Professor James E. Alleman
Civil, Construction, and Environmental Engineering
Iowa State University
Email: jea@iastate.edu
Page Date: 24 December 1998 / JEA

Free Ammonia-Nitrogen Calculator & Information

Enter: Total (NH3+NH4)-Nitrogen Concentration (mg/L)
Enter: pH
Enter: Temperature (degrees C)
Free Ammonia-Nitrogen Concentration (mg/L)

Background Information Regarding Free Ammonia-Nitrogen
What is 'free' versus 'ionized' ammonia? Free ammonia (NH3-N) and ionized-ammonia (NH4+-N) represent two forms of reduced inorganic nitrogen which exist in equilibrium depending upon the pH and temperature of the waters in which they are found. Of the two, the free ammonia form is considerably more toxic to organisms such as fish and, therefore, we pay considerable attention to the relative concencentration of this particular contaminant. Lastly, this free ammonia is a gaseous chemical, whereas the NH4+ form of reduced nitrogen is an ionized form which remains soluble in water.
What is the overall importance of NH3 and NH4+? In either case, these chemical species are generally viewed as indicators that a given water has been contaminated, usually in relation to the direct discharge of an ammonia-bearing waste (e.g., wastewater effluent, stormwater runoff, etc.). Granted, these reduced nitrogen species may be biochemically oxidized to nitrite and nitrate by a special group of nitrifying bacteria which are known as chemo-litho-autotrophs [i.e., they use 'chemical' oxidation as their source of energy, they oxidize inorganic substrates ('litho' means 'rock' in Greek), and they use inorganic carbon to build new cell mass (following an autotrophic lifestyle, as opposed to heterotrophic)]. In turn, one important problem with the presence of reduced nitrogen in waters is that its oxidation may impose an oxygen demand by these nitrifying bacteria (otherwise known as a nitrogenous oxygen demand , or NOD), which might then deplete the available dissolved oxygen (DO) concentration to a level which imposes stress on aquatic life. However, there is yet another problem stemming from the presence of the free ammonia form (NH3), in that it may also impose its own level of stress on fish at rather low (sub-part-per-million) levels. This gaseous form of reduced nitrogen is the same chemical as what you would find objectionable when using ammonia-based window cleaners, but in the case of fish, most of them are extremely sensitive to even minute levels of NH3 contamination.
Why is free ammonia toxic to fish? Unlike humans, which excrete reduced nitrogen within their urine waste stream, fish release reduced ammonia-nitrogen through the gill structures. As with the transfer of oxygen into, and carbon dioxide out of, our lungs, this transfer of ammonia across the surfaces of fish gill's is driven by a concentration gradient (i.e., moving progressively from a high concentration on one side of the gill [inside] to a low concentration on the outside surface). However, this transfer inevitably slows down as the magnitude of this gradient decreases. Indeed, as the external concentration of free ammonia rises, a fish will accordingly have a harder time releasing ammonia...at which point its internal blood level of free ammonia will then rise.
Are the other forms of toxic nitrogen? Yes...the oxidzed nitrite and nitrate forms produced by the oxidation of reduced nitrogen (via nitrification) can also impose a toxic impact, although these forms are considerably less potent. First, although nitrites can not only be toxic but also mutagenic, this partially oxidized compound rarely reach levels sufficiently high to cause any problems (i.e., because it is readily oxidized by Nitrobacter bacteria to form nitrates). Nitrates may also impose their own form of toxicity, but they are many times (i.e., about 10 to 100) times less dangerous to fish than is free ammonia. Even then, if the levels of nitrates does reach excessively high levels, it can still kill the fish. Fortunately, though, nitrates are the form of nitrogen that plants love to eat....and nearly all plants love nitrates. Next to carbon dioxide, nitrogen is the highest element on their list of essential growth ingredients. Without nitrogen (nitrates), therefore, these plants simply won't grow. Give a plant plenty of nitrogen (along with plenty of light, water, CO2, and about a dozen other trace elements), and it will then grow to be big and strong. It also locks that nitrogen up in its leaves and stems, removing them from the food chain.
What is the chemical relationship between NH3 and NH4+? The free (NH3) and ionized (NH4+) forms of reduced nitrogen exist in a chemical equilibrium whose relative distribution is governed by the water's pH and temperature. For example, as the pH of a water drops (i.e., the H+ ion concentration becoming higher), free ammonia (NH3) will tend to combine with this additional, thereby shifting this chemical equilibrium towards the ionized, NH4+, form, as follows:

NH3 + H+ -> NH4+

However, given that this reaction is transformation is maintained as an equilibrium reaction, the ionized ammonium form (NH4+) may also drop a proton (H+) as the pH increases...thereby reforming free ammonia (NH3), as follows:
NH4+ -> NH3 + H+

The relative equilibrium between these two forms is determined by what is known as an equilibrium constant, Keq, which at room temperature is approximately 10^-9.25. In addition, you should known that this constant is dependent upon temperature. In turn, the relative distribution of the free (NH3) and ionized (NH4+) forms can be mathematically determined as a function of the pH and temperature of any given water. This relationship is, unfortunately, quite complex, but the 'calculator' given at the top of this page will make things a lot simpler in terms of determining the actual free ammonia concentration relative to pH, temperature (degrees Celsius) and the total ammonia (i.e., free plus ionized) nitrogen concentration (in mgs per liter). You will note that NH3 is much more dependent on pH than temperature. Within the pH range shown, an increase of one pH unit will increase the NH3 concentration about 10-fold. The USEPA publishes water quality criteria for aquatic organisms. They base these criteria on published studies on fish and other aquatic life and focus on lethal concentrations, typically the concentration at which 50 percent of the test animals die. Other studies have examined the effects at lower "sublethal" concentrations. Although most of the studies on fish deal with food fish (trout, salmon, etc.), some were based on aquarium fish such as oscars and guppies. Among the food fish, salmonids are the more sensitive, so there are separate published criteria for these fish. Lastly, it must be emphasized that as pH and temperature decrease, more total ammonia can be tolerated. However, less un-ionized NH3 will be needed at lower pH to be lethal.
Just how toxic is free ammonia? The U.S. EPA's criteria for free ammonia toxicity are presented in terms of system pH and temperature for both total ammonia and un-ionized ammonia (NH3) according to both 1-hr values and 4-day averages (i.e., the EPA does not publish a single number). The total and free concentrations correspond to the values you would have entered into the calculator. The U.S. EPA recommends that these levels should not be exceeded more than once in three years...which would enable a system to recover from the stress which would have been caused by ammonia pollution. This approach implicity recognizes that some degree of fish mortality is acceptable in order to protect most ecosystems, and it will the be obvious that these criteria are inappropriate when there are highly sensitive organisms present within the subject ecosystem. Therefore, for aquaculture systems, you would probably want to build into your calculations a certain degree of safety. (NOTE: published recommendations for this margin of safety can reach as low as one-tenth of the U.S. EPA's recommended concentrations in order to avoid killing fish).
How does this NH3 calculator work? If you would like to learn more about the mathematical basis behind the derivation of the free ammonia concentration, please click here to obtain this information!