Phenolphthalein is used as an indicator for the titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change that is observed at an alkaline pH close to the equivalence point during this titration is :
D. colourless to pink
In the standardisation of sodium hydroxide using a standard solution of oxalic acid, oxalic acid (a primary standard) is taken in the conical flask and sodium hydroxide is taken in the burette. Phenolphthalein is used as the indicator, which is colourless in an acidic medium and pink in an alkaline medium. Initially, the solution in the conical flask contains oxalic acid, so it remains colourless. As sodium hydroxide is added, the pH of the solution increases. At the equivalence point, the salt formed is sodium oxalate, which is a salt of a weak acid and a strong base. It undergoes anionic hydrolysis to give an alkaline solution. At this alkaline pH (in the range of 8.2 to 10.0), the phenolphthalein indicator changes its colour to pink. Thus, the observed colour change at the equivalence point is colourless to pink. Answer: colourless to pink
Related: Chemistry — Ionic Equilibrium · All PYQ Banks