Title: Limit Test for Heavy Metals
- Objective: The limit test for heavy metals is provided to demonstrate that the content of metallic impurities that are coloured by hydrogen sulphide does not exceed the heavy metals limit given in the individual monograph in terms of micrograms of lead per gram of the test substance.
- Principle: Limit test of heavy metals is based on the reaction of metallic impurities with hydrogen sulphide in acidic medium to form colored solution. Metals that response to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum.
3. Procedure:
- Preparation of Standard Solutions:
Note: All solutions should be prepared in Distilled/Purified Water.
- Lead Standard Solution (1% Pb): Dissolve 0.4 g of lead nitrate in water containing 2 ml conc. nitric acid and add sufficient water to produce 250.0 ml.
- Lead Standard Solution (100 ppm Pb): Dilute 1 volume of lead standard solution (1% Pb) to 10 volumes with water immediately before use.
- Lead Standard Solution (20 ppm Pb): Dilute 2 volume of lead standard solution (100 ppm Pb) to 10 volumes with water immediately before use.
- Lead Standard Solution (10 ppm Pb): Dilute 1 volume of lead standard solution (100 ppm Pb) to 10 volumes with water immediately before use.
- Lead Standard Solution (2 ppm Pb): Dilute 1 volume of lead standard solution (10 ppm Pb) to 5 volumes with water immediately before use.
- Lead Standard Solution (1 ppm Pb): Dilute 1 volume of lead standard solution (10 ppm Pb) to 10 volumes with water immediately before use.
- Thioacetamide Reagent: Add 1 ml of a mixture of 15 ml of 1M sodium hydroxide, 5 ml of water and 20 ml of glycerol (85%) to 0.2 ml of thioacetamide solution, heat in a water bath for 20 seconds, cool and use immediately.
- Thioacetamide Solution (4% w/v): Dissolve 4g of Thioacetamide in water and dilute up to 100 ml with water.
- Hydrogen Sulphide Solution: A freshly prepared saturated solution of Hydrogen Sulphide in water.
- Acetate Buffer pH 3.5: Dissolve 25 g of ammonium acetate in 25 ml of water and add 38 ml of 7M Hydrochloric acid. Adjust the pH to 3.5 with either 2M Hydrochloric acid or 6M Ammonia and dilute to 100 ml with water.
- Dilute Ammonia Solution: Dilute 42.5 ml of strong ammonia solution to 100 ml with
- Dilute Sodium Hydroxide (5.0%w/v): Dissolve 5.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
- 1M sodium hydroxide: Dissolve 4.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
- 7M Hydrochloric acid: Dilute 59.5 ml of Conc. Hydrochloric acid to 100 ml with
- 2M Hydrochloric acid: Dilute 17 ml of Conc. Hydrochloric acid to 100 ml with
- 6M Ammonia: Dilute 45 ml of strong ammonia solution to 100 ml with
- Method:
The test consists of two consecutive operations: preparation of the test solution, and the colour development by reaction with hydrogen sulfide, followed by comparison of the colour obtained with that produced with standard lead solution.
Method A is generally applicable only when the amount of heavy metals in the weight of test substance used exceeds 5 μg.
- Method A:
Standard solution: Into a 50-ml Nessler cylinder pipette 1.0 ml of Lead Standard Solution (20 ppm Pb) and dilute with water to 25 ml. Adjust with dilute Acetic Acid or dilute Ammonia solution to a pH between 3.0 and 4.0, dilute with water to about 35 ml and mix.
Test solution: Into a 50-ml Nessler cylinder place 25 ml of the solution prepared for the test as directed in the individual monograph or dissolve the specified quantity of the substance being examined in sufficient water to produce 25 ml. Adjust with dilute acetic acid or dilute Ammonia solution to a pH between 3.0 and 4.0, dilute with water to about 35 ml and mix.
Procedure: To each of the cylinders containing the Standard solution and Test solution respectively add 10 ml of freshly prepared Hydrogen Sulphide solution, mix, dilute to 50 ml with water, allow to stand for 5 minutes and view downwards over a white surface.
Limit: The colour produced with the test solution is not more intense than that produced with the Standard solution.
- Method B:
Standard solution: Proceed as directed under Method A.
Test solution: Weigh in a suitable crucible the quantity of the substance specified in the individual monograph, add sufficient sulphuric acid to wet the sample, ignite carefully at a low temperature until thoroughly charred. Add to the charred mass 2 ml of Conc. Nitric Acid and 5 drops of Conc. Sulphuric Acid and heat cautiously until white fumes are no longer evolved. Ignite, preferably in a muffle furnace, at 500° to 600°, until the carbon is completely burnt off. Cool, add 4 ml of Hydrochloric Acid, cover, digest on a water-bath for 15 minutes, uncover and slowly evaporate to dryness on a water-bath. Moisten the residue with 1 drop of Hydrochloric Acid, add 10 ml of hot water and digest for 2 minutes. Add Ammonia solution drop wise until the solution is just alkaline to litmus paper, dilute to 25 ml with water and adjust with dilute acetic acid to a pH between 3.0 and 4.0. Filter, if necessary, rinse the crucible and the filter with 10 ml of water, combine the filtrate and washings in a 50-ml Nessler cylinder, dilute with water to about 35 ml and mix.
Procedure: Proceed as directed under Method A.
Limit: The colour produced with the test solution is not more intense than that produced with the Standard solution.
- Method C:
Standard solution: Into a 50-ml Nessler cylinder pipette 1.0 ml of Lead Standard solution (20 ppm Pb), add 5 ml of dilute Sodium Hydroxide solution, dilute with water to 50 ml and mix.
Test solution: Into a 50-ml Nessler cylinder place 25 ml of the solution prepared for the test as directed in the individual monograph or dissolve the specified quantity of the substance being examined in a mixture of 20 ml of water and 5 ml of dilute Sodium Hydroxide solution. Dilute with water to 50 ml and mix.
Procedure: To each of the cylinders containing the Standard Solution and the Test Solution respectively add 5 drops of Sodium Sulphide solution, mix, allow to stand for 5 minutes and view downwards over a white surface.
Limit: The colour produced with the test solution is not more intense than that produced with the Standard Solution.
- Method D:
Standard Solution: Into a small Nessler cylinder pipette 10.0 ml of either Lead Standard Solution (1 ppm Pb) or Lead Standard Solution (2 ppm Pb).
Test solution: Prepare as directed in the individual monograph and pipette 12 ml into a small Nessler cylinder.
Procedure: To the cylinder containing the Standard Solution add 2.0 ml of the Test solution and mix. To each of the cylinders add 2 ml of Acetate Buffer pH 3.5, mix, add 1.2 ml of Thioacetamide reagent, allow to stand for 2 minutes and view downwards over a white surface.
Limit: The colour produced with the test solution is not more intense than that produced with the standard solution.