Australia's Science Forum

When I went to use my universal indicator solution this morning which I purchased earlier this year I found it is distinctly red instead of green. This has never happened to me before. Is it still ok to use, how do I get it back to green? Any help would be much appreciated.

Anastasia

It is nice if your solution stays green but I have had bottles of stock solution which have been a reddish colour and they still show the correct Ph colour. Try some in water. If the water is a green colour your stock solution should be working OK. Having said that I too would be interested in others comments about this. Always willing to learn.
Lyn.

Just add a drop of 0.1M sodium hydroxide shake, repeat till green
I think it absourbs CO2 fromthe air that forms carbonic acid

JUst like Ocker, that is the way to go.

Also can make a solution of weak Sodium Hydrogen Carbonate (Bi Carb.Soda) or Boric Acid and slowly add to indicator until green colour is reached.
Cheers
Mother

A number of years ago I contacted Chem Supply after our universal was turning red. I was having a hard time convincing the staff here that it was fine to use even after I tested it for colour change against HCl and NaOH. They kept tipping it down the sink! The following is the reply from Chem Supply:

"We are happy to advise that the starting colour of our universal indicator solution does not affect its performance in any way.

We manufacture the product by dissolving a series of dyes in a mixture of ethanol and water. Once the dyes are added, the mixture is actually red (possibly because the ethanol has a small amount of residual acidity). In the last step of manufacturing the product, we add a very small amount of sodium hydroxide to our mixture so that the pH is 7 (giving a green colour).

The tests you performed with the HCl and NaOH solutions proved that the indicator solutions gave the same colour, regardless of the starting colour, when added to an excess of sample. When we manufacture the product, we carry out a similar test using pH buffer solutions so you can be assured that our product will be accurate.

Finally, you may be aware that water left standing in air actually becomes slightly acidic due to the absorption of carbon dioxide, forming a dilute carbonic acid solution; this may account for the colour change in your bottles.

Best regards,

Josh Krieg

Chemist

Chem-Supply Pty Ltd"