Heavy Metals Testing

Heavy Metals:

Definition:

In general, metallic type of impurities are detected by standard procedure of inorganic qualitative analysis which involve colour and precipitation reaction.

It detects elements with insoluble sulfides [lead (Pb), mercury (Hg), bismuth (Bi), arsenic (As), antimony (Sb), tin (Sn), cadmium (Cd), silver (Ag), copper (Cu), molybdenum (Mo)], it does not identify which is element is present.

This test is based on principle that traces of lead salts if present are converted to lead sulphide by the addition of Na2S to a slightly alkaline solution buffered by a high concentration of ammonium acetate. The brown colour obtained due to the presence of colloidal PbS in the sample solution is compared with that obtained from a known amount of lead.

Pb²⁺  + H₂S  →  PbS¯  + 2H⁺

Reagents:

Acetate Buffer pH 3.5
1 N Acetic Acid
6 M Ammonia Solution
Lead Standard Solution
Thioactamide reagent or hydrogen sulphide solution
Nitric acid
Hydrochloric Acid
Sulphuric Acid

Instrument:

• Analytical Weighing Balance
• Muffle Furnace

Preparations:

Lead Nitrate Stock Solution

Dissolve 159.8 mg of lead nitrate in 100 mL of water to which has been added 1 mL of nitric acid, then dilute with water to 1000 mL. Prepare and store this solution in glass containers free from soluble lead salts.

Standard Lead Solution

On the day of use, dilute 10.0 mL of Lead Nitrate Stock Solution with water to 100.0 mL. Each mL of Standard Lead Solution contains the equivalent of 10 µg of lead. A comparison solution prepared on the basis of 100 µL of Standard Lead Solution per g of substance being tested contains the equivalent of 1 part of lead per million parts of substance being tested.

pH 3.5 Acetate Buffer

Dissolve 25.0 g of ammonium acetate in 25 mL of water, and add 38.0 mL of 6 N hydrochloric acid. Adjust, if necessary, with 6 N ammonium hydroxide or 6 N hydrochloric acid to a pH of 3.5, dilute with water to 100 mL, and mix.

Standard Preparation

Into a 50-mL color-comparison tube pipet 2 mL of Standard Lead Solution (20 µg of Pb), and dilute with water to 25 mL. Adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, using short-range pH indicator paper as external indicator, dilute with water to 40 mL, and mix.

Test preparation

Transfer accurately weighed test sample into 50 ml of Nessler’s cylinder. Dissolve in and dilute to 25 ml with water.
Adjust the pH of this solution with 1N Acetic acid or 6M Ammonia solution to a pH between 3.0 & 4.0 using short range pH indicator paper as external indicator.
Dilute with water to 40 ml and mix.

Procedure:

Dilute with water to 40 ml and mix. Transfer accurately weighed test sample into 50 ml of Nessler’s cylinder. Dissolve and dilute to 25 ml with water & add 2 ml of 20 ppm standard lead solution.

Adjust the pH of this solution with 1N Acetic acid or 6M Ammonia solution to a pH between 3.0 & 4.0 using short range pH indicator paper as external indicator.

Monitor preparation:

To each of the 3 Nessler’s cylinders containing the standard preparation, test preparation and monitor preparation, add 2 ml of Acetate buffer of pH 3.5, then add 1.2 ml of Thioacetamide reagent, dilute with water to 50 ml and mix. The colour of the solution from the test preparation is not darker than that of the solution from the standard preparation and the intensity of the colour of the monitor preparation is equal to or greater than that of the standard preparation.

Precautions:

Use glass cylinders of the same diameter and matched as closely as practicable in other respects. Use the same quantities of the same reagents for both the solution under test and the control solution. Results should be observed by viewing downwards over a white surface. Allow to stand for 2 mins and view downward over a white surface.

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