# What is the etymology of lower case p as the operator for the negative of the common logarithm?

In high school we were taught that the formula for pH is the negative of the common logarithm of hydrogen ion concentration: pH = -log[H+].

It wasn't until I took organic chemistry that the "acid dissociation constant" (pKa) was introduced. It was at that point that the textbook explained that "p" was used as a mathematical operator, meaning the negative of the common logarithm. (See https://web.mst.edu/~gbert/logs/pH.html.)

Why didn't they teach us that in high school? There would have been no rote memorization of the pH formula: the equation is right there in the name!

Is this operator restricted to chemistry? I've never seen it used in a (purely) mathematical context and have always wondered how the usage originated. Does anyone know its etymology or the first time it appeared in a publication?

The source [6] cited (doi:10.1021/ed800002c) disputes "power" since the article was published in German, French and Danish, but not English. It prefers the p and q suggestion for the test and reference cells, citing https://doi.org/10.1016/S0968-0004(99)01517-0. This final paper shows that there were equations in Sørensen's papers using the symbols $p_H^+, C_p, C_q, π_p$ and $π_q$.