Clinical Chemistry, Vol 31, 267-270, Copyright © 1985 by American Association for Clinical Chemistry

Apparent binding of ionized calcium by various buffers

JA Crowell and GN Bowers Jr

Using different substance concentrations in an aqueous 1.25 mmol/L solution of CaCl2 plus NaCl to a final solution ionic strength of 160 mmol/L, we tested six buffers for their effect on measurements of ionized calcium (Ca2+). Measured Ca2+ decreased with increasing ionic strength and pH. Increasing concentrations of Tris caused a positive Ca2+ electrode bias; the other five buffers caused a negative Ca2+ electrode bias with increasing concentration. 4-(2-Hydroxyethyl)-1- piperazineethanesulfonic acid ("HEPES") at 10 mmol/L maintained pH 7.39 in the aqueous reference solutions containing 0.75 to 2.50 mmol of CaCl2 per liter, and showed a linear Ca2+ response among three different suppliers of Ca2+ instruments; it is thus a suitable buffer for a reference solution.