I use Phenolphthalein indicator for titrations in making biodiesel, which needs to be done at approx pH 8.5, so the phenophalein is perfect for this
purpose.
However, from time to time, I'd love to have other indicators for different pH's. I realize I can buy wide range indicators, other indicators, etc,
but this has been something I've been thinking about - I have enough phenophalein to do me forever. The stuff is only good for a year or so before it
becomes inaccurate, from what I understand.
Is it possible to change/modify Phenolphthalein indicator to change color at another ph value other than at 8.xx?
Edit byChemoleo: it's spelled Phenolphthalein...to make search easier
[Edited on 4-5-2006 by chemoleo]12AX7 - 3-5-2006 at 09:33
Quote:
Originally posted by ktw_100
I have enough phenophalein to do me forever.
Are you sure such contact is a good thing? It is a laxative after all.
Timmidnite13 - 4-5-2006 at 14:24
Somone correct me if I'm wrong, but aren't all indicators weak acids? So if you wanted to change the point where it changes colour, you'd have to
change
its chemical properties...
If you are in dire need of accurate resuls, just use a pH meter (which lasts you a lifetime )Fleaker - 4-5-2006 at 14:39
Yes, they are all weak acids. You can select an inidicator for the specific pH range so long as it is not too extreme. pH meters need constant
calibration, but if maintained properly, they will work well.chemoleo - 5-5-2006 at 05:09
You can try and modify it by direct chlorination, or bromation. I remember that eosin is tetrabromo-fluorescein
Although phenolphthalein has a different structure,
I am sure it can be treated in a similar manner, to yield a new product with different properties.
Alternatively, you can nitrate it with HNO3, the product will almost certainly have a different absorption spectrum. Then this could be reduced to
amine...which, via Sandmeyer, could lead to a large host of new compounds.
[Edited on 5-5-2006 by chemoleo]nitroglycol - 6-5-2006 at 03:53
Quote:
Originally posted by ktw_100
The stuff is only good for a year or so before it becomes inaccurate, from what I understand.
Is this true? That sucks, because I've got a nice bottle of crystalline phenolphthalein that's rather old. I'll have to try it out on a few solutions
and see what the deal is. Anyone know what the decomposition reaction is?unionised - 6-5-2006 at 07:49
If the stuff was only stable for a year before it became unusable as an indicator then it would be even more of a problem using it as a drug because
drugs need decent shelf-lives. Since it is used medically, it must be stable. The solutions might not last too well, (particularly, I suspect, in
alkaline conditions) but the solid should be OK for years.
