Why phenolphthalein is colourless in acidic solution but pink in acidic medium?
Answers
Hey!
ANSWER :-
Phenolphthelene exist in two different structural forms:
(i) Acidic media
(ii) Basic media
Enol form which is present in acidic solution is colourless so u will not find any colour in this medium while Keto form which is preseent in basic medium appears pink so it is pink in basic medium often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It is a weak acid, which can lose H+ ions in solution.
The pink colour of the solution is due to the concentration of In− ions in the solution.
Under acidic conditions, the concentration of In− in the solution is very low and concentration of H+ is high, hence it is colourless.
Similarly, under basic conditions, the concentration of H+ ions is very low and concentration of In− is high, hence the solution is pink coloured.