Determination of Ammonia Nitrogen

Aim
To determine the ammonia nitrogen of the given sample of water.

Principle
Colorimetric method, using Nessler s reagent is sensitive to 20mg/L of ammonia N and may be used up to 5mg/L of ammonia N. Turbidity, colour and substances precipitated by hydroxyl ion interfere with the determination. The sample containing ammonia must be analysed immediately after collection; if not 0.8 M conc. H₂SO₄/L should be added to the sample stored at 4°C.

Direct Nesslerisation
Direct Nesslerisation is used only for purified water, natural water and highly purified effluents, which have low ammonia concentration. In samples that have been properly clarified by a pretreatment method using zinc sulphate and sodium hydroxide, it is possible to obtain a measure of the amount of ammonia N by treatment with Nessler's reagent, which is strongly alkaline solution of potassium mercuric iodide (K₂HgI₄). It combines with NH3 in alkaline solution to form a yellowish brown colloidal dispersion, whose intensity of colour is directly proportional to the amount of NH₃ present. The yellow colour or reddish brown colour typical of ammonia N can be measured in a spectrophotometer in the wavelength of 400 500 nm with a light path of 1cm.

Apparatus

1. Spectrophotometer, or Nessler tube tall form (50 mL or 100 mL capacity)
2. pH meter

Reagents
(click to check the preparation of reagents)

1. Zinc sulphate solution
2. EDTA reagent as stabiliser
3. Nessler s reagent
4. Stock ammonium solution 1.00 mL = 1.00mg

Procedure

1. Residual chlorine is removed by means of a dechlorinating agent (one or two drops sodium thiosulphate solution)
2. 100 mL ZnSO₄ solution is added to 100 mL sample and to it is added 0.5 mL of NaOH solution to obtain a pH of 10.5. This is mixed thoroughly.
3. The floc formed is allowed to settle and the clear supernatent is taken for Nesslerisation.
4. If the sample contain Ca or Mg, EDTA reagent is added to 50 mL of sample.
5. To this is added 2 mL of Nessler s reagent (proportional amount to be added (if the sample volume is less).
6. A blank using distilled ammonia free water is treated with Nessler s reagent as above. The absorbance is fixed as zero.
7. Then the sample is put in 1cm standard tubes of spectrophotometer and the absorbance noted at 400 - 500 nm wavelengths.
8. A calibration curve is prepared as follows:
   With 0, 0.2, 0.4, 0.7, 1.0, 1.4, 1.7, 2.0, 2.5, 3.0, 4.0, 5.0 mL of standard NH₄Cl solution in 50 mL distilled water standard diluted samples are prepared.
9. Each sample is Nesslerised as indicated earlier and the absorbance is noted down.
10. A graph with mg of NH₃ along x-axis and absorbance along y-axis is plotted and a straight-line graph is drawn.
11. From the absorbance of a solution of unknown concentration, the µg of NH₃ present can be read from the calibration curve.

Calculation

ammonia N in mg/L =	A
	mL of sample

where, A = µg N found colorimetrically

Observation
The observation is presented in Tables A and B respectively.

Table A: Observation for calibration

Table B:


Results