Acid Rain
Given is an acid rain sample with the following composition:
| 0.02 mM HNO3 | (nitric acid) |
| 0.03 mM H2SO4 | (sulfuric acid) |
| 0.01 mM HCl | (hydrochloric acid) |
| 0.02 mM NH3 | (ammonia) |
We are looking for the pH of this solution for three different scenarios:
| Scenario 1 | without CO2 gas exchange | |
| Scenario 2 | with CO2 gas exchange (open system) | |
| Scenario 3 | with CO2 gas exchange and in contact with calcite |
Scenario 1
We start with pure water (button H2O). This and all subsequent tasks are solved with the reaction module (button Reac). Activate the checkbox “More reactions” and enter the four components as shown in the right screenshot. The order of reactants is arbitrary. Click on Start.
The calculated pH is immediately displayed (as in the right screenshot):
| pH = 4.16 |
All other parameters and species are presented in aqion’s subsequent panels/tables.
Scenario 2
All proceeds like in Scenario 1, but before you click on Start, click on Setup and activate “Open CO2 System” as shown in the right window. The default value (pCO2 = 3.408) corresponds to normal conditions in the atmosphere. We adopt it. Then click on Start.
The calculated pH is immediately displayed (as in the right screenshot):
| pH = 4.16 |
In fact, the pH remains the same, even when 0.013 mM DIC is added to the system here. Under the given acidic conditions, DIC mainly exists in form of the neutral species CO2(aq).
Scenario 3
All proceeds like in Scenario 2, but before you click on Reac you should incorporate at least 1 mM calcite. For this purpose activate the checkbox Minerals (in the main window) and enter an initial amount of 1 mmol/L calcite. An example is given here.
The calculation shows that 0.56 mM calcite dissolves (= 1 mM start value – 0.44 mM precipitates). Consequently, the pH increases significantly:
| pH: 4.16 ⇒ 8.21 |
The result is independent of the initial amount of calcite (as long it is greater than 0.56 mM). You can check it by repeating the calculation with 5 mM or, say, 30 mM calcite.
[Note: The initial amount of calcite remains in the mineral list until a new input water is taken, either via the button H2O or Open. Only then, the mineral inventory is reset to zero.]
Nitrogen Speciation and Redox Reactions
The initial solution contains 0.04 mM nitrogen (= 0.02 mM HNO3 + 0.02 mM NH3), i.e. nitrogen exists in two redox states: N(5) and N(-3). The total amount of nitrogen does not change in the reactions, but what is changed is the equilibrium distribution of the nitrogen species (disproportionation).
In practice, however, redox reactions seldom achieve full equilibrium state — even if the reaction is thermodynamically possible (favorable). Depending on the system under study the user is able to allow or disallow specific redox transitions here. In the present calculations we ignored all redox transitions (default settings).