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Clinical Chemistry 28: 1936-1945, 1982;
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Clinical Chemistry, Vol 28, 1936-1945, Copyright © 1982 by American Association for Clinical Chemistry

Establishing the direct-potentiometric "normal" range for Na/K: residual liquid junction potential and activity coefficient effects

JD Czaban, AD Cormier and KD Legg

The observed reference ranges for sodium and potassium as determined by direct potentiometry vary from instrument to instrument, depending on the composition of the calibration standards. To resolve the existing confusion as to which reference intervals are most appropriately considered "normal," we propose a straightforward convention (based on plasma-water concentration units) in which the difference between direct (undiluted sample) and indirect (diluted sample) methodologies is accounted for by the volume displacement effect of proteins, lipids, and other dissolved substances in a typical plasma sample. Thus, the proposed reference intervals for sodium and potassium are approximately 7% greater by direct potentiometry than by procedures involving dilution. Data consistent with this convention can be obtained with a variety of aqueous-based calibrants, provided care is taken to minimize the errors resulting from activity coefficient and liquid junction potential effects. Additional experimental results are presented to show that these effects also account for the apparent suppression of the sodium ion concentration observed in the presence of bicarbonate ion.


The following articles in journals at HighWire Press have cited this article:


Home page
 Clin. Chem.Home page
F. Zoppi and C. Cristalli
Ionized Magnesium in Serum and Ultrafiltrate: pH and Bicarbonate Effect on Measurements with the AVL 988-4 Electrolyte Analyzer
Clin. Chem., March 1, 1998; 44(3): 668 - 671.
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Copyright © 1982 by the American Association for Clinical Chemistry.