pH DefinitionDate: 10/20/2001 at 22:22:14 From: Lily Subject: pH definition Dr. Math, A few of my science/math teachers were having a discussion about pH levels one day and the question of what the 'p' in pH is arose. Nobody seemed to know the answer. Could you help us out? Date: 10/20/2001 at 23:05:54 From: Doctor Peterson Subject: Re: pH definition Hi, Lily. I just learned something. I always guessed it meant "proportion," but I was wrong! The bottom line is that we really don't know for sure where the p comes from. My first reaction to a question like this is to look it up in a dictionary. The old Merriam-Webster next to me doesn't say, but I went downstairs to get the latest edition, and it does. It says [G, fr. _P_otenz power + _H_ (symbol for Hydrogen)] (1909) (The underscores mean italicized letters.) In other words, this is an abbreviation of the German words "Potenz Hydrogen", or "power of hydrogen". Merriam-Webster can also be found on-line, at m-w.com. The other most likely place to find the information, Russ Rowlett's _How Many? A Dictionary of Units of Measurement_ at: http://www.unc.edu/~rowlett/units/dictP.html doesn't give the meaning of the "p" but agrees on the date: The pH scale was invented by S. P. L. Sorenson in 1909. He doesn't sound German, though. I should mention that the German word for hydrogen is not hydrogen, but "Wasserstoff." In checking that, I found that "Potenz" is not listed in my German dictionary; the normal words for "power" are "Kraft" and "Macht." I guessed that "Potenz" would be related to "potency," and would mean "power" only in the sense of "effectiveness." But then, looking for the word online, I found that it is used in a mathematical sense for an exponent, which of course is exactly what a logarithm is. So pH really means "exponent of hydrogen concentration" in German. And in fact, I found a reference that gives the full story: The pH scale - University of Waterloo http://www.science.uwaterloo.ca/~cchieh/cact/c123/ph.html According to the Oxford Dictionary, the pH scale was originally introduced by the Danish biochemist S.P.L. Sorensen in 1909 using the symbol pH. Other symbols such as p_H have been used in the past. The letter p is derived from the German word potenz meaning power or exponent of, in this case, 10. You may argue that what have we learned by looking into historical origin of the term? Well, the origin of concept is interesting in that we sometimes need to develop concepts ourselves. A concept or tool becomes important if many people find it convenient and elegant. The reality is that many chemists have used the pH scale and the p scales for many other quantities that they often take it for granted, without realizing the logic behind their usages. For example, we have used the pK_a, pK_b, pK_w, notations by analogy to the pH notations with out asking a question. Now that you know the pH is an exponent, the following relationship is obvious: [H+] = 10^-pH But then I searched the Web for the words "ph potenz" to check the story. I found a slightly different version at pH - Gondar Design Science http://www.purchon.com/chemistry/ph.htm The "p" stands for "potenz" (this means the potential to be) and the "H" stands for Hydrogen. "Potential" sounds like a good guess, but it's not quite right. Again, pH Explained - Swimming Pool Owners' Guide to Water Chemistry http://www.ftech.net/~jshep/ph.htm pH is the power (German 'potenz') of a solution to yield hydrogen ions [H+]. But then I found this: pH - SHiPS Resource Center http://www1.umn.edu/ships/words/pH.htm Jens Norby (2000) has recently clarified the origin and meaning of the little 'p' in pH. The H, of course, refers to H+, the hydrogen ion, whose concentration contributes to acid strength. Hence, many have supposed that the 'p' refers to power (or the French puissance or German Potenz). Some have taken this to mean the power of 10, referring to the logarithmic calculation of pH. Similarly, others construe the 'p' as denoting the potential, or intensity, of hydrogen (or in Latin, potentia). But Norby traces the 'p' to a simple mathematical convention of naming variables. Danish chemist Soren Peter Lauritz Sorensen proposed the pH scale in 1909. But he did not give an explicit reason for choosing 'p' in his original two papers. Sorensen was primarily concerned with determining the H+ concentration electrometrically. His central equation involved values for measurements at two electrodes, which he arbitrarily designated p and q. To develop a standard, he set the non-hydrogen component, Cq, at 1.0 and solved for the hydrogen ion concentration, Cp, or 10-p. The number p he suggested calling p+H. Sorensen ultimately defined pH as the negative logarithm of a factor for hydrogen concentration relative to a normality of 1.0, not (as many texts today do) as the negative logarithm of the hydrogen ion concentration. (As Norby notes, one cannot take a logarithm of a measurement with units.) You can learn and unlearn a lot on the Web in a short time! - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ Date: 10/21/2001 at 09:31:46 From: Lily/Sheela s. Subject: Re: pH definition Thank you! |
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