FIG. 2.

Potential response of a ${\text{CO}}_3^{2-}$-sensitive ISM to changes in CO₂ tension in NaHCO₃. Record shown is the millivolt (mV) response of an ISM to changes in CO₂ within a 10 mM (concentration) solution of NaHCO₃. Such an alteration in gas tension will cause a small (i.e., µM) increase in ${\alpha }_{{\text{HCO}}_3^{-}}$ but more than a 200% increase in ${\alpha }_{{\text{CO}}_3^{2-}}$. This simple, physicochemical fact can be used to determine if the ISM principally measures ${\alpha }_{{\text{HCO}}_3^{-}}\text{or}{\alpha }_{{\text{CO}}_3^{2-}}$ in a NaHCO₃ solution. Here a 10 mM NaHCO₃ solution was initially aerated with N₂ (top left). Exposure to CO₂ caused a prompt fall in solution pH to <6.4 (bottom left). At the same time ISM potential dropped by more than 50 mV, indicating that the ISM primarily measured ${\alpha }_{{\text{CO}}_3^{2-}}$ and not ${\alpha }_{{\text{HCO}}_3^{-}}$. Finally, solution aeration was changed back to N₂ (lower left of chart record). As solution CO₂ tension dropped from this latter maneuver pH rose from <6.4 to >8.7 (as noted by specific solution pH measurements indicated below vertical lines). ${\alpha }_{{\text{CO}}_3^{2-}}$ is shown above vertical lines.