UNIT-7
SYSTEMATIC QUALITATIVE
ANALYSIS
A
NALYSIS always does not mean breaking of substance into its ultimate
constituents. Finding out the nature of substance and identity of
its constituents is also analysis and is known as qualitative analysis.
Qualitative analysis of inorganic salts means the identification of cations and
anions present in the salt or a mixture of salts. Inorganic salts may be obtained
by complete or partial neutralisation of acid with base or vice-versa. In the
formation of a salt, the part contributed by the acid is called anion and the part
contributed by the base is called cation. For example, in the salts CuSO4 and
2+ 2– –
NaCl, Cu and Na+ ions are cations and SO4 and Cl ions are anions. Qualitative
analysis is carried out on various scales. Amount of substance employed in
these is different. In macro analysis, 0.1 to 0.5 g of substance and about 20 mL
of solution is used. For semimicro analysis, 0.05 g substance and 1 mL solution
is needed while for micro analysis amount required is very small. Qualitative
analysis is carried out through the reactions which are easily perceptible to our
senses such as sight and smell. Such reactions involve:
(a) Formation of a precipitate
(b) Change in colour
(c) Evolution of gas etc.
Systematic analysis of an inorganic salt involves the following steps:
(i) Preliminary examination of solid salt and its solution.
(ii) Determination of anions by reactions carried out in solution (wet tests)
and confirmatory tests.
(iii) Determination of cations by reactions carried out in solution (wet tests)
and confirmatory tests.
Preliminary examination of a salt often furnishes important information, which
simplifies further course of analysis. Although these tests are not conclusive but
sometimes they give quite important clues for the presence of certain anions or
cations. These tests can be performed within 10-15 minutes. These involve noting
the general appearance and physical properties, such as colour, smell, solubility
etc. of the salt. These are named as dry tests.
Heating of dry salt, blow pipe test, flame tests, borax bead test, sodium
carbonate bead test, charcoal cavity test etc. come under dry tests. Some of these
tests are given later in this unit.
, LABORATORY MANUAL CHEMISTRY
Solubility of a salt in water and the pH of aqueous solutions give important
information about the nature of ions present in the salt. If a solution of the salt is
acidic or basic in nature, this means that it is being hydrolysed in water. If the
solution is basic in nature then salt may be some carbonate or sulphide etc. If
the solution shows acidic nature then it may be an acid salt or salt of weak base
and strong acid. In this case it is best to neutralise the solution with sodium
carbonate before testing it for anions.
Gases evolved in the preliminary tests with dil. H2SO4/dil. HCl and conc. H2SO4
also give good indication about the presence of acid radicals (See Tables 7.1
and 7.3). Preliminary tests should always be performed before starting the
confirmatory tests for the ions.
EXPERIMENT 7.1
Aim
To detect one cation and one anion in the given salt from the following ions:
2+ 2+ 3+ 3+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+
Cations - Pb , Cu , As , Al , Fe , Mn , Ni , Zn , Co , Ca , Sr , Ba ,
2+ +
Mg , NH 4
2– 2– 2– 2– – – – – – 3– 2–
Anions - CO3 , S , SO3 , SO 4 , NO2 , NO3 , Cl , Br , I , PO 4 , C 2 O 4 ,
–
CH3COO .
(Insoluble salts to be excluded)
Theory
Two basic principles of great use in the analysis are:
(i) the Solubility product; and
(ii) the Common ion effect.
When ionic product of a salt exceeds its solubility product, precipitation takes
place. Ionic product of salt is controlled by making use of common ion effect
which you have studied in the textbook of chemistry.
Material Required
• Boiling tube : As per need
• Test tubes : As per requirement
• Measuring cylinder : One
• Test tube stand : One • Reagents : As per need
• Test tube holder : One
• Delivery tube : One
• Corks : As per need
• Filter paper : As per need
50
, SYSTEMATIC QUALITATIVE ANALYSIS
SYSTEMATIC ANALYSIS OF ANIONS
Step - I : Preliminary Test with Dilute Sulphuric Acid
In this test the action of dilute sulphuric acid (procedure is given
below) on the salt is noted at room temperature and on warming.
2– 2–
Carbonate ( CO 3 ), sulphide (S2–), sulphite ( SO3 ), nitrite ( NO 2– )
–
and acetate (CH3COO ) react with dilute sulphuric acid to evolve
different gases. Study of the characteristics of the gases evolved
gives information about the anions. Summary of characteristic
properties of gases is given in Table 7.1 below.
Procedure
(a) Take 0.1 g of the salt in a test tube and add 1–2 mL of
dilute sulphuric acid. Observe the change, if any, at
room temperature. If no gas is evolved, warm the
Fig. 7.1 : Testing a Gas
content of the test tube. If gas is evolved test it by using
the apparatus shown in Fig.7.1 and identify the gas
evolved (See Table 7.1).
Table 7.1 : Preliminary test with dilute sulphuric acid
Inference
Observations
Gas Evolved Possible Anion
A colourless, odourless gas is evolved
with brisk effervescence, which turns CO2 2–
Carbonate ( CO 3 )
lime water milky.
Colourless gas with the smell of rotten
2–
eggs is evolved which turns lead acetate H2S Sulphide (S )
paper black.
Colourless gas with a pungent smell, like
burning sulphur which turns acidified SO2 2–
Sulphite ( SO 3 )
potassium dichromate solution green.
Brown fumes which turn acidified
potassium iodide solution containing NO2 –
Nitrite ( NO 2 )
starch solution blue.
Colourless vapours with smell of
–
vinegar. Vapours turn blue litmus red. CH3COOH vapours Acetate, (CH3COO )
51
, LABORATORY MANUAL CHEMISTRY
2- 2– 2- – –
Confirmatory tests for CO3 S , SO3 , NO2 and CH3COO
Confirmatory (wet) tests for anions are performed by using water extract when
salt is soluble in water and by using sodium carbonate extract when salt is
2–
insoluble in water. Confirmation of CO3 is done by using aqueous solution of
the salt or by using solid salt as such because sodium carbonate extract contains
carbonate ions. Water extract is made by dissolving salt in water. Preparation of
sodium carbonate extract is given below.
Preparation of sodium carbonate extract
Take 1 g of salt in a porcelain dish or boiling tube. Mix about 3 g of solid sodium
carbonate and add 15 mL of distilled water to it. Stir and boil the content for about
10 minutes. Cool, filter and collect the filtrate in a test tube and label it as sodium
carbonate extract.
Confirmatory tests for acid radicals, which react with dilute sulphuric acid
are given below in Table 7.2.
2– 2– 2– – –
Table 7.2 : Confirmatory tests for CO3 , S , SO3 , NO3 , CH3COO
Anion Confirmatory Test
2 - Take 0.1 g of salt in a test tube, add dilute sulphuric acid.
Carbonate ( CO3 )
CO2 gas is evolved with brisk effervescence which turns lime
water milky. On passing the gas for some more time, milkiness
disappears.
2–
Sulphide (S ) Take 1 mL of water extract and make it alkaline by adding
ammonium hydroxide or sodium carbonate extract. Add a drop of
sodium nitroprusside solution. Purple or violet colouration appears.
*Sulphite ( SO3 )
2–
(a) Take 1 mL of water extract or sodium carbonate
extract in a test tube and add barium chloride
solution. A white precipitate is formed which dissolves
in dilute hydrochloric acid and sulphur dioxide gas
is also evolved.
(b) Take the precipitate of step (a) in a test tube and add
a few drops of potassium permanganate solution
acidified with dil. H 2 SO 4 . Colour of potassium
permanganate solution gets discharged.
– (a) Take 1 mL of water extract in a test tube. Add a few
Nitrite ( NO2 )
drops of potassium iodide solution and a few drops of
starch solution, acidify with acetic acid. Blue colour
appears.
(b) Acidify 1 mL of water extract with acetic acid. Add
2-3 drops of sulphanilic acid solution followed by 2-3
drops of 1-naphthylamine reagent. Appearance of red
colour indicates the presence of nitrite ion.
* Like CO2 sulphur dioxide also turns lime water milky. But CO2 is odourless gas and SO2 has
a characteristic smell.
52
SYSTEMATIC QUALITATIVE
ANALYSIS
A
NALYSIS always does not mean breaking of substance into its ultimate
constituents. Finding out the nature of substance and identity of
its constituents is also analysis and is known as qualitative analysis.
Qualitative analysis of inorganic salts means the identification of cations and
anions present in the salt or a mixture of salts. Inorganic salts may be obtained
by complete or partial neutralisation of acid with base or vice-versa. In the
formation of a salt, the part contributed by the acid is called anion and the part
contributed by the base is called cation. For example, in the salts CuSO4 and
2+ 2– –
NaCl, Cu and Na+ ions are cations and SO4 and Cl ions are anions. Qualitative
analysis is carried out on various scales. Amount of substance employed in
these is different. In macro analysis, 0.1 to 0.5 g of substance and about 20 mL
of solution is used. For semimicro analysis, 0.05 g substance and 1 mL solution
is needed while for micro analysis amount required is very small. Qualitative
analysis is carried out through the reactions which are easily perceptible to our
senses such as sight and smell. Such reactions involve:
(a) Formation of a precipitate
(b) Change in colour
(c) Evolution of gas etc.
Systematic analysis of an inorganic salt involves the following steps:
(i) Preliminary examination of solid salt and its solution.
(ii) Determination of anions by reactions carried out in solution (wet tests)
and confirmatory tests.
(iii) Determination of cations by reactions carried out in solution (wet tests)
and confirmatory tests.
Preliminary examination of a salt often furnishes important information, which
simplifies further course of analysis. Although these tests are not conclusive but
sometimes they give quite important clues for the presence of certain anions or
cations. These tests can be performed within 10-15 minutes. These involve noting
the general appearance and physical properties, such as colour, smell, solubility
etc. of the salt. These are named as dry tests.
Heating of dry salt, blow pipe test, flame tests, borax bead test, sodium
carbonate bead test, charcoal cavity test etc. come under dry tests. Some of these
tests are given later in this unit.
, LABORATORY MANUAL CHEMISTRY
Solubility of a salt in water and the pH of aqueous solutions give important
information about the nature of ions present in the salt. If a solution of the salt is
acidic or basic in nature, this means that it is being hydrolysed in water. If the
solution is basic in nature then salt may be some carbonate or sulphide etc. If
the solution shows acidic nature then it may be an acid salt or salt of weak base
and strong acid. In this case it is best to neutralise the solution with sodium
carbonate before testing it for anions.
Gases evolved in the preliminary tests with dil. H2SO4/dil. HCl and conc. H2SO4
also give good indication about the presence of acid radicals (See Tables 7.1
and 7.3). Preliminary tests should always be performed before starting the
confirmatory tests for the ions.
EXPERIMENT 7.1
Aim
To detect one cation and one anion in the given salt from the following ions:
2+ 2+ 3+ 3+ 3+ 2+ 2+ 2+ 2+ 2+ 2+ 2+
Cations - Pb , Cu , As , Al , Fe , Mn , Ni , Zn , Co , Ca , Sr , Ba ,
2+ +
Mg , NH 4
2– 2– 2– 2– – – – – – 3– 2–
Anions - CO3 , S , SO3 , SO 4 , NO2 , NO3 , Cl , Br , I , PO 4 , C 2 O 4 ,
–
CH3COO .
(Insoluble salts to be excluded)
Theory
Two basic principles of great use in the analysis are:
(i) the Solubility product; and
(ii) the Common ion effect.
When ionic product of a salt exceeds its solubility product, precipitation takes
place. Ionic product of salt is controlled by making use of common ion effect
which you have studied in the textbook of chemistry.
Material Required
• Boiling tube : As per need
• Test tubes : As per requirement
• Measuring cylinder : One
• Test tube stand : One • Reagents : As per need
• Test tube holder : One
• Delivery tube : One
• Corks : As per need
• Filter paper : As per need
50
, SYSTEMATIC QUALITATIVE ANALYSIS
SYSTEMATIC ANALYSIS OF ANIONS
Step - I : Preliminary Test with Dilute Sulphuric Acid
In this test the action of dilute sulphuric acid (procedure is given
below) on the salt is noted at room temperature and on warming.
2– 2–
Carbonate ( CO 3 ), sulphide (S2–), sulphite ( SO3 ), nitrite ( NO 2– )
–
and acetate (CH3COO ) react with dilute sulphuric acid to evolve
different gases. Study of the characteristics of the gases evolved
gives information about the anions. Summary of characteristic
properties of gases is given in Table 7.1 below.
Procedure
(a) Take 0.1 g of the salt in a test tube and add 1–2 mL of
dilute sulphuric acid. Observe the change, if any, at
room temperature. If no gas is evolved, warm the
Fig. 7.1 : Testing a Gas
content of the test tube. If gas is evolved test it by using
the apparatus shown in Fig.7.1 and identify the gas
evolved (See Table 7.1).
Table 7.1 : Preliminary test with dilute sulphuric acid
Inference
Observations
Gas Evolved Possible Anion
A colourless, odourless gas is evolved
with brisk effervescence, which turns CO2 2–
Carbonate ( CO 3 )
lime water milky.
Colourless gas with the smell of rotten
2–
eggs is evolved which turns lead acetate H2S Sulphide (S )
paper black.
Colourless gas with a pungent smell, like
burning sulphur which turns acidified SO2 2–
Sulphite ( SO 3 )
potassium dichromate solution green.
Brown fumes which turn acidified
potassium iodide solution containing NO2 –
Nitrite ( NO 2 )
starch solution blue.
Colourless vapours with smell of
–
vinegar. Vapours turn blue litmus red. CH3COOH vapours Acetate, (CH3COO )
51
, LABORATORY MANUAL CHEMISTRY
2- 2– 2- – –
Confirmatory tests for CO3 S , SO3 , NO2 and CH3COO
Confirmatory (wet) tests for anions are performed by using water extract when
salt is soluble in water and by using sodium carbonate extract when salt is
2–
insoluble in water. Confirmation of CO3 is done by using aqueous solution of
the salt or by using solid salt as such because sodium carbonate extract contains
carbonate ions. Water extract is made by dissolving salt in water. Preparation of
sodium carbonate extract is given below.
Preparation of sodium carbonate extract
Take 1 g of salt in a porcelain dish or boiling tube. Mix about 3 g of solid sodium
carbonate and add 15 mL of distilled water to it. Stir and boil the content for about
10 minutes. Cool, filter and collect the filtrate in a test tube and label it as sodium
carbonate extract.
Confirmatory tests for acid radicals, which react with dilute sulphuric acid
are given below in Table 7.2.
2– 2– 2– – –
Table 7.2 : Confirmatory tests for CO3 , S , SO3 , NO3 , CH3COO
Anion Confirmatory Test
2 - Take 0.1 g of salt in a test tube, add dilute sulphuric acid.
Carbonate ( CO3 )
CO2 gas is evolved with brisk effervescence which turns lime
water milky. On passing the gas for some more time, milkiness
disappears.
2–
Sulphide (S ) Take 1 mL of water extract and make it alkaline by adding
ammonium hydroxide or sodium carbonate extract. Add a drop of
sodium nitroprusside solution. Purple or violet colouration appears.
*Sulphite ( SO3 )
2–
(a) Take 1 mL of water extract or sodium carbonate
extract in a test tube and add barium chloride
solution. A white precipitate is formed which dissolves
in dilute hydrochloric acid and sulphur dioxide gas
is also evolved.
(b) Take the precipitate of step (a) in a test tube and add
a few drops of potassium permanganate solution
acidified with dil. H 2 SO 4 . Colour of potassium
permanganate solution gets discharged.
– (a) Take 1 mL of water extract in a test tube. Add a few
Nitrite ( NO2 )
drops of potassium iodide solution and a few drops of
starch solution, acidify with acetic acid. Blue colour
appears.
(b) Acidify 1 mL of water extract with acetic acid. Add
2-3 drops of sulphanilic acid solution followed by 2-3
drops of 1-naphthylamine reagent. Appearance of red
colour indicates the presence of nitrite ion.
* Like CO2 sulphur dioxide also turns lime water milky. But CO2 is odourless gas and SO2 has
a characteristic smell.
52
