On Wed, 2 Feb 2005 15:46:58 -0000, "Brian Reay"
wrote:

I thought the pH scale came from "per Hydronium"- H30+ ?

If the 'extra' H came from the water, then there would also be OH- to
neutralise it.

pH is defined as the negative of the logarithm of the hydrogen ion
concentration.

In pure water the product of [H+][OH-] is very close to 10exp-14,
where square brackets indicate concentrations.

Hence in pure water pH = 7 because the concetrations of the two
species are equal.

An acid solution, where [H+] is greater than that in water, has [H+]7

There is a more exact definition involving hydrogen ion activity.
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from
Aero Spike