Editing PH (section)

=== Strong acids and bases ===
[[Strong acid]]s and [[Strong base|bases]] are compounds that are essentially fully dissociated in water. This means that in an acidic solution, the concentration of hydrogen cations (H<sup>+</sup>) can be considered equal to the concentration of the acid. Similarly, in a basic solution, the concentration of hydroxide ions (OH<sup>−</sup>) can be considered equal to the concentration of the base. The pH of a solution is defined as the negative logarithm of the concentration of H<sup>+</sup>, and the pOH is defined as the negative logarithm of the concentration of OH<sup>−</sup>. For example, the pH of a 0.01&nbsp;M solution of hydrochloric acid (HCl) is equal to 2 (pH = −log<sub>10</sub>(0.01)), while the pOH of a 0.01&nbsp;M solution of sodium hydroxide (NaOH) is equal to 2 (pOH = −log<sub>10</sub>(0.01)), which corresponds to a pH of about 12.

However, self-ionization of water must also be considered when concentrations of a strong acid or base is very low or high. For instance, a {{val|5|e=−8|u=M}} solution of HCl would be expected to have a pH of 7.3 based on the above procedure, which is incorrect as it is acidic and should have a pH of less than 7. In such cases, the system can be treated as a mixture of the acid or base and water, which is an [[amphoteric]] substance. By accounting for the self-ionization of water, the true pH of the solution can be calculated. For example, a {{val|5|e=−8|u=M}} solution of HCl would have a pH of 6.89 when treated as a mixture of HCl and water. The self-ionization equilibrium of solutions of sodium hydroxide at higher concentrations must also be considered.<ref>{{cite web |last=Maloney |first=Chris |title=pH calculation of a very small concentration of a strong acid. |url=http://sinophibe.blogspot.com/2011/03/ph-calculation-of-very-small.html |url-status=live |archive-url=https://web.archive.org/web/20110708062942/http://sinophibe.blogspot.com/2011/03/ph-calculation-of-very-small.html |archive-date=8 July 2011 |access-date=13 March 2011}}</ref>