Question

What indicator is clear in an acid and pink in a base?

Answer 1

Phenolphthalein is another commonly used indicator for titrations, and is another weak acid.

In this case, the weak acid is colourless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colourless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink.

The half-way stage happens at pH 9.3. Since a mixture of pink and colourless is simply a paler pink, this is difficult to detect with any accuracy!

Answer 2

phenolphtalein is colorless in acids and pink in basic solutionj