SOP: Determination of Mercury by Flameless Atomic Absorption Spectroscopy
GIFSQ — Global Institute for Food Safety & Quality
Principle
The sample is digested with H₂SO₄, HNO₃ and HClO₄ (perchloric acid). The digest is treated with a reducing solution; mercury vapours are flushed into the gas flow-through cell of an atomic absorption spectrophotometer; light from a mercury hollow cathode lamp is passed through the cell, and the mercury content of the sample is determined at 253.7 nm.
Apparatus required
- Atomic absorption spectrophotometer equipped with mercury hollow cathode lamp and gas flow-through cell
- Digestion apparatus: 250-ml flat-bottom boiling flask with 24/40 standard joint; water condenser of 12–18 mm internal diameter and 400 mm length in borosilicate glass with 24/40 standard joint, modified to hold 6 mm Raschig rings, filled with Raschig rings to a height of 100 mm with a 20 mm layer of 4 mm glass beads on top
- Diaphragm pump with internal parts coated with acrylic paint and 16-gauge Teflon tubing for all connections
- Gas inlet adapter with 24/40 standard joint
- Variable transformer
Reagents
a. Reducing solution: mix 50 ml of H₂SO₄ with 300 ml of water; cool to room temperature and dissolve 15 g of NaCl, 15 g of hydroxylamine sulphate and 25 g of SnCl₂ in the solution; dilute to 500 ml
b. Diluting solution: to a 1000-ml volumetric flask containing 300–500 ml of water, add 58 ml of HNO₃ and 67 ml of H₂SO₄; dilute to volume with water
c. Magnesium perchlorate: placed as a drying agent in the filter flask and replaced as needed — caution: Mg(ClO₄)₂ is explosive in contact with organic substances
d. 2% sodium molybdate solution
e. Mercury standard solution: dissolve 0.1354 g of HgCl₂ in 100 ml of water (1 ml = 1000 μg); dilute 1.0 ml of this solution to 1 litre with 1 N H₂SO₄ before use (1 ml = 1 μg)
Sample preparation
Weigh 5 g of sample into the digestion flask; add 25 ml of 18 N H₂SO₄, 20 ml of 7 N HNO₃, 1 ml of 2% sodium molybdate solution and 5–6 boiling chips; connect the condenser to the flask
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Circulate water through the condenser; heat the flask gently for one hour; remove heat and allow to stand for 15 min
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Add 20 ml of HNO₃–HClO₄ (1+1) mixture through the condenser; stop the water circulation and boil vigorously until white fumes appear in the flask; continue heating for 10 min
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Cool the flask and cautiously add 10 ml of water through the condenser while swirling; again boil for 10 min; remove heat and wash the condenser with three 15-ml portions of water
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Cool the digest to room temperature and make up to 100 ml in a volumetric flask with water; transfer a 25-ml aliquot to another digestion flask and adjust the volume to about 100 ml with the diluting solution (reagent b).
Procedure
Adjust the pump output to about 2 litres of air per min by regulating the pump speed with the variable transformer; connect the apparatus as for the measurement, except for the gas inlet adapter
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With the pump working and the spectrophotometer set to zero, add 20 ml of reducing solution to the diluted aliquot; immediately connect the gas inlet adapter and aerate for about 3 min (adjust the aeration time to obtain maximum absorption)
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Record the absorption; disconnect the pressure on the out side of the pump and open the vent on the filter flask to flush the system
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Prepare a reagent blank and the standard curve: add 0.0, 0.02, 0.5, 1.0, 1.5 and 2.0 μg of mercury to a series of digestion flasks; to each add 100 ml of diluting solution; add the reducing solution and aerate the standards as for the sample and note the absorbance
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Plot absorbance against μg of Hg; determine the μg of Hg in the sample from the curve.
Calculation
Mercury, ppm = μg of Hg ÷ g of sample
Where,
- μg of Hg = mercury read from the calibration curve (μg)
- g of sample = weight of sample taken (g)
Precautions
- Magnesium perchlorate is explosive in contact with organic substances — replace as needed and keep away from organics
- Prepare the reagent blank through the same operations as the sample