Limit test for Heavy Metals

Aim: To carry out the limit test for Heavy metal for the given sample.

Apparatus & Chemical Requirements: Nessler's cylinder, glass rod, pipette, hydrogen sulphide solution, sodium sulphide.

Principle:

The limit test for heavy metal is based upon the reaction between heavy metal ion and hydrogen sulphide solution( freshly prepared) or sodium sulphide to produce black or brown color precipitate of heavy metal sulphide at suitable pH around 3-4 which could be maintained by adding dilute acetic acid or ammonia..

The color produce by the solution is compared with that of standard color produced by specific amount of lead nitrate solution under the same reaction condition.

For the sample to pass the limit test for heavy metal the sample color should not be more than that of standard colour.

Reaction:

Procedure:

Method A-

This method is applied for those substances which gives clear colorless solution under the specified condition.

Standard Color	Sample color
Pipette out 2ml of lead nitrate solution in to a clean Nessler's cylinder and dilute to 25 ml with distilled water. The pH of the solution is adjusted between 3-4 by adding dilute acetic acid and ammonia. Make up up the volume to about 35ml by adding water. Add 10 ml freshly prepared hydrogen sulphide solution and make up the volume to 50ml with distilled water.	Dissolve the specified amount of sample with 25 ml distilled water in a clean Nessler's cylinder. The pH. of the solution is adjusted between 3-4 by adding dilute acetic acid and ammonia. Make up the volume to about 35 ml by adding water. Add 10 ml freshly prepared hydrogen sulphide solution and make up the volume to 50 ml with distilled water.

Reason: 

• The pH is maintained 3-4 because in this range the heavy sulphide precipitate is more stable.
• The hydrogen sulphide solution is freshly prepared because on keeping the hydrogen sulphide gas escape into atmosphere.

Method B-

This method is applied for those substances which does not give clear colorless solution under the specified condition as per method A.

The sample solution has to be made in a different way but the standard solution is same as method A.

Preparation of sample solution for method B

• Specified quantities of sample as per monograph are taken in a crucible.
• Moistened with Sulphuric acid and ignited until charred.
• A few drops of nitric acid is added and the mixture is heated about 500 degree celcius.
• It is then allowed to cool and the residue is digested with  10 ml of Dil. HCl for 2-3min.
• The excess acid is neutralised by NH3 solution and dilute with water and filtered.
• 35ml of the above solution is taken in a Nessler's cylinders. Add 10 ml of freshly prepared H2S solution and make up the volume with water up to 50 ml.

Reason:

Heating the sample with acids is done to remove non-metallic substances which will interfere with the limit test

Method C - 

It is for those substances which gives clean colorless solution in NaOH

Standard Color	Sample color
Pipette out 2ml of lead nitrate solution in to a clean Nessler's cylinder. The pH of the solution is adjusted between 3-4 by adding Add 5 ml 10% NaOH and 5 drops of Na2S solution and make up the volume to 50 ml with distilled water.	Dissolve the specified amount of sample with 25ml distilled water in a clean Nessler's cylinder. Add 5ml 10% NaOH and drops of Na2S solution and make up  the volume to 50 ml with distilled water.

Comparison:

Compare the sample color with standard color by viewing the Nessler's cylinder against white back ground.

Observation & Report: 

When viewed transversely against a dark background, the color produced in the sample is less/more than the standard Color. Hence the given sample passes/fails the limit test for Heavy Metals.