How does Phenolphthalein acid base indicator work…and in what titrations should it be used for?
Phenolthalein is a weak acid. Let us call it HPh. When you add a solution of HPh to an acidic solution, the acid HPh does not ionise. Because the HPh molecule is colourless, the acidic solution is colourless. If yo now add a solution of a base (as in a titration) when all the acid has been neutralise, the HPh is able to dissociate HPh <=> H+ + Ph- The Ph – ion is bright pink in clour, so the base solution becomes pink in colour. The ionisation of the HPh occurs at a pH above 8.3. The indicator is therefore used to titrate a strong base with a strong acid. I have a very good paper on the chemistry of indicators. If you send me a contact email address I will forward to you.
