Every couple years someone asks me about these concepts. I've just found a great site
which talks clearly and in detail about these ideas, and I'm putting it here just for my own reference.
At the same time I'm curious, and am ordering a pH meter because I want to do some testing. A bit ago a friend of mine was just being an empiricist, did some pH measurements on RO water from the grocery store, and found the water to be an alarming pH 5. Pure water is supposed to be pretty darned close to pH 7. I talked with the guy who services the RO machines at the local stores, and was, umm, unimpressed with his knowledge of chemistry, shall we say. Nice guy, known him for years, he just lacks much of any background in Chemistry.
Contamination of the water would be necessary to shift it's pH. Even a little. and a jump on a logarithmic scale from 7 to 5? well that's pretty big, considering that the water is supposed to be Reverse Osmosis.
Doubtless I'll have more thoughts to follow on this. And if any of you are just curious enough, have a bit of litmus paper or a pH meter, I'd love to have you test your own water, just to see. "Curiouser and curiouser", claimed Alice.