Читать книгу Geochemistry - William M. White - Страница 149

While many redox reactions are straightforward, balancing more complex redox reactions for pε–pH diagrams can be a bit more difficult, but a few simple rules make it easier. Let's take as an example the oxidation of ammonium to nitrate. We begin by writing the species of interest on each side of the reaction:

The next step is to balance the oxygen. We don't want to use O₂ gas to do this. We used O₂ at a partial pressure of 1 to define the top boundary for the water stability region. Within the region of stability of water, the O₂ concentration will be lower and we don't necessarily know its value. This is usually best done using water:

Next balance the hydrogen using H⁺:

Finally, we use electrons to balance charge:

As a check, we can consider the valance change of our principal species and be sure that our reaction makes sense. In ammonium, nitrogen is in the 3− state, while in nitrate it is in the 5+ state, a net change of 8. This is just the number of electrons exchanged in the reaction we have written.