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I'm just wondering how long should universal indicator stay "fresh" for?? I have bottles of it dated 2005.
I've had problems with it being contaminated twice today.
The first lot was red so I discarded that and made up some new dropper bottles from a fresh bottle, but the teacher using it now has just called me in as it is quite yellow!!
Any ideas will be appreciated
Thanks

Have found this post by Robdean. Have a read and next time don't throw out the red stuff. Were the dropper bottles you used quite clean? Or the beaker you used to transfer liquid from the bottle wasn't contaminated in any way? It might be worth your while to have a browse through the search function looking at "universal indicator solution" as others have had similar problems. You might be able to make the yellow go back to green by using some of the methods used by others.

Re: Universal Indicator solution

Quote

Postby Robdean » Mon Nov 11, 2013 9:28 am

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"

thanks for that Lyn!