Limit Test

 Limit Test

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.

Limit test is nothing but
to identify the impurities present in the substance and compare it with standard. 

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

Test = to examine or
to inv
estigate

Impurities = a foreign matter
present in a compound


Importance of Limit
tests

To find
out
the harmful amount of
impurit
ies

To find
out
the avoidable/unavoidable amount of impurities.


Limit test for Chlorides

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

Specific weight of compound is dissolved

 

in water or solution is prepared as
directed in the pharmacopoeia
and
transferred in Nessler
cylinder

Take 1ml of
0.05845 % W/V solution of sodium

 

chloride in Nessler cylinder

Add 1ml of nitric acid

Add 1ml 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

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.


Limit test for Sulphates


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 water or solution
is prepared as directed
in the pharmacopoeia and transferred in Nessler
cylinder

Take 1ml of
0.1089 % W/V solution of
potassium

 

sulphate in Nessler cylinder

Add 2ml of dilute hydrochloric
ac
id

Add 2ml of dilute hydrochloric
ac
id

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.

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
for
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 colour.

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

 

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 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

Earlier aamoniumthiocyanate 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 helps precipitation of iron by ammonia by forming
a complex with it.


Limit test for 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
three methods
for the limit test of heavy metals.

Method I: 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
25 ml is transferred in

 

Nesslers cylinder

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’

Adjust the pH between 3 to
4 by adding dilute

 

acetic acid ‘Sp’ or dilute ammonia solution ‘Sp’

Dilute with water to 35
ml

Dilute with water to 35
ml

Add freshly prepared
10 ml of hydrogen

 

sulphide
solution

Add freshly prepared
10 ml of hydrogen
sulphide

 

solution

Dilute with water to 50
ml

Dilute with water to 50 ml

Allow to stand for
f
ive minutes

Allow to stand for
f
ive 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. 

Limit test for Lead

 

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


Dithizone is green in color in chloroform and leaddithizone complex is violet in color, so the resulting color at the end of process is
red.

Procedure:

Test sample

Standard compound

A known quantity of
sample solution is

 

transferred
in a separating funnel

A standard lead solution is prepared equivalent 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 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

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.


Pharmaceutical Impurities and Limit Test Notes PDF


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