The correct answer is B) Titration with standard alkali.
After the hydrolysis of an ester, it results in the formation of carboxylic acid and alcohol. To determine the change in concentration of the acid at different intervals, one can use titration with a standard alkali solution.
In this process, a known volume of the acid sample is taken, and it is titrated against a standardized solution of alkali, typically sodium hydroxide (NaOH). The reaction between the acid and alkali is a neutralization reaction, resulting in the formation of water and a salt.
The titration is conducted by slowly adding the alkali solution to the acid sample until the endpoint is reached, which is determined by using an indicator or a pH meter to detect the neutralization point when the acid has been completely reacted. The volume of alkali solution used in the titration is then used to calculate the concentration of the acid.
By performing these titrations at different intervals during the hydrolysis process, one can track the change in concentration of the acid as the ester is being hydrolyzed.
Therefore, the most appropriate method to calculate the change in concentration of the acid after the hydrolysis of an ester at different intervals is titration with a standard alkali (option B).
I hope this helps.