|MadSci Network: Science History|
The familiar pH "scale" isn't really a scale at all, or at least it didn't start out that way. What we call "pH" is actually a mathematical function. "p" is a shorthand notation for "-log" (the negative logarithm). "H" refers to hydrogen ion concentrations in solution. Thus, pH = -log[hydrogen ion concentration] Now, how does that all turn into a "scale"? A solution's acidity or basicity is determined by how many excess hydrogen ions are floating around in solution. Those excess hydrogen ions come from the water itself, by a natural reaction called "dissociation" that goes on all the time, and from things like acids that are added to the water. Chemists love to talk about how acidic (or how basic) something is, but it is far too tedious to say, "Here I have a solution that contains 1 x 10 (exp)-3 hydrogen ions!" Taking the negative logarithm of that number gives us "3". It makes for a very useful shorthand notation. "I have a solution with a pH of 3" says the same thing, and is much easier to understand. It just so happens that water solutions that have more than 1 x 10 (exp)-7 hydrogen ions are acidic, and solutions with less than that concentration are basic. That's why "pH 7" is right in the middle of the normal range. So remember, the pH scale is not just a bunch of numbers put up to record how acidic something is. It is actually a very precise measurement of how acidic a solution is, based on the actual concentration of acid it contains. Who thought it up? I don't believe history records that. Probably some mathematician with too much time on his hands! ADMIN NOTE: Greta Hardin reports that: "Sorenson invented the term pH in 1909. See www.lapeer.lib.mi.us/Chem/Chem2Docs/pHFacts.html for more details."
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