pH

pH 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.

The concept of pH was first introduced by Danish chemist Soren Peder Lauritz Sorensen at the Carlsberg Laboratory in 1909.

It is unknown what the exact definition of p stands for. Some references suggest the p stands for “Power”, others refer to the German word “Potenz” (meaning power in German), and still others refer to “potential”.

Jens Norby published a paper in 2000 arguing that p is a constant and stands for “negative logarithm”; which has also been used in other works.

H stands for Hydrogen. Sorensen suggested the notation "PH" for convenience, standing for "power of hydrogen", using the cologarithm of the concentration of hydrogen ions in solution, p[H] Although this definition has been superseded p[H] can be measured if an electrode is calibrated with solution of known hydrogen ion concentration.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.