Balancing redox reactions in alkaline solution

In redox half equations, hydrogen and oxygen may be balanced using major species that are available in aqueous solution. The major species available depends on the pH of the solution.

In acidic aqueous solution H⁺ and H₂O are major species.
In alkaline aqueous solution OH^– and H₂O are major species.

To balance zO (z = any number)
in acidic solution add

z H₂O to the other side of equation

H₂O is the more oxygen-rich (higher mass% O) of H⁺ and H₂O, the major species in acidic aqueous solution.

in alkaline aqueous solution add

z OH^– to the other side of the equation

OH^– is the more oxygen-rich (higher mass% O) of OH^– and H₂O, the major species in alkaline aqueous solution.

To balance zH
in acidic aqueous solution, add

zH⁺to the other side of the equation

H⁺ is the more hydrogen-rich (higher mass% H) of H⁺ and H₂O, the major species in acidic aqueous solution.

in alkaline aqueous solution, add

zH₂O to the side of the equation deficient in H then add
zOH^– to opposite side of the equation.

This is the same as adding H⁺ to the hydrogen-deficient side.

Example of balancing in alkaline solution:
Balancing is done stepwise below. In practice it would be done on a single line.

Balance atoms other than O and H.
2Fe(OH)₂ → Fe₂O₃
Add OH^– to balance O.
2Fe(OH)₂ → Fe₂O₃+ OH^–
Balance 3H with 3H₂O and 3OH^– to other side.
3OH^– + 2Fe(OH)₂ → Fe₂O₃+ OH^– + 3H₂O
Balance charge with electrons.
2OH^– + 2Fe(OH)₂ → Fe₂O₃+ 3H₂O + 2e^–