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In chemistry, pH (), historically denoting "potential of hydrogen" (or "power of hydrogen"), is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of H+ ions) are measured to have lower pH values than basic or alkaline solutions.
The pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution.





pH

=
−
log
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a


H

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)
=
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log
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H

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M

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{\displaystyle {\ce {pH}}=-\log(a_{\ce {H+}})=-\log([{\ce {H+}}]/{\ce {M}})}
where M = mol dm−3. At 25 °C (77°F), solutions with a pH less than 7 are acidic, and solutions with a pH greater than 7 are basic. Solutions with a pH of 7 at this temperature are neutral (i.e. have the same concentration of H+ ions as OH- ions, e.g. pure water). The neutral value of the pH depends on the temperature – being lower than 7 if the temperature increases above 25 °C. The pH value can be less than 0 for very concentrated strong acids, or greater than 14 for very concentrated strong bases.The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode. The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, agronomy, medicine, water treatment, and many other applications.

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