Impurities in pharmaceutical substances.pdf

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Branch: B. Pharm.
Semester: I
Subject: Pharmaceutical Inorganic Chemistry
Subject code:BP104T
UNIT-I
Topic: Impurities in pharmaceutical substances
Dr. LUBNA AZMI
Assistant Professor
Department of Pharmaceutical Chemistry
Institute of Pharmaceutical Sciences
University of Lucknow

• In 17th century: Discoveries of glycerine, chlorine,
manganese, tartaric acid, Iodine and isolation of
mineral salts (sodium, calcium, sulphur, phosphorus
etc) were reported.
• More than 100 elements have been discovered till to this
date.
• Half of these elements are practically less important due
to their rare occurrence.
• Rest of the elements may be useful in the biological,
pharmacological, medical problems & in
pharmaceutical applications.

• The knowledge of atomic structure of an ATOM
(Atomic theory by Dalton in 1807) that represents
simplest unit of the mass helps to predict the physical
and chemical properties of an element
• As they differ due to the difference in their electronic
configuration.
• Frankland in 1853: significant connection between
atomic weight & equivalents.
Atomic Weight = Equivalent weight x Valency
(Valency was first mentioned by Lothar Meyer in
1864)

• Pharmaceutical chemistry deals with the structure,
chemical nature, composition, preparation, studies
of physical and chemical properties, methods of
quality control and conditions of their uses.
• Inorganic chemistry
• Organic chemistry
• Analytical chemistry
• Physical chemistry
• Phytochemistry
• Medicinal chemistry
• Biochemistry
.

Perfix Meaning
Hydro- Indicates a binary compound of hydrogen as hydrochloric acid
(HCl)
Hypo- Indicates lowest state of oxidation
Per- High state of oxidation
Iso- Isomeric form
Meta- Indicates the least hydrated form of an acid
Ortho- Indicates the fully hydrated form of an oxyacid
Para- Indicates the higher hydrated form of an acid
Peroxy- Indicates the presence of peroxide (-O-O-) group
Pyro- Indicates an acid resulting from loss of water (H2O)
Sub- Indicates low state of valence
Thio- Indicates substitution of an oxygen by a sulphur atom

Suffix Meaning
-ate Indicates the salt of an acid
-ic Indicates a higher valency state (e.g., ferric or Iron (III))
-ide Indicates binary compounds (e.g., hydrogen chloride)
-ite Indicates salt of an acid (Sulfite from Sulphuric acid)
-ous Indicates a lower valency state.

• The term Pharmacopoeia first appears as a distinct title in a
work published in Basel, Switzerland in 1561 by Dr A. Foes.
• But does not appear to have come into general use until the
beginning of the 17th century.
• Today’s pharmacopoeias focus mainly on assurance of quality
of products by various tools of analytical sciences.
• Pharmacopoeia: the word derives from the ancient Greek
“pharmakopoiia” from pharmako- ″drug″, followed by the
verb-stem poi- ″make″ and finally the abstract noun ending -
ia.
• These three elements together can be rendered as ″drug-mak-
ing″ or ″to make a drug″.
PHARMACOPOEIA

• A pharmacopoeia, pharmacopeia,or pharmacopoeais a
legally binding collection (legal document, monograph,
official publication/book) prepared by a national or
regional authority, of standards and quality
specifications for medicines used in that country or
region (e.g., IP, BP, USP).
• Quality specification: such as
identity
strength or amount
and purity of medicines

• The role of a modern pharmacopoeia is to furnish
quality specifications for pharmaceutical starting
materials, excipients, intermediates, active
pharmaceutical ingredients (APIs), and finished
pharmaceutical products (FPPs).
analytical methods
microbiological purity
dissolution testing
Stability etc.

International Pharmacopoeia (Ph. Int.)
• Issued by WHO.
• aim to provide international standards & to help achieve
a potentially global uniformity of quality specifications
for selected pharmaceutical products, excipients and
dosage forms.
History of these approaches goes back to :
1902–1925 - agreements established a ″Unified″
Pharmacopoeia.
1929 - "Brussels Agreement" stipulated the League of
Nations to carry out related administrative functions.

1937 - the meeting of the ″Technical
Commission of Pharmaceutical Experts″
1948* - First World Health Assembly (WHA) approved
the Expert Committee on Unification of
Pharmacopoeias
One year later in 1949, the WHA renamed it the Expert
Committee on International Pharmacopoeia.

Organization, region or
country
Name of pharmacopoeia
World Health Organization
The International Pharmacopoeia
(Ph. Int.)
India Indian Pharmacopoeia (IP)
European Union European Pharmacopoeia (Ph. Eur.)
United Kingdom (UK) British Pharmacopoeia (BP)
China
Pharmacopoeia of the People's
Republic of China
Japan The Japanese Pharmacopoeia (JP)
United States of America United States Pharmacopeia (USP)
• In addition to these some other documents prescribing standards are:
• British Pharmaceutical Codex (B.P.C.) & National Formulary in America
(U.S.N.F.)

• Drugs & Cosmetic Act 1949 and Rules 1945.
• IP is published by the Indian Pharmacopoeia
Commission, the Standards Setting Institution for
Drugs.
• It is an autonomous body under the Ministry of
Health & Family Welfare.
• To keep pharmacopoeia
pharmacopoeial commissions
up-to-date,
constantly
update pharmacopoeia Once in every four
years.

• Monograph refers to the study of published matter
of a subject.
• The medicinal substance are used to cure &
prevent diseases
• Hence, it is important to give a detailed written study
– appears as a monograph in the pharmacopoeia

• Main title/synonyms
• Chemical formula/formula weight
• Therapeutic category
• Dosage
• Description
• Solubility
• Minimum standards of purity
• Identification tests
• Tests for purity/limit tests for impurities
• Methods of assay
• Storage condition
• Packaging and

Impure Chemical Compound:
A compound is said to be impure if it is having foreign matter
i.eImpurities.
Pure Chemical Compound:
A pure chemical compound refers to that compound which is
having no foreign matter i.e impurities.
Chemical purity means freedom from foreign matter.
Analytically 100 % pure substances are not available and traces of
impurities must be present. Normally undesirable foreign materials are
present in the pharmaceutical substances.
What is impurity?
Any material that affects the purity of the material of interest.

❑ Presence of Impurities in the pharmaceutical substances may produce toxic effects on the
body and may also lower down the active strength of the pharmaceutical substance.
❑ Impurities commonly in chemical substances include small quantities of lead, Arsenic,,
Chloride and sulphate.
Impurities commonly found in medicinal preparations:
❖Impurities which have toxic effects on body and bring about
unpleasant reactions when present beyond certain limits. e.g
Lead and Arsenic salts.
❖The impurities which are able to make substance incompatible with other substances.
❖The impurities which if present beyond the limit, affect the storage property of the pharmaceuticals.
❖ The impurities which are harmless, but if present beyond the limit, it will lower the
active strength of the medicinal compound. E.g Sodium salt in potassium salt.
❖The impurities which may bring about technical difficulties in the use of the substance.
❖Impurities such as taste, odour, colour or appearance which can be easily detected by the senses
and make the substance unhygienic and unaesthetic. E.g. Sodium chloride becomes damp because
of the presence of traces of magnesium salts. Also phenolic impurities present in sodium salicylate
alters its odour.

Sources of Impurities in Pharmaceuticals
1) Raw material used in manufacture
2) Reagents used in manufacturing process
3) Method/ process used in manufacture or method of manufacturing
4) Chemical processes used in the manufacture
5) Atmospheric contamination during the manufacturing process
6) Intermediate products in the manufacturing process
7) Defects in the manufacturing process
8) Manufacturing hazards
9) Inadequate Storage conditions
10)Decomposition of the product during storage
11)Accidental substitution or deliberate adulteration with spurious or
useless materials

Raw materials employed in manufacture
• Impurities known to be associated with these
chemicals may be carried through the manufacturing
process and contaminate the final product.
• Example
Rock salt-------- Calcium Sulphate (CaSO4) + Magnesium Chloride (MgCl2)= NaCl

•Example:
Copper sulphate may be prepared by the action of sulphuric acid on copper turnings:
Cu+ 2 H2SO4------------------------→ CuSO4 + 2 H2O + SO2
Copper turnings are known to have Iron and Arsenic as impurities.
Due to this I.P. prescribes limit of tolerance for Arsenic as impurity to be not
more than 8 parts per million in copper sulphate. Similarly it prescribes a
limit of Iron as impurity.
Copper turnings

1) Reagents used in the manufacturing process:
❑ If reagents used in the manufacturing process are not completely removed by
washing, these may find entry into the final products.
Example:
Ammoniated mercury may be prepared by adding a solution of Mercuric chloride to dilute
ammonia solution.
HgCl2+ 2NH4OH-------------→NH2HgCl + NH4Cl + 2 H2O
Soluble Soluble Ammoniated mercury (ppt) (soluble)
The precipitate of Ammoniated mercury (Final Product)contains
ammonium hydroxide. Thus, this precipitate is washed with cold water to
remove ammonium hydroxide.
If it is not removed completely by washing with water, the final product may
contain in it
Ammonium hydroxide as impurity.

2) Method or the process used in the manufacture:
• Many drugs and chemicals (usually organic) are manufactured from different raw
materials, by using different methods or processes.
• Some impurities are incorporated into the materials during the manufacturing process.
• The type and amount of impurity present in the drug/ chemical varies.
• In certain drugs , a multiple-step-synthesis procedure is used , which produces
intermediate compounds.
• The purification of intermediates is also important, otherwise the impurities present in
the intermediate will get incorporated in the final product.
• Usually side reactions occur during the synthesis.
• Impurities of the product side reactions also occur in the substances.
• This may introduce new impurities due to contamination by reagents and
solvents at various stages of the process as described below:
✓Reagents employed in the process
✓Reagents added to remove other impurities
✓Solvents
✓Action of solvents and reagents on reaction vessels.

C) Solvents:
Water is the cheapest solvent available and has been used wherever possible.
Tap Water
It has Ca+2, Mg+2, Na+, Cl-, SO4 -2 and CO3 -2 as
impurities in small amounts
Softened water It is obtained by allowing the tap water to pass through the
sodium form of Zeolite which removes divalent cations like
Ca+2 and Mg+2 from tap water in exchange of sodium.
So, softened water contains Na+, Cl- ions as impurity.
De-mineralised
water
It is obtained by passing tap water through columns packed
with ion exchange resin.The
water obtained from this process is free from Ca+2, Mg+2,
Na+, Cl-, SO -2 and CO -2
4 3
Thus the final product is free from these
impurities.
The water obtained from this source may still contain organic
impurities and so final product contains organic impurities.
Distilled water It is considered the best but it is very costly.

3) Chemical process used in the manufacture:
❖For the synthesis of drugs, many chemical reactions such:
Nitration,
Halogenation,
Oxidation,
Reduction,
Hydrolysis
❖In these chemical processes, different chemicals are used.
❖Tap water is generally used in the various processes and it is often having Cl-
,Mg+2, Ca+2 ions, which are generally found in the substance which is
beingmanufactured.

4) Atmospheric contamination during the manufacturing
process
• In the industrial areas, the atmosphere is contaminated with dust particlesand
some gases like Hydrogen sulphide, Sulphur dioxide, and blacksmoke.
• During the manufacture or purification of the pharmaceutical products,these
impurities enter the final products.
• There are many pharmaceutical products which when manufactured are
contaminated with atmospheric CO2 and water vapour.
2NaOH + CO2 --------------------------------→ Na2CO3 + H2O
• Due to this reaction, NaOH should not be kept open for a longer time duringits
manufacture.
• Therefore, IP has prescribed that Sodium hydroxide should not contain morethan
3% of sodium carbonate.

5) Defects in the manufacturing process:
In many manufacturing processes, there are defects like imperfect mixing,
incompleteness, non-adherence to proper temperature, pressure, pH or
reaction conditions, which may give chemical compounds with impurities in
them.
Example:
• Zinc oxide may be prepared by heating metallic zinc to bright redness in a
current of air. The vapours of Zinc burn to form Zinc oxide which is
collected as a fine white powder.
• But if there is less heat or air or both, zinc metal is not completely
converted to zinc oxide.
• Thus the final product, Zinc oxide may still contain metallic zinc as
impurity.
• So, IP has prescribed a test for Zinc metal in zinc oxide.

6) Intermediate products in the manufacturing process:
❑ Example:
Potassium iodide is prepared by reacting Iodine with Potassium
hydroxide. 6KOH+ 3I2--------------------→ 5KI
+ KIO3 + 3H2O
The resulting solution is first evaporated and then heated with
charcoal.
KIO3 + 3C----------→ KI + 3CO
❑In this process if the intermediate product (KIO3) is not
completely converted into KI, then it may be carried through to
the final
product as an impurity.

Process errors
Cross
contamination
Microbial
contamination
Packing
errors
Manufacturing hazards:

Particulate contamination:
The presence of unwanted particulate matter can arise due
to dirt, dust, glass, porcelain or plastic fragments from
sieves, granulating or tableting machinesor from product
containers.
❑Ware and tare of equipment or improperly cleaned
equipment may also cause particulate contamination.
❑Clarity of solutions for injection is particularly important.
❑E.g Metal particles which have been found in eye ointments
packed in metal tubes.

Process errors:
❑Gross errors arising from incomplete
solution of a solute in a liquid preparation
must be detected readily by the normal
analytical control procedures.
❑Minor errors arise if the manufacturing tolerance for the
quantity of active ingredient in the product has been wide.
Cross contamination:
❑The handling of powders, granules, and tablets in large
bulk creates air-borne dust, which leads to cross
contamination of the product.
❑So, face masks and special extraction equipment are used
to protect operators from harmful effects of drugs.
❑E.g penicillin preparation requires special handling during
its manufacture.

Microbial contamination:
• Parenteral preparations and ophthalmic
preparations require special care against
microbial contamination.
• Many liquid preparations and creams are liable to bacterial and
fungal contamination. So care should be taken.
• Eg. Acacia, senna, tragacanth---→They should be controlled
for Salmonellae.
Packing errors:
• Products of similar appearance such as tablets of same size,
shape, colour packed in similar containers can constitute a
potential source of danger.
• Improper labelling or destruction of stock of unused labels also
constitutes a major packaging hazard.

Storage conditions:
❑ The chemical substances when prepared have to be stored
in different types of containers depending upon:
✓Nature of the material
✓Batch size
✓Quantity
❑ Many types of materials are used for storage purpose like
plastic, polythene, iron vessels, stainless steel and
aluminium.
The products formed
are found as impurities
in the stored materials
With materials of
storage vessels
Reaction of
these
substances

Inadequate storage and their effects are as follows:
a)Filth: Stored products may become contaminated with dust,
bodies of insects, animal and insect excreta.
b)Chemical instability: decomposition because of light, traces
of acid or alkali, air oxidation, water vapour, CO2 and traces
of metallic ions.
e.g light sensitive materials should be stored in amber colored
bottles.
c)Reactions with container materials: e.g salicylic acid ointment
must not be stored in metal tubes.
d)Physical changes: The occurance of changes in the physical
form of drug like change in crystal size can lead to change in
efficiency of product.
e)Temperature effect: Chemical and physical changes occur if
materials are not stored at proper temperature.

Accidental substitution or deliberate adulteration
with spurious or useless materials:
❑It is possible to avoid accidental substitution by
storing the toxic substances together separately or
in a locked cupboard.
❑Many pharmaceutical chemicals are adulterated
with cheaper substances.
❑E.g The expensive potassium may be adulterated
with sodium bromide.

Effect of Impurities:
The impurities present in the substances may give following effects:
❑ Impurities having toxic effects may
be injurious to health, if present
above certain limits.
❑ Traces of impurities, may exert a cumulative toxic effect after a
certain time.
❑ Impurities may lower the active strength of the substance.
❑ Impurity may decrease shelf life of substance.
❑ Impurity may cause incompatibility with other substances.
❑ Impurities may cause a physical or chemical change in the
properties of the substance, so making the substance medicinally
useless.
❑ May cause change in color, odour and taste.

Test for purity:
❖ Pharmacopoeia prescribes the “Test for purity” for pharmaceutical
substancesto check their freedom from undesirable impurities.
❖ Pharmacopoeia will decide and fix the limit of tolerance for theseimpurities.
❖ For certain common impurities for which pharmacopoeia prescribes the testof
purity are:
✓Colour, odour, taste
✓Physicochemical constants (Iodine value, saponification value, meltingpoint,
refractive index etc.)
✓Acidity, alkalinity, pH
✓Humidity (Estimation of moisture)
✓Cations and anions
✓Ash
✓Arsenic or lead
✓Loss on drying
✓Loss on ignition

Impurity means
undesired particles
What is impurity?
Any material that affects the purity of the material of interest.
❖ Presence of Impurities in the pharmaceutical substances may produce toxic
effects on the body and may also lower down the active strength of the
pharmaceutical substance.

Limit tests:
❖ Tests being used to identify the impurity.
❖ Tests being used to control the impurity.
❖ Definition: Limit tests are quantitative or semi quantitative test designed to
identify and control small quantities of impurities which are likely to be
present in the substances.

Factors affecting limit tests:
❑Specificity of the tests
❑Sensitivity
❑Control of personal errors (Analyst errors)
• Test in which there is no visible reaction
• Comparison methods
• Quantitative determination
Types:
Tests in which there is no visible reaction
❑Comparison methods
❑Quantitative determinations

Turbidity means cloudiness or haziness.

Limit test for IRON:
❖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 (Ferrous thioglycolate
complex) which produces pale pink to deep reddish purple
color in alkaline media.
❖Thioglycolic acid is used as reducing agent.

Procedure:
Test sample Standard compound
Sample is dissolved in specific amount
of water and then volume is made up to
40 ml
2 ml of standard solution
of iron diluted with water
upto 40 ml
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 and adjust the volume to 50 ml
Add ammonia to make the solution
alkaline and adjust the volume to 50 ml
Keep aside for 5 min Keep aside for 5 min
Color developed is viewed
vertically and compared with
standard solution
Color developed is
viewed vertically and
compared with standard
solution
Note: All the reagents used in the limit test for Iron should themselves be iron free.

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 forms complex with metal cation and helps precipitation
of iron by ammonia by forming a complex with it.
➢Thioglycolic acid helps to oxidize iron (II) to iron (III).
➢Ammonia is added to make solution alkaline. The pale pink color
is visible only in the alkaline media. The color is not visible in
acidic media as ferrous thioglycolate complex decomposes in high
acidic media.

Limit test for Chloride:
The test is used to limit the amount of Chloride as an impurity in inorganic
substances.
Limit test for 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.
❖ The silver chloride produced in the presence of dilute Nitric acid makes the test
solution turbid, the extent of turbidity depending upon the amount of Chloride
present in the substance is compared with the standard opalescence produced by
the addition of Silver nitrate to a standard solution having a known amount of
chloride and the same volume of dilute nitric acid as used in the test solution.
AgNO3 --------------------------→ AgCl
+ NO3
-
Cl-
+
Soluble
chloride
present as
impurity
silver chloride

Specific weight of compoundis dissolved
in water or solution is prepared as
directed in the
Take 1 ml of 0.05845 % W/V
solution of sodium chloride in
Nessler cylinder
Add 1 ml of nitric acid Add 1 ml of nitric acid
Dilute to 50 ml in Nessler cylinder Dilute to 50 ml in Nessler cylinder
Add 1 ml of AgNO3 solution Add 1 ml of AgNO3 solution
Observe the
Opalescence/Turbidity
Observe the Opalescence/Turbidity

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 as Dilute HNO3 is insoluble in AgCl
.

Limit test for 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.
•Then comparison of turbidity is done with a standard turbidity
obtained from a known amount of Sulphate and same volume
of dilute Hydrochloric acid have been added to both solutions.
•The barium chloride test solution in the IP has been replaced
by Barium sulphate reagent which is having barium chloride,
sulphate free alcohol and a solution of potassium sulphate.
Potassium sulphate has been added to increase the sensitivity of
the test.

Procedure:
Specific weight of
compound is dissolved in water
or solution is prepared as
directed in the pharmacopoeia
Take 1 ml of 0.1089 % W/V
solution of potassium
sulphate in Nessler cylinder
Add 2 ml of dilute hydrochloric acid Add 2 ml of dilute hydrochloric acid
Dilute to 45 ml in Nessler cylinder Dilute to 45 ml in Nessler cylinder
Add 5 ml of barium sulphate reagent Add 5 ml of barium sulphate reagent
Observe the Turbidity Observe the Turbidity

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

Limit test for 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.
❖ Substance + dil HCl -------------------------→ H3AsO4
(containsArsenic impurity) Arsenic acid
❖ H3AsO4 +
Arsenic acid
H2SnO2 -------------------→ H3AsO3
+H2SnO3
Arsenious acid
H3AsO3 + 6[H] ---------------------→ AsH3 +3H2O
Arsenious acid nascent hydrogen Arsine gas

Apparatus:
It is having a wide mouthed glass bottle of 120 mL
capacity having mouth of about 2.5 cm in diameter. This
bottle is fitted with a rubber bung through which passes a
glass tube, 20 cm long.
✓External diameter=0.8 cm
✓Internal diameter=0.65 cm
✓The tube is constricted at its lower end extremity to
about 1 mm diameter and there is blown a hole, not less
than 2 mm in diameter, in the side of the tube near the
constricted part.

✓The upper end of the glass tube is fitted with two rubber
bungs(25 mm x 25 mm), each having a hole bored
centrally and exactly 6.5 mm in diameter.
✓One of the bungs has been fitted to the upper end of the
tube, while the second bung has to be fitted upon the first
bung in such a way that the mercuric chloride paper gets
exactly sandwiched between the central perforation of the
two.
✓The bungs are kept in close contact by using rubber band
or spring clip in such a manner that the gas evolved from
the bottle must have to pass through the 0.65 mm internal
circle of mercuric chloride paper.
✓An important feature has been the standardization of the
area of Mercuric chloride paper which is exposed to the
reaction of arsine gas.

The test solution is prepared by dissolving
specific amount in water and stannated
HCl (arsenic free) and kept in a wide
mouthed bottle.
A known quantity of dilute arsenic
solution in water and stannated HCl
(arsenic free) is kept in wide mouthed
bottle.
1 g of KI 1 g of KI
5 ml of stannous chloride acid solution 5 ml of stannous chloride acid solution
10 g of granulated zinc is added (all this
reagents must be arsenic free).
Keep the solution aside for 40 min
10 g of zinc is added (all this reagents must
be arsenic free).
Keep the solution aside for 40 min
Inference: If the stain produced by the test is not deeper than the standard stain, then sample complies with the limit
test for Arsenic.

Reasons:
Stannous chloride is used for complete evolution of arsine.
Zinc, potassium iodide and stannous
chloride is used as a reducing agent.
Hydrochloride acid is used to make
the solution acidic
Lead acetate paper are used to trap any hydrogen sulphide which may
be evolved along with arsine.
Use of stannated Hydrochloric acid:
If pure zinc and HCl are used, the steady evolution of gas does
not occur. This produces improper stain (e.g slow evolution
produces short but intense stain while rapid evolution of gas
produces long but diffused stain.)
So, to get steady evolution of gas, stannated hydrochloric acid is used.

Use of Lead Acetate solution:
H2S gas may be formed during the experiment as zinc contains
sulphides as impurities. It gives black stain to HgCl2 paper and so
will interfere the test.
Hence, gases evolved are passed through cotton wool plug
moistened with lead acetate, where H2S gas is trapped as PbS.
Use of Potassium iodide:
KI is converted to HI which brings about reduction of
unreacted pentavalent arsenic to trivalent Arsenic. Thus,
reproducible results can be obtained. If it is not used
thensome pentavalent Arsenic may remain unreacted.

Limit test for heavy metals
❖The limit test for heavy metals is designed to determine the
content of metallic impurities that are coloured by hydrogen
sulphide or sodium sulphide under the condition of the test
should not exceed the heavy metal limits given under the
individual monograph.
❖The heavy metals (metallic impurities) may be iron, copper, lead,
nickel, cobalt, bismuth, antimony etc.. The limit for heavy metals
is indicated in the individual monograph in term of ppm of
lead i.e. the parts of lead per million parts of the substance
being examined
❖The quantity is stated as the heavy metal limit and is expressed
as parts of lead (by
weight) per million parts of the test substance.
❖These remain distributed in colloidal state, and give rise to a
brownish coloration.

Limit test for 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 red in color

Method:
• Sample solution is transferred to a separating funnel.
• To it 6 ml of ammonium citrate, 2 ml
potassium cyanide and 2 ml of
hydroxalamine HCl are added.
• 2 drops of phenol red
• Solution is made alkaline by adding ammonia solution.
• This is then extracted with 5 ml portions of dithizone
solution until it becomes green.
• The combined dithizone extracts are shaken for 30 seconds
with 30 ml of nitric acid and chloroform layer is discarded.
• To the acid solution 5 ml of standard dithizone solution is
added and 4 ml ammonium cyanide and solution is shaken
for 30 sec.
• Similarly prepare standard.

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

known quantity of sample solution is transferred in a
separating funnel
A standard lead solution is prepared equivalentto
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
hydroxylamine hydrochloride
Add 2 ml of potassium cyanide and 2 ml
of hydroxylamine hydrochloride
Add 2 drops of phenol red Add 2 drops of phenol red
Make solution alkaline by adding ammonia solution. Make solution alkaline by adding ammonia solution.
Extract with 5 ml of dithizone until it becomes green Extract with 5 ml of dithizone until it becomes green
Combine dithizone extracts are shaken for 30 mins
with 30 ml of nitric acid and the chloroform layer is discarded
Combine dithizone extracts are shaken for 30 mins
with 30 ml of nitric acid and the chloroform layer is discarded
To the acid solution add 5 ml of standard dithizone solution To the acid solution add 5 ml of standard 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

Aq. Ammonia is added in limit test of lead:
❖Pb+ S →PbS
❖In limit test of lead, PbS is produced by addition of standard
H2S , to the solution containing lead.
❖pH 3-4 is necessary for the precipitation of PbS. So aq.
Ammonia / acetic acid is added to maintain that pH.

Modified limit test for Chlorides
•Alkaline substances have to be dissolved in acid so that
effervescence ceases and much of the free acid is left in the
solution as is prescribed in the test.
•Salts of organic acids like sodium benzoate, sodium
salicylate, etc. liberate free water insoluble organic acid during
acidification which is filtered off and the filtrate is employed
for the test.
•Deeply coloured substances have to be decolourised before
test e.g., potassium permanganate is reduced by boiling with
alcohol and the filtrate is used.
•Reducing substances like hypophosphorus acid, which react
with silver nitrate in the limit test for chlorides should be
oxidized with nitric acid or some other oxidizing agents before
carrying out the test.

1.Determinate Errors (Systematic errors): Definite
value and underlying (undisclosed/hidden) cause.
•Can measure & account for these errors.
Sources:
Impurities in the crude material.
Excess of reagent used.
Salts from the water used when the preparation is
carried out in aqueous solutions.
Metallic contamination.
Contamination from exposure to the atmosphere.

2. Indeterminate errors:
• Result from extending a system of
measurements to its maximum.
• Neither identify the sources of these
errors nor predict the magnitudes of
individual errors.

• Impurities are always present in the
Pharmaceutical products.
• The following points are considered to permit the
impurities in these substances:
1. A level of harmful substances is to be
determined.
2. A permissible limit, based on the amount to be
tolerated of the toxic substances is prescribed.
e.g., Heavy metal (Toxic impurities) limits in India

Acceptance criteria for impurities in
drug substances:
Each identified specified
impurity
(Determinate errors)
Not more than 0.5 per cent
Each unidentified impurity
(Indeterminate errors)
Not more than 0.3 per cent
Total impurities Not more than 1.0 per cent
Provided it has been determined that the impurities are not toxic. Higher limits
may be set if scientifically justified.

The limit for harmless impurities are also prescribed
depending upon the nature, type and usefulness of the
substances.
E.g., colouring substances
flavouring agents
preservatives are the harmless compounds added to
the pharmaceutical preparations.

❖ Dr. K G Bothara, A Hand Book of Inorganic Pharmaceutical Chemistry; Nirali Prakashan, New Delhi
❖ WHO, Review of Pharamcopoeias;March,2013.
❖ Indian Pharmacopoeia
❖ P C Kamboj, Pharmaceutical Analysis, Vol.1, Vallabh Publications, New Delhi
❖ A.H. Beckett & J.B. Stenlake's, Practical Pharmaceutical Chemistry Vol I & II, Stahlone Press of
University of London, 4th edition.
❖ Anand & Chatwal, Inorganic Pharmaceutical Chemistry
❖ P. Gundu Rao, Inorganic Pharmaceutical Chemistry, 3rd Edition
❖ Indian Pharmacopia 2018, volume I,II,III .
❖ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC
❖ https://jamanetwork.com/journals/jama/article-abstract/430842
❖ https://www.tandfonline.com/doi/full/10.1080/00026980.2016.1227180

Impurities in pharmaceutical substances.pdf

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