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?
Answer
First of all, it should be noted that the stated water analysis is incomplete; it does not contain any anions. To correct this gap, we add e.g. 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
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 |
Optionally, you can also add a title, say, NH3 (as filename).
Click on Start.
2. The first message is the charge balance error.
Select “DIC” in the pull-down list to adjust the charge balance (by clicking on the button next ≫).
3. You get the message:
| DIC: 0.030 ⇒ 0.026 mM |
In fact, by decreasing DIC by 0.004 mM, the resulting solution is completely charge-balanced and in thermodynamic equilibrium.
4. The button next ≫ opens the main output table.
Then, click on Ions in the top menu bar. Here you find the complete speciation, and in particular the ammonia concentration:
| NH3: 0.00238 mM |
That is, about 8 of the total ammonia (0.03 mM) exists as NH3 (0.00238 mM).2
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 is:
| 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
In addition to the pH, there is a second factor that influences the free ammonia in natural waters — the temperature. Repeating the above calculations 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 concentration rises in a non-linear way. All in all, the higher the temperature and/or pH the worse for fish and environment.
Ammonia at other Temperatures — Plots
A shorter way to perform the above calculations is via the reac module. The only requirement is that the input water is already in charge balance. That is, we start with the input water, but now set DIC = 0.026 mM. Then button Reac.
1. In the reac panel we activate the two check boxes change of temperature and plot titration curves, etc.. Then set the temperature range from 5 to 40 and click on Start.
2. After 2-3 seconds, the diagram panel opens in the default view displaying the constant pH = 8.5 as a function of T (which is quite uninteresting). To get the diagram shown in the right screenshot, please proceed as follows.
There are two rows of drop-down lists below the diagram. Select “IONS” in the second list of the first row. Then select in the two left lists “NH4+” and “NH3”.
3. Finally, to get the numerical values for “NH4+” and “NH3”, please click Show data in the upper menu bar. The data table is shown in the right screenshot.
Remarks
-
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. ↩
-
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. ↩