Ammonium is a cation (i.e. positively charged ion) and has the chemical formula (NH4+).
Ammonia is an inorganic compound of hydrogen and nitrogen and is unionized (NH3). It is characterised by the strong smell it gives off, something which is absent from its cation counterpart. Ammonia is highly soluble in water.
News reports and popular science often talk about ‘Ammonia’ in water, with ammonia being well recognised as associated with urine, the decay of nitrogenous organic matter and, for some, the Haber-Bosch process by which synthetic ammonia is produced to use as nutrients in agriculture.
Ammonia is a natural compound that is often formed during the decay of nitrogenous organic matter, during gas exchange with the atmosphere, as a result of forest fires and in animal and human waste. While it is not unusual to find ammonium in groundwater, in surface waters it is an indication that there has been some level of contamination as it is not expected to be naturally occurring in these environments.
Ammonia is widely used across several industries, most notably as an ingredient in fertilizers across the agriculture industry. Ammonia is also used in water treatment where it is combined with chlorine in order to provide a longer-lasting disinfection for water that is for human consumption.
Ammonia can find its way into surface waters and groundwaters through accidental leaching from agriculture and industrial effluent. It can also be deposited directly through human and animal waste which contains nitrogenous compounds.
A build-up of ammonia in surface waters can have serious consequences for aquatic life. High levels of ammonia inhibit the ability of aquatic organisms to excrete toxicants causing toxins to build-up in their tissues, potentially leading to death. This is exacerbated at higher temperature and pH levels.
Despite the impact on smell and taste ammonia can have on water, it is not considered toxic to humans. There is concern where ammonia is used for treatment with chlorine over the formation of chloramines which can be harmful.
Ammonium has not yet been mentioned because it is basically harmless in water. The reason ammonium is measured is explained below.
Both Ammonium and ammonia often exist in water together, the ratio at which they exist is primarily dependent upon the pH of the water and also both temperature and ionic strength also play a role in determining the ratio.
Commonly, only ammonium will be tested for in water quality surveys as ammonia is notoriously tricky to measure using in-situ methods with the ammonia having to be first transformed from its aqueous phase to the gaseous phase; methods that achieve this often have the disadvantage of producing noxious gases.
However, the detection of ammonium in water can be used to also calculate the amount of ammonia in the sample. It requires the use of simultaneous pH readings for the sample and follows the established ammonium/ ammonia equilibrium reaction.
NH3 + H2O <-> NH4 + + OH –
When the pH is lower, the reaction favours ammonium and at pH <7.25, the sample is likely to be almost all ammonium. As pH increases, the amount of ammonia present will also increase with an equal weighting of both assumed at around pH 9.25. Above pH 9.25 the equation will swing to favour greater ammonia levels. Please note, this is based on a maintained temperature of 25ºC and different temperatures will affect the weighting of the equation.
Proteus uses a liquid-membrane ISE to measure ammonium directly in the water sample. Ammonia can also be calculated by the internal software, using the method defined above, and outputted in real-time with the rest of the parameters. It uses the pH, temperature and conductivity sensors in addition to the ISE. Proteus also offers a second calculated parameter of Ammoniacal Nitrogen or ‘Total Ammonia’ which is a sum of both ammonium and ammonia present in the water.
