Chemistry and Chemical Disasters. A logarithmic measure used to state the acidity or alkalinity of a chemical solution. The properties of a liquid solution we call "acid" are caused by the presence of hydrogen ions (H+). The pH of a solution is a measure of the concentration of these hydrogen ions. Technically, the pH of a solution is defined to be the negative logarithm of the concentration, measured in moles per liter. This unit is inverted in the sense that lower pH readings correspond to greater acidity, and therefore more hydrogen ions. Lowering the pH by 1.0 means multiplying the ion concentration by a factor of 10. Mathematically the scale is open at both ends, but in practice pH values usually fall in the range from 0 to 14. Pure water at 25ºC (77ºF) has a pH close to 7.0. Numbers below 7 indicate increasing acidity, while numbers above 7 indicate increasing alkalinity. The pH ("potential of Hydrogen") scale was invented in 1909 by the Danish chemist Søren Peter Lauritz Sørensen (1868-1939)