In order to know
when the equivalence point is reached, an indicator solution called
phenolphthalein is added to the vinegar at the beginning of the titration. Phenolphthalein is a pH
sensitive organic dye. Phenolphthalein is colorless in acidic solutions like vinegar, and deep pink
in basic solutions like sodium hydroxide. At the equivalence point of the titration, just one drop
of NaOH will cause the entire solution in the Erlenmeyer flask to change from colorless to a very
pale pink.