...It is unknown what the exact definition of 'p' in pH is. Some other references suggest the p stands for “Power”,[7] others refer to the German word “Potenz” (meaning power in German),[8] still others refer to “potential”. Jens Norby published a paper in 2000 arguing that p is a constant and stands for “negative logarithm”;[9] which has also been used in other works.[10] H then stands for Hydrogen. According to the Carlsberg Foundation pH stands for "power of hydrogen".[7] Other suggestions that have surfaced over the years are that the p stands for puissance (also meaning power but then the Carlsberg Laboratory was French speaking) or that pH stands for the Latin terms pondus Hydrogenii or potentia hydrogenii. It is also suggested that Sørensen used the letters p and q (commonly paired letters in mathematics) simply to label the test solution (p) and the reference solution (q).[11]
http://en.wikipedia.org/wiki/PHTo scientists it is a mathematical function meaning
"the negative of the logarithm (base 10) of the hydrogen ion concentration".
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