Redox Elements (Valence States)
The term redox is an abbreviation for ‘reduction-oxidation’ processes. In redox reactions electrons are transferred between two valence states of one chemical element. In other words, only multi-valent elements (i.e. elements with more than one valence state) can partake in redox reactions, such as: oxygen, carbon, nitrogen, sulfur, iron, manganese, and some other trace metals. In fact, these are just the major components of living matter (except phosphorus).
Redox Elements used in aqion
The following table lists all redox elements used in aqion (in alphabetical order). What we call ‘valence state’ is also known as ‘oxygen number’; for us these are synonyms.
Each valence state is represented by several aqueous species and/or mineral phases (sometimes in large number); the table contains very few examples only.
| valence state | name | aqueous species | mineral phases |
|---|---|---|---|
| As(3) | arsenite | AsO3-3, H3AsO3, … | – 1 |
| As(5) | arsenate | AsO4-3, H3AsO4, … | FeAsO4:2H2O, … |
| C(4) | carbonate | CO2, HCO3-, CO3-2 | calcite, siderite (FeCO3), … |
| C(0) | carbon, Corg | C, CH2O 2 | – |
| C(-4) | methane | CH4 | – |
| Cr(2) | chromous | Cr+2, … | Cr(OH)2 |
| Cr(3) | chromic | Cr+3, Cr(OH)2+, … | Cr(OH)3, FeCr2O4, … |
| Cr(6) | chromate | CrO4-2, Cr2O7-2, … | CrO3, Na2CrO4, … |
| Cu(1) | cuprous | Cu+, CuCl2-, … | chalcocite (Cu2S) |
| Cu(2) | cupric | Cu+2, CuOH+, … | Cu(OH)2, covellite (CuS), … |
| Fe(2) | ferrous | Fe+2, FeCl+, … | siderite (FeCO3), pyrite (FeS2), … |
| Fe(3) | ferric | Fe+3, FeCl2+, … | Fe(OH)3, goethite (FeOOH), … |
| H(0) | hydrogen | H2(g) | – |
| H(1) | hydrogen ion | H+, H2O | – |
| Mn(2) | manganeous | Mn+2, … | pyrochroite (Mn(OH)2), … |
| Mn(3) | manganic | Mn+3, … | manganite (MnOOH), … |
| Mn(6) | manganate | MnO4-2, … | – |
| Mn(7) | permanganate | MnO4-, … | – |
| N(-3) | ammonium | NH4+, NH3, … | – |
| N(0) | nitrogen | N2(g) | – |
| N(3) | nitrite | NO2-, … | – |
| N(5) | nitrate | NO3-, … | Cu2(OH)3NO3 |
| O(-2) | oxide ion | O-2, H2O | – |
| O(0) | oxygen | O2 | – |
| S(-2) | sulfide | H2S, HS-, S-2 | FeS, … |
| S(6) | sulfate | SO4-2, HSO4-, … | gypsum, barite (BaSO4), … |
| U(4) | uranium ion | U+4, … | uraninite (UO2), coffinite (USiO4) |
| U(6) | uranyl | UO2+2, … | schoepite (UO2(OH)2:2H2O), … |
Database. The redox reaction for each aqueous and mineral species is defined by the corresponding reaction formula (stoichiometry) and log-K value (equilibrium constant). This information is contained in the thermodynamic database that underlies the program.
Redox-Disequilibrium. In some cases the user is able to prohibit a particular redox reaction – see here.
A More User-Friendly Notation
For a non-specialist, the identification of sulfate by S(6), ammonium by N(-3), or inorganic carbon by C(4) might seem a little bit confusing. To keep the learning curve flat aqion translates those valence states into common chemical language:
| C(4) | → | DIC |
| C(-4) | → | CH4 |
| S(6) | → | SO4 |
| S(-2) | → | sulfide |
| N(5) | → | NO3 |
| N(3) | → | NO2 |
| N(0) | → | N2 |
| N(-3) | → | NH4 |
Footnotes