SOP: Determination of Ammonia Nitrogen in Water
GIFSQ — Global Institute for Food Safety & Quality
Principle
When a sample of water is made alkaline and distilled, the ammonia nitrogen present in the sample passes over in the distillate. The ammonia nitrogen in the distillate may be estimated by nesslerization (Method I) or the distillate may be collected in boric acid and the ammonia nitrogen determined by titration (Method II).
Apparatus required
- Distillation apparatus: 1-litre flask with provision for steam inlet and a splash bulb condenser
- Nessler cylinders marked to 50 ml
Reagents
a. Ammonia-free water: to distilled water, add a few drops of bromine; allow to stand overnight and redistil; discard the first 100 ml of the distillate and then collect.
b. Phosphate buffer, pH 7.4: dissolve 14.3 g of anhydrous potassium dihydrogen phosphate (KH₂PO₄) and 68.8 g of anhydrous dipotassium monohydrogen phosphate (K₂HPO₄) in ammonia-free water and dilute to 1 litre.
c. 0.02 N H₂SO₄.
d. Boric acid solution: dissolve 20 g of anhydrous boric acid (H₃BO₃) in ammonia-free water and make up to 1000 ml.
e. Standard ammonium chloride solution: dissolve 3.819 g of anhydrous ammonium chloride (NH₄Cl) in ammonia-free water and dilute to 1000 ml; dilute 10 ml of this solution to 1000 ml with ammonia-free water (1 ml = 0.01 mg of nitrogen = 0.0122 mg of NH₃).
f. Mixed indicator: dissolve 0.2 g of methyl red and 0.1 g of methylene blue in 95% alcohol, and dilute to 300 ml with 95% alcohol.
g. Nessler reagent: dissolve 100 g of mercuric iodide (HgI₂) and 70 g of potassium iodide (KI) in a small quantity of ammonia-free water; dissolve 160 g of NaOH in 500 ml of ammonia-free water and cool; to this solution add the KI and HgI₂ solution slowly with stirring, and dilute to 1000 ml. Store in an amber-coloured bottle.
Procedure
Blank distillation: set up the distillation apparatus using a 1-litre flask with provision for steam inlet and a splash bulb condenser; measure 500 ml of distilled water into the flask; add a few glass beads and 10 ml of phosphate buffer; pass steam and test the distillate for the presence of ammonia using Nessler reagent; continue distillation until the distillate is free from ammonia
↓
Empty the distillation flask; measure 500 ml of the sample; adjust the pH, if necessary, to 7 with dilute acid or alkali; add 10 ml of phosphate buffer; if the sample contains residual chlorine, dechlorinate with a trace of sodium thiosulphate (Na₂S₂O₃); if the sample contains H₂S, add a small amount of lead carbonate (PbCO₃) or zinc acetate [Zn(C₂H₃O₂)₂·2H₂O] to prevent the H₂S coming over in the distillate
↓
Transfer to the distillation flask; distil at the rate of 6 to 10 ml per min, collecting the distillate in Nessler cylinders marked to 50 ml
↓
Method I — Nesslerization: when the first 50 ml has distilled over, add 2 ml of Nessler solution and mix; after 10 min, compare with the standard (if the concentration of ammonia nitrogen is very low, allow a reaction time of 30 min)
↓
In the meanwhile, continue distillation and nesslerize subsequent 50-ml portions of the distillate until the distillate is free from ammonia; if the first 50 ml contains not more than 0.02 to 0.03 mg of ammonia, the second 50 ml is usually ammonia-free, but collect and nesslerize a second and third 50 ml of the distillate
↓
Alternatively, distil until almost 200 ml of the distillate are collected in a 200-ml volumetric flask; make up the distillate to 200 ml with ammonia-free water; take 50 ml of the distillate or an aliquot portion and dilute to 50 ml with ammonia-free water; nesslerize as before and compare with the standards
↓
Do not discard the residual sample after distillation; it may be used for determining albuminoid nitrogen. Carry out a reagent blank similarly, using ammonia-free distilled water and the same aliquot of buffer as used with the sample.
↓
Standards: pipette aliquots of the ammonium chloride solution from 0 to 6 ml, dilute to 50 ml with ammonia-free water, and nesslerize as before; keep the experimental conditions with regard to temperature and reaction time uniform for the blank, the sample, and the standard; compare the colour either visually or in a colorimeter at 400 to 425 nm, setting the instrument to 100% transmittance with the blank; plot optical density as ordinate and concentration of ammonia as abscissa
↓
Method II — Boric acid method: if the ammonia content in the first 50 ml of the distillate is more than 0.05 mg, it is more accurate to estimate ammonia nitrogen by absorbing the ammonia in the distillate in boric acid solution; collect the distillate in 50 ml of boric acid solution (each milligram of additional ammonia nitrogen requires the use of an added increment of 50 ml of boric acid solution); after distillation, rinse the tip of the condenser and make up to a known volume; determine the ammonia in an aliquot by titration with 0.02 N H₂SO₄, using the mixed indicator; the end point is indicated by a pale lavender colour
Calculation
Method I — Nesslerization (500 ml of sample taken):
mg/litre of ammonia = (mg of ammonia nitrogen × 1000) / 500
Where,
- mg of ammonia nitrogen = total ammonia nitrogen in the sample, from the sum of the ammonia found in each separate portion of the distillate
For the 200-ml distillate version:
mg/litre of ammonia = (mg of ammonia nitrogen in the aliquot × 200 × 1000) / (ml of aliquot × ml of sample taken)
Note: the printed layout of this second expression is partially garbled; it is carried here in the standard form (the “× 200” is the made-up volume of distillate, the “500” divisor is the 500-ml sample) and flagged.
Method II — Boric acid titration:
mg/litre of NH₃ = ((ml of H₂SO₄ for sample − ml of H₂SO₄ for blank) × Normality of H₂SO₄ × 17 × Volume of distillate × 1000) / (ml of distillate × ml of sample taken for distillation)
Where,
- ml of H₂SO₄ for sample and for blank = titration volumes of 0.02 N H₂SO₄
- Normality of H₂SO₄ = normality of the standard sulphuric acid
- 17 = equivalent weight of ammonia (NH₃)
- Volume of distillate = made-up volume of the distillate collected in boric acid
- ml of distillate = volume of the distillate aliquot taken for titration
- ml of sample taken for distillation = volume of the water sample distilled
Notes: to calculate as mg/litre of ammonia nitrogen, use 14 instead of 17, as the book states. The printed formula layout is partially garbled (the OCR reads “viewasbanin”); the ×17 and ×1000 factors are the standard titration equivalents required by the book’s own “14 instead of 17” instruction — without them the expression would compute milliequivalents per litre, not mg/litre. Carried here in the corrected standard form and flagged.
References
- Nichols, M.S. & M.E. Foote, Ind. Eng. Chem., Anal. Edn., 3, 311 (1931).
- Sawyer, C.N., Anal. Chem., 25, 816 (1953).
- Standard Methods for the Examination of Water and Wastewater, 13th edn., American Public Health Association, Washington D.C., p. 222 (1971).