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xkcd #2943: Unsolved Chemistry Problems
(imgs.xkcd.com)
Alt text:
I'm an H⁺ denier, in that I refuse to consider loose protons to be real hydrogen, so I personally believe it stands for 'pretend'.
For what it's worth, my job is as an analytical chemist, dealing with pH readings every single day, and I've always thought this was correct.
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