P’ceutical inorganic chemistry (BP104TP)
LIMIT TEST
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.





The quantity of any one impurity in an official substance is often small and
consequently the visible reaction response to any test for that impurity is also small.
The design of individual tests is therefore important if errors are to be avoided in the
hands of different operators.
It involves simple comparison of opalescence, turbidity or colour produced in test with
that of fixed standards.
Limit tests are normally performed using Nessler cylinder. They have nominal capacity
of 50 ml and the overall height of Nessler cylinder is 150 mm.
Comparison is made by placing the two Nessler cylinders side by side and viewing
transversely against proper background.
Importance:




Limit test is generally carried out to determine the inorganic impurities present in the
compound.
It is carried out to control & check if inorganic impurity present in the drug does not
exceed to prescribe limit.
Limit test is used to find put harmful impurities.
To find out avoidable/unavoidable amount of impurities.
LIMIT TEST FOR CHLORIDE
The chloride limit test is designed to determine the allowable limit of chloride contained in a
sample.
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. This white precipitates of silver chloride are insoluble in dilute nitric acid and
hence gives turbidity or opalescence to the solution. The extent of the turbidity produced
depends upon the amount of the chloride present in the sample and is compared with standard
opalescence produced by addition of silver nitrate to a standard solution having known amount
of chloride and the same volume of the dilute nitric acid as used in the test solution.
Cl- + AgNO3
Dhruvi Prajapati
→
𝐷𝑖𝑙𝑢𝑡𝑒 𝑁𝑖𝑡𝑟𝑖𝑐 𝑎𝑐𝑖𝑑
AgCl + NaNO3
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P’ceutical inorganic chemistry (BP104TP)
Role of nitric acid: Silver chloride precipitates are insoluble in presence of nitric acid so that
observable and comparable turbidity is produced. Another role of nitric acid is to prevent the
precipitation of silver as its carbonate and phosphate.
Procedure:
Test sample
Specific weight of compound is dissolved in
water or solution is prepared as directed in
the pharmacopoeia and transferred in Nessler
cylinder
Add 1 ml of dil. nitric acid
Dilute to 50 ml in Nessler cylinder
Add 1 ml of 0.1 M AgNO3 solution
Keep aside for 5 min protected from light
Observe the Opalescence/Turbidity
Standard compound
Take 1 ml of 0.05845 % W/V solution of
sodium chloride I Nessler cylinder
Add 1 ml of dil. nitric acid
Dilute to 50 ml in Nessler cylinder
Add 1 ml of 0.1 M AgNO3 solution
Keep aside for 5 min protected from light.
Observe the Opalescence/Turbidity
View both the solution transversely against black background and compare the turbidity
/opalescence produced in test solution with respect to standard solution. If the turbidity
developed in the test solution is less than the turbidity produced in the standard solution, the
sample passes the limit test for chloride and vice-versa.
LIMIT TEST FOR SULPHATE
The Sulphate Limit Test is designed to determine the allowable limit of sulphate contained in
a sample.
Principle:
It is a comparison method. It involves the comparison of opalescence or turbidity of test sample
with standard sample which contains the definite amount of sulphate impurities.
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.
The barium chloride test solution in the I.P. has been replaced by barium sulphate reagent
which is having barium chloride, sulphate free alcohol, and a solution of potassium sulphate.
The ionic concentrations in the reagent has been so adjusted that the solubility product of
barium sulphate gets exceeded, and the very small amount of barium sulphate present in the
reagent acts as seeding agent for precipitation of barium sulphate.
Potassium sulphate has been added to increase the sensitivity of the test. Alcohol helps to
prevent super saturation (i.e. the crystallization of sulphate with any other ion) and thus produce
a more uniform opalescence. Hydrochloric acid/acetic acid helps to make solution acidic.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
SO42-
+
BaCl2

BaSO4 + 2Cl-
Procedure:
Test solution
Specific weight of compound is dissolved in
water or solution is prepared as directed in
the pharmacopoeia and transferred in Nessler
cylinder
Add 2 ml of dilute hydrochloric acid/acetic
acid
Dilute to 45 ml in Nessler cylinder
Add 5 ml of barium sulphate reagent
Keep aside for 5 min
Observe the Turbidity in black background
Standard solution
Take 1 ml of 0.1089% W/V solution of
potassium sulphate in Nessler cylinder
Add 2 ml of dilute hydrochloric acid/acetic
acid
Dilute to 45 ml in Nessler cylinder
Add 5 ml of barium sulphate reagent
Keep aside for 5 min
Observe the Turbidity in black background
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.
MODIFIED LIMIT TEST
A specified amount of the substance is dissolved in distilled water, and the volume made up to
50 ml in a Nessler cylinder. Depending upon the nature of the substance, some modifications
have to be adopted for the preparation of the solution.
a) 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.
b) Insoluble substances are generally extracted with water and then filtered, and the filtrate
is used for the test, because the presence of insoluble substance modifies the
opalescence and colour.
c) 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.
d) Coloured substances like crystal violet, malachite green, etc. are carbonised and the ash
so produced is extracted in water.
e) Deeply coloured substances have to be decolourised before test e.g., potassium
permanganate is reduced by boiling with alcohol and the filtrate is used.
f) 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.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
Modified limit test for chloride
Principle:
Modified limit test is performed; if the limit tests for a sample (coloured compound) cannot be
done by normal method. For example, if potassium permanganate is used as a test sample, it
cannot be done by normal method. As potassium permanganate gives purple colour aqueous
solution that interferes in the comparison of opalescence or turbidity, therefore the aqueous
solution must first be decolorized.
Potassium permanganate is de-colourised by boiling with ethanol. Potassium permanganate
is oxidizing agent while ethanol is reducing agent. When potassium permanganate solution is
treated with ethanol in presence of heat the redox reaction will take place, i.e. potassium
permanganate gets reduced to manganese dioxide (precipitates). The filtrate of the reaction is
colourless that is subjected to proceed for limit test for chloride.
2KMnO4 + 3C2H5OH  3CH3CHO + 2KOH + 2MnO2 + 2H2O
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. This white precipitates of silver chloride are insoluble in dilute nitric acid and
hence gives turbidity or opalescence to the solution. The extent of the turbidity produced
depends upon the amount of the chloride present in the sample and is compared with standard
opalescence produced by addition of silver nitrate to a standard solution having known amount
of chloride and the same volume of the dilute nitric acid as used in the test solution.
Cl- + AgNO3
→
𝐷𝑖𝑙𝑢𝑡𝑒 𝑁𝑖𝑡𝑟𝑖𝑐 𝑎𝑐𝑖𝑑
AgCl + NaNO3
Role of nitric acid: Silver chloride precipitates are insoluble in presence of nitric acid so that
observable and comparable turbidity is produced. Another role of nitric acid is to prevent the
precipitation of silver as its carbonate and phosphate.
Procedure:
Preparation of test solution: Dissolve 1.5 g in 50 ml of distilled water, heat on a water bath
and add gradually 6 mL of ethanol (95%), cool, dilute to 60 ml with distilled water.
Test sample
Take 40 ml of the above test solution and
transfer in Nessler cylinder
Add 1 ml of dil. nitric acid
Dilute to 50 ml in Nessler cylinder
Add 1 ml of 0.1 M AgNO3 solution
Keep aside for 5 min protected from light
Observe the Opalescence/Turbidity
Dhruvi Prajapati
Standard compound
Take 1 ml of 0.05845 % W/V solution of
sodium chloride I Nessler cylinder
Add 1 ml of dil. nitric acid
Dilute to 50 ml in Nessler cylinder
Add 1 ml of 0.1 M AgNO3 solution
Keep aside for 5 min protected from light.
Observe the Opalescence/Turbidity
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P’ceutical inorganic chemistry (BP104TP)
View both the solution transversely against black background and compare the turbidity
/opalescence produced in test solution with respect to standard solution. If the turbidity
developed in the test solution is less than the turbidity produced in the standard solution, the
sample passes the limit test for chloride and vice-versa.
Modified limit test for sulphate
Principle:
Modified limit test is performed; if the limit tests for a sample (coloured compound) cannot be
done by normal method. For example, if potassium permanganate is used as a test sample, it
cannot be done by normal method. As potassium permanganate gives purple colour aqueous
solution that interferes in the comparison of opalescence or turbidity, therefore the aqueous
solution must first be decolorized.
Potassium permanganate is de-colourised by boiling with ethanol. Potassium permanganate
is oxidizing agent while ethanol is reducing agent. When potassium permanganate solution is
treated with ethanol in presence of heat the redox reaction will take place, i.e. potassium
permanganate gets reduced to manganese dioxide (precipitates). The filtrate of the reaction is
colourless that is subjected to proceed for limit test for sulphate.
2KMnO4 + 3C2H5OH  3CH3CHO + 2KOH + 2MnO2 + 2H2O
It is a comparison method. It involves the comparison of opalescence or turbidity of test sample
with standard sample which contains the definite amount of sulphate impurities.
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.
The barium chloride test solution in the I.P. has been replaced by barium sulphate reagent
which is having barium chloride, sulphate free alcohol, and a solution of potassium sulphate.
The ionic concentrations in the reagent has been so adjusted that the solubility product of
barium sulphate gets exceeded, and the very small amount of barium sulphate present in the
reagent acts as seeding agent for precipitation of barium sulphate.
Potassium sulphate has been added to increase the sensitivity of the test. Alcohol helps to
prevent super saturation (i.e. the crystallization of sulphate with any other ion) and thus produce
a more uniform opalescence. Hydrochloric acid/acetic acid helps to make solution acidic.
SO42-
+
BaCl2

BaSO4 + 2Cl-
Procedure:
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
Preparation of test solution: Dissolve 1.5 g in 50 ml of distilled water, heat on a water bath
and add gradually 6 mL of ethanol (95%), cool, dilute to 60 ml with distilled water.
Test solution
Take 40 ml of the above test solution and
transfer in Nessler cylinder
Add 2 ml of dilute hydrochloric acid/acetic
acid
Dilute to 50 ml in Nessler cylinder
Add 5 ml of barium sulphate reagent
Keep aside for 5 min
Observe the Turbidity in black background
Standard solution
Take 1 ml of 0.1089% W/V solution of
potassium sulphate in Nessler cylinder
Add 2 ml of dilute hydrochloric acid/acetic
acid
Dilute to 50 ml in Nessler cylinder
Add 5 ml of barium sulphate reagent
Keep aside for 5 min
Observe the Turbidity in black background
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.
LIMIT TEST FOR IRON
Principle:
Limit test of Iron is a comparison method where intensity of the pale pink to purple colour of
test solution is compared with standard solution which contain a definite amount of Iron
impurities.
It is based on the reaction of iron in ammonical solution of thioglycollic acid in presence of
citric acid to form iron thioglycolate (Ferrous thioglycolate complex) which produces pale pink
to deep reddish purple colour in alkaline media.
Thioglycolic acid (CH2(HS)COOH) is a useful analogue of glycolic acid (CH2(OH)COOH). It
is added as reducing agent and hence reduces Fe+3 (ferric ion) to Fe+2 (ferrous ion), when all
the ferric ions are reduced to ferrous ion, thioglycolic acid combines with ferrous ion to form
ferrous thioglycolate complex, which is pale pink to purple colour in alkaline media.
Citric acid is added to prevent the interference of metal cation by forming a complex with it
and also prevents the precipitation of iron by ammonia.
Ammonia is added to provide alkaline media because the pale to purple colour is visible only
in basic media. The colour is not visible in acidic media as ferrous thioglycolate complex
decompose in high acidic media.
The colour of the ferrous thioglycolate complex fades in the presence of air due to oxidation.
Also, the colour is destroyed in presence of oxidizing agents and strong alkalis.
Earlier, ammonium thiocyanate reagent was used for the limit test of iron. Since thioglycolic
acid is a more sensitive reagent for iron, it has replaced ammonium thiocyanate in the test.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
2Fe+3 + 2 CH2SHCOOH

Fe+2 + HOOCCH2-S-CH2COOH + 2H+
Procedure:
Test sample
Sample is dissolved in specific amount of
water and then volume is made up to 40 ml.
Add 2 ml of 20 % w/v of citric acid (iron
free)
Add 2 drops of thioglycollic acid
Add ammonia to make the solution alkaline
and adjust the volume to 50 ml
Keep aside for 5 min
Colour developed is viewed vertically and
compared with standard solution
Standard compound
2 ml of standard solution of iron diluted with
water up to 40 ml.
Add 2 ml of 20 % w/v of citric acid (iron
free)
Add 2 drops of thioglycollic acid
Add ammonia to make the solution alkaline
and adjust the volume to 50 ml
Keep aside for 5 min
Colour 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.
The purple colour produce in sample solution should not be greater than standard solution. If
purple colour produces in sample solution is less than the standard solution, the sample will
pass the limit test of iron and vice versa.
LIMIT TEST FOR ARSENIC
Arsenic is a well-known undesirable and harmful impurity which is present in medicinal
substances. All pharmacopoeias prescribe a limit test for it. Pharmacopoeial method is based
on the Gutzeit test. All the special reagents used in the limit test for Arsenic are marked and
distinguished by letter ‘As T’, which means that they all should be Arsenic free and should
themselves confirm to the test for Arsenic.
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P’ceutical inorganic chemistry (BP104TP)
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 × 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 perforations 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.
During the test, the evolved gases have been passing through the side hole, the lower hole
serving as an exit for water which condenses in the constricted part of the tube. An important
feature has been the standardization of the area of Mercuric chloride paper which is exposed to
the reaction of arsine gas.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
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 (H3AsO4) in the sample is reduced to arsenious acid (H3AsO3)
by reducing agents like potassium iodide, stannous acid, zinc, hydrochloric acid, etc. When the
sample is dissolved in acid, the Arsenic present in the sample gets converted to Arsenic acid.
Arsenious acid is further reduced to arsine (gas) (AsH3) by nascent hydrogen (which is
produced by Zn and HCl) and reacts with mercuric chloride paper to give a yellow stain.
The depth of yellow stain on mercuric chloride paper will depend upon the quantity of arsenic
present in the sample. (Mercuric bromide paper can also be used as an alternative for mercuric
chloride paper.)
Substance
+
Dil. HCl
(Contains Arsenic impurity)
H3AsO4
+

H3AsO4
Arsenic acid
H2SnO2
Arsenic acid
 H3AsO3
+
H2SnO
Arsenious acid
H3AsO3
+
6[H]
Arsenious acid
nascent hydrogen
 AsH3
+
3H2O
Arsine gas
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 then some pentavalent Arsenic may remain unreacted.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
Procedure:
Test sample
Standard compound
The test solution is prepared by dissolving
specific amount in water and 10 ml of
stannated HCl AsT is kept in a wide
mouthed bottle.
5 ml of 1 M of potassium iodide
10 g of granulated zinc AsT is added. Keep
the solution aside for 40 min
A known quantity of dilute arsenic solution
in water and 10 ml of stannated HCl AsT is
kept in wide mouthed bottle.
5 ml of 1 M of potassium iodide
10 g of granulated zinc AsT is added. Keep
the solution aside for 40 min
Observation: Stain obtained on mercuric chloride paper is compared with standard solution.
Standard stain must be freshly prepared as it fades on keeping. If the stain produced by the test
is not deeper than the standard stain, then sample complies with the limit test for Arsenic.
Important points:
1) Lead acetate pledger or papers are used to trap any hydrogen sulphide which may be
evolved along with arsine.
2) Stannous chloride is essential for the complete evolution of arsine. Ain the arsenic test,
preference is given to stannous salts because they reduce arsenic to arsenious state and
sometime to metallic state whereas cadmium salts in themselves are not reducing
agents.
3) Care must be taken that the mercuric chloride paper remains quite dry during test.
4) The most suitable temperature for running the test is generally about 40°C.
5) The tube must be washed with hydrochloric acid AsT, rinsed with water and dried
between succeeding tests.
LIMIT TEST FOR HEAVY METALS
The limit test for heavy metals is used to control the heavy metal impurities present in the
substance.
Principle:
It is designed to determine the content of metallic impurities that are coloured by hydrogen
sulphide or sulphide ion 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.
Limit test of heavy metals is based on the reaction of metallic impurities with hydrogen
sulphide (in Method I & Method II) or sodium sulphide (in Method III) in acidic medium to
form brownish colour solution.
Dhruvi Prajapati
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P’ceutical inorganic chemistry (BP104TP)
These remain distributed in colloidal state, and give rise to a brownish coloration. The colour
produced in the test solution is compared with that of standard solution having definite amount
of Lead (heavy metal).
Heavy metal
+
H2S/Na2S

Heavy metals sulphide (brownish in colour)
Procedure: I.P limit for heavy metals in 20 ppm: The test solution is compared with a standard
prepared using a lead solution (as the heavy metal). IP has adopted 3 methods for this:
Method I
: The method is applicable for the samples which give clear colourless solution
under specified conditions of test.
Method II
: The method is applicable for the samples which DO NOT give clear
colourless solution under specified conditions of test.
Method III
: Used for substances which give clear colourless solutions in sodium
hydroxide medium.
Method I:
Test sample
Solution is prepared as per the monograph
and 25 ml is transferred in Nessler’s
cylinder
Adjust the pH between 3 to 4 by adding
dilute acetic acid ‘Sp’ or dilute ammonia
solution ‘Sp’
Dilute with water to 35 ml
Add freshly prepared 10 ml of hydrogen
sulphide solution
Dilute with water to 50 ml
Allow to stand for five minutes
View downwards over a white surface
Standard compound
Take 2 ml of standard lead solution and
dilute to 25 ml with water
Adjust the pH between 3 to 4 by adding
dilute acetic acid ‘Sp’ or dilute ammonia
solution ‘Sp’
Dilute with water to 35 ml
Add freshly prepared 10 ml of hydrogen
sulphide solution
Dilute with water to 50 ml
Allow to stand for five minutes
View downwards over a white surface
Method II:
Test sample
Weigh specific quantity of test substance,
moisten with H2SO4 and ignite on a low
flame till completely charred. Add few
drops of HNO3 and heat to 500 °C. Allow
to cool & add 4 ml of HCl and evaporate
to dryness. Moisten the residue with 10
ml of HCl and digest for 2 min.
Neutralize with ammonia solution and
make acid with acetic acid.
Dhruvi Prajapati
Standard compound
Take 2 ml of standard lead solution and
dilute to 25 ml with water
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P’ceutical inorganic chemistry (BP104TP)
Adjust the pH between 3 to 4 and filter if
necessary
Dilute with water to 35 ml
Add freshly prepared 10 ml of H2S
solution
Dilute with water to 50 ml
Allow to stand for five minutes
View downwards over a white surface
Adjust the pH between 3 or 4 by adding
dilute acetic acid ‘Sp’ or dilute ammonia
solution ‘Sp’.
Dilute with water to 35 ml
Add freshly prepared 10 ml of H2S
solution
Dilute with water to 50 ml
Allow to stand for five minutes
View downwards over a white surface
Method III:
Test sample
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 drops of sodium sulphide solution
Mix and set aside for 5 min
View downwards over a white surface
Standard compound
Take 2 ml of standard lead solution
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
Mix and set aside for 5 min
View downwards over a white surface
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 thiocarbazone (dithizone) in
alkaline solution to form lead dithizone complex, which is red in colour. Dithizone in
chloroform, is able to extract lead from alkaline aqueous solutions as a lead dithizone complex
(Red in colour)
The original dithizone is having a green colour in chloroform while the lead - dithizone is
having a violet colour. So, resulting colour at the end of the process is red. The intensity of the
colour of complex is dependent upon the amount of lead in the solution.
The colour of the lead - dithizone complex in chloroform has been compared with a standard
volume of lead solution, treated in the same manner.
In this method, the lead present as an impurity in the substances, gets separated by extracting
an alkaline solution with a dithizone extraction solution. The interference and influence of the
other metal ions has been eliminated by adjusting the optimum pH for the extraction by
employing Ammonium citrate/ potassium cyanide.
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P’ceutical inorganic chemistry (BP104TP)
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 colour 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.
Procedure:
Test solution
Standard solution
A known quantity of sample solution is A standard lead solution is prepared
transferred in a separating funnel
equivalent to the amount of lead permitted in
the sample under examination
Add 6 ml of ammonium citrate
Add 6 ml 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
Add 2 drops of phenol red
Make solution alkaline by adding ammonia
solution.
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
To the acid solution add 5 ml of standard
dithizone solution
Add 4 ml of ammonium cyanide
Shake for 30 mins and observe the color
hydroxylamine hydrochloride
Add 2 drops of phenol red
Make solution alkaline by adding ammonia
solution.
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
To the acid solution add 5 ml of standard
dithizone solution
Add 4 ml of ammonium cyanide
Shake for 30 mins and observe the color
Observation: The intensity of the colour of complex, is depends on the amount of lead in the
solution. The colour produced in sample solution should not be greater than standard solution.
If colour produces in sample solution is less than the standard solution, the sample will pass
the limit test of lead and vice versa.
During the preparation of a sample solution, an appropriate preliminary treatment is given, so
as to get lead in the solution, without any interfering substance or ion. All reagents used under
the test (except for standard lead solution), must be free from lead, and have been designated
as PbT reagents in pharmacopoeias.
Important points:
1) All reagents and solutions used in the test must be free from lead.
2) The reagents are used in the test to buffer the solution of the sample to the optimum pH
for extraction of lead dithizonate and disallow interference with other metals which
produce similar colours with dithizone.
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P’ceutical inorganic chemistry (BP104TP)
Comments:
Q. 1) Why do pharmacopoeia not prescribe limit tests for chloride, sulphate and iron ?
Ans.: Pharmacopoeia does not prescribe any numerical value of limit test for chlorides,
sulphate and iron because limit test is based on the simple comparison of opalescence or colour
between the test and standard solution prescribed according to pharmacopoeia. The variation
in the permissible limits for the various substances are obtained by taking varying quantities of
the substances under test.
In this type of limit test, the extent of turbidity or opalescence or colour produced in influenced
by the presence of other impurities present in the substance and also by variation in time and
method of performance of test. Thus, the pharmacopoeia does not prescribe any numerical
value of the limit test.
Q.2) Why nitric acid but not hydrochloric acid is used in limit test for chloride ?
Ans.: As in limit test for chloride the pass or fail of the sample is justified on the basis of white
opalescence that is produced as a result of reaction between soluble chloride present in the
sample and silver nitrate reagent which ultimately form silver chloride white precipitates.
Silver chloride precipitates are much more insoluble in presence of nitric acid so that
observable and comparable turbidity is produced.
Another role of nitric acid is to prevent the precipitation of silver as its carbonate and phosphate
(carbonate and phosphate impurities might also present in substance and interfere in final
comparison). While hydrochloric acid itself contain chloride and definitely it gives same
reaction with silver nitrate to produce silver chloride and hence wrong result may be predicted.
“Successful and unsuccessful people do not vary greatly in their abilities. They vary in their
desires to reach their potential.”
– John Maxwell
Dhruvi Prajapati
Page 14