The Universal Mystery

[ri]

Hi All,
here in Science we have a mystery!
We buy the Chem-Supply Universal Indicator and use it undiluted.
From time to time we find the contents in the small 25mL dropper bottle has gone from a nice deep green to red. This is despite ensuring all of the bottle and dropper assembly are clean & rinsed in (usually) distilled water.
Can anyone offer an explanation for this? It is very frustrating especially in a prac test!
It is easy to keep the changed solution and bring back to green with dilute sodium hydroxide but wish this wasn't necessary.
Cheers,
~Robyn

[Sassi]

Could it maybe be a student having fun??? It used to happen all the time at my old school...

[ri]

Hello Sassi,
very possible and probably does happen, however it is happening to bottles that the students had not access to as yet. Very frustrating!
~Robyn

[Sassi]

HuH??? Thats just weird... I hope we didnt blame the poor students for something they didnt actually do. I hope someone else will be able to shed some light on this one

[cheltie]

Hi Robyn,
We have had this problem too and I have always blamed the students for contaminating it then we got a bottle that hadn't been used that had turned red. When I spoke to the supplier he said it should still work the same and it did but he couldn't explain the colour change.
Weird hey!
See if yours still works OK.
Helen

[Slartibartfast]

This happens here when our students contaminate the droppers with the solution they are checking the pH of. Happens a fair bit with the younger years. But as you mention, some Q Stores batches have 'gone off' rather quickly. They may have had them on their shelves for some time.

[ri]

Hi Helen,
the teacher tried the tainted one against the green one and found the resultant pH colour was different. They had to give the student who had not the knowledge that she had a different one to the others the benefit of the doubt.
Haven't had a chance to test the red one myself yet but will soon.....
~Robyn

[dime]

I have made our Universal Indicator for at least 14 years now and when you make it you have to put in the NaOH to make it change from the red to the green. It is quite fun to watch as you add the NaOH, the drops turn green but then it is absorbed by the whole of the liquid and turns red again. You just keep adding the NaOH till the whole solution stays green. My point is that sometimes, on standing, the whole litre of green can turn red if not quite enough NaOH is added. The company maybe correct when they say it will still work OK, but like you, I like it to green to start with. I think sometimes there might be a tiny bit of acidic stuff, left in the little bottles, or a drop of the tested liquid gets onto the dropper. I just tip the little bottle of red back into the main bottle, if it hasn't been to class, mix it around to turn green and refill. So I think it is all down to the balance of the mixture, and occasionally on standing a bottle prefers to be red!

[Ian]

Some time in the distant past, one of my predecessors must have ordered litres and litres of UI. I have 6 or 8 unopened bottles in my store dating back to 199? (or something) Most are green, but some are red in the unopened bottles. Not being a chemist, I tend to just ignor the fact and hope that no body notices. So far, no body has complained,and it still seems to work OK. Adding a little NaOH sounds like a good idea, though.

regards
Ian

[ri]

Thanks all for your comments,
I notice that the bible, The Laboratory, has the recipe for Universal Indicator. I have been tempted to make my own again after getting in the necessary 4 stains years ago (and guessing the 4th decimal place with methyl red).
When I made it up, it looked so 'watery' compared to the bought stuff.
As it is made up in ethanol, I wonder if the red colour appears if the ethanol evaporates past a certain level?? When a drop of Universal Indicator falls to the bench, it quickly changes to red.
Oh well, onwards...............

[dime]

I've used methylated spirits a couple of times when a bit light on in supplies of ethanol. Worked just fine and a lot cheaper.

[Ocean Breeze]

I reckon that the odd red ones are due to
a) mainly some student contamination. They just dont have te technical awareness... eg they 'touch' the end of the dropper on to their sample and dont even notice
b) Something trace in the bottle that has reacted with the UI
c) we live in an acid world and its absorbed H+ from the air
d) The UI have been 'on the edge' of turning or
e) the elves got in in the night to turn the world pretty colours

Whatever the cause.. I just tip them all back into the one beaker, return back to green with the Na OH, give a good mix, and refill the droppers so that every student group has the same result as everyone else