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.Â
 {tocify} $title={Table of Contents}
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.
Prepare 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.
REAGENTS OF MGO:
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.
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
Where,
V = Volume of 0.05 M EDTA solution
M = Molarity of EDTA solution, and
W = Weight of talc taken for fusion in g