Here's the explanation:
Acid-Base Reaction:
Oxalic acid (H2C2O4) is a diprotic acid, meaning it can donate two protons (H+) per molecule. Sodium hydroxide (NaOH) is a strong base that dissociates completely in water, releasing hydroxide ions (OH-). The reaction between oxalic acid and sodium hydroxide can be represented as follows:
H2C2O4 + 2NaOH → Na2C2O4 + 2H2O
Equivalence Point:
The equivalence point in titration is reached when the moles of acid and base are stoichiometrically equal. In the case of oxalic acid and sodium hydroxide, the equivalence point is reached when all the oxalic acid molecules have reacted with an equal number of hydroxide ions. At this point, the solution is neutral, meaning it has neither an acidic nor a basic pH.
Phenolphthalein as an Indicator:
Phenolphthalein is a widely used acid-base indicator that undergoes a color change in the pH range of approximately 8.3 to 10.0. In its acidic form, phenolphthalein is colorless, and when the pH rises above 8.3, it turns pink.
In the titration of oxalic acid with sodium hydroxide, phenolphthalein can effectively indicate the equivalence point. As you add sodium hydroxide to the oxalic acid solution during titration, the pH gradually increases. Initially, the solution will be acidic, and the phenolphthalein indicator will remain colorless. However, as the pH approaches the equivalence point, the solution becomes less acidic and eventually reaches neutrality (pH ≈ 7). At this point, the addition of even a small amount of excess sodium hydroxide will cause a significant pH increase due to the strong basicity of NaOH. This sudden change in pH causes the phenolphthalein indicator to turn pink.
Therefore, the appearance of a persistent pink color in the solution indicates that the equivalence point has been reached, signaling the completion of the titration. Phenolphthalein's color change in the pH range close to neutrality makes it a suitable indicator for titrations involving weak acids like oxalic acid and strong bases like sodium hydroxide.
