Re: Physical Interpretation of Negative pH.
- From: Uncle Al <UncleAl0@xxxxxxxxxxxxx>
- Date: Mon, 29 Aug 2005 11:21:50 -0700
Farooq W wrote:
>
> Why are most general chemistry text simply silent on negative pH values
> and values greater than 14? In the elementary classes chemistry
> students learn that pH are defined from 0-14, but later they are unable
> to accept the negative pH concept when by accident they _calculate_ pH
> of > 1 M HCl, but of course activity comes into play.
> (i) My question is what are common examples (leave aside super-acids)
> of solutions having negative pH values and how are they experimentally
> measured. Though I never tried, in fear of damaging it, but if I dip
> the glass electrode in concentrated 12 M HCl would the pH-meter show
> negative values?
>
> (ii) Some people claim that the dissociation is controlled by self
> ionization of water and the product [H+][OH-] = 10^-14 always holds
> true.
> See for example the answer 1 here:
> http://www.newton.dep.anl.gov/askasci/chem99/chem99230.htm
>
> (iii) A short note in J.Chem.Ed has a procedure for making a pH
> -47 to +47 HCl solution which I think is meant to be a joke.
>
> Suppose we have 1 liter of 1x10^-6 M HCl, add about 99 L of water, the
> [HCl] is 1x10^-8 M and pH= 8. Now suppose you have single molecule of
> HCl in 10^23 L of water, so that
> [HCl] = 1.7x10^-47, the pH is then 47!
>
> Regards,
> Farooq.
You might want to cease omitting the autoprotolysis background,
[H+][OH-] = 10^(-14)
Adding a single proton to a liter of pure water gives you pH 7. You
need not even bother with a counterion.
In water you cannot have an acid more acidic than hydronium or a base
more basic than hydroxyl, pH 0-14. If you did, it would react with
water to give hydronium or hydroxyl - the Leveling Effect. You could
melt a hydrated salt containing an oxophilic cation to get
ridiculously acid Arrhenius pH values vs. aqueous solution because
there would not be any unbound water in "solution."
In DMSO you cannot have an acid more acidic than protanted DMSO or a
base more basic than dimsyl. That is a *much* wider pH, -log[H+],
range. In anhydrous H2SO4 you can tolerate immensely strong acids,
for H2SO4 does not easily protonate. CF3SO3H even less so.
A proton is a special case of a Lewis acid. You can create
solvo-cation solvo-anion pairs in which the anion is large,
delocalized, symmetric, and a very weak nucleophile indeed -
polychlorinated dodecahedral carboranes. Aqueous pH value limits are
then a poor joke as protonated silylonium ions obtain as isolable
species.
Similarly, one can play thermodynamics against kinetics. Bu-Li/TMEDA
is an immensely strong base that will multiply deprotonate ferrocene.
It will deprotonate THF to acetaldehyde enolate, slowly. Keep the
solution cold and don't take a coffee break. Proton Sponge is hugely
more basic than aniline, but don't be in a rush. Proton Sponge takes
hours or days to come into equilibrium with naked protons. It's a
steric hindrance problem.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
.
- References:
- Physical Interpretation of Negative pH.
- From: Farooq W
- Physical Interpretation of Negative pH.
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