Apparent binding of ionized calcium by various buffers.
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.
D002021:Buffers; D002118:Calcium / Q000032:analysis*; D004553:Electric Conductivity; D004566:Electrodes; D006531:HEPES; D009994:Osmolar Concentration; D014325:Tromethamine
D002021:Buffers; D014325:Tromethamine; D006531:HEPES; D002118:Calcium