What is Qualitative Inorganic Analysis?
The qualitative inorganic analysis is a method of analytical
chemistry that seeks to find out the elemental composition of inorganic
compounds through various reagents. It is mainly focused on detection of ions
in an aqueous solution, so the substances in other states need to be converted
into an aqueous solution before starting the experiment.
The procedure for detecting ions (cations and anions) in aqueous
solutions are called Cation Analysis and Anion Analysis.
Let's discuss the Qualitative Analysis of Anions.
Qualitative Analysis
of Anions
Preliminary Tests
Some preliminary tests are done before going to the anion
analysis.
(A) Physical
Examination: Colour and Smell
The physical examination of the unknown salt involves the study of
colour, smell and density. Characteristic smell helps to identify some anions
such as acetate, sulphide etc.
(B) Dry Heating Test
This test is performed by heating a small amount of salt in a dry
test tube. Quite valuable information can be gathered by carefully performing
and noting the observations. On heating, some salts undergo decomposition, thus
evolving the gases.
(C) Identification of
Anions (Acid Radicals)
The identification of the radicals is first done on the basis of
the preliminary tests. The 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 on the fact that:
· CO32-, S2-, NO2- and SO32- react with dil.
H2SO4 to give out CO2, H2S, NO2 and SO2 gases
respectively. These gases on identification indicate the nature of the anion
present in the salt.
· Cl-,Br-,I-,NO3- and C2O42- and CH3COO- react with
conc. H2SO4 but not with dil. H2SO4 to produce characteristic gases.
· SO42- and PO43- react neither
with dil H2SO4 nor with conc. H2SO4. These are, therefore, identified by
individual tests.
Thus, these anions may be identified by performing the following
tests below:
1) Dil. H2SO4 Tests
Treat a pinch of the salt with dil. H2SO4 and identify
the gas evolved.
Chemical Reactions
Involved in Dil.H2SO4 Test:
Dilute H2SO4 decomposes carbonates, sulphides, sulphites and nitrites in
cold to give gases. These gases on identification indicate the nature of the anion
present in the salt.
(a) Carbonate: On treating the
solid carbonate, CO2 is given off in the cold with brisk effervescence.
(b) Sulphide: Sulphides when treated with dil.
H2SO4 give H2S gas.
(c) Sulphite: On heating
solid sulphite with dil.H2SO4, SO2 gas is evolved
(d) Nitrite: On treating solid nitrite with dil. H2SO4, nitric oxide (NO)
gas is evolved which readly gives brown fumes of NO2with the oxygen of
the air.
2) Conc. H2SO4 Test
If no action takes place with dil. H2SO4, warm a pinch of the
salt with conc. H2SO4 and identify the gas evolved.
Chemical Reactions
Involved in Conc. H2SO4 Test
(a) Chlorides: Chloride salts
react with conc. H2SO4 to evolve hydrogen chloride (HCl) gas.
(b) Bromides: Bromide salts react with conc. H2SO4 to evolve
bromine gas.
(c) Iodides: Iodide salts react with conc. H2SO4 to evolve
vapours of iodine.
(d) Nitrates: Upon reaction with conc.H2SO4 nitrates evolve
NO2 gas.
(e) Acetates: Acetates react with conc. H2SO4 to produce
vapours of acetic acid.
(f) Oxalates: Oxalates salts react with conc. H2SO4 to evolve a
mixture of carbon dioxide and carbon monoxide.
3) Independent Group.
(SO42- and PO43-) Test
If the salt does not react with dil H2SO4 as well as with
conc. H2SO4 test for SO42-and PO43- by performing
their individual tests.
4) Potassium
permanganate Test
This test is performed by using dilute sulphuric acid and
potassium permanganate as reagents. This test helps in the detection of Cl-, Br-, I- and C2O42-.
Chemical reactions
involved in Potassium permanganate test
Wet Tests for Acid Radicals (Anions)
Let us discuss the chemical reactions involved in the confirmation
of Anions:
Confirmation of
Carbonate (CO32-)
(a) Reaction with
di.l HCl
Carbonate on reaction with dil. HCl gives CO2 gas that reacts
with lime water to produce a white precipitate of calcium carbonate that turns
lime water milky. In case of soluble carbonate, this test is performed with
water extract and in case of insoluble carbonates, this test is performed with
the solid salt.
(b) Magnesium
sulphate test
This test is performed in the case of soluble carbonates only.
Soluble carbonates react with the magnesium sulphate solution to form a white
precipitate of magnesium carbonate.
Confirmation of
Sulphite (SO32-)
(a) Barium chloride
test
Sulphites on reaction with barium chloride to form a white
precipitate of barium sulphite. Barium sulphite dissolved in dil. HCl with the
evolution of sulphur dioxide gas.
(b) Potassium permanganate test
The colour of potassium permanganate is discharged when it reacts
with sulphite. The reaction is given below.
(c) Potassium
dichromate test
A green colour is obtained when sulphites react with potassium
dichromate solution.
Confirmation of
Sulphide (S2-)
(a) Sodium
nitroprusside test
Sulphides give a violet or purple colouration with the sodium
nitroprusside solution due to the formation of Na4[Fe(CN)5NOS].
(b) Lead acetate test
Sulphides react with lead acetate to form a black precipitate of
lead sulphide.
(c) Cadmium carbonate test:
Sulphides react with a suspension of cadmium carbonate to form a
yellow precipitate of cadmium sulphide.
Confirmation of
Nitrite (NO2-)
(a) Ferrous sulphate
test
Nitrites give a dark brown or black colouration in Ferrous
sulphate test due to the formation of FeSO4.NO.
(b) Starch - Iodide
test
Nitrites react with potassium iodide in the presence of dilute
sulphuric acid to liberate iodine. Iodine forms a blue-black complex with
starch.
(c) Diphenylamine
test
In the presence of nitrites, diphenylamine is oxidised, giving a
blue colouration.
Confirmation of Nitrate (NO3-)
(a) Diphenylamine
test
In the presence of nitrates, diphenylamine is oxidised, giving a
blue colouration.
(b) Copper chips test
In this reaction copper chips reduces nitrates to reddish brown NO2 gas.
(c) Brown-ring test
This test can be performed by adding a solution of iron (II)
sulphate to a solution of nitrate, followed by the slow addition of
concentrated sulphuric acid, such that the sulphuric acid forms a layer below
the aqueous solution. The formation of a brown ring at the junction of two
layers indicates the presence of nitrate.
The overall reaction is the reduction of nitrate ion by iron (II)
which reduced to iron (I) and formation of a nitrosonium complex where nitric
oxide is oxidised to NO+.
Confirmation of
Chloride (Cl-)
(a) Silver nitrate
test
Chlorides on reaction with silver nitrate solution to form a white
precipitate of silver chloride which is soluble in ammonium hydroxide.
(b) Manganese dioxide
test
When chloride salts react with manganese dioxide and concentrated
sulphuric acid, chlorine gas is liberated.
(c) Chromyl chloride
test
When chloride salts react with potassium dichromate and conc.
sulphuric acid red fumes of chromyl chloride is formed which reacts with sodium
hydroxide to form yellow solution of sodium chromate. Sodium chromate reacts
with lead acetate in presence of dil. acetic acid to form yellow precipitate of
lead chromate.
Confirmation of
Bromide (Br-)
(a) Silver nitrate
test
Bromides on reaction with silver nitrate solution forms a pale
yellow precipitate of silver bromide which is sparingly soluble in ammonium
hydroxide.
(b) Manganese dioxide
test
When bromide salts react with manganese dioxide and concentrated sulphuric
acid, bromine gas is liberated.
(c) Chlorine water
test
Bromine liberated in this test being soluble in carbon disulphide
imparts an orange colour to the carbon disulphide layer.
Note: Carbon tetrachloride, cyclohexane,
chloroform etc can be used instead of carbon disulphide.
Confirmation of
Iodide (I-)
(a) Silver nitrate
test
Iodides on reaction with silver nitrate solution forms an yellow
precipitate of silver iodide which is insoluble in ammonium hydroxide.
(b) Manganese dioxide
test:
When iodide salts react with manganese dioxide and concentrated
sulphuric acid, iodine gas is liberated.
(c) Chlorine water
test
Iodine liberated in this test being soluble in carbon disulphide
imparts a violet colour to the carbon disulphide layer.
Note: Carbon tetrachloride, cyclohexane,
chloroform etc can be used instead of carbon disulphide.
Confirmation of
Acetate (CH3COO-)
(a) Oxalic acid test
Oxalic acid reacts with acetate salt to form acetic acid which has
a characteristic vinegar like smell.
(b) Ester test
Acetate salts react with conc. sulphuric acid and ethyl alcohol to
form the ester, ethyl acetate which has a fruity smell.
(c) Ferric chloride
test
The reaction takes place in the ferric chloride test is given by
the following equations.
Confirmation of
Oxalate (C2O42-)
(a) Calcium chloride
test
Oxalate salts react with calcium chloride to form white
precipitate of calcium oxalate.
(b) Potassium permanganate test
In this test, the pink colour of potassium permanganate is
decolourised with the evolution of carbon dioxide gas.
Confirmation of
Sulphate (SO42-)
(a) Barium chloride
test
Sulphates react with barium chloride to form white precipitate of
barium sulphate.
(b) Match stick test
Violet streaks are produced during this test.
(c) Lead acetate test
Sulphates react with lead acetate to form white precipitate of
lead sulphate.
Confirmation of
Phosphate (PO43-)
(a) Ammonium
molybdate test
Phosphate salts react with ammonium molybdate solution to form a
deep yellow precipitate of ammonium phosphate molybdate. The chemical reaction
is as follows:
(b) Magnesia mixture
test
Phosphate salts react with magnesia mixture to form white
precipitate of magnesium ammonium phosphate.
Note: To prepare magnesia mixture, add solid NH4Cl to magnesium
chloride solution. Boil, cool and add NH4OH till a strong smell of ammonia ia
obtained.