The ionic strength was increased by additions of calciumnitrate in the solutions. Sensitivities obtained, when the ionic strength varies, are presented in Fig. 3. In the normal mode, the sensitivity decreases with a slope of 4 mVrpCa. In the differential mode, the sensitivity remains in the range of repeatability: 25"5 mVrpNHq. The 4effect of the ionic strength remains in the range of repeatability in the differential mode which is not the case in the normal mode