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Summary: Details of the formation of potentials a t a metal-oxide/H20 interphase are described.On the one hand the whole potential drop consists of one fixed (HELMHOLTZ layer) and two diffuse parts (space charge in the oxide phase and G o w layer in the electrolyte), on the other hand it is given as sum of free charges and dipols. Besides the PZC the charge distribution is determined above all by the pH value of the electrolyte under conditions without an external current. The following cases can be qualitatively distinguished in connection with the important pH22 oc =7 : > > PHelectrolyte 2 PHPZC 5 7.

The present paper deals with the minima of conductivity in aqueous solutions, which occur due to the hydrolysis reaction with added bases. The minima show lower conductivities than the intrinsic conductivity of pure water. The minimum is a function of the molar conductivity of the added ions. There exists a limiting condition of <75.825 ⋅ 10−4 S ⋅ m2 ⋅ mol−1 for the occurrence of a minimum in the real (positive) concentration area. Values higher than 75.825 ⋅ 10−4 S ⋅ m2 ⋅ mol−1 lead to minimas in the virtual (negative) concentration area. Connecting all the minima, a curve with a maximum is observed. This point is given by 75.825 ⋅ 10−4 S ⋅ m2 ⋅ mol−1 and the intrinsic conductivity of pure water. The effect is independent of whether the added substances come from a strong or weak base. So far, the phenomenon should not influence measurements of conductivity under usual circumstances, but might be more of academic interest. Interestingly, we found that the effect for Rubidium and Cesium ions is different compared to other alkali metal ions. No minimum conductivity is predicted for these ions.