Riehl-Kall-mann-Vogel

r We has any meaning at all, it is the activation energy to form solvated electrons at an infinite distance from the metal. Electrons can be solvated rather closer to the metal than this; the activation energy can be expected, therefore, to be rather smaller than that suggested by Matthews. We believe that the closest point to the metal at which an electron may be considered to be solvated lies in the plane through the center of the first layer of water molecules which are in contact with the metal surface. In this position the electron will lie approximately in the inner Helmholtz plane and the work required to separate an electron from a metal by a distance a, the radius of a water molecule, may be estimated on the basis of a model for the work function of a metal discussed by Loeb. 33 The work required to remove an electron is considered in two parts. (a) at points closer to the metal than a critical distance Xo the electron moves in a constant field e/4xo 2. The work done in taking an electron from the metal to Xo is e2/4Xo; (b) at distances larger than xo the electron moves against a mirror image force. The work required to take an electron from Xo to ~ is e2/4xo. The work function is the sum of these two parts and equals e2/2xo. For metals, with ~# ~ 5 ev, Xo > a and the work required to separate the electron from the metal by a distance a becomes ae2/4Xo 9. The activation energy for the solvation of an electron in this position becomes ae2/4Xo 2 --We. Metals with r > 5 ev have xo < a and the activation energy is given by r --e2/4a --We. Experimental values of the heats of activation for the deposition of hydrogen at copper, ~4 mercury, s4 nickel, 34 platinum, 34 and cobalt 35 surfaces in aqueous electrolytes are compared, in The agreement between the experimental heats of activation for hydrogen evolution and the calculated heats for the formation of solvated electrons is encouraging. Experimental verification of the role played by the solvated electron in the evolution of hydrogen is only available for mercury, however the calculated values of AH do suggest that it may also play a dominant role in the evolution of hydrogen at copper, nickel, and cobalt surfaces. Platinum has a large work function and the activation energy for the formation of solvated electrons at the platinum-electrolyte interface is therefore large. Consequently it appears improbable that hydrogen evolution via the formation of solvated electrons will be significant at a platinum surface. However, in general AH for the formation of a solrated electron at the inner Helmholtz plane is not large and the formation of a solvated electron as a step in the evolution of hydrogen should not be discounted on the grounds that it will lead to negligible rates of hydrogen evolution. earth borates 37 but did not report on the cathodoluminescence. We should like to point out that in section 5 of the paper cited by the authors we did in fact give cathode-ray efficiencies (p. 491), namely a radiant efficiency (energy conversion efficiency) of about 2%. Moreover in another paper ~s we gave a radiant efficiency of 2.2% for 2CaO 9 Na20_ 9 B203-0.15 Tb. This means that these phosphors have an efficiency of about 25-30% of that of willemite (Zn2SiO4-Mn). a9 They are, therefore, comparable with those described in the paper under consideration. In a third paper 40 we have given data on more efficient Tb-activated phosphors under cathode-ray excitation, namely on LnPO4-Tb (where Ln = Y, La, Gd). For GdPO4-Tb we found an efficiency of 5%, i.e., about 65% of that of willemite. F. 3. Avella: The author is grateful to Dr. Wanmaker and Dr. Bril for supplying the information on the cathodoluminescence efficiency of their borate as well as their phosphate phosphors. References to the formet 37,3s were inadvertently omitted, while the latter reference 40 was not available during preparation of the paper under discussion. It should be noted, however, that their statement of comparable performance applies only to the Tb-activated borates containing the alkaline earths. The data given in The Repeatability of the Anode Effect in The apparent overvoltage on these, also before the intervention of the anode effect, includes a large ohmic contribution due to the presence of the gaseous phase in the anodic layer and on the anode surface. This contribution depends (besides on the alumina content of the baths) on the circumstances controlling the gas evacuation and thus on the shape of the anodes (being obviously greater on the anodes of the types: B, C, F, on which the permanence of the gases is favored). This statement is confirmed by the influence of mechanical vibrations impressed on the anode assembly which, by enhancing the gas evolution, reduces the ohmic contribution to the apparent overvoltage. The ohmic character of this contribution is also confirmed by the oscillographic recordings of the electrode voltage at the current inlet and outlet. Thus the amount and configuration of the gaseous phase in the anodic ~TW. L. Wanmaker and A. Bril, P/t/lips Res