Sørensen did not explain why he used the letter p, and the exact meaning of the letter is still disputed.
[5][6] Sørensen described a way of measuring pH using
potential differences, and it represents the negative
power of 10 in the concentration of hydrogen ions. The letter
p could stand for the French
puissance, German
Potenz, or Danish
potens, all meaning "power", or it could mean "potential". All of these words start with the letter
p in
French,
German, and
Danish, which were the languages in which Sørensen published: Carlsberg Laboratory was French-speaking; German was the dominant language of scientific publishing; Sørensen was Danish. He also used the letter
q in much the same way elsewhere in the paper, and he might have arbitrarily labelled the test solution "p" and the reference solution "q"; these letters are often paired.
[7] Some literature sources suggest that "pH" stands for the
Latin term pondus hydrogenii (quantity of hydrogen) or
potentia hydrogenii (power of hydrogen), although this is not supported by Sørensen's writings.
[8][9][10]