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CALCULATION OF FREE UN-IONIZED AMMONIA

Ammonia is among the most toxic metabolic waste product excreted by fish. Ammonia will be present in two forms; ionized and un-ionized. It is the un-ionized form that is believed to be the most toxic as given in the literature, this is however a lot of nonsense and you should always base your calculations on total ammonium in the water. However if you wish to base the water quality limit only on the un-ionized fraction, then the ratio of ammonium to ammonia is dependent on the pH, hardness, salinity and temperature of the water, the pH however has the greatest influence. An increase of the pH by 0.3 units from 7.0 to 7.3 would double the concentration of unionized ammonia present.

The Environmental Protection Agency of the USA and the European Inland Fisheries Advisory Commission state that the maximum safe level of un-ionized ammonia to be 0.02 and 0.25 mg/1 respectively. The above values were determined by multiplying the concentration of ammonia that would kill 50% of the fish in 48 or 96 hours (LC50), by an application factor. Since these factors are not supported empirically it is advisable to maintain the concentration of un-ionized ammonia below 0.01mg/1.

There is also a body of opinion that it is not the concentration of un-ionized ammonia that is of importance but rather the total ammonium concentration which will comprise principally of ionized ammonium at neutral pH values.

Fish and aquatic animals (especially freshwater species) will become acclimatized to the environment as long as it remains stable, for example salmonids have been know to tolerate ammonium concentration as high as 10mg/l on a continuous basis. However the same fish if exposed to a fluctuating concentration of ammonium from 0.1 to 2 mg/l mv not survive the conditions.

NH3-N = Ammonia Nitrogen

NH4+- N = Ammonium Nitrogen

pKa = Acidity constant

The pKa Constant at Different Temperatures.

Temperature
Degrees			pKa
Centigrade
5			9.90
10			9.73
15			9.56
20			9.40
25			9.25
30			9.09

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