Why should starch solution be used as the indicator in iodometic titration?
Jenkin replies: If you are adding sodium thiosulphate solution to iodine in a titration, the iodine colour, brown to begin with, gradually becomes paler as the iodine reacts. It goes through a yellowish range but eventually, at the end point, the solution is colourless. So it is not absolutely essential to use an indicator at all. In practice, the end point is sharpened, becoming more distinct and therefore resulting in a more accurate titration, if a little starch solution is added. But this should not be done until most of the iodine has reacted; the solution will be a pale yellow (straw-like) colour at this stage. The starch will form a deep blue/black complex but this colour will disappear at the end point. Always carry out a risk assessment and check with your teacher before starting any practical work. Risk assessment Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from the
