Limit test for heavy metals

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BACKGROUND

In this limit test, the heavy metals are lead, bismuth, antimony, tin, manganese etc but the heavy metals are expressed in terms of lead. The IP had prescribed the limit test for heavy metals for many drugs. The purpose of testing these heavy metals under one group is for their presence in pharmacopoeias. The reaction is based upon the reaction of the metal ion with hydrogen sulphide which gives a brownish colouration.¹

The test is compared against standard by parts per million of lead solution and the limit is 20 ppm. The IP prescribes three methods for the better understanding and determination of heavy metals in a solution.

Aim: Aim of this experiment is to carry out the limit tests for heavy metals of the given samples.

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REQUIREMENTS

Sample:     4gm Dextrose (C₆H₁₂O₆)

Apparatus: Analytical Balance,

Glass Rod,

Nessler’s cylinder

Chemicals: Dilute acetic acid solution,

Dilute ammonia solution,

Freshly prepared hydrogen sulphide solution,

Sulphuric acid,

Hydrochloric acid,

Standard lead solution,

Distilled water

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PROCEDURE

Preparation of standard solution

2ml of standard iron solution (prepared by adding 0.173g of ferric ammonium sulphate to 1.5ml of HCl and sufficient water were added to produce 1000ml of solution, each ml of which contains 0.02 mg of iron) was taken and dissolved in few ml of distilled water. To this 2ml of 20% of citric acid followed by 0.1ml of thioglycollic acid was added and the solution was made alkaline with iron free ammonia solution. Then the volume was made upto 50ml with distilled water.

Preparation of test solution

Accurately weighed quantity of test sample was taken and dissolved in few ml of distilled water, to this 2ml of 20% w/v citric acid was added. Then 0.1 ml of thioglycollic acid was added. The solution was made alkaline with iron free ammonia solution and volume was made upto 50 ml with distilled water.

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CONCLUSION

The colour intensity observed in the test solution was less than the standard solution. Hence the given samples had passed the limit test for iron and those are of pharmaceutical grade.

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REFERENCES

1. Chatwal GR. Pharmaceutical inorganic chemistry (vol- 1); 2016.
   
2. Indian pharmacopoeia; 2016.