To determine the ammonia nitrogen of the given sample of water.
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 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.
- Spectrophotometer, or Nessler tube tall form (50 mL or 100 mL capacity)
- pH meter
Reagents (» click to check the preparation of reagents)
- Zinc sulphate solution
- EDTA reagent as stabiliser
- Nessler s reagent
- Stock ammonium solution 1.00 mL = 1.00mg
- Residual chlorine is removed by means of a dechlorinating agent (one or two drops sodium thiosulphate
- 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.
- The floc formed is allowed to settle and the clear supernatent is taken for Nesslerisation.
- If the sample contain Ca or Mg, EDTA reagent is added to 50 mL of sample.
- To this is added 2 mL of Nessler s reagent (proportional amount to be added (if the sample volume
- A blank using distilled ammonia free water is treated with Nessler s reagent as above. The absorbance
is fixed as zero.
- Then the sample is put in 1cm standard tubes of spectrophotometer and the absorbance noted at 400
500 nm wavelengths.
- 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.
- Each sample is Nesslerised as indicated earlier and the absorbance is noted down.
- A graph with mg of NH3 along x-axis and absorbance along y-axis is plotted and a straight-line graph
- From the absorbance of a solution of unknown concentration, the µg of NH3 present can be read from
the calibration curve.
|ammonia N in mg/L =
|mL of sample
where, A = µg N found colorimetrically
The observation is presented in Tables A and B respectively.
Table A: Observation for calibration