2. Titration of free iodine with thiosulfate: free iodine is
consumed by sodium thiosulfate in the titration step.
The amount of thiosulfate used is proportional to the
amount of free iodine liberated from the salt. Starch is
added as an external (indirect) indicator of this
reaction and reacts with free iodine to produce a blue
colour. When added towards the end of titration (that
is, when only a trace amount of free iodine is left) the
loss of blue colour, or end-point, which occurs with further titration, indicates that all remaining free iodine
has been consumed by thiosulfate.