Now, let us proceed with the calculation.

The standard Gibbs energy of formation for each substance in the half reaction (④') is noted below, and the total difference between the product form and the original form ⊿G_{f}^{0} is obtained.

original form product form

half reaction(S) SO_{4}^{2-}
+ 9H^{+} + 8e^{-} = HS^{-} + 4H_{2}O

G_{f}^{0} (kJ/mol) －744.5 0 12.08 －237.2

Total difference between the standard Gibbs energy of formation of the substance in its original and product forms.：⊿G_{f}^{0}

⊿G_{f}^{0} (J)
= 12.08×10^{3} + 4(－237.2×10^{3}) － (－744.5 ×10^{3}) = －192220

From the condition that the energies (including electrons) of the original and the product forms are equal, we obtain the standard electrode potential E_{0}.

E_{0} = －⊿G_{f}^{0} /(n・F) = －(－192220) / (8・96485) = 0.249 (V)（≒0.25）

Applying Nernst equation,

E = 0.25 － 0.003208・Ln {([HS^{－}][H_{2}O]^{4})
/ ([SO_{4}^{2-}][H^{+}]^{9})}

As a rule of physical chemistry, let [H_{2}O] = 1.

E = 0.25 + 0.003208・Ln([SO_{4}^{2-}]
/ [HS^{－}]) + 0.003208・9・Ln [H^{+}]

= 0.25 +
0.003208・Ln([SO_{4}^{2-}]
/ [HS^{－}]) ＋
0.003208・9・Log[H^{+}] /
Log(e)

= 0.25 + 0.003208・Ln([SO_{4}^{2-}]
/ [HS^{－}]) － 0.06648×pH

Hydrogen ion concentration rewritten in pH（ pH = -Log_{10}[H^{+}] )

If we attach a boundary condition ([SO_{4}^{2-}] / [HS^{-}]=1) where SO_{4}^{2-} and HS^{-} are present in the same ratio, E becomes

E = 0.25 － 0.06648×pH. (equation 1)

For example, if we substitute seawater pH = 8, the boundary between the greater and lesser abundance ratios of [SO_{4}^{2-}] and [HS^{-}] is E = -0.28 (V).

The ratio of [SO_{4}^{2-}] >> [HS^{-}] is reached when the redox potential of the ambient water is E >-0.28 (V),
the ratio of [SO_{4}^{2-}] << [HS^{-}] is reached when the redox potential of the ambient water is E <-0.28 (V).

**It is important to note that** this is the boundary condition obtained from the thermodynamic constant, and it has been confirmed that sulfate reduction (HS- generation) occurs at E<-0.1 (V) in actual sediments. Due to the action of organisms (local reducing environment within the organisms and enzymes), sulfate reduction occurs in a more oxidizing environment than the boundary conditions in the thermodynamic calculations. The rate of sulfate reduction is also believed to be much faster due to microbial action.