It's quite simple.

If the titration flask is washed out with the tablet solution, some of the contents or alkaline substance (and hence OH- ions ) would remain on the inner walls of the flask.

i.e the flask would now have more than the amount that is present in 25.0cm³ of the solution.

So you'll need more hydrochloric acid to neutralise this extra amount (that wasn't supposed to be there).

So the titre value goes up.

This is why it is best to rinse the titration flask (usually a conical or otherwise known as an Erlenmeyer flask) with water first, then ethanol or acetone.


This washes out any impurities (water soluble ones get carried away as well by the water, then the second wash with ethanol flushes out the water and those impurities that dissolve in organic solvents)

Any remaining ethanol/acetone would evaporate quickly, leaving the flask very dry (unlike water which could take some time evaporate)