  Charge regulation is a fundamental mechanism in most chemical, geochemical,
and biochemical systems. Various minerals surfaces and proteins are well-known
to change their charge state as a function of the activity of the hydronium
ions, i.e., the pH. Besides being modulated by the pH, the charge state is
sensitive to salt concentration and composition due to screening and ion
correlations. Given the importance of electrostatic interactions, a reliable
and straightforward theory of charge regulation would be of utmost importance.
This paper presents such a theory that accounts for salt screening, site and
ion correlations. Our approach shows an impeccable agreement compared to Monte
Carlo simulations and experiments of 1:1 and 2:1 salts. We furthermore
disentangle the relative importance of site-site, ion-ion and ion-site
correlations. Contrary to previous claims, we find that ion-site correlations
are subdominant to the two other correlation terms.
