SOP: Determination of Nitrate Nitrogen in Water by the Phenoldisulphonic Acid Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
The reaction between nitrate and 1,2,4-phenoldisulphonic acid produces 6-nitro-1,2,4-phenoldisulphonic acid, which upon conversion to the alkaline salt yields a yellow colour. This colour is compared with standards similarly prepared.
Apparatus required
- Evaporating dish
- Nessler tubes
Reagents
a. 1,2,4-Phenoldisulphonic acid solution.
b. Standard nitrate solution (prepared as under the brucine method: 0.7216 g of anhydrous KNO₃ made to 1000 ml; working standard 1.0 ml = 0.01 mg of N).
c. Strong KOH solution.
d. 0.02 N H₂SO₄.
Procedure
Remove interfering substances before the determination: if nitrite is present in the sample in excess of 0.2 mg per litre as nitrogen, remove it by heating the sample for a few minutes with a few drops of H₂O₂, or add dilute KMnO₄ solution until a faint pink colouration persists (subtract the nitrogen equivalent of the nitrite thus oxidized from the final nitrogen reading); if the sample has a colour over 10, decolourize by adding 3 ml of aluminium hydroxide suspension to 150 ml of sample, stirring thoroughly, allowing to stand for a few minutes, filtering and discarding the first portion of the filtrate; add sufficient 0.02 N H₂SO₄ to the filtrate to neutralize the methyl orange alkalinity
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If the chloride is above 10 mg per litre, add 1 ml of glacial acetic acid and sufficient standard silver sulphate, Ag₂SO₄, solution to the sample to precipitate all but 0.1 mg of chloride (or 1 mg per litre); the acetic acid is used to prevent the absorption of the nitrate ions by the AgCl precipitate; filter or centrifuge, and evaporate the filtrate or the supernatant liquid to dryness
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Evaporate 100 ml of the (treated) sample to dryness in an evaporating dish
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Cool the residue and add 2 ml of phenoldisulphonic acid solution; rub thoroughly with a glass rod to dissolve the residue
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Dilute cautiously with 20 ml of distilled water and add KOH solution with stirring until the yellow colour is developed
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Transfer the solution to a Nessler tube, filtering if necessary, and make up to 50 ml with distilled water
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Prepare the standards: evaporate aliquots of the standard solution, add 2 ml of phenoldisulphonic acid solution and a volume of strong KOH solution equal to that used for the sample to each, and dilute to 50 ml in Nessler tubes; the standards may be kept for several days without deterioration
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Compare the colour of the sample with those of the standards
Calculation
mg/litre nitrate nitrogen = (mg of nitrate nitrogen × 1000) / (ml of sample taken)
Where,
- mg of nitrate nitrogen = nitrate nitrogen matched against the standards
- ml of sample taken = volume of the water sample taken
References
- Chamot, E.M., D.S. Pratt & H.W. Redfield, J. Am. Chem. Soc., 33, 366 (1911).
- Taras, M.J., Anal. Chem., 22, 1020 (1950).
- Standard Methods for the Examination of Water and Wastewater, 13th edn., American Public Health Association, Washington D.C., p. 234 (1971).