Anyone who has studied any chemistry even at junior high school level will have come across the odd little abbreviation, pH, with its lower case p and its upper case H. The pH scale is a numeric scale, a logarithmic scale in fact, used to specify the acidity or basicity (alkalinity) of an aqueous solution. A solution that has a pH of 0 is very acidic with a high concentration of hydrogen ions (although things can be of greater acidity), a pH of 7 is a neutral solution, equal concentrations of hydrogen ions and their basic counterpart hydroxy ions. While pH 14 represents a high concentration of hydroxy ions, again you can have a pH beyond that, but for most common purposes, pH 0 to 14 covers pretty much every normal situation.
The pH scale was devised by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and later modified to its modern form in 1924 to accommodate definitions and measurements in terms of electrochemical cells. The pH of a solution approximates to the negative of the base 10 logarithm of the concentration of hydrogen atoms present in the solution measured in units of moles per litre (molar concentration). Sørensen was born 150 years ago this year, January 9 1868.
The H in pH stands for hydrogen. But, there is some controversy as to what the p represents. The Carlsberg Foundation itself says pH means "power of hydrogen". However, German chemists claim it stands for Potenz (also meaning power), whereas the French say it is their word for power, puissance. Ancient Romans would have it that it's a Latin phrase, pondus hydrogenii (meaning quantity of hydrogen), or perhaps potentia hydrogenii (capacity of hydrogen). The Brits would say it's nothing more complicated than potential hydrogen. However, there is some suggestion that Sørensen used the letters p and q to label his electrodes: the positive, hydrogen electrode, being p, the negative, next in the alphabet, thus q.
Nature Chemistry's Chief Editor Stuart Cantrill stirred up his twitter followers recently by posing the simple-seeming question asking them what the p in pH stands for. Many thought he was taking the p others told alluded to him that he should mind his p's and q's. However, the majority of the guesses were those cited above and Cantrill claimed he was just having a little pHun...the truth, maybe, lies in a 2010 article in his journal written by Michelle Francl, to which, in the end, he points his loyal followers:
Nature Chemistry 2010, 2, 600–601; DOI: 10.1038/nchem.750
In this article, Francl goes back to Sørensen's original papers and corroborates the p and q electrodes theory, wherein Sørensen discusses hydrogen ion concentration in the solution and gives it the variable Cp and then derives an equation that linearly relates log10(1/Cp) to πp (the potential of the hydrogen electrode). He then proposes that the latter quantity be given the symbol p+H. This was later simplified to the pH we all know and love, whether one has an acid tongue or a caustic wit.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".
pH scale graphic By OpenStax College [CC BY 3.0 via Wikimedia Commons