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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 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 Nessler’s 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 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.

Method II: 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 with sulphuric acid and ignite on a low flame till 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 		Take 2 ml of standard lead solution and dilute to 25 ml with water
Adjust the pH between 3 to 4 and filter if necessary 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 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.

Method III: 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 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 Take 2 ml of standard lead 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.
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