Field of Science

Chemical Urban Legends: pH

What does the p in pH stand for?

The term pH has been in use for more than a century. It is a logarithmic measure of the hydrogen ion concentration ([H+]): pH = -log10[H+]. (Technically, there aren't bare protons (H+) floating around in solutions, but that wasn’t known when pH was introduced!) The original symbol used by Sorensen was pH+.

Theories vary as to the origin of the p - most agree it means power but whether in Latin, French or German, seems in dispute. Thinking it would be either French or Latin as the original paper was published in French, I was surprised to find that it's neither, though the legend is both old and persistent. By 1920, many authors were assuming that it meant “power”, but Jens Norby returned to the original sources and points out that it was the arbitrary choice of the letters p and q for two variables in the work-up of the experimental data. The variable p eventually ends up in the formula arrived at for the concentration of the hydrogen ion.

The modern form pH was introduced in 1920, "as a matter of typographical convenience".

For the full explanation, see Jens G. Norby, The origin and the meaning of the little p in pH, Trends in Biochemical Sciences 25, 36-37 (2000). The illustration is a selection from the original paper: Sorensen, Compt. redn. du Lab. de Carlsberg 8 1-168 (1909).

5 comments:

  1. There is, of course, also pKa, the negative log of the dissociation constant of a weak acid.

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  2. @ AYDIN ÖRSTAN

    I am pretty sure that the p in Pka is derived from is use in the pH.

    And I think to remember from a lecture I went to last year that some of the confusion over the p comes from difference in the German and the French version of the Sørense Paper.

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  3. A nice write-up in Nature Chemistry...was surprised that this blog post was written almost half a year before the paper :)

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  4. I originally learned it as "potential." Go figure.

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  5. The “p” stands for potential and the “H” stands for Hydrogen. Okay, so that makes it as clear as mud. What is potential Hydrogen? A scientific explanation would state that pH refers to the plant’s ability to attract hydrogen ions. A less scientific explanation says pH is the acid/alkaline balance.

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