Name: james gordon
I'm thinking about a conceptual problem I have in regards to teaching
about pH at the high school level. To my knowledge there are no
negative pH values, implying that the highest hydrogen-ion concentration
occurs at pH = 0 (1 M HCl, for example). In my experience with reactions
involving hydrochloric acid a 6 M HCl solution reacts somewhat more
quickly than a 1 M HCl solution, implying more hydrogen ions. Doesn't
that suggest that there might be negative pH numbers (10 M HCl would be
pH = -1, for example. Right?) What do you think?
An excellent question...we have a partial answer by considering
the fact that all strong acids are fully ionized in dilute
aqueous solution...but are NOT necessarily fully ionized in
a CONCENTRATED (6M) solution. Such a solution is said to
be "nonideal..." so the hydronium concentration of a solution
made by dropping 6 moles of HCl into 1 L of solution is not
Then we must also consider the fact that water autoionizes, and
that therefore H3O+ is the most acidic species that can
exist in water. This autodissociation is controlled by the
k_w = [H3O+][OH-]
is an approximate form, although for such concentrated solutiions
these need to be replaced by activities, perhaps using the Debye-
Huckel theory. Anyway, HCl is not the most acidic thing out there;
there are stronger proton-donators, called "superacids," which
are characterized by a function called the "Hammett acididity function."
Bottom line; negative pH values aren't possible because of the
autodissociation of water (H3O+, not H+, is the form in which
protons exist in solution and the concentration of H3O+ is
controlled by k_w, which is fixed at about 14 at 25 degrees celsius).
I am, 100%, WRONG.
I got a note from Frank Brown gently reminding me that the
first step in the autodissociation of water does not restrict
the pH to be positive. As a result I have gone and done some reading.
My confusion arose from the fact that the autodissociation
of water can be a controlling factor in certain regimes...but
definitely not in the strong acid / strong base regime.
As it turns out, commercial concentrated HCl (37% by weight)
has a pH of approximately -1.1....and saturated NaOH
solution has a pH of about 15.0 [source: Chemical Principles,.
4th edition, by Dickerson/Gray/Darensbourg/Darensbourg].
So, the answer is, although pH TENDS to range between 1 and 14
for most household chemicals and substances encountered in natural
earth conditions, there is nothing which fundamentally restricts it
to this range even at 25C.
Thanks for the sanity check Frank. And please, all accept my
apologies for any confusion.
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Update: June 2012