pH

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pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25 °C with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). The pH of 7.00 is considered neutral at 25 °C because at this pH the concentration of H3O+ approximately equals the concentration of OH− in pure water. pH is formally dependent upon the activity of hydronium ions (H3O+), but for very dilute solutions, the molarity of H3O+ may be used as a substitute with little sacrifice of accuracy. (H+ is often used as a synonym for H3O+.) Because pH is dependent on activity, a property which cannot be measured easily or predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways of doing this.

The concept of pH was first introduced by Danish chemist S. P. L. Sørensen at the Carlsberg Laboratory in 1909. The name, pH, has been purported to come from a variety of places including: pondus hydrogenii (Latin), potentiel hydrogène (French), and potential of hydrogen (English). However pH is actually a shorthand for its mathematical approximation: in chemistry a small p is used in place of writing − log10+], the concentration of hydrogen ions.