Calculated and measured [Ca2+] in buffers used to calibrate Ca2+ macroelectrodes

The ionized concentration of calcium in physiological buffers ([Ca2+]) is normally calculated using either tabulated constants or software programs. To investigate the accuracy of such calculations, the [Ca2+] in EGTA [ethylene glycol-bis(β-aminoethylether)-N,N,N|,N|-tetraacetic acid], BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N|,N|-tetraacetic acid], HEDTA [N-(2-hydroxyethyl)-ethylenediamine-N,N|,N|-triacetic acid], and NTA [N,N-bis(carboxymethyl)glycine] buffers was estimated using the ligand optimization method, and these measured values were compared with calculated values. All measurements overlapped in the pCa range of 3.51 (NTA) to 8.12 (EGTA). In all four buffer solutions, there was no correlation between measured and calculated values; the calculated values differed among themselves by factors varying from 1.3 (NTA) to 6.9 (EGTA). Independent measurements of EGTA purity and the apparent dissociation constants for HEDTA and NTA were not significantly different from the values estimated by the ligand optimization method, further substantiating the method. Using two calibration solutions of pCa 2.0 and 3.01 and seven buffers in the pCa range of 4.0–7.5, calibration of a Ca2+ electrode over the pCa range of 2.0–7.5 became a routine procedure. It is proposed that such Ca2+ calibration/buffer solutions be internationally defined and made commercially available to allow the precise measurement of [Ca2+] in biology.