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30
Viva Voce in
Pharmaceutical Chemistry
LIMIT TEST FOR HEAVY METALS
Q 182 Name five heavy metals.
Ans: Lead, iron, copper, nickel, cobalt, and bismuth are some heavy
metals.
Q 183 Why limit test for heavy metals is performed?
Ans: Limit test for heavy metals is performed to check that the
quantity of heavy metals present in sample is below the
threshold level which causes harmful effects.
Q 184 What is the chemical basis for limit test for heavy metals?
Ans: The limit test for heavy metals is indicated in terms of the
parts of lead (Pb), per million parts (by weight) of the
substance under examination. When the metallic impurities
react with hydrogen sulphide, coloured metallic sulphides are
precipitated.
Q 185 Why does the colour of solution ranges between black and
brown?
Ans: Intensity of the colour produced depends upon the concen-
tration of metallic impurity. Concentrated solutions of lead
salts produce black precipitate while dilute solutions produce
brown precipitate.
Q 186 Why pH of the solution is adjusted between 3.0 and 4.0?
Ans: Between pH 3.0 and 4.0, the metal sulphides are precipitated
in colloidal form, giving uniform colour to the solution, making
the comparison between test and the standard possible.
Q 187 How pH is adjusted between 3 and 4?
Ans: The pH is adjusted between 3 and 4 by use of either acetic acid or
ammonia solution and confirming the pH by use of pH paper.
Q 188 What harmful effects heavy metals are capable to cause?
Ans: Heavy metals like lead, cadmium and mercury are potent
nerve poisons.
Q 189 Why stringent limit is prescribed for heavy metals?
Ans: Stringent limit for heavy metals is prescribed because of severe
harmful effects of them. In addition the, cumulative effect of
such impurity can occur when patient is on long term/chronic
Limit Tests 31
therapy like anti-rheumatics, heart stimulants and anti-
hypertensives.
Q 190 Which salt is used for preparation of lead standard solution?
Ans: Lead nitrate is used for preparation of lead standard solution.
Q 191 Describe the procedure for preparing standard colouration
in limit test for heavy metals.
Ans:
• Take 1 ml of lead standard solution (20 ppm Pb) in a Nessler
cylinder.
• Dilute with water to 25 ml.
• Adjust the pH between 3.0 and 4.0 with dilute acetic acid or
ammonia solution.
• Dilute with water to about 35 ml, and mix. Add 10 ml of freshly
prepared hydrogen sulphide solution, and mix.
• Dilute to 50 ml with water.
• Allow to stand for 5 minutes.
Q 192 Why freshly prepared solution of hydrogen sulphide should
be used in limit test for heavy metals?
Ans: Hydrogen sulphide solution is a saturated solution of H
2
S in
cold water. There is a possibility that H
2
S gas is lost in storage.
To avoid this freshly prepared solution of hydrogen sulphide
should be used in limit test for heavy metals.
Q 193 How would you prepare test colouration in limit test for
heavy metals?
Ans:
• Dissolve prescribed quantity of sample in 10 ml of water in a
Nessler cylinder.
• Add 2 ml of dilute acetic acid and sufficient water to produce
25 ml.
• Adjust pH in between 3.0 and 4.0 with dilute acetic acid or
dilute ammonia solution.
• Dilute with water to about 35 ml and mix.
• Add 10 ml of freshly prepared hydrogen sulphide solution
and mix.
• Dilute to 50 ml with water.
• Allow to stand for five minutes.
Q 194 How limit test for heavy metals is performed?
Ans: Same as described in limit test for chlorides.
32
Viva Voce in
Pharmaceutical Chemistry
Q 195 What is the prescribed IP limit for heavy metals in the
sample of magnesium sulphate?
Ans: 10 ppm
Q 196 What is the prescribed IP limit for heavy metals in the
sample of sodium chloride?
Ans: 5 ppm
Q 197 What quantity of sodium chloride should be used in
preparing test colouration in limit test for heavy metals if
the limit for heavy metal is 5 ppm? The standard colouration
is produced using 2 ml of 20 ppm Pb solution. The limit for
heavy metals in sodium chloride is 5 ppm.
Ans: 1 g of sample 5 ppm of Pb
How many g of sample =
~
40 ppm of Pb
=
40
5
× 1 = 8 g of sample
Q 198 What is the limit set for heavy metals when 3 g of sample is
used for preparing test colouration? Standard colouration
is prepared using 2 ml of 15 ppm solution.
Ans: 3 g of sample 2 ml of 15 ppm Pb
30 ppm of Pb
1 g of sample = 10 ppm of Pb
Limit Tests 33
Q 199 What is the significance of performing identification tests?
Ans: Identification tests are performed to identify components of
the sample. Identification tests verify the qualitative
characteristics of a material under examination.
Q 200 Which is the criteria for identification of inorganic substances?
Ans: Identification tests of inorganic substances are based on the
reactions of anions and cations of the sample.
Q 201 Name few cations.
Ans: Ammonium, barium, calcium, ferrous, ferric, magnesium,
potassium, sodium, and zinc are cations.
Q 202 Name few anions.
Ans: Acetate, bicarbonate, citrate, chloride, gluconate, iodide,
sulphate, and thiosulphate are anions.
Q 203 Name ten inorganic substances.
Ans: Sodium chloride, sodium thiosulphate, magnesium sulphate,
ammonium chloride, sodium bicarbonate, potassium iodide,
barium sulphate, calcium gluconate, potassium citrate, iodine, etc.
Q 204 Write the meaning of organoleptic properties.
Ans: Organoleptic properties are the properties like nature, colour,
odour, solubility, etc. which can be perceived through our
sense organs.
TESTS FOR IDENTIFICATION OF CATIONS
AMMONIUM ION
Q 205 How do you test the presence of ammonium ion in a sample?
Give chemistry.
Ans: Three tests are performed for detecting presence of ammo-
nium ions:
Identification Tests
for Anions and
Cations
2
33
34
Viva Voce in
Pharmaceutical Chemistry
(i) Heat a few mg of the substance under examination with sodium
hydroxide solution, ammonia is evolved, which is recognisable
by its odour and by its action on moistened litmus paper, which
turns blue.
The chemistry involved is
(ii) Dissolve few mg of sample in water, add 0.2 g of light
magnesium oxide. Pass a current of air through the mixture
and direct the gas that is evolved to just beneath the surface
of a mixture of 1 ml of 0.1 M hydrochloric acid and 0.05 ml
of methyl red solution, the colour of the solution changes to
yellow.
The chemistry involved is
Ammonium gas neutralises HCl and thus methyl red changes
the colour from red to yellow.
(iii) Dissolve 10 mg of substance being examined in water. Add
1 ml of freshly prepared 10% w/v solution of sodium cobalti-
nitrite. A yellow precipitate is produced.
BARIUM ION
Q 206 Give the tests for detection of barium ions. Discuss the
reactions involved.
Ans: Two tests are performed for detection of barium ions.
(i) Barium salts impart a yellowish green colour to a non-
luminous flame which appears blue when viewed through a
green glass.
(ii) Dissolve 20 mg of substance under examination in 5 ml of
dilute hydrochloric acid and add 2 ml of dilute sulphuric acid,
a white precipitate insoluble in nitric acid is formed. The
chemistry involved is
Identification Tests for Anions and Cations 35
CALCIUM ION
Q 207 What are the identification tests for calcium containing
compounds? Elaborate chemistry.
Ans: IP 2007 gives three tests for the identification of calcium ions.
i. Dissolve 20 mg of the substance under examination in 5 ml of
5M acetic acid. Add 0.5 ml of potassium ferrocyanide solution,
the solution remains clear. Add 50 mg of ammonium chloride;
a white crystalline precipitate is formed.
ii. To 5 ml of a 0.4% w/v solution of the substance under
examination, add 0.2 ml of a 2% w/v solution of ammonium
36
Viva Voce in
Pharmaceutical Chemistry
oxalate. A white precipitate is obtained that is only sparingly
soluble in dilute acetic acid but is soluble in hydrochloric acid.
The chemistry involved in this test is:
1.
2.
iii. Dissolve 20 mg of sample in the minimum quantity of dilute
HCl and neutralise with dilute sodium hydroxide solution.
Add 5 ml of ammonium carbonate solution, a white
precipitate is formed which, after boiling and cooling the
mixture, is only sparingly soluble in ammonium chloride
solution.
The chemistry is
MAGNESIUM ION
Q 208 What are the identification tests for detection of magnesium
ions? Give the chemistry.
Ans: Two tests are prescribed for identification of magnesium ions
in IP 2007:
1. Dissolve about 15 mg of the substance under examination in
2 ml of water. Add 1 ml of dilute ammonia solution. A white
precipitate forms.
Identification Tests for Anions and Cations 37
Add 1 ml of 2 M ammonium chloride. The precipitate
redissolves.
Add 1 ml of 0.25 M disodium hydrogen phosphate. A white
crystalline precipitate is produced.
2. To 0.5% of a neutral or slightly acid solution of the substance
under examination, add 0.2 ml of 0.1 percent w/v solution of
titan yellow and 0.5 ml of 0.1 M sodium hydroxide, a bright
red turbidity develops which gradually settles to give a bright
red precipitate.
Titan yellow is a water soluble yellow dye. It is absorbed
by magnesium hydroxide producing a deep red colour or
precipitate.
POTASSIUM ION
Q 209 How would you identify the presence of potassium ions?
What is the chemistry of the tests?
Ans: Three tests are given by IP 2007 for detection of potassium
ions.
1. Dissolve about 50 mg of substance under examination in
1 ml of water. Add 1 ml of dilute acetic acid and 1 ml of
freshly prepared 10% w/v solution of sodium cobaltinitrite.
A yellow or orange yellow precipitate is produced
immediately.
2. Dissolve 0.1 g of the substance under examination in 2 ml of
water. Heat the solution with 1 ml of sodium carbonate
solution. No precipitate is formed. Add 0.05 ml of sodium
38
Viva Voce in
Pharmaceutical Chemistry
sulphide solution. No precipitate is formed, cool in ice. Add
2 ml of 15 % w/v solution of tartaric acid and allow to stand.
A white crystalline precipitate is produced.
3. Ignite few mg of the substance under examination, cool and
dissolve in the minimum quantity of water. To this solution,
add 1 ml of platinic chloride solution in presence of 1 ml of
hydrochloric acid. A yellow precipitate is produced.
SODIUM ION
Q 210 Describe the identification tests along with chemistry for
detection of sodium ions.
Ans: IP gives two tests for identification of sodium ions.
1. Dissolve 0.1 g of the substance in 2 ml of water. Add 2 ml of a
15% w/v solution of potassium carbonate and heat to boiling,
no precipitate is produced. Add 4 ml of freshly prepared
potassium antimonate solution and heat to boiling. Allow to
cool in ice and scratch the inside of the test tube with a glass
rod. A dense white precipitate is formed.
2. Acidify a solution of the substance under examination with
1 M acetic acid and add a large excess of magnesium uranyl
acetate solution. A yellow crystalline precipitate is formed.
Identification Tests for Anions and Cations 39
TESTS FOR IDENTIFICATION OF ANIONS
BICARBONATE ION
Q 211 Describe the identification tests for bicarbonates along with
chemistry.
Ans: IP 2007 gives three tests for identification of bicarbonates.
i. Dissolve the sample in water and heat, liberation of carbon
dioxide can be observed by passing through the limewater.
The limewater turns milky.
ii. Treat a solution of the substance under examination with a
solution of magnesium sulphate. No precipitate is formed.
Boil, a white precipitate is formed.
When carbonates are treated with magnesium sulphate,
white precipitate forms before heating.
iii. Dissolve 0.1 g of substance in 2 ml of water. Add 2 ml of 2 M
acetic acid, close the tube immediately using a stopper fitted
with a glass tube bent at two right angles. Heat gently and
collect the gas in 5 ml of barium hydroxide solution. A white
precipitate forms, that dissolves in dilute hydrochloric acid.