One-Hundred Years of pH

The idea of expressing the hydrogen-ion concentration on a log arithmetic scale was presented by S. P. L. Sorensen in 1909. The symbol that he used was the letter p and a smaller H appearing almost as a subscript. Typographical convenience led journals to adopt the current symbol. It has been common to assume that the p represented a word such as power, but in 2000, it was asserted that the p came from the way that Sorensen designated the cells that he used to measure pH. Biological workers quickly adopted the pH symbol to specify buffer conditions, but chemists were slow to adopt it. Today this symbol is widely used, and the little p has assumed a life of its own. We trace some of the history of buffers, indicators, and the pH meter. We also show the slow incorporation of pH into "Chemical Abstracts" and into Introductory Chemistry in Berkeley. The modern definition of pH is a purely operational one. Buffers that have been carefully calibrated to closely follow the activity of the hydrogen ion are used to standardize pH meters, and these meters are then used to interpolate between these standards. The teaching problems caused by the definition pH in terms of a single-ion activity are also discussed.