Australia's Science Forum

Our 2M NaOH solution has everyone including the Snr Chem teacher baffled. When universal indicator is added to it, the solution turns purple (as expected) & then changes to a teal colour (~ph8) within minutes. I've tried making fresh batches with distilled water, tap water, different batches of NaOH pellets and still get the same results. When I tested the Universal ind. with a buffered pH11 solution it developed a purple colour and did not change.
Does anyone have any ideas or theories as to why this is happening?
Thanks.

Josie,
this makes no sense at all!!!!!
Love to know what the problem is once you've worked it out. contamination with glassware?? sounds like you've tested all the obvious things. anything dripping from above to contaminate??


Good luck.
Sharon

No year 8 boys around with gaseous problems???

I just tried again using NaOH kindly donated by another school. Problem still persisting. Funny thing is, when diluted down to a 0.25M solution it is holding the correct colour.
I've ruled out glassware contamination. Air contaminants? OMG! I'm breathing the same air so I hope not.

Grrr! trime to order a new bottle of Universal Indicator.

As you know sodium hydroxide is hydroscopic and readily absorbs CO2 from the air,that is why we should store it in air tight container. If your NaOH is old and stored incorectly than
2 NaOH + CO2 → Na2CO3 + H2O.
2M NaOH has pH of 14 while sodium carbonate is salt of carbonic acid with pH lower than NaOH app 10ish I think.

Interesting thought, Jazz... Josie, is it super humid at the moment?

I have experienced the same problem. I don't think it is a consequence of the NaOH forming Na2CO3 as the diluted solution would potentially have the same issue. I have put it down to the phenolphthalein in the universal indicator fading with time.
"At a pH above 10 the pink color slowly fades as P2- reacts further with NaOH, (OH -), to give the colorless POH3- ion according to the equation:
P2-(aq) + OH-(aq) → POH3-(aq)"
pink colorless

The kinetics of this reaction is dependent on the concentration of OH- ions and is therefore much slower with diluted solutions of NaOH.

If anyone has a better explanation let me know...

I have the same experience. If teachers want NaOH just to show colour of indicator, must use 0.1M. Not sure why that is , but I made myself and any other labbie that steps in a note to use very dilute solution. For 2M solution have to use phenolphthalein.
I dont think it is the formation of carbonate.
Lada

Thanks everyone for your input.
My NaOH pellets are dry so I ruled out storage issues.
JelJane, I like your explanation & I'm relieved to hear I'm not the only one who has come across this.