Determination of Heavy Metals in Talc and Magnesium

These testing methods in the determination of heavy metals such as lead in talc determination Magnesium as MGO, and heavy metal analysis in water. The brown colour produced by the material with hydrogen sulphide solution is matched against that produced with a standard lead solution. 

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Apparatus: Nessler Cylinders - 50 ml capacity

Reagents of heavy metals:

Perchloric Acid - 60 per cent (m/m)

Hydrofluoric Acid - 40 per cent (m/m)

Citric Acid - Solid

Ammonium Hydroxide(Purity) - Dilute one volume of liquor ammonia (relative density 0.90) with 10 volumes of water.

Hydrogen Sulphide Solution - Saturated solution (freshly prepared)

Standard Lead Solution - Dissolve 1.600 g of lead nitrate in water and make up the solution to exactly 1000 ml. Pipette out 10 ml of the solution and dilute it again with water to 1000 ml. One millilitre of the final solution contains 0.01 mg of lead (Pb). The solution should be freshly prepared.

Testing Procedure of heavy metals

1. Place 2.0 g of the material, accurately weighed, in a platinum dish and incinerate for about 2 hours at 525 to 550°C.

2. Cool and treat with 5 ml of perchloric acid. Take to fumes on a hot plate with three successive portions of hydrofluoric acid.

3. Cool and dilute with water. Filter the solution, if necessary, with suction, through a fine-fritted glass filter catching the filtrate in a 500 ml glass-stoppered conical flask.

4. Transfer the solution to a Nessler cylinder. Add 10 g of citric acid and adjust to pH 3.0 to 3.4 by adding dilute ammonium hydroxide (yellow to purple colour with bromophenol blue indicator).

5. Add 10 ml of hydrogen sulphide solution and mix.

6. Carry out simultaneously a control test using 4 ml of standard lead solution and the same quantities of other reagents as used with the material.

7. Allow the cylinders to stand for 10 minutes and compare the colour in the two cylinders.

Note: The material shall be taken as having satisfied the requirement prescribed if the intensity of the colour produced in the test with the material is not greater than that produced in the control test.

Determination of Magnesium as MGO

On fusion with anhydrous sodium carbonate, magnesium silicate gets converted into magnesium oxide which is then titrated against a standard EDTA solution.


Concentrated Hydrochloric Acid

Strong Ammonia Solution

Anhydrous Sodium Carbonate

Standard EDTA solution - 0.05 M

Strong Ammonia - Dissolve 6.75 g of ammonium chloride in 74 ml of strong Ammonia solution and dilute to 100 ml with water.

Determination of heavy metals as lead in talc and Magnesium as MGO

Testing Procedure of Magnesium as MGO

1. Weigh accurately a clean, dry platinum crucible containing approximately about 0.2 g of anhydrous sodium carbonate.

2. Weigh-in accurately about 0.12 g of the Talc sample, followed by a further quantity of approximately 0.4 g of anhydrous sodium carbonate.

3. Fuse the mixture by heating the crucible with the lid over a flame for about 30 minutes.

4. Extract the fused mass with instalments of about 3-4 ml of concentrated Hydrochloric acid (crush the fused mass with a glass rod to facilitate the extraction) and transfer quantitatively to

150 ml beaker.

5. Wash the crucible and the lid with water and transfer them to the beaker.

6. Dissolve by stirring and gently crushing with the glass rod and boil, if required for complete digestion.

7. Cool and neutralize by slowly adding ammonia solution till a white precipitate appears.

8. Add a little excess ammonia. Cool and filter through a wet filter paper into a 500 ml conical flask. Give washings with distilled water.

9. Add 10-12 ml strong ammonia-ammonium chloride solution, a pinch of solo chrome black indicator, and titrate against standard 0.05 M EDTA solution to a blue endpoint.

CALCULATION of Magnesium as MgO

 V x 0.002015 x M x 100

Magnesium as MgO, % by mass = -------------W x 0.05


V = Volume of 0.05 M EDTA solution

M = Molarity of EDTA solution, and

W = Weight of talc taken for fusion in g


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