Limit test for heavy metals

Learn the limit test for heavy metals through 3 different methods as a laboratory experiment or practical.

Aim:

To perform the limit test for heavy metals of the given samples as per IP.

Reference: Indian Pharmacopoeia (IP) 1996, 2006.

Table of Contents: Principle Preparation of chemicals Procedure as per method I Procedure as per method II Procedure as per method III

Requirements:

Glasswares and miscellaneous: Nessler cylinder, measuring cylinder, pipette, dropper, rubber stopper, test tube stand, pH paper, balance, and glass rod, etc.

Chemicals: Lead nitrate (Pb(NO3)2), nitric acid (HNO3), acetic acid (CH3COOH), hydrogen sulphide solution, and ammonia solution (NH4OH), etc.

Principle of limit test for heavy metals:

The reaction of metallic impurities with hydrogen sulphide to produce a brownish solution in an acidic medium is the basis for the heavy metals limit test. Lead, arsenic, bismuth, copper, mercury, antimony, silver, tin, cadmium, and molybdenum are among the metals that respond to this test. Parts of lead per million parts of the substance are used to measure metallic impurities. The usual limit, according to the IP, is 20 ppm.

Preparation of chemicals:

Preparation of dilute acetic acid:

Take 05.70 ml of concentrated glacial acetic acid and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved and cool, make up the volume to 100 ml.

Preparation of dilute ammonia:

Take 42.50 ml of strong ammonia solution and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved and cool, make up the volume to 100 ml.

Preparations of 0.1% lead standard solution:

Take 0.40 gm of lead nitrate and dissolve in 150.00 ml of distilled water containing 2 ml of nitric acid in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume to 250.00 ml.

Preparations of 100 ppm lead standard solution:

Take 01 volume 0.1% lead standard solution and dilute to 10 volumes with distilled water.

Preparations of 20 ppm lead standard solution:

Take 01 volume 100 ppm lead standard solution and dilute to 05 volumes with distilled water.

The Indian Pharmacopoeia offers three methods for determining heavy metal limits.

Method I:

It refers to and is used for a substance that, under certain conditions, produces a clear, colorless solution.

Method II:

It refers to and is used for a substance that, under certain conditions, does not produce a clear, colorless solution.

Method III:

It refers to and is used for a substance that, under certain conditions, produces a clear, colorless solution in sodium hydroxide medium.

Procedure of limit test for heavy metals as per method I:

Standard:

• Take 02.00 ml of prepared standard lead solution using a pipette into a Nessler cylinder, dilute to 25.00 ml with water and label it as standard.
• Adjust the pH of this solution to between 03.00 and 04.00, using dilute acetic acid or dilute ammonia solution.
• Then, dilute it with water to 35.00 ml.
• Add 10.00 ml of freshly prepared hydrogen sulphide solution.
• Add sufficient water to make 50.00 ml and allow standing for five minutes.
• Observe this standard preparation under a white background or over a white surface.

Test:

• Take 25.00 ml of the prepared solution according to the monograph using a pipette into a Nessler cylinder and label it as a test.
• Adjust the pH of this solution to between 03.00 and 04.00, using dilute acetic acid or dilute ammonia solution.
• Then, dilute it with water to 35.00 ml.
• Add 10.00 ml of freshly prepared hydrogen sulphide solution.
• Add sufficient water to make 50.00 ml and allow standing for five minutes.
• Observe this test preparation under a white background or over a white surface.

Observations:

The test solution produces less or same or more opalescence than the standard solution.

Results:

The sample passes/fails the limit test as per Indian Pharmacopoeia.

Procedure of limit test for heavy metals as per method II:

Standard:

• Take 02.00 ml of prepared standard lead solution using a pipette into a Nessler cylinder, dilute to 25.00 ml with water and label it as standard.
• Adjust the pH of this solution to between 03.00 and 04.00, using dilute acetic acid or dilute ammonia solution.
• Then, dilute it with water to 35.00 ml.
• Add 10.00 ml of freshly prepared hydrogen sulphide solution.
• Add sufficient water to make 50.00 ml and allow standing for five minutes.
• Observe this standard preparation under a white background or over a white surface.

Test:

• Weigh a specific amount of test substance, moisten it with H2So4, then ignite it over a low flame until it is charred.
• Add few drops of nitric acid and heat to 500 °C.
• Allow cooling before adding 04.00 ml of HCL and evaporate to dryness.
• Moisten the residue with 10.00 ml of HCL and digest for 2 minutes.
• Neutralize it using ammonia solution and just acidified with acetic acid.
• Adjust the pH of this solution to between 03.00 and 04.00, and filter if necessary
• Then, dilute it with water to 35.00 ml.
• Add 10.00 ml of freshly prepared hydrogen sulphide solution.
• Add sufficient water to make 50.00 ml and allow standing for five minutes.
• Observe this test preparation under a white background or over a white surface.

Observations:

The test solution produces less or same or more opalescence than the standard solution.

Results:

The sample passes/fails the limit test as per Indian Pharmacopoeia.

Procedure of limit test for heavy metals as per method III:

Standard:

• Take 02.00 ml of prepared standard lead solution using a pipette into a Nessler cylinder and label it as standard.
• Add 05.00 ml of dilute NaOH solution and make up the volume to 50.00 ml with water.
• Add five drops of freshly prepared hydrogen sulphide solution and allow standing for five minutes.
• Observe this standard preparation under a white background or over a white surface.

Test:

• Take 25.00 ml of the prepared solution according to the monograph using a pipette into a Nessler cylinder and label it as a test. Or weigh a specific amount of test substance, dissolve it in 20 ml of water, then add 5 ml of dilute sodium hydroxide solution.
• Add sufficient water to make 50.00 ml and allow standing for five minutes.
• Observe this test preparation under a white background or over a white surface.

Observations:

The test solution produces less or same or more opalescence than the standard solution.

Results:

The sample passes/fails the limit test as per Indian Pharmacopoeia.