Principle:
The limit test for heavy metals relies on the reaction of metallic impurities with hydrogen sulfide in an acidic environment, resulting in the formation of a brownish-colored solution. Metals that react to this test include lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum. The concentration of metallic impurities in substances is typically expressed as parts per million of lead in the substance. As per the Indian Pharmacopoeia, the standard limit is 20 parts per million (ppm).
Procedure of limit test for Heavy metal
The Indian Pharmacopoeia has adopted three techniques to conduct limit test for heavy metals.
Method-I:
This method is use for the substance which produces clear colorless solution under the specific condition.
Sl. No. | Test sample | Standard sample |
1. | 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 |
2. | 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’ |
3. | Dilute with water to 35 ml | Dilute with water to 35 ml |
4. | Add freshly prepared 10 ml of hydrogen sulphide solution | Add freshly prepared 10 ml of hydrogen sulphide solution. |
5. | Dilute with water to 50 ml | Dilute with water to 50 ml |
6. | Allow to stand for five minutes | Allow to stand for five minutes |
7. | View downwards over a white surface | View downwards over a white surface |
Observation:
Intensity of color produce in sample solution should not be greater than standard solution. If intensity of color produces in sample solution is less than the standard solution, the sample will pass the limit test for heavy metal and vice versa.
Method-II:
This method is use for the substance which do not developed clear colorless solution under the specific condition.
Sl. No. | Test sample | Standard sample |
1. | 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 |
2. | 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’ |
3. | Dilute with water to 35 ml | Dilute with water to 35 ml |
4. | Add freshly prepared 10 ml of hydrogen sulphide solution | Add freshly prepared 10 ml of hydrogen sulphide solution |
5. | Dilute with water to 50 ml | Dilute with water to 50 ml |
6. | Allow to stand for five minutes | Allow to stand for five minutes |
7. | View downwards over a white surface | View downwards over a white surface |
Observation:
Intensity of color produce in sample solution should not be greater than standard solution. If intensity of color produces in sample solution is less than the standard solution, the sample will pass the limit test for heavy metal and vice versa.
Method-III:
This method is use for the substance which produces clear colorless solution in sodium hydroxide solution.
Sl. No. | Test sample | Standard sample |
1. | 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 |
2. | 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 |
3. | Add 5 drops of sodium sulphide solution | Add 5 drops of sodium sulphide solution |
4. | Mix and set aside for 5 min | Mix and set aside for 5 min |
5. | View downwards over a white surface | View downwards over a white surface |
Observation:
Intensity of color produce in sample solution should not be greater than standard solution. If intensity of color produces in sample solution is less than the standard solution, the sample will pass the limit test for heavy metal and vice versa.
Also read: Limit test for lead
Also read: Limit test for sulphate
Also read: Limit test for Iron
Also read: Limit test for arsenic
Also read: Limit test for chloride