SOP: Determination of Arsenic by the Gutziet Method (AOAC)
GIFSQ — Global Institute for Food Safety & Quality
Principle
Arsenic in the wet digest of the sample is reduced to arsine (arsene) by nascent hydrogen, which produces a stain on a strip of mercuric bromide paper; the length of the stain is proportional to the arsenic concentration and is estimated by comparison with the stains produced by standard arsenic solutions of different concentrations.
Apparatus required
- Gutziet generator and absorption tubes: 2-oz wide-mouthed bottle as generator; glass tube 1 cm in diameter and 6 to 7 cm long with a constricted end fitted to the stopper; a small wad of glass wool in the constricted bottom end; 3.5 to 4.0 g of prepared sand in the tube; narrow glass tube 2.6–2.7 mm internal diameter and 10 to 12 cm long for the mercuric bromide strip
Reagents
a. Stannous chloride solution: dissolve 40 g of arsenic-free SnCl₂·2H₂O in HCl and make up to 100 ml with HCl
b. Zinc: arsenic-free granulated zinc
c. Ammonium oxalate [(NH₄)₂C₂O₄·H₂O] — saturated solution
d. Potassium iodide (KI) solution: dissolve 15 g of KI in water and dilute to 100 ml
e. Sand: clean 30-mesh (through 30 mesh but not 40 mesh) white sea sand, prepared by washing successively with hot 10% NaOH solution, hot HNO₃ and hot water; dry the clean sand
f. Standard arsenic solution: dissolve 1 g of arsenic trioxide (As₂O₃) in 25 ml of 20% NaOH solution; saturate the solution with CO₂ and dilute to 1 litre with recently boiled and cooled water (1 ml = 1 mg of As₂O₃); dilute 40 ml of this solution to 1 litre, then make 50 ml of this diluted solution to 1 litre (1 ml = 0.002 mg of As₂O₃); use for preparing standard stains; prepare fresh dilute solutions at frequent intervals
g. Mercuric bromide (HgBr₂) paper: cut No. 40 Whatman filter paper into strips 2.5 mm wide and 12 cm long; soak the strips for 1 h in a fresh 5% solution of HgBr₂ in 95% alcohol; dry and store in amber bottles covered on the outside with black paper; use as soon as possible; handle the sheets by the paper attached to either end and cut in half just before use; strips must be clean and free of contamination
Sample preparation
Destroy the organic matter in the sample by wet digestion and make up the digest to a known volume with water.
Procedure
Set up the generator: place a small wad of glass wool in the constricted bottom end of the tube and add 3.5 to 4.0 g of the sand, taking care to have the same quantity in each tube; moisten the sand with 10% lead acetate solution and remove the excess by light suction; clean the sand when necessary without removing it from the tube by treatment with HNO₃, rinse with water under suction and treat with the lead acetate solution; if the sand has dried, clean and remoisten; connect the tube by a rubber stopper to the narrow glass tube; clean and dry the tube; introduce the clean end of the HgBr₂ paper strip into the narrow tube without curling, which causes uneven staining
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Determine the H₂SO₄ present in a definite volume of sample solution by titration; place aliquots (not exceeding 30 ml) containing 0.01 to 0.03 mg of As₂O₃ in the Gutziet generator (if the arsenic is outside these limits, repeat with a proper aliquot)
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Add sufficient 25% arsenic-free NaOH solution to exactly neutralize the H₂SO₄ present; then add 5 ml of HCl and cool if necessary
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Add 5 ml of potassium iodide reagent followed by 4 drops of the stannous chloride solution
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Prepare standards corresponding to 0.01, 0.02 and 0.03 mg of As₂O₃ from the standard arsenic solution; the standards must contain the same kind and quantities of acid as the samples — add 5 ml of HCl as in the sample; if the H₂SO₄ has been neutralized, add an equivalent quantity of arsenic-free Na₂SO₄ to the standards
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Mix and allow to stand for 30 min at not less than 25 °C, or 5 min at 90 °C; dilute with water to 40 ml
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Add to each of the standards and samples 2 to 5 g of granulated zinc (the same quantity must be added to each generator); if sheets of strips are used, prepare sample and standard strips from the same strip
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Keep at room temperature (20–25 °C) or immerse the apparatus to within 1 in. of the top of the narrow tube in a water bath (20–25 °C) and allow evolution to proceed for 1.5 h
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Remove the strips and note the average length of the stains on both sides of the paper in mm; plot the length of the stain in mm as ordinate against mg of As₂O₃ as abscissa; from the graph, note the quantity of arsenic in the sample (take smaller or larger aliquots when the stain is longer or shorter than the highest or lowest standard); report as ppm.
Run frequent blanks with reagents of suitable quality; the blank should not exceed 0.001 mg of As₂O₃.
Calculation
As₂O₃, mg/100 g = (mg of As₂O₃ of the sample as read from graph × Total volume of digest × 100) ÷ (ml of digest taken × Wt of sample)
As₂O₃, ppm = mg of As₂O₃ per 100 g × 10
Where,
- mg of As₂O₃ of the sample as read from graph = arsenic read from the stain-length graph (mg)
- Total volume of digest = volume to which the digest was made up (ml)
- ml of digest taken = aliquot taken into the generator (ml)
- Wt of sample = weight of sample taken (g)
References
- Official Methods of Analysis, Association of Official Analytical Chemists, 11th edn., p. 399 (1979). (Carried as printed; the 11th edition pairing with 1979 is flagged as a source inconsistency.)