Ammonium ions occur primarily in domestic waste waters, but
frequently also in industrial waste waters. In surface and ground waters ammonium ions may indicate decomposition of animal or vegetable substances. Control of the ammonium values is therefore important to the water supply. In a wastewater treatment works, the outfall may be tested for ammonia content before discharge to a watercourse. Ammonia concentrations in water vary according to pH strength and temperature, and is present in two forms: NH3 (ammonia or free ammonia) and NH4+(ammonium).
Assuming a stable temperature of 25 degrees C:
At pH 7.25 the ammonia in a sample will be almost all NH4+
At pH 9.25 the ammonia will be equally split between the two forms
At pH 11.25 the ammonia will be virtually all NH3
Ammonia monitoring may be done using various methods, including ion-selective and gas-sensing electrodes which give a millivolt response correlating to levels of ammonia.
Other methods include colorimetric devices and UV absorbance.
The colorimetric method basically uses a light intensity meter to measure colour intensity of the sample after a reagent has been added. The indophenol blue method is a commonly accepted reliable method of ammonia monitoring. There may be characteristics such as turbidity and colouration that could have an impact on Ammonia monitoring using this method.
Ion-selective electrodes and Gas sensitive electrodes have a membrane which can be permeated to create a change in the electrolyte proportional to the ammonia concentration, for accuracy, pH and temperature should be taken into consideration. In some analysers using Gas-sensitive electrodes, Sodium Hydroxide is dosed into the sample to raise the pH and convert all the NH to NH3. There may be characteristics such as chemical interference that could have an impact on Ammonia monitoring using this method. Potassium is a known interference with Ion-sensing (but not gas-sensing) electrodes.
A UV absorbance ammonia analyser generally has a light source on one side of the measurement chamber, and a spectrophotometer opposite to measure the absorbance in the range of 200-450nm. Reagents are added to treat the sample, and the analyser measures the difference in the light emitted from the light source and the light detected by the spectrophotometer. There may be characteristics such as turbidity and colouration that could have an impact on Ammonia monitoring using this method.