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Here, we have a 100 ml of tap water, and we will add some orthotolidine.
Five drops of this reagent, is adequate to get good colour change in this volume of sample.
And we will wait a few minutes.
This is after about one minute.
After about two minutes.
And after three minutes.
This is about the concentration of swimming pool water.
And at four minutes, we'll add some Javex. One drop.
Stir.
And there you go!
Let's add another one.
It gets lighter instead of darker, because the chlorine is bleaching the dye.
So...that's why Javex is used as bleach.