Charge regulation is fundamental in most chemical, geochemical,
and biochemical systems. Various mineral surfaces and proteins are
well-known to change their charge state as a function of the activity
of the hydronium ions, that is, 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 Article presents a theory that
accounts for salt screening, site, and ion correlations. Our approach
shows an impeccable agreement as 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 for the studied cases are subdominant to the two other
correlation terms.