The Charlot equation, named after Gaston Charlot, is used in analytical chemistry to relate the hydrogen ion concentration, and therefore the pH, with the formal analytical concentration of an acid and its conjugate base.
It can be used for computing the pH of buffer solutions when the approximations of the Henderson–Hasselbalch equation break down.
For an acid-base equilibrium such as HA ⇌ H+ + A−, the Charlot equation may be written as where [H+] is the equilibrium concentration of H+, Ka is the acid dissociation constant, Ca and Cb are the analytical concentrations of the acid and its conjugate base, respectively, and Δ = [H+] − [OH−].
The equation can be solved for [H+] by using the autoionization constant for water, Kw, to introduce [OH−] = Kw/[H+].
This results in the following cubic equation for [H+], which can be solved either numerically or analytically: The solution to this equation may also be given in explicit form, although this is inconvenient to use Considering the dissociation of the weak acid HA (e.g., acetic acid):