ION EFFECTS IN MEASUREMENT OF IONIZED CALCIUM WITH A CALCIUM-SELECTIVE ELECTRODE

  • 1 January 1980
    • journal article
    • research article
    • Vol. 26  (5) , 640-644
Abstract
The performance characteristics of an improved electrode for measurement of ionized Ca with respect to interferences by Na+, K+, Li+, Mg2+ and H+, as well as the effect of changes in ionic strength and osmolality were examined. At physiologically observed concentrations all ions except Na+ insignificantly affected ionized-Ca measurements. The decrease in ionized Ca2+ with increased Na+ concentration is caused by the increase in ionic strength and the associated decrease in Ca2+ activity. Na had no significant direct effect in concentrations below 250 mmol/l. A previously published equation for the Na+ correction is invalid for this improved electrode. Changes in osmotic pressure negligibly affect electrode response. Significant changes in ionic strength, as would be observed in severe hypo- and hypernatremic sera, increase or decrease, respectively, values for ionized Ca by changing Ca2+ activity. With aqueous Ca2+ solutions, changes of .ltoreq. 3% were observed for Na+ concentrations that deviate from 140 mmol/l by not more than 20 mmol/l. Because the Ca ion activity is considered to be the physiologically important variable, no correction need be made for ionic strength effects.