Detection of heavy metals

Qualitative estimation of heavy metals (Reinsch's Test)
Aim
To detect the presence of heavy metals in the given sample.
Principle
Copper displaces from the solution elements below it in the electrochemical series. The displaced element coats the copper giving a characteristic colour, which indicates a positive result.
Materials and chemicals
Beaker, conc. HCl, copper foil or wire, test tube and spirit lamp.
Test procedure
1. Place 10 g of tissue (finely minced) in a beaker or test tube.
2. Add 10 ml of 10% HCl.
3. Introduce copper wire into the test tube.
4. Boil for 10-15 min (never inhale fumes).
5. Remove the copper wire and place it on a filter paper.
6. Observe the colour of the copper wire.
Inference:
Black colouration - Arsenic or Bismuth
Shining silver deposit - Mercury
Dull white deposit - Silver
Dark colour with purple to blue-violet sheen - Antimony.
Depending on the colour of copper wire the confirmatory test is conducted.
If the deposit is black it can be confirmed whether it is due to arsenic or bismuth by placing the copper strip in 1-2 ml of 10 per cent potassium cyanide. If the deposit is due to arsenic, it will dissolve, but if it is due to bismuth or antimony, it will persist.
In mercury, colour of the deposit ranges from greyish (50 mg) to shiny silver (100 mg)
Confirmation test for mercury (Cuprous Iodide test)
Procedure
1. Dissolve 5 g of cupric sulphate and 3 g of ferrous sulphate in 10 ml of water.
2. Slowly add 7 g of KI in 50 ml of water while stirring. A precipitate is formed. Filter and wash the precipitate with water (by decantation) to remove the excess iodide. Mix 1 and 2. The cuprous iodide so formed is stored in a brown bottle in 10 ml of water and may be stirred and used in the form of suspension.
3. Place the copper wire used in Reinsch's test in few drops of cuprous iodide suspension. Cover with a watch glass and allow it to stand for about one hour. The copper wire turns to light pink.
Inference
Presence of mercury (30 μg) turns cuprous iodide to mercury iodide that is light pink; the intensity of colour is proportional to the concentration of mercury present.

Qualitative Test for Lead (Grunwald test)
Aim
To detect the presence of lead in the given sample.
Principle
The determination of lead in biological material is done by wet digestion. Nitric acid is used to digest the organic matter. Addition of potassium iodide imparts yellow colour due to formation of lead iodide.
Materials and chemicals
Conc. nitric acid, 10% potassium iodide solution, test tube and spirit lamp.
Procedure
1. Mince the liver/kidney piece or collect a small amount of scraping from stomach wall.
2. Add few drops of conc. nitric acid and heat gently till dry (never over heat the sample as it turns black, making reading difficult).
3. Add few drops of water and two drops of 10 % potassium iodide solution.
Inference
Development of yellow colour indicates the presence of lead.

Other methods for detection of lead
To the test solution/ sample add the following: -
With dilute sulphuric acid: Lead ions in dilute H2SO4 produce an insoluble white precipitate of PbSO4, which is insoluble in acids but soluble in hydroxides (alkalis).
Further, filter the precipitate and to the residue on filter paper add sodium sulphide solution, formation of black coloured lead sulphide is seen.
With potassium chromate: - To the test sample, add few drops of dilute potassium chromate solution. Lead chromate (yellow in colour) is formed in presence of lead. Lead chromate is insoluble in water, dilute acetic acid and ammonia, but soluble in dilute mineral acids and hydroxides.