Question

A theoretical question: If you had to perform the ascorbic acid/sodium hydroxide titration without a pH meter, what indicator could be used to detect an endpoint as close as possible to the equivalence point?

If you did not have a pH meter reading equipment, how else would you detect equivalence point while performing this titration?

Answer 1

Answer

The process of slowly adding acid to basic solution or base to acidic solution until the reaction is complete is called as titration. This neutralization reaction can be monitored using acid-base indicator by noting down the color change of the solution depending on the pH of the solution.

In the above titration, acid is ascorbic acid (HC6H7O6) and the base is sodium hydroxide (NaOH).

For the above acid base titration, phenolphthalein can be used as an indicator. Phenolphthalein is a weak acid, which is colorless in acidic and neutral solution and gives pink color in basic solution.

During titration, 2 or 3 drops of phenolphthalein is added to the unknown concentration of ascorbic acid solution (remains colorless) in the conical flask. Then the known concentration of sodium hydroxide in the burette is slowly added to the colorless mixture until the end point. The endpoint is when the indicator changes the color of the ascorbic acid solution to light pink. This color change indicates that the reaction between ascorbic acid and sodium hydroxide is complete.

To avoid errors in the result and to get a value as close as possible to the equivalence point, the addition of sodium hydroxide should be stopped immediately once the solution changes its color to light pink (endpoint).

Difference between Equivalence point and End point: The equivalence point in a titration is a point when the titrant (sodium hydroxide) is chemically equivalent to the analyte (ascorbic acid). The endpoint in a titration is a point when the indicator changes the color of the solution.

Therefore, the acid base titration can be carried out without pH meter by using Phenolphthalein as an indicator. Important note is the phenolphthalein gives pink color when there is a little excess of sodium hydroxide.