In acidic medium, H ions and water are added to balance each half-reaction.
For example, when manganese reacts with sodium bismuthate.
:Unbalanced reaction: Mn + NaBiO → Bi + 
:Oxidation: 4 HO + Mn →  + 8 H + 5 e
:Reduction: 2 e + 6 H +  → Bi + 3 HO
Finally, the reaction is balanced by multiplying the stoichiometric coefficients so the numbers of electrons in both half reactions match
:8 HO + 2 Mn → 2  + 16 H + 10 e
:10 e + 30 H + 5  → 5 Bi + 15 HO
and adding the resulting half reactions to give the balanced reaction:
:14 H + 2 Mn + 5 NaBiO → 7 HO + 2  + 5 Bi + 5 Na