pH is a number used by scientists to indicate the concentration of hydrogen ions in a solution. pH generally ranges from 0 to 14. The letters pH stand for potential hydrogen. At 77 °F (25 °C), a pH below 7 indicates that a solution is acidic. A pH above 7 indicates that a solution is basic (alkaline). A neutral solution, such as pure water, is neither acidic nor basic. It has a pH of 7. Human blood has a pH of about 7.4.
The Danish biochemist Soren Sorensen invented the pH system in 1909. A solution’s pH is defined as the negative logarithm, to the base 10, of its hydrogen-ion concentration. This concentration is expressed in moles of hydrogen ions per liter of solution (see Mole ). A solution with a pH of 6 contains one millionth of a mole of hydrogen ions per liter.
pH is often measured with an electronic pH meter or with special dyes called acid-base indicators. The color of an indicator depends on the concentration of hydrogen ions. pH paper contains several indicators that change color at different pH’s. When dipped into a solution, the paper’s color indicates the approximate pH of the solution. pH is used to analyze body secretions, to test soil suitability for certain crops, and for various industrial purposes.
