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Encyclopedia of Astrobiology
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Hydrogen ion concentration index


pH describes the activity of hydronium ions (H3O+, or more precisely the hydrated form of the proton H+ in solution) in aqueous solution. pH characterizes the acidity or basicity of aqueous solutions. It is formally defined by the relation:

$$ \mathrm{p}\mathrm{H}=-{ \log}_{10}{a}_{\mathrm{H}} $$

in which a H is the activity of the solvated hydronium ion. The pH of an acidic solution is less than 7, and the pH of basic solution is greater than 7 at 25 °C, but pH values change as a function of temperature.

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Correspondence to Kensei Kobayashi .

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© 2014 Springer-Verlag Berlin Heidelberg

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Kobayashi, K. (2014). pH. In: Amils, R., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg.

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