Figure 4

The MC3-ISMs are insensitive to changes in pH or \({{\rm{HCO}}}_{3}^{-}\). (A) The Cl⁻ responsiveness of an MC3-ISM was not affected by pH changes from 6.5 to 8.5 units. The left panel shows the three-point calibration response of V_Cl for [Cl⁻] of 1, 10, and 100 mM is identical at pH 7.4 or pH 6.5 (depicted in the upper plot). Similarly, the right panel shows that the Cl⁻ calibration response was not affected by a pH change from 7.4 to 8.5 units. (B) The left portion of the V_Cl trace (0 to 300 s) shows the MC3-ISM response in 10 mM Cl^- remained constant, as the [\({{\rm{HCO}}}_{3}^{-}\)] was varied in step increments from 1 mM to 50 mM. The pH for these NaCl: NaHCO₃ mixtures varied from 7.2 to 7.68, indicating the pCO₂ of the solution had not fully equilibrated with room air. The right part of the record (>350 sec) is the response of this same electrode to changes in [Cl⁻] in the absence of \({{\rm{HCO}}}_{3}^{-}\). The changes in \({{\rm{HCO}}}_{3}^{-}\) and Cl⁻ are depicted in the upper panel by the red and black traces, respectively. For all panels, the different [Cl⁻] were obtained by mixing Cl-based and SO₄-based solutions (Methods). The “spikes” in the records are due to flux-dependent artifacts during solution switching.