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Author: Subject: pH probe for toluene
angelhair
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[*] posted on 6-11-2008 at 16:28
pH probe for toluene


What type of pH probe can best be used to measure and monitor the HCl gassing of substanes in polar or very weakly polar solvents like toluene?
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not_important
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[*] posted on 6-11-2008 at 17:45


Probably best to just look for increased escaping gas, overshooting a bit in the process.
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Klute
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[*] posted on 7-11-2008 at 01:17


I remember some debate here (i think) on this issue, i think the bottom end was that pH-electrodes cannot give a accurate measurement of the "pH" of a non-poalr solution, as it can't measure any H3O+ (there isn't any), it could be used to roughly determine the point where all the base had been titrated, but needed good stirring: a single "drop" of neutralized salt with some trace of water would give an acidic response even if some non-poalr freebase was still around...

I persoanlly think that wetted ph-paper is just fine, just need a little patience. Bteer know roughly how much base you have in solution in the first place, and then add just under the required amount of acid, and titrate slowly, testing with a spatula and some moistened ph-paper.

Depending on your use, you could even add a minute amount of phenolphatleine in solution, or another colored indicator.




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stoichiometric_steve
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[*] posted on 9-11-2008 at 23:54


There apparently are some newer pH electrodes ("solvotrodes") that work with organic solvents, albeit they do really need good stirring.



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chemrox
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[*] posted on 9-11-2008 at 23:58


and don't give a pH as such by definition



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angelhair
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[*] posted on 10-11-2008 at 00:48


I will check that out thanks.

Just on a slight side note. I read that if you overshoot with HCl your salt will start to solute back into the solvent. That does the amount that solutes do so proportionally to the pH and say that your already < pH1 will more and more solute if you continue to gass the hell out of it?
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[*] posted on 7-12-2008 at 20:24


Quote:

and don't give a pH as such by definition


Can you please explain. The Metrohm web site say's it measures 0 - ph 14 and it looks like it conects to any old meter.
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[*] posted on 8-12-2008 at 02:19


pH is related to the concentration of hydrogen ions, in non-polar solvents the concentration of H+ is low to none. A pH probe is more likely reporting the concentration of hydrogen ions within itself, which would bear some relationship to the concentration of the non-ionised acid in the bulk non-polar solvent.
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Blind Angel
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[*] posted on 8-12-2008 at 09:47


Aren't they actually giving the concentration of H3O+ ions? I had the impression that the concept of free H+ was a simplification. But you can also use the proton giver concept, which in that case cannot be analyzed by electrode no?



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