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) the maximum number of eight electrons are needed:

(1) NO3-   →   NO2-   →   N2(gas)   →   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°C.

Redox Disequilibrium – Configuration Panel

In natural environments redox reactions seldom achieve full equilibrium state – even if the reaction is thermodynamically possible (favourable). Depending on the system under study the user is able to allow or disallow specific redox transitions.

aqion_mineral_phases_config

The right screenshot shows the configuration pannel to exclude/include one or more redox reactions defined in Eqs.(2a) to (2c). This panel opens via the menu Settings in the main input window.

The example shown here exludes all redox transitions – except the equilibrium between nitrate and nitrite (controlled by Eq.(2a)).

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

  1. A list of all redox elements used in aqion is given here

  2. The idea how to handle redox reactions of nitrogen (in the thermodynamic database) was adopted from D.L. Parhurst in http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/faq.html.

[last modified: 2014-01-27]