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
Suggested readings:
Recommended readings:
- Pharmaceutics I Practical
- BIOSTATISTICS AND RESEARCH METHODOLOGY Theory
- Pharmaceutical Analysis I Theory
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