## Free Ammonia in Water

Problem

Un-ionized ammonia (NH3) is highly toxic to fish. Total ammonia, as the sum of NH3 and NH4+, is what is measured analytically. Let’s assume, a water sample is characterized by the following parameters:

 pH  =  8.5 T  =  15 Total ammonia  =  0.03 mM

What is the corresponding NH3 concentration?

Please note that the given water analysis is incomplete; it does not contain any anions. To fill this gap we add, say, 0.03 mM DIC (with HCO3- as the major anion). We don’t know the exact value of DIC, but the program sets it automatically (when charge balance is required).1

We start with pure water (button H2O) and switch to molar units (upper checkbox mol). Then enter the following parameters:

 pH = 8.5 T = 15 Amm = 0.03 mM DIC = 0.03 mM

Click on Start and adjust the charge balance with DIC in the pull-down list. You will get the message:

 DIC:   0.030  ⇒  0.026 mM

In fact, by diminishing DIC by 0.004 mM the resulting solution is fully charge-balanced as well as in thermodynamic equilibrium.

The button next opens the main output table. Now, click on Ions in the top menu bar. Here you find the speciation, and in particular the ammonia concentration:

 NH3:   0.00238 mM

About 8 % of the total ammonia (0.03 mM) exists in form of NH3 (0.00238 mM). From the numerical point of view, the obtained result is rather ‘stable’2 and independent of the chosen anion (DIC or chloride).

Ammonia at higher pH

It is easy to repeat the calculation for higher pH values (keeping total ammonia fixed at 0.03 mM). The result for T = 15:

 pH 8.5:     0.00238 mM pH 8.6:     0.00294 mM pH 8.7:     0.00360 mM pH 8.8:     0.00440 mM pH 8.9:     0.00534 mM pH 9.0:     0.00643 mM

Note that with increasing pH the concentration of NH3 rises exponentially.

Ammonia at other Temperatures

Aside from pH there is a second factor that affects free ammonia in natural waters – the temperature. Repeating the above calculation yield the following concentrations of NH3 at pH 8.5:

 5 °C:     0.00113 mM 10 °C:     0.00166 mM 15 °C:     0.00238 mM 20 °C:     0.00333 mM 25 °C:     0.00456 mM

Also here the concentrations rises in a non-linear way. All in all, the higher the temperature and/or pH the worse for fish and environment.

Remarks

1. The result will remain the same if another anion is chosen, for example 0.03 mM chloride. The result also will remain the same if we start with another initial value for DIC or chloride, e.g. 0.1 mM.

2. The same value of NH3 is displayed in the first table column, which is the column containing the data before charge-balance adjustment. Indeed, the effect of the DIC adjustment is so small that you have to enhance the number of digits from 3 to 5 (drop down list in the upper menu bar) to observe any difference.