Why does phenolphthalein turn the solution pink in a acid-base reaction?

Question

Why does phenolphthalein turn the solution pink in a acid-base reaction?

Answer 1

Phenolphthalein is a weak acid. For brevity, let us call it HPh. Being an acid, it would be expected to ionise   HPh <=> H+  +  Ph-

If an acid is added, which ionises to produce more H+ ions, the dissociation of the HPh is forced to the left, ie undissociated HPh results. The undissociated HPh is colourless.

Conversely, if HPh is introduced to a basic solution, there are very little H+ ions present in the solution. The HPh is then able to ionise as before, producing the Ph- ion. This Ph- ion has a pink colour, which colours the solution pink. This makes the product a good indicator in acid/ base titration, because the end point especially when titrating strong acid with a strong base, is very sharp.

Answer 2

Bases remove the phenolic hydrogen to form a cyclohexadienone which produces a pink color when light is absorbed.

See this cite for color and reaction products:http://64.233.169.104/search?q=cache:F8I...

Answer 3

It is a crystalline organic compound used medicinally as a laxative and is widely employed in the laboratory as an indicator of the acidity or basicity of solutions. In pure water it is insoluble, but it is soluble in basic solutions or in ethanol or acetone. In a solution with pH greater than 8 (pH 8 is mildly basic) phenolphthalein is pink to red in color, but at pH less than 8 it is colorless. Phenolphthalein is a phenol derivative of phthalic anhydride.