Are sodium bicarbonate and potassium bicarbonate fully dissociated under physiological conditions?

Abstract

In solutions containing 160 mmol/1 Na⁺ and K⁺, respectively, measurements with an ion-selective electrode system (KNA1, Radiometer), showed apparent falls in the respective Na⁺ and K⁺ concentrations when CI- was replaced by HCO-₃. After correction for the change in liquid junction potential, the fall was 9.2 mmol/1 for Na₊ and 7.3 mmol/1 for K₊. On the basis of these findings we conclude that sodium bicarbonate and potassium bicarbonate are not fully dissociated in solution, and that NaHCO₃d` and KHCO<sub>3</sub>d` do exist as chemical components with association constants of 0.72 and 0.55, respectively. Using these association constants, normal plasma will contain 1.2 mmol/1 NaHCO₃d` and 0.03 mmol/1 KHCO<sub>3</sub>d`. Thus NaHCO₃d` accounts for virtually the same amount of CO₂ as the physically dissolved fraction. A review of all the currently known CO₂ species in plasma suggests that there may be a residue of about 2 mmol/1 of unknown CO² species in normal plasma.

Key Words:

• activity coefficients
• ion-pairing
• ion-selective electrodes
• liquid junction potential