Introduction: In 1909, SPL Sorensen published an article in Biochem Z in which he discussed the effect of H1+ ions on enzyme activity. In this paper he coined the term pH to describe this effect and defined it as -log[H1+]. In 1924, Sorensen realized that the pH of a solution is a function of the “activity” of the H1+ ion and not the concentration and published a second article on the subject. A better definition would be pH=-log[aH1+ ], where aH1+ denotes the activity of the H1+ ion. The activity of an ion depends on many variables, including concentration. It is unfortunate that chemistry texts use a definition of pH that has been obsolete for over 50 years. Due to the difficulty of accurately measuring the activity of the H1+ ion for most solutions, the International Union of Pure and Applied Chemistry (IUPAC) and the National Bureau of Standards (NBS) have defined the pH as the reading on a pH meter that has been standardized against standard buffers. The following equation is used to calculate the pH of all solutions: The activity of the H1+ ion is determined as accurately as possible for the standard solutions used. The identity of these solutions varies from one authority to another, but all give the same pH values ​​to ±0.005 pH units. The historical definition of pH is correct for solutions so dilute and so pure that H1+ ions are not influenced by anything except solvent molecules (usually water). In most solutions, the pH ...