Determination of band positions in TiO₂ colloids using pH tuning. The radical cation MV⁺˙ produced by the reaction is detected at the indicator electrode by re-oxidation to the dication. Significant amounts of MV⁺˙ are detected when the TiO₂ electron quasi-Fermi level is aligned with the methylviologen redox Fermi level. R and O represent the reactant and product respectively in reaction (1.21a).