Monday, July 28, 2008

pH crash course

Herein, in as few words as possible, I will attempt to describe what pH is and why every aqueous solution has a pH.

pH is mathematically defined as the negative log of the hydronium concentration. Or:

pH = –log[H3O+]
So, what is H3O+? Simply put, it is the active ingredient of acid. A more concentrated acid will produce more H3O+.
Water spontaneously reacts with itself to form hydronium (H3O+) and hydroxide (OH-):
2H2O <----> H3O+ + OH-
At 25°C, the concentration of hydronium times the concentration of hydroxide is always 1x10-14 moles per liter (M). Mathematically:
[H3O+][OH-]= 1x10-14
To make a long story short, [H3O+] cannot be zero and will never be zero. So, any and all aqueous solutions must have a pH value. Besides, the log of 0 is really big. I mean really, really big.

A neutral solution has equal amounts of hydronium and hydroxide:
[H3O+] = [OH-] = 1x10-7 M (at 25°C)
The pH of a neutral solution is 7 since -log(1x10-7) = 7.

So, it is inaccurate to say a solution does not have a pH.

In addition:

A solution can have a negative pH. For example a pH of -1.0 would correlate to a hydronium concentration of 10M.

Every common usage of pH is at 25°C. pH is temperature dependent. For example a neutral solution of pure water at 100°C has a pH of 6.14!!! Even though the pH is <7, this is neutral solution!!!!


Liquidcarbon said...

Redefine Kw. Product of two concentrations can't be in M.

Chemgeek said...

Well, that was a pretty stupid mistake. I cut and pasted and didn't drop the units.

It's fixed now.


Anonymous said...

errr... the magnitude of log(0) is really huge... regrettably log(0) is really really small.

That's what my math degree is good for because it certainly doesn't pay bills.