IODINE/ THIOSULPHATE TITRATIONS

Iodine, I₂, is sparingly soluble in water but dissolves in potassium iodide solution, KI(aq), in which it forms a complex ion, KI₃ , potassium iodide.

I₂ (s) + I^- (aq)       ---> I₃^- (aq)

This dissociates so readily that it can be regarded simply as a solution of iodine in potassium iodide.  It is possible to use the colour of the iodine solution as an indicator, but this becomes very faint towards the end point.  It is better to use a little starch, which forms an intensely blue compound with free iodine.  The colour disappears suddenly when the last trace of iodine has been reduced.  If the starch is added too early iodine is strongly adsorbed onto it and accuracy is reduced.