Correction of proton and Ca association constants of EGTA for temperature and ionic strength

The association constants of ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) for protons and Ca can be used to calculate the apparent association constant of EGTA for Ca (K'(CaEGTA)). This value is often used in calculating the free [Ca²⁺] of complex solutions such as those used to determine the Ca sensitivity of skinned muscle fibers. As association constants are usually measured at 0.1 M ionic strength and between 20 and 25°C, these constants must first be adjusted for conditions different from those at which they were measured, before calculation of K'(CaEGTA). The proton and Ca association constants (and their ΔH values) from A.E. Martell and R.M. Smith (Critical Stability Constants, New York; Plenum, vol. 1, 1974) adjusted for ionic strength and temperature using a semiempirical Debye-Huckel formalism and Van 't Hoff isochore, respectively, closely describe the influence of both ionic strength and temperature on K'(CaEGTA). Errors in the adjustment or calculation of association constants can dramatically affect the calculated value of K'(CaEGTA) and hence the calculated free [Ca²⁺] of complex solutions, such as those used to mimic the intracellular environment.