Metal-solution potentials of nickel in foreign ion solutions were investigated. It was found that nickel may show stationary potentials in potassium hydroxide and phosphate buffer solutions, if oxygen was excluded. The stable potentials were reversible with respect to a change of pH. It was noted that nickel was unable to displace hydrogen from a 0.1 M phosphoric acid solution; nickel dissolved only on admission of air. The possibilities of interpreting the observed potentials are discussed.

Phase boundary potentials of nickel in various aqueous solutions, practically free from nickel ions, have been measured. The nickel-solution potentials in air drift and reach high positive (noble) potentials. The potentials of nickel vs. solution in an inert gas reach stationary values after several hours, depending upon the pH. The final potentials remain negative with respect to the standard hydrogen electrode. The differences of the corresponding potentials in the same solution, in presence of air and in absence of air, range up to half a volt or more. The role played by oxygen in influencing the potentials has an interesting bearing on the initial stages of the corrosion of nickel.

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The properties of the metal-solution potentials of nickel in nickel free hydroxide and phosphate solutions are compared with the properties of unpolarizable electrodes and with the properties of polarizable metal-solution systems. The electrochemical equilibria of unpolarizable systems are analyzed in terms of transfer mechanisms between the phases present. In the absence of any electrochemical equilibria, the metal-solution potentials are interpreted as adsorption potentials. Chemisorption in the inner Helmholtz layer is discussed in connection with ``reversibility'' and polarizability of the system.

For the interpretation of potentials of plant roots in electrolyte solutions it is desirable to know the variations with pH of net charge on the non‐permeating material inside the root wall. Since this is a very complex mixture, it is proposed to use as a model a solution characterized by a continuous range of titration constants.