Limit test of Heavy metals IP Principle, Procedure

Principle:
The limit test of heavy metals is based on the reaction of metallic impurities with hydrogen sulfide in an acidic medium to form a brownish colour solution. Metals that respond 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 the substance which gives a clear colourless solution under the specific condition.

Test sampleStandard compoundSolution is prepared as per the monograph and 25 ml is transferred in Nessler’s cylinderTake 2 ml of standard lead solution and dilute to 25 ml with waterAdjust 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 mlDilute with water to 35 mlAdd freshly prepared 10 ml of hydrogen sulphide solutionAdd freshly prepared 10 ml of hydrogen sulphide solutionDilute with water to 50 mlDilute with water to 50 mlAllow to stand for five minutesAllow to stand for five minutesView downwards over a white surfaceView 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. ^[7-9]

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 standing for five minutes	Allow standing for five minutes
View downwards over a white surface	View downwards over a white surface

Observation:

The colour produced in the sample solution should not be greater than the standard solution. If the colour produces in the 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 a clear colourless 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 colour produced in the sample solution should not be greater than the standard solution. If colour produces in the sample solution is less than the standard solution, the sample will pass the limit test of heavy metals and vice versa

Limit test of Heavy metals IP Principle, Procedure