The titration is done in the presence of phenolphthalein, an indicator that is colorless in acid solution but
turns pink in basic solution. For the titration to give an accurate result, one must stop adding titrant
exactly at the equivalence point, when the indicator changes color. At the equivalence point, all of the
analyte has reacted, and only a tiny excess of titrant has been added, just enough to change the color
of the indicator.