MadSci Network: Science History

Re: Who made up the ph scale?

Date: Sun May 2 10:05:19 1999
Posted By: Samuel Conway, Senior Scientist, Message Pharmaceuticals, Aston, PA
Area of science: Science History
ID: 924100167.Sh

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

Who thought it up?  I don't believe history records that.  Probably some
mathematician with too much time on his hands!

Greta Hardin reports that:
"Sorenson invented the term pH in 1909.
See for more details."

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