Elimination of the erythrocyte effect on the liquid junction potential in potentiometric measurements on whole blood using mixed salt bridge solutions.

An effect of erythrocytes on the liquid junction potential of the junction "concentrated KCl whole blood" has previously been described, the effect being a positive bias of +1,4 mV with a high erythrocyte volume fraction (0.75). We have studied the erythrocyte effect with different concentrated salt solutions, and found a similar positive bias with KBr, KJ, KNO3, K2SO4, but a negative bias with NaCl, NaNO3, LiCl and CaCl2. The erythrocyte bias may be eliminated by mixing salts with opposite effect, e.g. KCl + NaCl (1.0 + 7.0 mol/l), KNO3 + NaNO3 (1.0 + 3.0 mol/l) KNO3 + Li-acetate (0.15 + 4.0 mol/l). The use of a mixed salt bridge solution may be advantageous for eliminating the erythrocyte bias when measuring ionic activity (e.g. Na+, K+, Ca2+) directly in whole blood.