Redox Reactions of Nitrogen
Nitrogen exists in several oxidation state ranging from +5 (in nitrate) to -3 (in ammonium).1 To climb the oxidation-state ladder from N(5), via N(3) and N(0), to N(-3) eight electrons are needed:
| (1) | NO3- ⇒ NO2- ⇒ N2(g) ⇒ NH4+ |
The corresponding three redox reactions are:
| (2a) | NO3- + 2H+ + 2e- | = | NO2- + H2O | log K = 28.57 | ||
| (2b) | NO3- + 6H+ + 5e- | = | ½ N2 + 3H2O | log K = 103.54 | ||
| (2c) | NO3- + 10H+ + 8e- | = | NH4+ + 3H2O | log K = 119.077 |
Here the equilibrium constants K refer to standard conditions at 25.
Redox Disequilibrium – Configuration Panel
In natural environments redox reactions seldom achieve full equilibrium state – even if the reaction is thermodynamically possible. Depending on the system under study the user is able to allow or disallow specific redox transitions.
The right screenshot shows the configuration panel to exclude/include one or more redox reactions defined in 2a to (2c). This panel opens via the menu Settings in the main input window.
The screenshot shows the default setup, which excludes all redox transitions. For most cases it is a good starting point.
Unfortunately, there is no universal recipe to select the best strategy. One thing you can do is to compare the outcome of different scenarios. The exclusion of a redox transition is a rather crude way to simulate redox kinetics. But it improves the understanding of the particularly tricky topic of redox processes.2
Note. The selection in the configuration panel remains valid until program exit. After restart the default setting (see above screenshot) is established.
Footnotes