Handbook of Inorganic Impurities in Pharmaceuticals

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By

Parjanya Kumar Shukla

Dr. Amita Verma

1

Handbook of

Inorganic Impurities in Pharmaceuticals

Parjanya Kumar Shukla
M. Pharm, Ph D (Pursuing)

Assistant Professor
Krishnarpit Institute of Pharmacy, Allahabad

Dr. Amita Verma
M. Pharm, Ph D

Head & Associate Professor
Department of Pharmaceutical Sciences,
SHIATS Deemed University, Allahabad

Handbook of Inorganic Impurities in Pharmaceuticals

2

Preface

Inorganic Impurities, their sources, control and limit test are the important topics of Inorganic
Pharmaceutical chemistry syllabus for all universities. The impurities in pharmaceuticals are
unwanted chemicals that remain with the active pharmaceutical ingredients (APIs) or develop
during formulation or upon aging of both API and formulation. The presence of these unwanted
chemicals even in trace amount may influence the efficacy and safety of pharmaceutical product.
So in this book we discuss the sources of impurities in pharmaceutical products and their limit
test according to the Indian Pharmacopeia 2007.

Point wise and tabular presentation of the topic is the main features of this small book. Hope this
book will help B. Pharm & D. Pharm students as well as chemistry students to understand
Inorganic Impurities and their limit test procedures in easy way.

Parjanya Kumar Shukla
Dr. Amita Verma

Handbook of Inorganic Impurities in Pharmaceuticals

3

Contents

Topic Page No.

1. Impurities & their Sources 4

2. Limit test 9

3. Limit test of Chloride 10

4. Limit test of Sulphate 11

5. Limit test of Iron 12

6. Limit test of Lead 13

7. Limit test of Arsenic 16

8. Limit test of Heavy Metals 18

9. Important Questions 21

Handbook of Inorganic Impurities in Pharmaceuticals

4

Impurities & their Sources

Introduction:

 It is virtually impossible to have absolutely pure chemical compounds and even

analytically pure chemical compounds contain minute trace of impurities.

 Impurities = a foreign unwanted matter present in a compound which are differ from the

actual molecular formula.

 Chemically a compound is impure if it contains undesirable foreign matter i.e. impurities.

Thus chemical purity is freedom from foreign matter.

Sources of Impurities:

The various sources of impurities in pharmaceutical substances are as follows:

Source of
Impurities

Raw Materials Manufacturing
Method of Hazards

Manufacturing Instability of
*Contamination from

*Reagents Used Matter Products
*Intermediate *Cross Contamination * C h e mical Instabilities

Products *Contamination by *Physical Instabilities
*Reagents used to Microbes *Reaction with
eliminate impurity * E r r o rs in Container

*Solvents Used M anufacturing * Temprature
*Atmospheric *Errors in Storage &
Contamination Packaging

Flow chart of Sources of Impurities

Handbook of Inorganic Impurities in Pharmaceuticals

5

1. Raw Materials:

 Pharmaceutical substances are either isolated from natural sources or synthesized from

chemical starting materials which have impurities.

 Impurities associated with the raw materials may be carried through the manufacturing

process to contaminate the final product.

2. Method of Manufacture:

 The Process or method of manufacture may introduce new impurities.

 Due to impure reagents, catalysts and solvents, reaction vessels and reaction

intermediates employed at various stages.

(A) Reagents employed in the manufacturing process:

 Calcium carbonate contains ‘soluble alkali’ as impurity
– -2

 Anions like Cl and SO are common impurities in many substances because of the
4

use of hydrochloric acid and sulphuric acid respectively

 Barium ion may be an impurity in hydrogen peroxide

(B) Regents used to eliminate other impurities:

 Barium is used to remove sulphate from potassium bromide, which can be found

itself (barium) as impurity at the end of process.

(C) Solvents:

 Small amounts of solvents employed in preparation, and purification of the product

may also result in the contamination of the pharmaceutical substances.

Handbook of Inorganic Impurities in Pharmaceuticals

6

 Water is the cheapest solvent which can be the major source of impurities as it
2+ 2+ + – -2 -2

contains different type of impurities like Ca , Mg ,Na , Cl , CO and SO in
3 4

trace amounts.

(D) Intermediates:

 Sometimes, an intermediate substance produced during the manufacturing process may

contaminate the final product

 e.g. Sodium bromide is prepared by reaction of sodium hydroxide and bromine in slight

excess.

o 6 NaOH + 3 Br → NaBrO + 5 NaBr + 3 H O …(1)
2 3 2

 The sodium bromate an intermediate product is reduced to sodium bromide by heating

the residue with charcoal.

NaBrO + 3 C → NaBr + 3 CO …(2)
3

 If sodium bromate is not completely converted to the sodium bromide then it is

likely to be present as an impurity.

(E) Atmospheric contamination during the manufacturing process:

 Atmosphere may contain dust (aluminium oxide, sulphur, silica, soot etc.) and some

gases like carbon dioxide, sulphur dioxide, arsine and hydrogen sulphide.

 These may contaminate the final product during the manufacturing process.

 e.g. sodium hydroxide readily absorbs atmospheric carbon dioxide when exposed to

atmosphere.

 2 NaOH + CO → Na CO + H O
2 2 3 2

(3) Manufacturing hazards:

 If the manufacturer is able to control and check impurities from the all above mentioned

sources there exists certain manufacturing hazards which can lead to product

contamination.

Handbook of Inorganic Impurities in Pharmaceuticals

7

(A) Contamination from the particulate matter:

 The unwanted particulate matter can arise by accidental introduction of dirt or glass,

porcelain, plastic or metallic fragments from sieves, granulating, tabletting and filling

machines and the product container.

(B) Cross-contamination of the product:

 Cross-contamination of product can occur by air-born dust arising out of handling of

powders, granules and tablets in bulk.

 If 2 or more Products are manufactured in same time this type of contamination is

possible.

(C) Contamination by microbes:

 Many products, like liquid preparations and creams intended for topical

applications are liable to contamination by microbes from the atmosphere during

manufacturing.

 Microbes like Bacteria, fungi, Algae etc can contaminate the final product.

(D) Errors in the manufacturing process:

 Sometimes in a liquid preparation, there is incomplete solution of the solute.

 A error on the efficiency of mixing, filling, tabletting, sterilization etc arise

impurity in final product.

(E) Errors in the packaging:

 Similar looking products, such as tablets of the same size, shape and colour,

packed in similar containers can result in mislabeling of either or both of the

products.

Handbook of Inorganic Impurities in Pharmaceuticals

8

(4) Instability of the product:

(A) Chemical instability:

 Impurities can also arise during storage because of chemical instability of the

pharmaceutical substance.

 Many pharmaceutically important substances undergo chemical decomposition

when storage conditions are inadequate.

 This chemical decomposition is often catalyzed by light, traces of acid or alkali,

traces of metallic impurities, air oxidation, carbon dioxide and water vapours.

(B) Changes in physical properties:

 Pharmaceuticals may undergo changes in physical properties during storage.

 There can be changes in crystal size and shape, sedimentation, agglomeration and caking

of the suspended particles.

(C) Reaction with container material:

 The possibility of reaction between the container material and the contents can be

possible.

 Preparations susceptible to reaction with metal surfaces

 e.g. salicylic acid ointment must not be packed in metal tubes.

 Plastic containers and closures have tendency to give undesirable additives, such as

plasticizers, particularly in the presence of non-aqueous solvents.

(D) Temperature:

 The rate of chemical decomposition and physical changes of stored products depends

upon the temperature.

 The susceptible substances may have temperature storage requirements assigned to them

in order to protect them against undesirable decomposition.

Handbook of Inorganic Impurities in Pharmaceuticals

9

Limit Test

 Limit = a value or amount that is likely to be present in a substance

 Test = to examine or to investigate

 Limit test is defined as quantitative or semi quantitative test designed to identify and control

small quantities of impurity which is likely to be present in the substance.

 Limit test is generally carried out to determine the inorganic impurities present in compound.

In short, limit test is nothing but to identify the impurities present in the substance and

compare it with standard.

Importance of Limit tests:

 To find out the harmful amount of impurities

 To find out the avoidable/unavoidable amount of impurities

Handbook of Inorganic Impurities in Pharmaceuticals

10

Limit Test of Chloride

Principle: Limit test of chloride is based on the reaction of soluble chloride with silver nitrate in

presence of dilute nitric acid to form silver chloride, which appears as solid particles

(Opalescence) in the solution.

Procedure:

–
Test sample Standard compound (25 ppm Cl )

Specific weight of compound is dissolved in Take 1ml standard solution of chloride in

water or solution is prepared as directed in the Nessler cylinder

pharmacopoeia and transferred in Nessler

cylinder

Add 10 ml of nitric acid Add 10 ml of nitric acid

Dilute to 50ml in Nessler cylinder Dilute to 50ml in Nessler cylinder

Add 1ml of AgNO3 solution Add 1ml of AgNO3 solution

Keep aside for 5 min Keep aside for 5 min

Observe the Opalescence/Turbidity Observe the Opalescence/Turbidity

Standard chloride solution: 5ml of 0.0824 % W/V solution of sodium chloride in 100 ml of

water.

Observation: The opalescence produce in sample solution should not be greater than standard

solution. If opalescence produces in sample solution is less than the standard solution, the sample

will pass the limit test of chloride and visa versa.

Reasons: Nitric acid is added in the limit test of chloride to make solution acidic and helps silver

chloride precipitate to make solution turbid at the end of process.

Handbook of Inorganic Impurities in Pharmaceuticals

11

Limit Test of Sulphate

Principle: Limit test of sulphate is based on the reaction of soluble sulphate with barium

chloride in presence of dilute hydrochloric acid to form barium sulphate which appears as solid

particles (turbidity) in the solution.

Procedure:

Test sample Standard compound

Specific weight of compound is dissolved in Take 1ml of 0.1089 % W/V solution of

water or solution is prepared as directed in the potassium sulphate in Nessler cylinder

pharmacopoeia and transferred in Nessler

cylinder

Add 2ml of dilute hydrochloric acid Add 2ml of dilute hydrochloric acid

Dilute to 45 ml in Nessler cylinder Dilute to 45 ml in Nessler cylinder

Add 5ml of barium sulphate reagent Add 5ml of barium sulphate reagent

Keep aside for 5 min Keep aside for 5 min

Observe the Turbidity Observe the Turbidity

Barium sulphate reagent contains barium chloride, sulphate free alcohol and small amount of

potassium sulphate.

Preparation of ethanolic sulphate standard solution (10 ppm SO 2-): Dilute 1 volume of a
4

0.181% w/v solution of potassium sulphate in ethanol (30%) to 100 volumes with ethanol (30%).

Preparation of sulphate standard solution (10 ppm SO 2-): Dilute 1 volume of a 0.181% w/v
4

solution of potassium sulphate in distilled water to 100 volumes with distilled water.

Handbook of Inorganic Impurities in Pharmaceuticals

12

Observation: The turbidity produce in sample solution should not be greater than standard

solution. If turbidity produces in sample solution is less than the standard solution, the sample

will pass the limit test of sulphate and vice versa.

Reasons:

 Hydrochloric acid helps to make solution acidic.

 Potassium sulphate is used to increase the sensitivity of the test by giving ionic

concentration in the reagent

 Alcohol helps to prevent super saturation.

Limit Test of Iron

Principle: Limit test of Iron is based on the reaction of iron in ammonical solution with

thioglycollic acid in presence of citric acid to form iron thioglycolate which is pale pink to deep

reddish purple in color.

Procedure:

Test sample Standard compound (20 ppm Fe)

Sample is dissolved in specific amount of 2 ml of standard solution of iron diluted with

water and then volume is made up to 40 ml water upto 40ml

Add 2 ml of 20 % w/v of citric acid (iron free) Add 2 ml of 20 % w/v of citric acid (iron free)

Add 2 drops of thioglycollic acid Add 2 drops of thioglycollic acid

Add ammonia to make the solution alkaline Add ammonia to make the solution alkaline

and adjust the volume to 50 ml and adjust the volume to 50 ml

Keep aside for 5 min Keep aside for 5 min

Color developed is viewed vertically and Color developed is viewed vertically and

compared with standard solution compared with standard solution

Handbook of Inorganic Impurities in Pharmaceuticals

13

Iron Standard Solution (20 ppm Fe): Dilute 1 volume of a 0.1726 per cent w/v solution of

ferric ammonium sulphate in 0.05 M sulphuric acid to 10 volumes with water. Contains iron in

ferric state.

Earlier aamonium thiocyanate reagent was used for the limit test of iron. Since thioglycolic acid

is more sensitive reagent, it has replaced ammonium thiocyanate in the test.

Observation: The purple color produce in sample solution should not be greater than standard

solution. If purple color produces in sample solution is less than the standard solution, the sample

will pass the limit test of iron and vice versa.

Reasons:

 Citric acid helpsin to prevent precipitation of iron by ammonia by forming a complex

with it.

 Thioglycolic acid helps to oxidize iron (II) to iron (III).

 Ammonia to make solution alkaline

Limit Test of Lead

Lead is a most undesirable impurity in medical compounds and comes through use of sulphuric

acid, lead lined apparatus and glass bottles use for storage of chemicals.

Principle: Limit test of lead is based on the reaction of lead and diphenyl thiocabazone

(dithizone) in alkaline solution to form lead dithizone complex which is read in color.

Dithizone is green in color in chloroform and lead-dithizone complex is violet in color, so the

resulting color at the end of process is red.

Handbook of Inorganic Impurities in Pharmaceuticals

14

Procedure:

Test sample Standard compound

A known quantity of sample solution is A standard lead solution is prepared equivalent

transferred in a separating funnel to the amount of lead permitted in the sample

under examination

Add 6ml of ammonium citrate Add 6ml of ammonium citrate

Add 2 ml of potassium cyanide and 2 ml of Add 2 ml of potassium cyanide and 2 ml of

hydroxylamine hydrochloride hydroxylamine hydrochloride

Add 2 drops of phenol red Add 2 drops of phenol red

Make solution alkaline by adding ammonia Make solution alkaline by adding ammonia

solution. solution.

Extract with 5 ml of dithizone until it becomes Extract with 5 ml of dithizone until it becomes

green green

Combine dithizone extracts are shaken for 30 Combine dithizone extracts are shaken for 30

mins with 30 ml of nitric acid and the mins with 30 ml of nitric acid and the

chloroform layer is discarded chloroform layer is discarded

To the acid solution add 5 ml of standard To the acid solution add 5 ml of standard

dithizone solution dithizone solution

Add 4 ml of ammonium cyanide Add 4 ml of ammonium cyanide

Shake for 30 mins Shake for 30 mins

Observe the color Observe the color

Handbook of Inorganic Impurities in Pharmaceuticals

15

Preparation of standard lead solution (1 ppm Pb): Dissolve 0.400 g of lead nitrate in water

containing 2 ml of dilute nitric acid and add sufficient water to produce 250.0 ml. This gives

standard lead solution (1% Pb). Standard lead solution (1 ppm Pb) is prepared by diluting 1

volume of standard lead solution (1% Pb) to 1000 volumes with water.

Preparation of dithizone extraction solution: Dissolve 30 mg of dithizone in 1000 ml of

chloroform and add 5 ml of ethanol (95%). The solution is stored in refrigerator. Before use, the

solution is shaken with about half of its volume of 1% v/v nitric acid solution and acid is

discarded.

Preparation of Dithizone standard solution: Dissolve 10 mg of dithizone in 1000 ml of

chloroform.

Observation: The intensity of the color of complex, is depends on the amount of lead in the

solution. The color produce in sample solution should not be greater than standard solution. If

color produces in sample solution is less than the standard solution, the sample will pass the limit

test of lead and vice versa.

Reasons:

 Ammonium citrate, potassium cyanide, hydroxylamine hydrochloride is used to make pH

optimum so interference and influence of other impurities have been eliminated.

 Phenol red is used as indicator to develop the color at the end of process

 Lead present as an impurities in the substance, gets separated by extracting an alkaline

solution with a dithizone extraction solution.

Handbook of Inorganic Impurities in Pharmaceuticals

16

Limit Test of Arsenic

Principle: Limit test of Arsenic is based on the reaction of arsenic gas with hydrogen ion to form

yellow stain on mercuric chloride paper in presence of reducing agents like potassium iodide.

It is also called as Gutzeit test and requires special apparatus.

Arsenic, present as arsenic acid in the sample is reduced to arsenious acid by reducing agents

like potassium iodide, stannous acid, zinc, hydrochloric acid, etc. Arsenious acid is further

reduced to arsine (gas) by hydrogen and reacts with mercuric chloride paper to give a yellow

stain.

The depth of yellow stain on mercuric chloride paper will depend upon the quality of arsenic

present in the sample.

Procedure:

Test solution: The test solution is prepared by dissolving specific amount in water and stannated

HCl (arsenic free) and kept in a wide mouthed bottle.

To this solution 1 gm of KI, 5 ml of stannous chloride acid solution and 10 gm of zinc is added

(all this reagents must be arsenic free)

Keep the solution aside for 40 min and stain obtained on mercuric chloride paper is compared

with standard solution.

Handbook of Inorganic Impurities in Pharmaceuticals

17

Standard solution:

A known quantity of dilute arsenic solution is kept in wide mouthed bottle and rest procedure is

followed as described in test solution.

Apparatus as per IP 1996:

This apparatus has following parts:

 Approximately 60 ml generator bottle with 40 ml indicating line.

 Glass tube with 6.5 mm inner diameter

 A ground joint glass tube with 6.5 mm inner diameter and 18 mm outer diameter at the

joint. Inner joint and the outer joint form a concentric circle.

 Rubber stopper

 Narrow part of the glass tube B. Glass wool is inserted up to this part.

 Lead acetate cotton plug or wool

Reasons:

 Stannous chloride is used for complete evolution of arsine

 Zinc, potassium iodide and stannous chloride is used as a reducing agent

 HCl is used to make the solution acidic

 Lead acetate pledger or papers are used to trap any hydrogen sulphide which may be

evolved along with arsine.

Handbook of Inorganic Impurities in Pharmaceuticals

18

Limit Test of Heavy Metals

Principle: Limit test of heavy metals is based on the reaction of metallic impurities with

hydrogen sulfide in acidic medium to form brownish colour solution.

Metals that response to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium,

silver, copper, and molybdenum.

The metallic impurities in substances are expressed as parts of lead per million parts of the

substance. The usual limit as per Indian Pharmacopoeia is 20 ppm

Procedure: The Indian Pharmacopoeia has adopted four methods for the limit test of heavy

metals.

Method A: Use for the substance which gives clear colorless solution under the specific

condition.

Test sample Standard compound

Solution is prepared as per the monograph and Take 2 ml of standard lead solution and dilute to

25 ml is transferred in Nessler’s cylinder 25 ml with water

Adjust the pH between 3 to 4 by adding dilute Adjust the pH between 3 to 4 by adding dilute

acetic acid or dilute ammonia solution acetic acid or dilute ammonia solution

Dilute with water to 35 ml Dilute with water to 35 ml

Add freshly prepared 10 ml of hydrogen Add freshly prepared 10 ml of hydrogen

sulphide solution sulphide solution

Dilute with water to 50 ml Dilute with water to 50 ml

Allow to stand for five minutes Allow to stand for five minutes

View downwards over a white surface View downwards over a white surface

Observation: The color produce in sample solution should not be greater than standard solution.

Handbook of Inorganic Impurities in Pharmaceuticals

19

If color produces in sample solution is less than the standard solution, the sample will pass the

limit test of heavy metals and vice versa.

Method B: Use for the substance which do not give clear colorless solution under the specific

condition.

Test sample Standard compound

Weigh specific quantity of test substance, moisten Take 2 ml of standard lead solution and

with sulphuric acid and ignite on a low flame till dilute to 25 ml with water

completely charred

Add few drops of nitric acid and heat to 500 °C

Allow to cool and add 4 ml of hydrochloric acid and

evaporate to dryness

Moisten the residue with 10 ml of hydrochloric acid

and digest for two minutes

Neutralize with ammonia solution and make just acid

with acetic acid

Adjust the pH between 3 to 4 and filter if necessary Adjust the pH between 3 to 4 by adding

dilute acetic acid or dilute ammonia

solution

Dilute with water to 35 ml Dilute with water to 35 ml

Add freshly prepared 10 ml of hydrogen sulphide Add freshly prepared 10 ml of hydrogen

solution sulphide solution

Dilute with water to 50 ml Dilute with water to 50 ml

Allow to stand for five minutes Allow to stand for five minutes

View downwards over a white surface View downwards over a white surface

Observation: The color produce in sample solution should not be greater than standard solution.

If color produces in sample solution is less than the standard solution, the sample will pass the

limit test of heavy metals and vice versa.

Handbook of Inorganic Impurities in Pharmaceuticals

20

Method C: Use for the substance which gives clear colorless solution in sodium hydroxide

solution.

Test sample Standard compound

Solution is prepared as per the monograph and Take 2 ml of standard lead solution

25 ml is transferred in Nessler’s cylinder or

weigh specific amount of substance and

dissolve in 20 ml of water and add 5 ml of

dilute sodium hydroxide solution

Make up the volume to 50 ml with water Add 5 ml of dilute sodium hydroxide solution

and make up the volume to 50 ml with water

Add 5 drops of sodium sulphide solution Add 5 drops of sodium sulphide solution

Mix and set aside for 5 min Mix and set aside for 5 min

View downwards over a white surface View downwards over a white surface

Observation: The color produce in sample solution should not be greater than standard solution.

If color produces in sample solution is less than the standard solution, the sample will pass the

limit test of heavy metals and vice versa.

Method D :

Standard Solution: Pipette 10.0 ml of either standard lead solution (1 ppm Pb) or standard lead

solution (2 ppm Pb) into a small Nessler cylinder labeled as “Standard”. Add 2.0 ml of the test

solution and mix.

Test Solution: Prepare as directed in the individual monograph and pipette 12 ml into a small

Nessler cylinder labeled as “Test”.

Procedure: Add 2 ml of acetate buffer pH 3.5 to each of the above Nessler cylinders, mix, add

1.2 ml of thioacetamide reagent and allow to stand for 2 minutes. Compare the colour by viewing

vertically downwards over a white surface. The colour produced with the test solution is not

more intense that than produced with the standard solution.

Handbook of Inorganic Impurities in Pharmaceuticals

21

Important Questions

I. very short answer

1. Name 5 Sources of Impurities in pharmaceutical preparation.

2. Limit test is …………………………….. test designed to identify and control small

quantities of impurities.

3. Limit test for chloride has been based open Rx b/w…………and………..to obtain silver

chloride.

4. Limit test for sulphate has been basedupon the ppt of sulphate with……..in the presence

of……..

5. In limit test for sulphate to prevent the supersaturation of BaSO4 a small amount

of……….has been added in the reagent.

6. Limit test for iron is based upon reaction of Fe with ………in of a solution buffered with

ammonium citrate.

7. Limit test for iron purple color is due to formation of……………..

8. In limit test for iron …………………. prevent the precipitate of iron as Fe(OH)3NH3

solution.

9. In limit test for iron Ferrous thioglycolate has stable pink to reddish purple

colour in …………………… medium.

10. Limit test for Pb has been based upon Rxn b/w………. and……….. to form complex.

11. The structure of dithiazone …………

12. The limit test for Arsenic is based upon……………. test.

13. In limit test for Arsenic ……….. is converted into Arsenous acid/Arsinegas.

14. Arsine gas is carried and comes into contact with…………in produces a yellow or

brown stain.

15. In limit test for Arsenic, temp, should be maintained to……………..

16. The function of granulated Zn in limit test for Arsenic is………………………

17. If the test solution color, turbidity oropalesence is less than the standard solution it

……… …………. the limit test.

Handbook of Inorganic Impurities in Pharmaceuticals

22

18. The main sources of …….. impurities are sulphuric acid and lead apparatus.

19. The standard and test solution used for limit test are prepared in………………..

20. Why nitric acid is added in the limit test of choride ?

21. What is the role of citric acid, thioglycolic acid & ammonia in the limit test of Iron ?

22. Limit test of Arsenic is also known as ?

23. Why lead acetate cotton is used in the limit test of Arsenic?

24. What is the use of stannous chloride in limit test of Arsenic?

Short Answer

1. Discuss limit test of Iron.

2. Discuss the Principle and procedure for the limit test of sulphate.

3. Discuss the apparatus used in limit test of Arsenic.

4. Describe the principle of limit test of sulphate & Arsenic & chloride.

Long answer

1. Write a brief Essay on sources of Impurities in Pharmaceuticals.

2. Give complete Principle and Method as per IP of Limit test of following:

a. Lead

b. Sulphate

c. Chloride

d. Arsenic

e. Iron

f. Heavy Metal

Handbook of Inorganic Impurities in Pharmaceuticals

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