In a basic glucose solution, methylene blue acts as a reversible redox mediator. When dissolved oxygen is present (after shaking), methylene blue is oxidized and appears blue. As the solution sits, glucose reduces methylene blue to its leuco (colorless) form, consuming dissolved O₂ in the process. With less oxygen available, the dye stays reduced and the blue color fades. At the same time, phenolphthalein reports the solution’s pH. In strongly basic conditions it is pink; when the effective basicity drops (as redox processes and dissolved CO₂ subtly shift the equilibrium), the pink can fade toward colorless. Shaking reintroduces oxygen, restoring the oxidized blue form of methylene blue and re-establishing conditions where phenolphthalein is visibly pink. Net effect: • Shake → oxygen in → methylene blue oxidized (blue), phenolphthalein pink in base • Let stand → glucose reduces dye (colorless), oxygen depleted → color shifts back Key chemistry concepts: • Redox cycling (oxidation...