paw_20
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Titration exponent, pI...?
So to avoid studying calculus, I started flipping through Lange's Handbook of Chemistry and stumbled upon this nifty little section in Organic
Chemistry entitled "Mixed Indicators"
"Mixed indicators give sharp color changes and are especially useful in titrating to a given titration exponent (pI)."
It goes on to give a detailed table describing the color changes and pI's of these mixed indicator systems (which I suppose I could scan in
if anyone's interested).
My question is, what in the sweet chocolate Christ is a titration exponent? It seems as though they are referring to the pKa of the indicator combo,
just curious if anyone else has seen this terminology before. Never saw it in analytical.
[Edited on 5-8-2013 by paw_20]
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blogfast25
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This must simply be referring to pH of the endpoint, as different acid-base titrations show different pH values at endpoint. But who would use mixed
indicators in acid base titrations is mystery to me: there's no need for a mixed indicator, only one indicator that changes colour (or goes from clear
to coloured or vice versa) at the anticipated pH of the endpoint will do.
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paw_20
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The table is taken from a 1940s book on volumetric analysis, so maybe it was more common back then.
It does seem to indicate that some of the mixed indicator systems have sharper endpoints, for example 3 parts bromocresol green (0.1% in alc.) to 1
part methyl red (0.2% in alc.) is supposed to have a very sharp endpoint at pH=5.1 (wine-red to green), which if true is definitely better than
bromocresol green on it's own, which can be a little muddy in my (limited) experience. Some of the others indicate more easily visible color changes,
like violet to green vs. blue to green.
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Hexavalent
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Perhaps they're referring to screened indicators (to both adjust the colours for better contrast, and adjust the transition point?) - for instance,
screened methyl orange, an indicator consisting of a solution of methyl orange and xylene cyanol, changes from grey to green as the solution becomes
more basic.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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paw_20
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That would make sense. One of the listed mixtures is just that, methyl orange and xylene cyanol; violet in acidic, gray at pH 3.8, green in alkaline.
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Acidum
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pI stands for "isoelectric point".
Most prominent examples are amino acids, and pH value at which they are electroneutral (sum of all ionized groups like -COO- and
-NH4+ equals zero). The same goes for any ionizable multifunctional molecule.
...and then I disappeared in the mist...
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blogfast25
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Acidum:
I read pI in paw's post as "p forward slash", not as "pI". pI is indeed the pH of the isoelectric point but that's far from clear from the
provided context.
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Acidum
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Quote: Originally posted by blogfast25  | Acidum:
I read pI in paw's post as "p forward slash", not as "pI". pI is indeed the pH of the isoelectric point but that's far from clear from the
provided context. |
No, it's I in italic. If you copy only problematic letter in simple text editor (like notepad) it will be pasted as normal I. Also, in title
you an clearly see I. Finally, there is no "forward slash" or "root" in chemistry...
As for context, it is not clear at all. I haven't heard of "titration exponent", but I am not from english speaking area...
...and then I disappeared in the mist...
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blogfast25
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Language and texts evolve, I though "p forward slash" (as I read it) might have stood for '"p whatever", like "pH", "pK", "pC", at the time of
writing. Somefink like that...
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