reduces nitric acid to NO2 (Reddish brown gas).

Chemical Reactions Involved in cone. H2S04 Test
qualitative-analysis-identification-acid-radicals-anions-7
qualitative-analysis-identification-acid-radicals-anions-8

Tests For Independent Radicals (SO42- and PO43-)

As already discussed these radicals are not detected by dilute or concentrated H2S04. They are tested individually.
1. SULPHATE (SO42-)
Boil a small amount of salt with dilute HCl in a test tube. Filter the contents, and to the filtrate add few drops of BaCl2 solution. A white precipitate, insoluble in cone. HCl, indicates presence of sulphate.
2. PHOSPHATE (PO43-)
Add cone. HNO3 to the salt in a test tube. Boil the contents and add excess of ammonium molybdate solution. A yellow precipitate indicates presence of phosphate.

Wet Tests Foe Basic Radicals (Cations)

Preliminary tests such as dry heating test, charcoal cavity test, flame test and borax bead test may give us some indication about the cation present in the salt. However, the cation is finally detected and confirmed through a systematic analysis involving wet tests. For the sake of qualitative analysis the cations are classified into six groups (Table 9.10).

Table 9.10. Classification of Cations

Group Cations
Group zero NH4+
Group I Pb2+
Group IIA Pb2+, Cu2+
Group IIB As3+
Group III Fe3+, Al3+
Group IV CO2+, Ni2+, Mn2+, Zn2+
Group V Ba2+, Sr2+, Ca2+
Group VI Mg2+

Before carrying out the wet tests for the analysis of cation, the salt has to be dissolved in some suitable solvent to prepare its solution.