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

  1. For example, Zn(As2O5 (leiteite) and Pb(As2O5 (paulmooreite) are arsenite minerals, but neither of them are implemented in the program.

  2. CH2O (i.e. 1/6 glucose as an ‘abbreviated Redfield formula’ for organic matter) is included as reactant only.

[last modified: 2014-01-26]