Question

Phenolphthalein does not act as an indicator for the titration between:

• A. $HClandN{H}_{4}OH$
• B. $Ca(OH{)}_{2}andHCl$
• C. $NaOHand{H}_{2}S{O}_4$
• D. $KOHandC{H}_{3}COOH$

Answer 1

The correct option is A $HClandN{H}_{4}OH$

The equivalence point of a titration is where you have mixed the two substances in exactly equation proportions. You obviously need to choose an indicator which changes colour as close as possible to that equivalence point. That varies from titration to titration. Phenolphthalein indicator distinguishes the pH change between $pH$ $8to10$. Therefore it is used for strong acid and strong base titrations or weak acid and strong base titrations. In the given options, $HCl$ and $N{H}_{4}OH$ is a strong acid and weak base titration where phenolphthalein is useless as an indicator.