A Complete Civil Engineering Study Solutions: Measurement of Nitrates in Water

Measurement of Nitrates in Water

Aim: Measurement of Nitrates in Water

THEORY: Generally the ground water has high nitrate concentration because of the percolating
sewage, industrial waste, chemical fertilizers, leaches from solid waste landfills, septic tank effluents etc. Whatever may be the reason the high concentration of nitrate is harmful to human beings, particularly for infants. The low acidity in the infants’intestine permits the growth of nitrate reducing bacteria that converts the nitrate to nitrite that is then absorbed in the blood stream. The nitrite has a great affinity for hemoglobin than the oxygen and it replaces oxygen in the blood. The deficiency of oxygen causes suffocation. The colour of the skin of the infants becomes blue so it is termed as blue baby disease. The medical name is ‘mathemoglobinemia’. This disease is a fatal disease and it takes place when the concentration of nitrates is more than 45 ppm. So it is important to find the amount of
nitrate in drinking water though it is a difficult task and requires spectrophotometer also.

Apparatus
(i) Spectrophotometer with a range of 300 – 700 nm.
(ii) Nessler tubes capacity 100 ml

Reagents
(i) Standard silver sulphate
(ii) Phenol disulphonic acid
(iii) Ammonium hydroxide
(iv) Stock nitrate solution: Dry potassium nitrate (KNO3) in an oven at 105°C for 24 hours. Dissolve 0.1631 g in water and dilute to 1000 ml. 1.0 ml = 100 micro gram NO3- N. Preserve with 2 ml CHCl3/l
(v) Standard nitrate solution

Procedure
(i) Take 50 ml of filtered sample in a flask.
(ii) Add an equivalent amount of silver sulphate to remove chlorides. So chloride determination is done prior to the nitrates. 1 mg/lCl = 1 ml Ag2SO4 solution.
(iii) Slightly warm and filter the precipitated AgCl.
(iv) Evaporate the filtrate in a porcelain dish to dryness.
(v) Cool and dissolve the residue in 2 ml phenoldisulphonic acid and dilute to 50 ml.
(vi) Add 10 ml of liquid ammonia to develop a yellow colour. Actually nitrate reacts with disulphonic acid and produces a nitro- derivative that in alkaline medium produces a yellow colour..
(vii) Observe the colour developed at 410 nm with a light path of 1 cm.
(viii) Calculate the concentration of nitrate from the standard curve.
(ix) Prepare the standard curve using suitable aliquots of standard nitrate solution in
the range of 5 to 500 mg NO3 by following the above procedure.