Why did my iodine clock reaction not work?

For the iodine clock reaction, I'm using Potassium iodide, sodium thiosulfate ($\text{0.05 M}$) and sodium peroxodisulphate ($\text{0.04 M}$) and starch as an indicator. I am varying the concentration of $\ce{KI}$ ($\text{0.01, 0.02, 0.04, 0.08 M}$). Should the concentration of the starch be $\text{0.1 M}$?


I ran the experiment but it did not work:
In my procedure, I first mix 10 mL of $\ce{KI}$ with 1 mL sodium thoisulfate, then add 5 drops of starch and then add $\text{5 ml}$ sodium peroxodisulpahte, however no black solution was formed.




Try repeating without the thiosulfate - you should get an instant blue/black colour.


If you do, then there's probably too much thiosulfate meaning the concentration is not 0.05 but higher.


If you don't get the instant blue/black without the thiosulfate the it's one or both of the other two solutions.


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