pH is a measure of the acidity or alkalinity of a solution. The variability of pH can have a dramatic effect on geochemical processes (e.g., weathering processes).

The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868–1939) in 1909 and is generally presented as ranging from 0 to 14, although there are no theoretical limits on the range of the scale (there are substances with negative pH's and with pH's greater than 14, although for most substances the range of 0–14 suffices). A solution with a pH of less than 7 is acidic and a solution with a pH of greater than 7 is basic (alkaline). The midpoint of the scale, 7, is neutral. The lower the pH of a solution, the more acidic the solution is and the higher the pH, the more basic it is. Mathematically, the potential hydronium ion concentration (pH) is equal to the negative logarithm of...