The reaction involves ethanedioic acid (a weak acid, also known as oxalic acid) and potassium hydroxide (a strong base, KOH).

The titration of a weak acid with a strong base results in an equivalence point at a pH greater than 7 (in the alkaline range). Phenolphthalein is the most suitable indicator for this type of titration because its colour change range (pH 8.3-10.0) falls within the steep, alkaline portion of the titration curve. It changes from colourless in acid to pink in alkaline solution, which provides a sharp and distinct endpoint.