There are two major ways to describe the concentrations of ammonia, nitrite, and nitrate in water. The "nitrogen" weight of
these molecules describes the weight of only the nitrogen atoms within them. On the other hand, the "ion" weight of these
molecules describes the weight of the entire molecule.
For example, the term nitrate-nitrogen (NO3-N) refers to the weight of only the nitrogen atom within the nitrate molecule; as
opposed to nitrate-ion (NO3), which describes the weight of the entire nitrate molecule. Note that a given nitrate-nitrogen
value will always be lower than the associated nitrate-ion value. Conversion between the two forms is as simple as applying
a constant (see graph below).
Scientific literature often uses the "nitrogen" form rather than the "ion" form to describe the concentration for these
molecules. The "nitrogen" form is more appropriate when discussing nitrification/denitrification cycles, as it simplifies
various equations and flow-charts. For example, it is easy to see that 100 ppm of nitrite-nitrogen (NO2-N) can go on to form
100 ppm of nitrate-nitrogen (NO3-N). Using the "ion" form here would be more cumbersome; 100 ppm of nitrite (NO2) goes on
to form approximately 135 ppm of nitrate (NO3).
Note that a total ammonia value (NH3 + NH4) cannot be trivially converted from "nitrogen" to "ion" form or vice versa. The
total ammonia value must first be broken into component NH3 and NH4 values, then those converted individually to NH3-N and NH4-N
and added together. The Free Ammonia Calculator can be used to do this.