PH

pH is a scale of acidity from 0 to 14. It tells how acidic or alkaline a substance is. More acidic solutions have lower pH. More alkaline solutions have higher pH. Substances that aren't acidic or alkaline (that is, neutral solutions) usually have a pH of 7. Acids have a pH that is less than 7. Alkalis have a pH that is greater than 7.

pH is a measure of the concentration of protons (H⁺) in a solution. S.P.L. Sørensen introduced this concept in the year 1909. The p stands for the German potenz, meaning power or concentration, and the H for the hydrogen ion (H⁺).

The most common formula for calculating pH is:

${\displaystyle {\mbox{pH}}=-\log _{10}\left[{\mbox{H}}^{+}\right]}$

[H⁺] indicates the concentration of H⁺ ions (also written [H₃O⁺],^[1] the equal concentration of hydronium ions), measured in moles per litre (also known as molarity).

However, the correct equation is actually:

${\displaystyle {\mbox{pH}}=-\log _{10}\left[a_{\mathrm {H^{+}} }\right]}$

where ${\displaystyle a_{\mathrm {H^{+}} }}$ indicates the activity of H⁺ ions.^[2] But, this equation in most cases provides the same value as the more common formula, so in introductory chemistry the previous equation is given as the definition of pH.

Most substances have a pH in the range of 0 to 14, although extremely acidic or alkaline substances may have pH < 0, or pH > 14.

Alkaline substances have, instead of hydrogen ions, a concentration of hydroxide ions (OH^-).

1. ↑ Cite error: The named reference explan was used but no text was provided for refs named (see the help page).
2. ↑ Hawkes, Stephen J. (2000-09-01). "Easy Derivation of pH Is Approximately Equal To (pKa1 + pKa2) / 2 Using Autoprotolysis of HA-: Doubtful Value of the Supposedly More Rigorous Equation". Journal of Chemical Education. 77 (9): 1183. doi:10.1021/ed077p1183. ISSN 0021-9584.