consider a iodometric titration. why we add starch indicator just before the end point????. why cannot we add it before we start the titration???
simply because iodine will readily react with starch to form a dark blue solution, and therefore interfere with the end point detection. we therefore add starch indicator just as the iodine colour in solution pales i.e. pale yellow indicating that iodine concentration is very low. Near the endpoint, the iodine color is very pale that it can be overlooked. adding starch to a very pale solution gives a clear end point indication by a visual deep blue colour solution.
hope that helpsposted by bonjo