Potenz, or Danish potens, meaning "power", or it could mean "potential". All the words for these start with the letter p in French, German, and Danish—all languages Sørensen published in: Carlsberg Laboratory was French-speaking, German was the dominant language of scientific publishing, and Sørensen was Danish. He also used the letter q in much the same way elsewhere in the paper. He might also have labelled the test solution "p" and the reference solution "q" arbitrarily; these letters are often paired. Some literature sources state that the "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. Currently in chemistry, the p stands for "decimal logarithm of", and is also used in the term pKa, used for acid dissociation constants and pOH, the equivalent for hydroxide ions. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues (of whom she was one) with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little, Clark and colleagues knew about Sørensen's work a few years prior. She said: In these studies [of bacterial metabolism] Dr. Clark's attention was directed to the effect of acid on the growth of bacteria. He found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined the quantity, not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration. These methods replaced the inaccurate titration method of determining the acid content in use in biologic laboratories throughout the world. Also they were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934. It was in response to local citrus grower Sunkist that wanted a better method for quickly testing the pH of lemons they were picking from their nearby orchards. Definition and measurement pH pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. For example, for a solution with a hydrogen