Effect of pH and NaCl on measurements of ionized calcium in matrices of serum and human albumin with a new calcium-selective electrode.
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.
D002118:Calcium / Q000097:blood*; D004563:Electrochemistry / Q000295:instrumentation*; D004566:Electrodes; D005069:Evaluation Studies as Topic; D006801:Humans; D006863:Hydrogen-Ion Concentration; D012709:Serum Albumin; D012965:Sodium Chloride
D012709:Serum Albumin; D012965:Sodium Chloride; D002118:Calcium