In any solution of water, a small proportion of the water molecules exist as protons (H+) and hydroxide ions (OH^-). The pH of a solution measures the proton concentration ([H^-]), which is important as the pH influences many chemical reactions and biological processes. It is calculated by the following formula:

pH = –log [H+]

The pH of a solution can range from 0 to 14. Since the pH scale is logarithmic, a difference of one pH unit corresponds to a tenfold difference in hydrogen ion concentration. A solution is neutral (pH = 7) when the concentrations of protons (H+) and hydroxide ions (OH^-) are equal. When the concentration of protons is higher than that of hydroxide ions, the pH is lower than 7 and the solution is acidic. When the concentration of protons is lower than that of hydroxide ions, the pH is higher than 7 and the solution is basic. An acid can therefore be described as a molecule that releases a proton (H+), and a base is a molecule that accepts a proton in aqueous solution.

Pure water has a pH of 7—that is, it is neutral, with an equal concentration of protons and hydroxide ions. The pH of most cells is approximately 7 and is tightly regulated, as most chemical reactions can be carried out only in a narrow pH range. Certain cellular compartments, however, have a much lower pH. The pH of blood is slightly basic, with a pH around 7.4. This value is sometimes referred to in medicine as physiological pH. Freshwater lakes, ponds, and rivers tend to be slightly acidic because carbon dioxide from the air dissolves in the water and forms carbonic acid (Chapter 6).