Phenolphthalein is used as an indicator for titration of sodium hydroxide solution against standard solution of oxalic acid. The colour change observed at an alkaline pH close to equiva lence point during this titration is:

Phenolphthalein is a pH indicator that is colourless in acidic and neutral solutions and turns pink in basic solutions. Its colour change occurs around pH ≈ 8.2 – 10, which corresponds to the deprotonation of its phenolic groups.

In the titration described, a strong base (NaOH) is added to a weak diprotic acid (oxalic acid, H₂C₂O₄). The overall neutralisation reaction is:

$H₂C₂O₄\; +\; 2 NaOH\; \to \; Na₂C₂O₄\; +\; 2 H₂O$

Because oxalic acid is weak, the solution at the equivalence point is not neutral; the oxalate ion (C₂O₄²⁻) hydrolyses slightly, producing a modestly alkaline pH (typically around 8.3 – 9.0). This pH lies within the phenolphthalein transition range.

Before the equivalence point the solution is acidic to near‑neutral, so phenolphthalein remains colourless. As the titration approaches the equivalence point and the pH rises into the alkaline region, the indicator begins to develop its pink colour.

Therefore, the observable colour change at the alkaline pH close to the equivalence point is from colourless (acidic side) to pink (basic side).