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.