A student carries out a titration to determine the end point of a reaction between ethanoic acid and potassium hydroxide solution. Explain why the student should perform a rough titration first, and then describe how concordant titre values are obtained from subsequent accurate titrations.

A rough titration is carried out first to locate the approximate end point and to estimate the volume of base required. This allows the student to set the burette close to the expected end point and to avoid wasting time and reagent.

In the subsequent accurate titrations the base is added very slowly, dropwise, as the volume approaches the end point. The end point is recognised by a permanent colour change of the indicator.

If two or more accurate titre values agree within 0.10 cm³ of each other, they are considered concordant and can be used to calculate the concentration of the acid.