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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5708_Библиотеки_им_академика_М_И_Перельмана.pdf
Chapter
4
Qualitative Analysis of
Anions and Cations
The tests performed for the identification of chemicals are based, generally, on
chemical reactions of an anion or a cation as referred in the Indian Pharmacopoeia.
Identification tests of some anions or cations feasible in the laboratory are
presented in this chapter.
Experiment
1 To perform identification tests with a given sample containing
acetates
CH
3
COO
Chemical Test
S. no. Experiment Observation Inference
1. Heat the given sample
with an equal quantity of
oxalic acid.
Evolution of gas having a
characteristic odour of acetic
acid was observed.
Indicates the presence of
acetate ions.
2. Dissolve 1 gm of the
sample in 3 mL of ethanol.
To it further add 1 mL of
sulphuric acid and heat.
Apple
-like or banana-like odour
is observed.
Indicates the formation of an
ester (ethyl acetate) due to
the presence of acetate ions.
Reaction involved
S. No. 1
2CH COONa + (COOH) 2CH COOH
3
Sodium acetate
2
Oxalic acid
3
Ace
⎯→⎯⎯⎯
ttic acid
2
Sodium
+ (COONa)
oxalate
S. No. 2
C H OH + CH COOH CH COOC H
25
Ethanol
3
Acetic acid
HSO
325
Ethyl
24
⎯→⎯⎯⎯⎯
acetate
2
Water
+HO
Result The given sample was found to contain acetate as anion.
138
Practical Pharmaceutical Analytical Chemistry
Experiment 2 To perform identification tests with a given sample containing
chlorides
Cl
Chemical Test
S. no. Experiment Observation Inference
1. To the solution of sample
containing chloride ion
equivalent to about 2 mg of
chloride ion in water (2 mL)
add silver nitrate solution
(0.5 mL).
Acidify the solution with
nitric acid.
Add excess of nitric acid to
the solution.
Add dilute ammonia solution.
The silver chloride
precipitate is filtered off,
washed with distilled water,
and then shaken with sodium
arsenite solution.
A curdy white precipitate
is formed.
Precipitate is insoluble.
Precipitate gets solubilized.
Yellow silver arsenite
produced.
Indicates the formation of silver
chloride.
Due to the formation of complex
This test serves in distinction of
chloride from silver bromide and
silver iodide, which are unaffected
by this treatment. Presence
chloride confirmed.
Reaction involved
Cl + Ag AgCl
AgCl + 2NH [Ag Cl
32
+
2
+
−+
⎯→⎯⎯⎯ ↓
↓⎯→⎯⎯⎯
()
+NH
Ag(NH
3
3
]
[)]
++ Cl + 2H AgCl + 2NH
+
4
⎯→⎯⎯⎯ ↓
+
The solution obtained after dissolving in ammonia is explosive and should be
discarded quickly.
3AgCl Ag 3Cl
3
+⎯→⎯⎯⎯ ↓ +AsO AsO
3
3
3
Result The given sample was found to contain chloride as anion.
139
Qualitative Analysis of Anions and Cations
Experiment 3 To perform identification tests with a given sample containing
bicarbonates
HCO
3
Chemical Test
S. no. Experiment Observation Inference
1. Boil the solution of bicarbonate. Effervescence is observed. Indicates the liberation
of carbon dioxide.
2. To the solution of the substance,
add magnesium sulphate solution.
Boil the reaction mixture.
No precipitation is observed.
Formation of a white
precipitate.
Indicates the formation
of magnesium
hydroxide.
3. To the solution of the sample, add
2 N acetic acid (2 mL). Close the
tube below using a stopper fitted
with a glass tube bent at two right
angles. Heat gently and pass the
gas formed to barium hydroxide
solution (5 mL)
Further, add excess of dilute
hydrochloric acid.
A white precipitate is
formed.
Precipitate gets dissolved.
Indicates the formation
of barium carbonate.
Due to the formation
of barium chloride
Reaction involved
S. No. 1
2NaHCO Na CO
3
Sodium bicarbonate
Heat
23
Sodium carbonat
⎯→⎯⎯⎯⎯
ee
2
Water
2
Carbon dioxide
+HO+ CO↑
S. No. 2
2NaHCO Na CO
3
Sodium bicarbonate
Heat
23
Sodium carbonat
⎯→⎯⎯⎯⎯
ee
2
Water
2
Carbon dioxide
+HO+ CO↑
Na CO + H O 2Na + 2OH
23
Sodium carbonate
2
Water
+
Sodium ion
-
Hyd
⎯→⎯⎯⎯
rroxide ion
2
Carbon dioxide
+CO↑
MgSO + 2OH Mg(OH)
4
Magnesium sulphate
-
Hydroxide ion
2
Magne
⎯→⎯⎯⎯
ssium hydroxide
(White precipitate)
4
2-
Sulphate ion
+SO
S. No. 3
NaHCO + CH COOH CH COONa
3
Sodium
bicarbonate
3
Acetic acid
3
So
⎯→⎯⎯⎯
ddium acetate
2
Water
2
Carbon
dioxide
+HO+CO↑
140
Practical Pharmaceutical Analytical Chemistry
Ba OH + CO BaCO
2
Barium hydroxide
2
Carbon dioxide
3
Bariu
()
↑⎯ →⎯⎯⎯ ↓
mm carbonate
2
Water
+HO
BaCO + 2HCl BaCl
3
Barium
carbonate
Hydrochloric
acid
2
Barium
⎯→⎯⎯⎯
cchloride
2
Water
2
Carbon
dioxide
+HO+CO↑
Result The given sample was found to contain bicarbonates as anion.
Experiment 4 To perform identification tests with a given sample containing
iodides
I
Chemical Test
S. no. Experiment Observation Inference
1. To solution of the substance
containing an equivalent
amount of about 5 mg of
iodide ions per mL, add
2 N sulphuric acid (0.5 mL),
potassium dichromate
solution (0.15 mL), water
(2 mL), and chloroform
(2 mL) and shake for a
few seconds. Allow to
stand.
The violet or violet-red
colour appears in the
chloroform layer.
Indicates the formation of
iodine.
Reaction involved
S. No. 1
Cr O + 6I + 14H
2 7
2-
Dichromate
-
Iodide
+
Proton
(Sulphuric
acid)
⎯→⎯⎯⎯⎯
()
2Cr + 7HO +
3+
2
Water Iodine
Violet
3
2
I
Result The given sample was found to contain iodides as anion.
141
Qualitative Analysis of Anions and Cations
Experiment 6 To perform identification tests with a given sample containing
phosphates
HPO
4
3
Experiment 5 To perform identification tests with a given sample containing
nitrates
NO
3
Chemical Test
S. no. Experiment Observation Inference
1. To the solution of substance containing
nitrates add sulphuric acid (1 mL). Cool the
solution and add ferrous sulphate solution
(0.5 mL) without mixing.
A brown colour forms
at the interface of the
two liquids.
Indicates the
presence of nitrates.
2. To the substance add a mixture of nitrobenzene
(0.1 mL) and sulphuric acid (0.2 mL). Cool in
ice water. Then add water (5 mL) and sodium
hydroxide solution (5 mL) with stirring. To it,
add acetone (5 mL), shake and allow to stand.
The upper layer shows
an intense violet
colour.
Indicates the
presence of nitrates.
Chemical Test
S. no. Experiment Observation Inference
1. Neutralize the sample solution and
to it add silver nitrate solution.
Boil the solution.
Add dilute ammonia solution and
dilute nitric acid.
A light yellow precipitate
forms.
The colour of the
precipitate does not change.
Precipitate gets solubilized.
Indicates the formation of
silver phosphate.
Due to the formation of
silver diamino complex.
2. Mix the prescribed solution
(1 mL) with ammonical magnesium
sulphate solution (1 mL).
A white crystalline
precipitate is produced.
Indicates the presence of
phosphate ion.
Reaction involved
S. No. 1
3Fe + NO + 4H
2+
Ferrous ion
3
3-
Nitrate
+
Proton
(Sulphuric acid)
⎯→⎯⎯⎯⎯ 3Fe + 2H O + NO
Fe + NO
3+
Ferric ion
2
Water
Nitric
oxide
2+
Ferrous ion NNitric oxide
2+
Brown colour
Fe NO⎯→⎯⎯⎯
()
⎡
⎣
⎤
⎦
Result The given sample was found to contain nitrates as anion.
142
Practical Pharmaceutical Analytical Chemistry
Reaction involved
S. No. 1
3Ag + 2HPO Ag PO + H PO
+
Silver ion
4
-
Phosphate ion
34 24
-
⎯→⎯⎯⎯
Ag PO 3Ag + PO
34
+
4
3-
⎯→⎯⎯⎯
Ag + 2NH Ag NH
+
Silver ion
3
Ammonia
3
2
+
Silverdiamino
c
⎯→⎯⎯⎯
()
⎡
⎣
⎢
⎤
⎦
⎥
oomplex soluble
PO + H O HPO + OH
4
3-
2
Water
4
2- -
Hydroxide ion
⎯→⎯⎯⎯
H+OH HO
+
Hydrogen ion
-
Hydroxide ion
2
Water
⎯→⎯⎯⎯
Mg + HPO + NH + 6H O
2+
Magnesium
ion
4
3-
Phosphate
ion
3
Ammonia
2
Water
⎯→⎯⎯⎯⎯ ⋅MgNH PO 6H O
44 2
Result The given sample was found to contain phosphates as anion.
Experiment 7 To perform identification tests with a given sample containing
barium
Ba
Chemical Test
S. no. Experiment Observation Inference
1. Burn the sample in nonluminous
flame.
Yellowish-green colour to a
flame, appearing blue when
viewed through a green glass.
Indicates presence of
barium.
2. Dissolve the substance (20 mg) in
dilute hydrochloric acid (5 mL).
Add dilute sulphuric acid (2 mL).
Then add nitric acid.
A white precipitate forms.
Precipitate gets insolubilized.
Due to the formation
of barium sulphate.
Reaction involved
S. No. 2
Ba + SO
2+
Barium ion
4
2-
Sulphate ion
HCl
Hydrochloric acid
⎯→⎯⎯⎯⎯⎯⎯⎯ BBaSO
4
Barium ulphates
Result The given sample was found to contain barium as cation.
143
Qualitative Analysis of Anions and Cations
Experiment 8 To perform identification tests with a given sample containing
calcium
Ca
Chemical Test
S. no. Experiment Observation Inference
1. To a solution of substance
containing calcium ion (20 mg)
in 5 M acetic acid (5 mL) add
potassium ferrocyanide solution
(5 mL).
Add ammonium chloride (50 mg).
The solution remains clear.
A white crystalline
precipitate is formed.
Indicates formation
of calcium potassium
ferrocyanide.
2. To a solution of the substance
containing calcium ion, add few
drops of ammonium oxalate
solution.
Add dilute acetic acid.
Further, add hydrochloric acid.
A white precipitate is
obtained.
Precipitate is only sparingly
soluble.
Precipitate gets solubilized.
Indicates formation of
calcium oxalate.
Indicates formation of
calcium chloride.
3. To the solution of substance in
dilute hydrochloric acid, add
ammonium carbonate solution
(5 mL) after neutralization with
dilute sodium hydroxide solution.
Boil, and after cooling add barium
hydroxide solution.
Further, add hydrochloric acid
solution.
White precipitate is formed.
Precipitate gets solubilized.
Due to the formation of
barium carbonate.
Indicates the formation
of barium chloride.
Reaction involved
S. No. 1
Ca + Fe CN + 2K
2+
Calcium
ion
6
4-
Ferrocyanide
+
Pottasium
ion
()
⎡
⎣
⎢
⎤
⎦
⎥
CCH COOH
Acetic acid
2
6
4-
3
CaK Fe CN⎯→⎯⎯⎯⎯⎯⎯
()
⎡
⎣
⎢
⎤
⎦
⎥
S. No. 2
NH CO 2NH + CO
4
2
24
Ammonium oxalate
4
+
Ammonium ion
24
()
22-
Oxalate ion
144
Practical Pharmaceutical Analytical Chemistry
CO + Ca CaCO
24
2-
Oxalate ion
2+
Calcium ion
24
Calcium oxalate
Wh
⎯→⎯⎯⎯
iite precipitate
()
CaC O + 2HCl CaCl
24
Calcium oxalate
Hydrochloric acid
2
Calcium
⎯→⎯⎯⎯
cchloride
2
Oxalic acid
+ COOH
()
S. No. 3
CaCl + NH CO 2NH Cl
2
Calcium
chloride
4
2
3
Ammonium
carbonate
4
()
⎯→⎯⎯⎯
AAmmonium
chloride
3
Calcium
carbonate
+CaCO
CaCO CaO + CO
3
Calcium carbonate
Calcium oxide
2
Carbon dioxide
⎯→⎯⎯⎯
CO + Ba OH BaCO
2
Carbon dioxide
2
Barium hydroxide
3
Barium car
()
⎯→⎯⎯⎯
bbonate
2
Water
+HO
2HCl + BaCO BaCl
Hydrochloric
acid
3
Barium
carbonate
2
Barium
c
⎯→⎯⎯⎯
hhloride
2
Water
2
Carbon
dioxide
+HO+CO
Result The given sample was found to contain calcium as cation.
Experiment 9 To perform identification tests with a given sample containing
ferric
Fe
Chemical Test
S. no. Experiment Observation Inference
1. To the solution of the substance
add 5% w/v solution of potassium
ferrocyanide solution (1 mL).
Add dilute hydrochloric acid.
An intense blue precipitate
is formed.
Precipitate is insoluble.
2. To 3 mL of a solution add 3 N;
1 mL hydrochloric acid and 1 mL
ammonium thiocyanate solution.
Divide the reaction mixture into
two portions; to one portion
(1 mL), add solvent ether (5 mL),
shake and allow to stand.
To the other portion, add 0.2 M
mercuric chloride (3 mL).
Blood red colour appears.
The ether layer becomes
pink.
The red colour disappears.
Due to the formation
of ferric thiocyanate.
Indicates the formation
of mercuric thiocyanate,
which is colourless.
145
Qualitative Analysis of Anions and Cations
Reaction involved
S. No. 1
4FeCl + 3K Fe CN
3
Ferric chloride
4
6
Pottassium ferrocyanide
()
⎯→⎯⎯⎯ FFe Fe CN + 12KCl
43
6
3
Blue colour
Potassium
chloride
()
⎡
⎣
⎢
⎤
⎦
⎥
S. No. 2
FeCl + 3NH SCN Fe SCN
3
Ferric chloride
4
Ammonium thiocyanate
⎯→⎯⎯⎯
()
33
Ferric thiocyanate
Red colour
4
Ammonium
chloride
+3NHCl
2Fe SCN + 3HgCl 3Hg SC
3
Ferric thiocyanate
Mercuric chloride
()
⎯→⎯⎯⎯ NN + 2FeCl
2
Mercuric thiocyanate
colourless
3
Ferric chloride
()
Result The given sample was found to contain ferric as cation.
Experiment 10 To perform identification tests with a given sample
containing ferrous
Fe
Chemical Test
S. no. Experiment Observation Inference
1. To 1 mL of a solution of iron, add
potassium ferricyanide solution (1 mL).
Add dilute hydrochloric acid.
Add sodium hydroxide solution.
A dark blue precipitate is
formed.
Precipitate is insoluble.
Precipitate gets decomposed.
2. To 1 mL of a solution containing not
less than 1 mg of iron, add potassium
ferrocyanide solution (1 mL).
Allow to stand for some time.
A white precipitate is formed.
Colour changes to Prussian
blue.
Due to the
formation
of ferrous
ferrocyanide.
Reaction involved
S. No. 1
Fe + 2K + Fe CN
++
Ferrous ion
+
Potassium
6
4-
Ferricyanide
ion
()
⎡
⎣
⎢
⎤
⎦
⎥
⎯⎯→⎯⎯⎯
()
⎡
⎣
⎢
⎤
⎦
⎥
KFeFeCN
2
6
S. No. 2
2FeSO +K Fe CN
4
Ferrous
sulphate
4
6
Potassium
ferrocyanide
()
⎡
⎣
⎢
⎤
⎦
⎥
⎯⎯→⎯⎯⎯
()
⎡
⎣
⎢
⎤
⎦
⎥
Fe Fe CN .2K SO
2
6
+
24
Ferrous ferrocyanide
White pre(ccipitate
4
6
3
Prussian blue
Fe Fe CN
)
OAir
[]
⎯→⎯⎯⎯⎯
()
⎡
⎣
⎢
⎤
⎦
⎥
Result The given sample was found to contain ferrous as cation.
146
Practical Pharmaceutical Analytical Chemistry
Judge Yourself
1. Why does acetates give banana-like odour with ethanol and
sulphuric acid?
2. Why does reprecipitation occurs on addition of nitric acid in
chloride test?
3. White precipitate of barium carbonate formed in the test of acetates
gets solubilized on addition of hydrochloric acid. What does it
indicate?
4. Write the reaction involved in the chemical test of iodides.
5. What is the colour given by barium with nonluminous flame?
6. Differentiate between ferrous and ferric ions based on their chemical
tests.