ABSTRACT
Figure 7.1 shows an experiment in which a Ca2+ -selective electrode/saturated KCl reference electrode pair was placed in a nominal 1 mmol.l−1 CaCl2+ 140 mmol.l−1 NaCl solution buffered to pH 7.0 with 10 mmol.l−1 HEPES. The left axis shows the recorded p.d., ECa, the right hand axis a ‘[Ca2+]’. The ECa values were recorded to an accuracy of 0.1mV, which represents about 2% of the total drift in this experiment. The 1 mmol.l−1 value on the [Ca2+] scale is placed opposite the final time point when the electrode p.d. had stabilised to a constant level. The remaining ‘[Ca2+]’ values are calculated from the earlier ECa values, using a calibration curve previously obtained by the electrode in the test solution. The important point to note is that the drift in the ECa values initially overestimates the true [Ca2+] by as much as 45%. The cause of the drift in such
experiments cannot be determined from such observations, but the most likely sources are drift of the potential at the Ag/AgCl interface with the filling solution and at the ionselective membrane with the test solution. The latter phenomenon is especially evident when a new electrode is placed in a test solution.
