LIMIT TEST FOR IRON

1. LIMIT TEST FOR
IRON
BY
PROF. TAUFIK MULLA
ASSISTANT PROFESSOR
DEPARTMENT OF PHARMACEUTICS
SPBC COLLEGE OF PHARMACY.

2. LIMIT TEST FOR IRON
The iron Limit Test is designed to determine the allowable limit of iron contained in a sample.
• Principle:
• This test is based on the formation of purple colour by reaction of iron with thioglycolic acid to form
ferrous thioglycollate in the presence of ammonium citrate.
• Iron present as an impurity in the sample reacts with thioglycollic acid to form ferrous thioglycollate
which is responsible for the appearance of purple colour in the solution. This reaction will only occur
if the given sample contains iron as an impurity even in trace amount. The intensity of colour
produced by test solution is compared with the standard soluton obtained by taking standard iron
solution containing 0.04 mg of iron.

3. • Note:
• Citric acid reacts with ammonia solution giving ammonium citrate, which act as a buffer.
The citrate ion also prevents the precipitation of iron by ammonia by forming a complex
with it.
• Thioglycolic acid not only react with ferrous ions, but it also reduces ferric ions to ferrous
ions.

4. PROCEDURE
TEST SAMPLE STANDARD SAMPLE REASON
Dissolve 1gm of sample in 40 ml of
distilled water and transfer in
Nessler cylinder and marked it as ‘T’
Take 2ml of standard iron solution
in Nessler cylinder and marked it as
‘S’ and add 40 ml of distilled water
The aqueous solution will leach out
all iron present in sample
Add 2ml of 20%w/v iron free citric
acid
Add 2ml of 20%w/v iron free citric
acid
Citric acid prevents the
precipitation of iron with ammonia
and keeps iron in solution even in
presence of ammonia by forming
complex with iron as iron citrate.
Add 0.1 ml thioglycolic acid Add 0.1 ml thioglycolic acid To convert ferric ion to ferrous ion.
And further react with ferrous ion
and form ferrous thioglycolate
Add ammonia to make alkaline
solution
Add ammonia to make alkaline
solution
Ferrous thioglycolate give color
only in alkaline media
Make volume with distilled water to
50 ml
Make volume with distilled water to
50 ml
Equal volume of sample and
standard is needed to identify
intensity of color

5. CONCLUSION
• The purple color produced in the sample solution should not be greater than standard
solution.
• If purple color produced in the standard is more than that of test, the sample complies
with the limit test of sulphate as per I.P. 1996