A neutral ligand ion-exchanger type of ion-selective electrode is used in the current version of the Orion Model SS-20 calcium analyzer. We evaluated it over the pH range 4.0 to 8.5, using albumin solutions containing 150 mmol/L NaCl and pooled patients' serum. Within this pH range response was nernstian, and the relationship between potential (in millivolts) and log [Ca2+] at each pH was linear. Variation in Ca2+ with increasing ionic strength (because of added NaCl) was not significantly different between standards and albumin solutions; it decreased in both. In contrast, the proportion of calcium in serum increased with increasing NaCl concentrations. Ca2+ was not bound in whole serum albumin at a pH less than about 3.9. We observed a substantial difference between the degree of Ca2+ binding in solutions of fatty-acid-free human albumin and serum.

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