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Whence came the symbol pH

People who are curious about the symbol pH may well enjoy the history of the symbol by William B. Jensen of Oesper Collections in the History of Chemistry, Department of Chemistry, University of Cincinnati [J. Chem. Ed., 81, 21 (2004)].

Jensen says that the Danish biochemist Søren P. L. Sørensen (1868–1939) introduced the pH concept in 1909. At the time, he was investigating the use of a normal hydrogen electrode, H⁺/Pt/H₂, to measure the acidity of solutions. If the pressure of the H₂ is fixed at 1 atm, the potential of the overall cell is proportional to –log[H⁺]. Consequently, Sørensen found it convenient to use that term as an indirect measure of acidity in his work. This was particularly so because most of the H⁺ concentrations he was working with were much smaller than 1 and could be written "in the form of a negative power of 10 ... [and] I will employ the name 'hydrogen ion exponent' and the symbol pH for the numerical value of the exponent of this power."

In other words, Sørensen expressed his hydrogen ion concentrations in the form 10^-p. In this form, p represented the numerical power (English), puissance (French), or potenz (German) of the exponent. Variations on Sørensen's symbol soon appeared. They included p_H+, P_h, and pH; pH eventually became dominant, largely because of its official adoption by the Journal of Biological Chemistry during the decade 1910–19.

Not everyone was pleased with Sørensen's proposal, Jensen writes. For example, W. Mansfield Clarke, whose monograph, "The Determination of Hydrogen Ions," was the major American reference for Sørensen's work during the early decades of the 20th century, was certainly convinced of the importance of the H⁺ ion in biochemical phenomena. Clarke, however, was a good deal more ambivalent about the wisdom of using the pH concept to represent variations in its concentration. He wrote: "Both convenience and the nature of the physical facts invite us directly or indirectly to operate with some logarithmic function of [H⁺]. It is unfortunate that a mode of expression so well adapted to the treatment of various relations should conflict with a mental habit. [H⁺] represents the hydrogen ion concentration, the quantity usually thought of in conversation when we speak of increases or decreases in acidity. pH varies inversely as [H⁺]. This is confusing."

Nevertheless, Jensen explains, Clarke believed that he had to bow to the trends in the chemical literature, which showed an exponential increase in the use of the [pH] concept between 1910 and the appearance of the third edition of his book in 1928. But even in his acquiescence, Jensen notes, Clarke could not resist making the somewhat sarcastic observation that, "like the Greeks who personified the virtues, we, having embodied the acidic and basic properties, have lifted to our Olympus the hydrogen and hydroxyl ions."

A porcine memory

A story appeared here recently (C&EN, Jan. 19, page 96) about Robert Millikan, whose work on the famous oil-drop experiment required him to eyeball an oil drop for hours on end. Mrs. Millikan, the story went, once explained her husband's absence to a dinner guest in the [misinterpreted] words that he had "washed and ironed for an hour and a half."

This yarn reminded Martin P. Hughes of Bothell, Wash., of another story of misunderstood words. Hughes was reared on a hog farm in Idaho. The farm specialized in the breeding and birth of hogs, or farrowing. After the piglets were weaned, the farmer sold them to another farmer, who specialized in rearing them for market. Female pigs, Hughes adds, are called gilts before they give birth and sows thereafter.

Hughes's youthful brother one day was picking up a load of feed at the local co-op when he was quizzed by an old-timer about why he needed 10 tons of hog feed. Asked the old timer: "How many pigs you got out there?" Hughes's brother did the calculation in his head, adding the sows they had in the farrowing barn to the new gilts that they had just bought, and told the old-timer, "We've got 19 sows and gilts." He couldn't quite understand the old-timer's amazement. Then, a week or so later, a neighbor stopped by with the secret. He said, "The old guy down at the feed store was telling everyone who came by that 'there's this young kid out on the prairie that's got 19,000 gilts! Can you imagine?' "