1pH is a measure of the acidity or basicity of a solution.
It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+).
Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations.
The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement.
Some references suggest the p stands for “Powerâ€[2], others refer to the German word “Potenz†(meaning power in German)[3], still others refer to “potentialâ€.
Jens Norby published a paper in 2000 arguing that p is a constant and stands for “negative logarithmâ€[4]; which has also been used in other works[5]. H stands for Hydrogen.
Sørensen suggested the notation "PH" for convenience, standing for "power of hydrogen",[2] using the cologarithm of the concentration of hydrogen ions in solution, p[H][6] Although this definition has been superseded p[H] can be measured if an electrode is calibrated with solution of known hydrogen ion concentration.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 (77 °F) 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. pH measurements are important in medicine, biology, chemistry, food science, environmental science, oceanography and many other applications.