Introduction:In 1909 SPL Sorensen published an article in Biochem Z in which he discussed the effect of H1+ ions on the activity of enzymes. In the article he invented the term pH to describe this effect and called it -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 paper on the topic. A better definition would be pH=-log[aH1+], where aH1+ denotes the activity of the H1+ ion. The activity of an ion is a function of many variables, one of which is the concentration. It is unfortunate that chemistry textbooks use a definition of pH that has been outdated for over 50 years. Because of the difficulty in accurately measuring H1+ ion activity for most solutions, the International Union of Pure and Applied Chemistry (IUPAC) and the National Union Bureau of Standards (NBS) have defined pH as the pH meter reading standardized compared to 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 they all give the same pH values ​​up to ±0.005 pH units. The historical definition of pH is correct for those solutions that are so dilute and so pure that they are not affected by H1+ ions except solvent molecules (usually water). In most solutions the pH ...