Is the difference between 2 pH values linear or exponential?
ie. Is the difference between a pH 6 and 7 the same as the difference between pH 2 and 3?
It’s logarithmic.
pH 7 is ten times the propensity of Hydrogen of pH 6.
Each integer is ten time higher than the last.
Ugh.
“Each integer value of pH represents a value ten times higher than the last”
So 7.0 is ten times 6.0
pH = -Log[H[sup]+[/sup]]
For the layperson (like me), that means the pH is equal to the negative log of the concentration of hydrogen atoms in a given solution.
OK. Thanks, I have a pH meter and I’m trying to get the hot tub balanced and it seemed that it was not linear.
(bolding mine)
[nitpick]That should be hydrogen ions (or usually hydronium ions, depending on which definition of pH you’re using–technically, if I’m not mistaken, the “true” definition used by hard-core physical chemists is something like “the negative log of the ionic strength of hydrogen” or something like that, but my p-chem is WAY too rusty to remember how to word that correctly), not hydrogen atoms.[/nitpick]
Sorry, this is the SDMB. We can’t let you use such a sloppy definition 
Oh, pH is even messier than that. While it technically has something to do with hydronium, it’s actually a matter of the free electrons available in the solution, usually an inverse of the hydronium, but when one starts to throw Lewis acids into the matter, things get wacky.
I think this is called the ‘activity’ if I recall analytical chem correctly. There are huge tables which are needed to accurately calculate pH (based on the ions), but like you said what we use now is generally a good approximation.
Carver, keep in mind that a pH electrode is temerature-dependent (but I’m not sure how much an effect his has), so you should probably test your water both hot and cold.
Yes, activity is precisely the word I was looking for.
I actually was working on an animation on pH a few months back for a general chemistry text, and had to force myself to stick to the textbook and not go off on a tangent (so I never quite got to the point of re-learning the complex definition, I just reminded myself that the complex definition existed). I almost put a couple questions in there about negative pH’s and pH’s greater than 14, but found that it really really depended on how you defined pH, so I think I ended up avoiding it completely.
even with the lewis acids, the definition -log(H_3O+) would work… for example, NH_4+ + H_20 -> NH_3 + H_3O (i can’t make an equillibrium sign…)