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What is 'free'
versus
'ionized' ammonia?
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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.
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What is the overall
importance of
NH3 and NH4+?
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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.
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Why is free ammonia
toxic to fish?
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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.
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Are the other forms
of toxic nitrogen?
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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.
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What is the chemical
relationship between
NH3 and NH4+?
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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.
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Just how toxic
is free ammonia?
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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).
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