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Name: james gordon
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
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?



Replies:
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 6M.

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 equilibrium constant....

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).

rtopper


WHOOPS!

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.

-topper



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