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 °C
Total ammonia  =  0.03 mM

What is the corresponding NH3 concentration?

Answer

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

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

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

Click on Start and adjust the solution 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 mmol/L the obtained solution is both in full charge balance and 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 even rather ‘stable’2 and independent of the chosen anion1.

Ammonia at higher pH

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

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:     0.00642 mM

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

Ammonia at higher Temperature

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, say 0.1 mM. 2

  2. Note that the same value of NH3 is displayed in the first column already, i.e. the column containing the data before charge-balance adjustment. Indeed, the effect of this 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.

[last modified: 2014-02-02]