Figure 1. Experimental determination of pK_a of Tyrode solution.

Tyrode solution saturated with 5 % CO₂-95 % O₂ was titrated with NaHCO₃ (4 mM aliquots) at 37 °C, and steady-state pH was measured. Each pH value was used to compute apparent pK_a from the Henderson-Hasselbalch equation (see text for details). This value is then plotted versus computed solution ionic strength. •, normal CO₂-buffered Tyrode solution composition except for the varied [HCO₃⁻]. ○, as above, but without divalent cation salts CaCl₂ and MgCl₂. See ‘Solutions’ in Methods for Tyrode solution composition. Line fitted by least-squares linear regression; n= 10 for all samples. No significant difference between • and ○, P > 0.05. Normal Tyrode solution containing 22 mM HCO₃⁻, pH 7.40, has an ionic strength of 0.16 M, at which the graph indicates a pK_a of 6.15.