INTRODUCTION TO PHARMACEUTICAL CHEMISTRY, INTRODUCTION TO LIMIT TESTS, LIMIT TEST OF IRON, CHLORIDE, SULPHATE, ARSENIC AND THERE DIAGRAMS WITHTHE PRINCIPAL AND PROCEDURE OF ALL THE LIMIT TEST WITH THEIR RESULTS

1. TOPIC
INTRODUCTION TO PHARMACEUTICAL
CHEMISTRY
and Limit tests
BY
MRS. SUJATA WANKHEDE
LECTURER
NAGPUR COLLEGE OF PHARMACY,
WANADONGRI, HINGNA ROAD, NAGPUR.
MAHARASHTRA

2. Que. 1: Define Pharmaceutical Chemistry
Ans: The chemistry which studies about the drug design and synthesis of
biologically active molecules is known as Pharmaceutical Chemistry.
The aim is to obtain new chemical molecules that could enable the
discovery of new pharmaceuticals or enhance already known drug
structures, so that to expand the selection of chemical drugs.
Que. 2: Explain 1) Limit tests and 2) Significant figures
Ans:
i) Limit tests: Limit tests are quantitative or semi-quantitative tests
designed to identify and control small quantities of impurity which are
likely to be present in the substance.
ii) Significant figures: The number of significant figures is the number
of digits which are necessary to express results which are consistent with
the precision of the measurement.

3. Que. 3: Explain the principle and reaction involved in Iron limit test I.P.
Ans: Principle: Limit test for iron depends upon the interaction of thioglycolic
acid with iron in the presence of citric acid and in the ammonical alkaline
medium. This results in the formation of purple colored ferrous salts of
thioglycolic acid. The limit test of iron is carried out in two Nessler’s Cylinders,
one for the ‘test’ and other for ‘standard’. The intensity of purple color
produced in the two is compared by viewing transversely. If the color intensity
of test solution is less than that of standard, the sample passes the test & If the
intensity of colour is more in the test sample than in the standard, it means that
the sample contains more quantity of iron impurity than the permissible limit
and hence sample is declared as not of standard quality.
Chemical Reaction:

4. Role of Thioglycolic acid-
Iron impurity may be present in trivalent ferric form or in the divalent
ferrous form. If it is in ferric form, thioglycolic acid convert ferric form
of impurity into ferrous form and then forms ferrous thioglycolate
complex.
Role of Citric acid: It prevents the precipitation of iron in presence of
ammonia
Role of Ammonia: It maintains alkaline pH for the formation of stable
purple colored ferrous thioglycolate complex.

5. Que. 4: Give procedure involved in the limit test in iron.
Ans: Procedure:
2) Add 2ml of a 20% w/v Citric Acid iron free solution to both.
3) Add 0.1ml of thioglycollic acid to both mix well & make alkaline with
iron free ammonia solution.
4) Dilute to 50ml with distilled water & allow to stand for 5 minutes.
5) Compare the intensity of colour produced in the test solution with the
standard.
6) Report accordingly.
Standard Solution Test Solution
1) Pipette out 2.0ml of iron
standard solution in a labeled
Nessler’s Cylinder & add 20ml of
distilled water.
1) Weigh 2gm of given sample &
dissolve In 20ml of distilled water
in a labeled Nessler’s Cylinder.

6. Que. 5: Explain the principle along with reactions involved in limit
test for sulphate I.P
Ans: Principle: This is based upon the interaction of sulphate with
barium chloride in presence of dilute hydrochloric acid. This results in
the precipitation of sulphate as barium sulphate. Hydrochloric acid is
added to prevent precipitation of other acid radicals. Alcohol prevents
super saturation & potassium sulphate increases sensitivity of the test
when very small quantity of sulphate ions is present. Barium sulphate
appears as turbidity. This is compared with standard turbidity. If turbidity
produced in test sample is less than standard, it means sample passes test.
Chemical Reaction:
BaCl2 + SO4 BaSO4 + 2Cl

7. Que. 6: Procedure for Limit test for sulphate IP
Ans:
Standard Solution Test Solution
1. Take 1 ml of 25% w/v of barium chloride
in Nesseler’s Cylinder.
1. Take 1 ml of 25% w/v of barium chloride
in Nesseler’s Cylinder.
2. Add 1.5 ml of ethanolic sulphate standard
solution (10 ppm SO4--) mix and allow to
stand for 1 min.
2. Weigh one gram of sample (Sodium
Bicarbonate) and add to labeled Nessler’s
cylinder.
3. Add 0.15 ml of 5 M acetic acid in
Nesseler’s cylinder
3. Add 1.5 ml of ethanolic sulphate standard
solution( 10 ppm SO4--) mix and allow to
stand for 1 min.
4. Dilute up to 50 ml with water and stir with
glass rod
4. Add 10ml distilled water and
neutralizewith hydrochloric acid and add 15
ml ofdistilled water.
5. Allow to stand for 5 min 5. Add 0.15 ml of 5 M acetic acid in
Nesseler’s cylinder
6. . Dilute up to 50 ml with water and stir
with
glass rod and allow to stand for 5 min.

8. Que. 7: Give the principle and reactions involved in Limit Test for
Chloride (I. P.)
Ans: Principle: Limit test for chlorides depend upon the interaction of
chlorides with silver nitrate in the presence of nitric acid. This results in
the precipitation of Silver chloride. When only very small quantity of
chloride ions are present, silver chloride appears as opalescence and not
as precipitate. Silver chloride appears as opalescence which is compared
with standard in Nessler cylinder. The standard turbidity is produced by
the action of silver nitrate solution with 1 ml of 0.05845 % w/w of
Sodium chloride (Chloride Standard Solution- 25 ppm Cl).
Nitric acid is added to prevent precipitation of other acid radicals such as
phosphate, sulphate etc. with silver nitrate solution.
Reaction:
Cl - + AgNO3 AgCl ↓ + NO3 –
NaCl + AgNO3 AgCl ↓ + NaNO3

9. Que. 8: Procedure involved in Limit Test for Chloride (I. P.)
Ans: Procedure:
2. Add 10 ml of dilute Nitric acid.
3. Dilute to 50 ml mark with distilled water.
4. Add 1 ml of 0.1M solution of Silver nitrate.
5. Stir each solution with glass rod and allow to stand for 5 minutes &
compare the opalescence transversely against a dark background.
Test Standard
1. Dissolve 1 gm of sample in
about 10 ml of distilled water and
transfer it to a Nessler cylinder
labeled as ‘Test’.
1. Place 10ml of Chloride
Standard solution ( 25 ppm Cl) in
Nessler Cylinder labeled as ‘
Standard’.

10. Que. 9: Explain the principle and reactions involved in limit test for
arsenic along with well labeled diagram of apparatus used in it.
Ans: Principal:
Step-1
Limit test for arsenic is based on semi quantitative determination of
arsenic impurities in the test sample of drug. The sample is dissolved in
stannated acid which converts the arsenic impurities to arsenious acid or
arsenic acid depending upon valency state of arsenic impurity present in
the test sample.

11. Step- 2
When acidic solution of sample treated with reducing agent (Stannous
chloride) converts pentavalent arsenic acid into trivalent arsenious acid.
Step- 3
Step-4
The arsenious acid is then converted into gaseous hydride (Arsine gas)
with the help of nascent hydrogen which is produced by Zinc and
hydrochloric acid.

12. Step -5
Arsine gas is carried through the tube by the steam of
hydrogen and out through the mercuric chloride paper. A
reaction occurs between arsine and mercuric chloride, which
produce yellow colour stain.
Step-6
The stain produced by test sample compared with that of
standard arsenic solution. The limit of arsenic is expressed as
part per million.

13. Diagram: Gutzeit apparatus

14. Que. 10: Explain the role of lead acetate cotton plug and mercuric
chloride paper in arsenic limit test.
Ans:
Role of lead acetate cotton plug :
Lead acetate cotton plug is used to trap any hydrogen sulphide which
may be evolved along with arsine gas. It prevents the interaction of
hydrogen sulphide gas and mercuric chloride paper.
Role of Mercuric chloride paper :
Conversion of arsenious acid to arsine gas which reacts with mercuric
chloride paper gives yellow stain; depth of yellow stain on mercuric
chloride paper will depend upon quantity of
arsenic present in the sample.

15. Que. 11: Define the term impurity. Explain various sources of
impurities present in pharmaceuticals.
Ans: Definition: It is the undesirable foreign material which may be
toxic or may not be toxic, present in the pharmaceutical substances.
Impurity is the substance or the matter which does not form part of the
medicinal or pharmaceutical substances or drugs.
Sources of Impurities:
1. Raw material
2. Reagents used in manufacturing process
3. Intermediate products in manufacturing process
4. Defects in manufacturing process / manufacturing hazards
5. Solvents
6. Action of solvent and reagents on reaction vessel
7. Atmospheric contamination during manufacturing process
8. Defective storage of final products
9. Adulteration

16. 1. Raw Materials: When substances or chemicals are
manufactured; the raw materials from which these are
prepared may contain impurities which get incorporated
into final product. Example- sodium chloride prepared
from rock salt contains traces of Ca & Mg compounds.
2. Reagents used in manufacturing process: Synthesis of
drugs involves many chemical reactions like nitration,
halogenation, oxidation and hydrolysis. Different chemicals &
solvents are used in these chemical processes. When chemical
reactions are carried out in reaction vessel, the material of
these vessels (Iron, tin, copper, aluminium etc.) is reacted
upon by the solvents, chemicals & reaction products are
formed. These reaction products then occur as impurities in
the final product. Thus impurities of iron, lead, and heavy
metals, copper are due to the above mentioned reasons.

17. 3. Defects in the manufacturing process: Defects such as
imperfect mixing, incompleteness of reaction, non-adherence
to proper temperature, pressure, pH or reaction condition etc.
may result in the production of chemical compounds with
impurities in them.
4. Storage condition: The chemical when prepared is stored
in different types of containers. Various types of materials are
used for storage purpose. Reaction of substance with material
of the storage vessel may take place. The reaction may take
place directly or leaching out effect on the storage vessel. This
acts as impurity. Also, rodents & insects may add impurities to
the products.

18. 5. Solvents: Water is the solvent easily available & cheap and
is used in the manufacture of inorganic chemicals. This can
give rise to trace impurities such as sodium, calcium,
magnesium, carbonate & sulphate ions.
6. Decomposition: Decomposition is caused by light, air,
oxygen & causes contamination of final product. A number of
organic substances get spoiled because of decomposition on
exposure to the atmosphere. E.g. amines, phenol, potent drugs.
7. Atmospheric contaminants: Atmospheric contamination
may take place through dust, sulphur dioxide, hydrogen
sulphide & arsenic. Carbon dioxide & water vapour are
possible contaminants of substances which are affected by
their action.