The standard measure of proton activity in solutions is pH, a term suggested by the Danish chemist S. P. L. Sørenson in 1909. It’s essentially a more convenient version of the minuscule percentages of molecules involved (for some details, see box). The pH scale runs from 0 to 14. The pH of neutral, pure water, with equal numbers of protons and OH ions, is set at 7. A pH lower than 7 indicates a greater concentration of protons and so an acidic solution, while a pH above 7 indicates a greater prevalence of proton-accepting groups, and so a basic solution. Here’s a list of common solutions and their usual pH.