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pH is a measure of the acidity or basicity of a solution. It was first introduced by Danish chemist Soren Peder Lauritz Sorensen at the Carlsberg Laboratory in 1909. Sorensen suggested the notation "PH" for convenience, standing for "power of hydrogen", using the cologarithm of the concentration of hydrogen ions in solution, pH. Although this definition has been superseded pH can be measured if an electrode is calibrated with solution of known hydrogen ion concentration.

Pure water is said to be neutral. The pH for pure water at 25 °C is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline.