or alkali concentration. It can be defined as follows: where is an infinitesimal amount of added base, or where is an infinitesimal amount of added acid. pH is defined as -log₁₀[H⁺], and d(pH) is an infinitesimal change in pH.

With either definition the buffer capacity for a weak acid HA with dissociation constant K_a can be expressed as

where [H⁺] is the concentration of hydrogen ions, and is the total concentration of added acid. K_w is the equilibrium constant for self-ionization of water,

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