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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  Nitrate-Nitrogen

Nitrate-Nitrogen

Collection

Collect 100ml of water (filtered using Whatman GF/F) in an acid washed polythene bottle and refrigerate.

Cadmium Reduction

This procedure is for soluble nitrate using filtered water samples (GF/F). Samples should be diluted to an approximate concentration of 500µg/L NO3-N. (test with nitrate strips).

Principal

Nitrate is reduced to nitrite on exposure to cadmium chips. The nitrite is then combined with reagents to form a red/pink azo dye which can be measured spectrophotometrically.

Reagents

For Cadmium reduction only...

  • Spongy cadmium: cadmium granules can be purchased as 0.3-1.5 mm chips. Otherwise place zinc rods in a 20% w/v solution of cadmium sulphate overnight. Scrape off the cadmium and divide into small particles with a spatula. The cadmium is then washed with 2% HCl followed by thorough rinsing in distilled water. The cadmium is then stored under water in a well sealed vessel. Re-wash with acid 10 min. before use and then with distilled water immediately before use [WARNING - Cadmium is highly toxic]
  • Ammonium chloride: dissolve 2.6g in 100ml of water.
  • Borax (Disodium tetraborate) Na2B4O7.10H2O: dissolve 2.1g in 100ml of water.
  • Sulphanilamide: dissolve 1.00g in 100ml of 10% v/v HCl (i.e. 10ml conc. HCl added to 100ml of water).
  • N-1-naphthylethylene diamide dihydrochloride: dissolve 1.00g in 1 litre of water
  • Hydrochloric acid, 2% v/v: add 2ml conc. HCl to 100ml of water.
  • Standard nitrate solution: dissolve 7.218g of potassium nitrate, KNO3 in water and make up to 1 litre.

1.0ml = 1.00mg NO3-N

This solution can be preserved with 2ml of chloroform and will keep for 6 months. For the working solution dilute 1.0ml of the stock solution to 1000ml with deionised water.

1.0ml = 1.0mg/L NO3-N

General procedure

The calibration curve should be prepared simultaneously with the samples.

Calibration

From the working stock solution prepare a range of dilutions, make up each standard to 10ml with deionised water.

  • 10ml of deionised water as a laboratory blank
  • 0.50ml = in 10ml H2O = 0.05mg/L
  • 1.00ml = in 10ml H2O = 0.10mg/L
  • 2.00ml = in 10ml H2O = 0.20mg/L
  • 3.00ml = in 10ml H2O = 0.30mg/L
  • 4.00ml = in 10ml H2O = 0.40mg/L
  • 5.00ml = in 10ml H2O = 0.50mg/L
  • 7.50ml = in 10ml H2O = 0.75mg/L
  • 10.0ml = in 10ml H2O = 1.00mg/L

Samples

  1. Pipette 10ml of each sample (and the above calibration dilutions + blanks) into 30ml sterilin universal containers.
  2. To each sample add 3.0ml of ammonium chloride solution (b).
  3. To each sample add 1.0ml of borax solution (c).
  4. To each sample add 0.5-0.6g of spongy cadmium (a). N.B. extreme care should be taken at all times when handling cadmium.
  5. Screw on the caps and shake on a mechanical shaker for 20 minutes (timing is critical).
  6. Transfer 1.4ml of each sample to a 10ml test tube and add 0.2ml of sulphanilamide reagent (d) and swirl to mix.
  7. After 4-6 minutes add 0.2ml of N-1-naphthylethylene diamide dihydrochloride solution (e) and swirl to mix.
  8. Make up to 10ml with deionised water.
  9. After 10 -120 min. measure the absorbance on a spectrophotometer at 543nm in a 1cm cell using the deionised water sample as a blank.
  10. From the calibration samples, plot absorbance against concentration and determine, from the nearest fit line, the concentration (0-1mg/L) per unit absorbance.
  11. For the water samples multiply the absorbance by the concentration per unit absorbance (determined in step 10) to give nitrate concentration. If sample was diluted scale up by dilution factor.

Cadmium Recovery

Due to the toxicity of cadmium great care should be taken to retain all trace of the metal. Any solutions which have come in contact with cadmium should be kept for safe disposal. The spongy cadmium used can be recovered by filtration and then washed with 2% (v/v) HCl, followed by thorough rinsing in distilled water. Storage should be in a clearly labelled sealed vessel.