Chemistry:

The persulfate - iodide reaction was run in the presence of a delayed indicator solution of sodium thiosulfate (Na₂S₂O₃). The reaction of S₂O₈^2- with I^- produces I₂. As we see in the second reaction, the I₂ is rapidly reduced to I^- by S₂O₃^2-. When all the thiosulfate solution was consumed, there was an excess of I₂ and the blue starch-I₂ complex formed ,and you observed the formation of a blue solution.

		(rxn 1)
		(rxn 2)
		(rxn 3)

The time it take for the solution in the mixing beaker to turn blue is a measure of the rate of reaction.