The effect of acid and base on universal indicator.

I happen to see a video from steve spangler which demonstrate on the effect of milk of magnesia (a kind of liquid antacid) on acid with universal indicator. The colour change of the solution from red to purple was awesome. The video can be found in steve spangler website. The colour change slowly is because the substance in milk of magnesia dissolve slowly in acid, so it will neutralise the acid slowly instead of neutralizing the acid straight away when added.

As I was preparing for the experiment on the effect of pH on the universal indicator so I think the experiment steve spangler did will be good to show the student as a demonstration. I cannot get milk of magnesia in Singapore so I tried those antacid pill by crushing them into powder and mix in water.

To my surprise, antacid pill is acidic. I don't know why, may have to ask the manufacturer. The 2 antacid pill I tried is Wei Sen U & Gelusil Antacid. So I created something quite similar to milk of magnesia which is calcium hydroxide solution. It gives a milky appearance (like milk of magnesia) as it does not dissolve fully in water and is alkaline. Somehow it works and it can be seen in this video that is taken during the experiment. The change is quite fast, so in order to see the yellow colour in between, I reverse the reaction by adding HCl drop by drop so the colour will change slowly from purple to grey to green then yellow and finally red.

From the video, you can see that when the acid is not mixed properly, the middle is red while the side is yellow.

However calcium hydroxide do not work every time due to their solubility. So they may not be able to dissolve fully to neutralise the solution and make it turn purple. Then I tried using sodium hydroxide pellet (which is a stronger base). It works and change the universal indicator from red to purple, see this video.

It's cool right! Enjoy the video as I took a few tried in order to get such great result.

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.