PH

pH is a measure of the activity of hydrogen ions (H+) in a solution and, therefore, its acidity or alkalinity. In aqueous systems, the hydrogen ion activity is dictated by the dissociation constant of water (Kw) = 1.011 × 10−14 at 25 °C) and interactions with other ions in solution. Due to this dissociation constant a neutral solution (hydrogen ion activity equals hydroxide ion activity) has a pH of approximately 7. Aqueous solutions with pH values lower than 7 are considered acidic, while pH values higher than 7 are considered alkaline.

pOH

There is also pOH, in a sense the opposite of pH, which measures the concentration of OH− ions. Since water self ionizes, and notating as the concentration of hydroxide ions, we have

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:K_{w} = left left = 10^{-14} (*)

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where Kw is the ionization constant of water.

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Now, since

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:log_{10}K_{w} = log_{10} left + log_{10} left

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by logarithmic identities, we then have the relationship

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:-14 = log_{10} left + log_{10} left (*)

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and thus

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:mbox{pOH} = -log_{10} left = 14 + log_{10} left = 14 - mbox{pH} (*)

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(*) Valid exactly for temperature = 298.15 K (25 °C) only, acceptable for most lab calculations.

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