In the titration of sodium thiosulphate with iodine, the colour change occurs due to the reaction between these two substances. The endpoint of the titration is reached when all of the thiosulphate has reacted with the iodine. At this point, the solution will be colourless. However, if more iodine is added after the endpoint has been reached, the solution will turn blue due to the formation of triiodide ions.

The reaction between thiosulphate and iodine can be represented by the following equation:

2S2O3^2- + I2 → S4O6^2- + 2I^-

At the endpoint of the titration, all of the thiosulphate has reacted with the iodine, so there will be no more S2O3^2- ions in the solution. This means that any additional iodine that is added will not react with the thiosulphate and will instead remain in solution as I2 molecules. These I2 molecules will absorb light in the visible spectrum, giving the solution a blue colour.

The formation of triiodide ions can be represented by the following equation:

I2 + I- → I3^-

The I3- ions are also responsible for the blue colour of the solution.