This type of titration is carried out between acetic acid and sodium hydroxide. The free H+ ion from the weak acid is neutralized by OH- ions from the base and there is a small increase in pH. Around the equivalence point large increase in pH is observed.
However, Sodium acetate formed after neutralization reacts with OH- ions of water (hydrolysis) and so at the equivalence point the pH is not 7. The H+ ion concentration decreases and the pH is about 8.7 at the equivalence point. By the addition of small amount of NaOH, pH change is observed from about 8.0 to 10.0 near the equivalence point. It is therefore necessary to use an indicator with pH range on alkaline side.
Phenolphthalein is commonly used. Methyl orange cannot be used because it would give inaccurate indication of equivalence point.