Phenolphthalein is an indicator which is used in acid-base titrations. In acidic solution, phenolphthalein turns the solution into colourless solution whereas in basic solution, phenolphthalein turns the solution into pink.
Phenolphthalein is a weak acid which has tendency to lose protons. The molecule of phenolphthalein is colourless and its ion is pink in colour.
There is no colour change in the phenolphthalein in the acidic pH. Therefore, it is cannot be used to determine the pH of an acid.