Comprehensive Guide to Inorganic Semimicro Qualitative Analysis Techniques

UNIT V
INORGANIC SEMIMICRO QULATITAIVE ANALYSIS

UNIT-V
Principles and techniques of inorganic qualitative analysis: Semi-micro
qualitative analysis - anions (interfering and non-interfering), reactions of some common
anions (carbonate, sulphide, sulphate, nitrate, halides, oxalate, borate and phosphate),
principle of preparation of sodium carbonate extract and elimination of interfering
anions. Classification of cations into groups, reaction of Pb2+, Cu2+, Al3+, Ni2+, Ca2+, Ba2+,
Mg2+and NH4
+cations, group reagents. Applications of Solubility, solubility product
principle and common ion effect in group separation of cations.
Self-study:
Preparation of solution for cation testing on semimicro scale.

In qualitative analysis the given mixture is analysed for acidic and basic
radicals i.e. for anions and cations that it contains.
Depending upon the quantity of sample made available it is classified as :
Macro Analysis 0.1 – 1g
Semi-micro Analysis …..10-100 mg
Micro Analysis ….. 1-10 mg

The common procedure for testing an unknown sample is to make its
solution and then test this solution for different ions present. Steps involved
in the systematic analysis of a given sample are
Preliminary Tests :
i. Colour and smell
ii. Flame Test
iii. Borax Bead Test
Tests for Acid Radicals (Anions):
i. Dil. Acid test (dil. H2SO4)
ii. Conc. Acid test (conc. H2SO4)
iii. Wet tests for acid radicals
Wet Tests for Basic Radicals

(a) Preliminary Tests
Colour and smell
Colour Ion Indicated
Blue/Bluish Green Cu2+ or Ni2+
Greenish Ni2+
Light Green Fe2+
Dark Green Cr3+
Pink Co2+
Light Pink, Flesh colour or earthy
colour
Mn2+
Dark Brown Fe3+

(a) Preliminary Tests
(i) smell
Smell Ion Indicated
Ammonical smell NH4
+
Vinegar like smell CH3COO-
Smell of rotten eggs S2-
(i) Flame Test
Colour of Flame Ion Indicated
Brick Red Ca++
Crimson Red Sr++
Grassy Green Ba++
Bright-Bluish Green Cu++

(iii) Borax Bead Test
Colour of bead in oxidizing flame Ion Indicated
Green in hot, light brown in cold Copper (Cu++)
Pinkish violet in both hot and cold Manganese (Mn)
Yellowish brown in hot and pale yellow in
cold
Iron (Fe+++)
Brown in hot and pale brown in cold Nickel (Ni++)
Colour of bead in oxidizing flame Ion Indicated

Anions - Interfering and non interfering

Anions - non interfering
1. Carbonates
2. Sulphite
3. Sulphide
4. Sulphate
5. Thiosulphate
6. Chloride
7. Bromide
8. Iodide
9. Nitrate
10. Borate
11. Acetate
12. Cynaide
13. Cyanate
14. Hypochlorite
15. Silicate

Anions - interfering
Oxalate,
Borate,
Fluorlde and
phosphate are known as interfering acidic radicals.

Reactions of some common anions
1.Carbonate
2.Sulphide
3.Sulpahte
4.Nitrate
5.Halides
6.Oxalate
7.Borate
8.Phosphate

Reaction of some common anions:
(a) Carbonate (CO3
2-) :
Reaction with dilute HCl.
A pinch of the substance is treated with 3 mL of dil. HCl in a test tube. -
Brisk effervescence of colourless gas is evolved. It turns lime water milky.
CaCO3 + 2HCl CaCl2 + CO2 + H2O
CO2 gas when passed in lime water
Ca(OH)2 + CO2 CaCO3 + H2O

(b) Sulphide (S-)
A little of the substance is heated with a pinch of tin metal and a few drops of
conc. H2SO4.
- Colourless rotten egg smell gas turning lead acetate paper black.

(c) Sulphate (SO4
2-).
Barium Chloride Test :
To about 1 mL of the extract, dil. HCl is added drop by drop till the
effervescence stops. The contents are heated, cooled and barium
chloride solution is added.
Na2SO4 + BaCl2 BaSO4 + 2NaCl
White ppt.
Lead Acetate Test :
Na2SO4 + Pb(CH3COO)2 PbSO4 + 2CH3COONa

(d) Nitrate (NO3
-):
(i) Brown Ring Test:
About 2 mL of the extract is acidified with dil. H2SO4 and mixed with 3 mL
of freshly prepared ferrous sulphate solution. Conc. H2SO4 is added
along the sides of the tube.
NaNO3 + H2SO4 NaHSO4 + HNO3
6FeSO4 + 3H2SO4 + 2HNO3 3Fe2(SO4)3 + 4H2O + 2NO
FeSO4 + NO + 5H2O [Fe(NO)(H2O)5]SO4
Brown ring

(d) Nitrate (NO3
-):
(ii) Copper Test :
A pinch of substance is heated with 2 mL of conc. H2SO4 and pieces of
copper turning are added and warmed.
2NaNO3 + H2SO4 Na2SO4 + 2HNO3
4HNO3 + Cu Cu(NO3)2 + 2NO2 + 2H2O
Reddish brown

(i) Chloride (Cl-)
1. A pinch of the substance is mixed with a pinch of MnO2 and 2 mL of
conc. H2SO4 and warmed gently - Greenish yellow gas evolves which
turns starch iodide paper blue.

2. AgNO3 Test :
To about 1 mL of the extract, dil. HNO3 is added drop by drop till
the effervescence stops. The contents are heated, cooled and Silver
nitrate solution is added – A curdy white precipitate soluble in ammonium
hydroxide is formed.

3. A pinch of the substance is warmed with a few amounts of potassium
dichromate and conc. H2SO4.
- Red brown gas is evolved forming yellow precipitate with lead acetate
solution.

(e) Oxalate (C2O4
2-):
About 2 mL of the extract is acidified with acetic acid, boiled with 5 mL of
calcium chloride solution. - A white precipitate is obtained
If a white precipitate obtained in the above experiment, it is filtered off.
The white precipitate is dissolved in 3 mL of dil. H2SO4, heated and to
the clear solution; a dil. solution of KMnO4 is added drop by drop. -
1. KMnO4 solution is decolorized.

(f) Borate (BO3
3-)
1. Ethyl Borate test:
A pinch of substance is mixed with 2 mL of ethyl alcohol and 10 drops of
conc. H2SO4 in a test tube. It is heated and the vapor is ignited. - The
vapor burns with a green edged flame.
2. Boron Trifluoride test:
A pinch of the substance is mixed with calcium fluoride and a drop of
conc. H2SO4 is added and made into a paste. It is taken at the end of
charred splinter and introduced into the edge of non-luminous flame. - A
bright green colour is imparted to the flame.

(g) Fluoride (F-):
A pinch of the substance is heated with 2 mL of conc. H2SO4. -
Colourless gas giving white precipitate with a glass rod dipped in water.
Oily appearance at the top of the test tube.

(h) Phosphate:
Ammonium Molybdate Test:
About 1 mL of the extract is boiled with few drops of conc. HNO3 and
cooled. Then it is shaken with 10 mL of ammonium molybdate solution.

Sodium Carbonate Extract (Soda Extract):
•An extract of the mixture obtained after boiling the mixture with an
excess of sodium carbonate solution is called Soda extract or
Sodium carbonate extract.
•It is usually prepared for testing some of the acid radicals and also
for identifying the basic radicals of insoluble substance.
Preparation of Sodium Carbonate Extract:
A small amount of given substance taken in a 50 mL of beaker and
mixed with three times of sodium carbonate and one test tube of
distilled water. Then the beaker was boiled for 10 min and then
filtered using filter paper. The filtrate was sodium carbonate extract,
which was used for the following reactions.

Why sodium carbonate extract?
Sodium carbonate extract (Soda extract) gives an expedient way of bringing all
the anions of the sample into solution.
Otherwise, they might be insoluble and couldn’t carry out the experiment
properly. Also, it prevents the interference of cations.
(i) The preparation of sodium carbonate extract affords a convenient method
for bringing the anions of the mixture into solution which were otherwise
insoluble with caution of salt.
(ii) It removes the basic radical (usually coloured) which interferes in the usual
tests of some of the acidic radicals.
(iii) The residue (can be used for the tests of basic radicals of I to VI groups. Such
a solution does not involve the problem of removing interfering radicals like
oxalate, fluoride, borate and phosphate.

•Removal of Oxalate:
•Removal of Borate and fluoride:
•Removal of Phosphate:

Removal of Oxalate:
•Take 1 g of the mixture in a porcelain dish and heat it strongly for 5-
10 minutes. Now add 3-4 mL of conc. nitric acid and evaporate to
dryness.
• The residue dissolved in little cone. HCI, diluted with water and it
can be used as the original solution.

Removal of Borate and fluoride:
Take 1 g of the mixture in a porcelain dish and add 2 mL of conc. HCI.
Evaporate the solution to almost dryness.
Repeat this operation five to six times.
The residue is dissolved in little conc. HCI, diluted with water and it
can be used as the original solution.

Removal of Phosphate:
A portion of given substance is added with zirconyl chloride (ZrOCl2)
or zirconyl nitrate, the obtained precipitate got removed.
The process is repeated (at least 3-4 times), till no precipitate is
obtained.

Separations of Basic Radicals into Groups

Reactions of Some common Cations

Copper: Cu2+
Centrifugate
Centrifugate
Centrifugate

Common Ion Effect
The degree of ionization of weak electrolyte is suppressed by the addition of
strong electrolyte containing a common ion.
For example, ammonium hydroxide ionizes in solution, thus:
On applying the law of mass action,
On the addition of ammonium chloride, ammonium ions are added to the
solution. The concentration of NH4+ increases, and since K is constant at any fixed
temperature, there must be an increase in the concentration of NH4OH and a
decrease in the concentration of OH-.
Thus the ionization of NH4OH is diminished by the addition of NH4CI which
furnishes the common ion, NH4
+ .

Common Ion Effect
The principle of common ion effect has a great importance in qualitative
analysis.
Ionization of hydrogen sulphide is suppressed in the presence of hydrochloric
acid:

Solubility Product and Precipitation

Comprehensive Guide to Inorganic Semimicro Qualitative Analysis Techniques

Comprehensive Guide to Inorganic Semimicro Qualitative Analysis Techniques

More Related Content

Similar to Comprehensive Guide to Inorganic Semimicro Qualitative Analysis Techniques

Recently uploaded