where the subscript 'eq' denotes that these are equilibrium concentrations, K 1 is the equilibrium constant for the reaction CO 2 + H 2 O ⇌ H + HCO 3 (i.e. the first acid dissociation constant for carbonic acid), K 2 is the equilibrium constant for the reaction HCO 3 ⇌ H + CO 3 (i.e. the second acid dissociation constant for carbonic acid), and DIC is the (unchanging) total concentration of dissolved inorganic carbon in the system, i.e. [CO 2 ] + [ HCO 3 ] + [ CO 3 ]. K 1, K 2 and DIC each have units of a concentration, e.g. mol / L.