The effect of bleach on universal indicator.

I was preparing normal household reagent for the student to test on its pH. One of the reagent is bleach. I have searched in the net that bleach will gives a purple colour on the universal indicator since it is alkaline. When I tested it, it shows purple initially, but it changes from purple to green to yellow then finally colourless. This will mislead the student as they may not know what colour to record down since the colour keep on changing.

So I decided to find out why from the internet and realised that the universal indicator and any other liquid pH indicator are destroyed by bleach, rendering them useless for testing the pH. This applies to litmus paper and phenolphthalein too.

The only way to solve the problem is to adds sodium thiosulfate to the bleach solutions, it will neutralize the colour-removing effects of bleach and allow it to test with the liquid indicators. Thiosulfate reduces the hypochlorite (active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:

4 NaClO + Na₂S₂O₃ + 2 NaOH → 4 NaCl + 2 Na₂SO₄ + H₂O

I have tried it out and it works. So in future if bleach is needed to test for its pH, neutralize it with sodium thiosulphate first before adding the liquid indicator.