Australia's Science Forum

I have some recently purchased universal indicator pH 3- pH11 double checked the bottle!!
The solution is green pH7 colour according to the chart. When added to
Tap water pH6 yellow
Deionised water is pH6 yellow
Sodium hydroxide wont go to pH11 purple but pH11 blue??

Can anyone explain or having the same problem?
The colours are not matching the charts..... (same brand even)???

THAT SHOULD READ pH9 BLUE

Perhaps your sodium hydroxide isn't strong enough? What molarity are you using? Maybe make up some really strong stuff in a tiny beaker and see if that goes purple? Can't say I've come across this before... I have always found it very hard to get blue!!

Hi bindi,
FYI
i have a very old book that has approx pH values.
.01M NaOH pH 12.0
.1M NaOH pH 13.0
1M NaOH pH 14.0
All these concentrations should show purple. You would need a very week soln to get something less than purple. i just tested my 0.1M NaOH and it goes purple.
Sorry I can't be more help!!
Sharon

Hey Bindi,

I think folks aren't reading you right. Did you say your UI is reading pH9 blue for NaOH, instead of pH11 purple, which is what it ought to be doing? In other words it seems to be reading a little low. Right?

Smiley your correct!
Im very busy, on my own first 2 terms and my emails are just 'wacked' out.
Sorry if I was unclear!

Yes, my stock bottles (x2) are giving low results. According to the charts on the stock bottle & supplier charts.
I have decided to order another batch and see what happens, thought they maybe adding a acid to give it a longer shelf life.?????
Have done all the usual, fresh solution, clean glass wear etc.