Why is it important in the iodine clock reaction that there is only a small quantity of sodium thiosulfate?

These are 2 questions .
Why is thiosulphate needed ? 
And why In small quantities ? 
I know that the thiosulphate reacts with the I2 to form I ions 
Why is that important though ? 




To look at why Thiosulfate is needed, it is helpful to first break down the Iodine clock reaction into two parts:


$$\ce{H2O2 + 2I- + 2H+ → I2 + 2H2O}$$


This is the slow part of the reaction and why there is a clock component to it.


$$\ce{2S2O3^2− + I2 → S4O6^2- + 2I-}$$


The conversion back into Iodide ions happens much more quickly than the first part of the reaction, so you don't see the solution change color as almost all of the Iodine is in its ionic form.


The Thiosulfate is required in this reaction to convert the Iodine back into its ionic form; if there was no Thiosulfate present, the solution would just very quickly turn black and it would lose its surprise value. Since the Thiosulfate is used to convert Iodine back into Iodide, if you added more, it would take longer for the quantity of Thiosulfate to react in the reaction. It wouldn't affect the mechanism of the reaction, only the duration until the solution would change color. This is why a small amount is typically used. 


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