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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  Ammonium Ions

Ammonium Ions

Ammonium ions react with phenol and hypochlorite in an alkaline solution to form indophenol blue: the reaction is catalysed by nitroprusside. The resulting absorbance is proportional to the concentration of ammonium ions present, and is measured spectrophotometrically at 640nm.


  1. Phenol solution: dissolve 20g phenol in 200ml of 95% ethanol
  2. Sodium Nitroprusside solution: dissolve 1g of sodium nitroprusside in 200ml of water [store solution in the dark]
  3. Alkaline solution : dissolve 100g tri-sodium citrate and 5g of Sodium hydroxide in 500ml of water
  4. Sodium Hypochlorite solution: take 600ml commercial sodium hypochlorite and dilute to 1 litre
  5. Ammonium Sulphate standard solution: dissolve 0.9433g of ammonium sulphate in water and make up to 1 litre (1ml = 200ugNH4-N)


Remember to set up your calibration curve at the same time as processing your samples

  1. Place 50ml of filtered water sample into a 100ml conical flask. Dilute samples likely to contain more than 400ug/L (effluent, anaerobic waters, water from ochreous dykes) appropriately - usually 1 in 5.
  2. Make up oxidising solution by mixing 4 parts solution C with 1 part solution D.
  3. Add 2ml of solution A to each sample and mix.
  4. Add 2ml of solution B to each sample and mix.
  5. Add 5ml of oxidising solution to each sample and mix.
  6. Store the samples in the dark . After 1.5-12 hours, measure the absorbance in a cm cell at 640nm against the standard blank.


Prepare a calibration curve using the following dilutions of the ammonia standard (E)

Take 1ml of solution E and dilute to 500ml. This is the working standard (X) (400 ug/L).

  • 400 ug/L - 50ml X
  • 280 ug/L - 35ml X made up to 50ml
  • 200 ug/L - 25ml X made up to 50ml
  • 120 ug/L - 15ml X made up to 50ml
  • 80 ug/L - 10ml X made up to 50ml
  • 0 ug/L - 50ml of water