J . Phys. Chem. 1987, 91, 1691-1692
1691
COMMENTS Comments on “The Absolute Potentlal of a Standard Hydrogen Electrode: A New Estlmate”, by H. Relss and A. Heller Sir: Reiss and Heller’ have proposed a new method for the experimental determination of the absolute potential of a standard hydrogen electrode (designated as &,+/H2) using the circuit vacuum/p-InP/Pt, sat. H2/aq HC104 solution/vapor/vacuum* (1) To be more precise, b H + H 2 is the electrochemical potential of the electron in the condensed phases of circuit 1 that are in equilibrium with one another, the point (*) in the vacuum near the solution surface being taken as the origin. Referring to earlier works of Lohmann,2 Gomer and T r y ~ o n Trasatti? ,~ and Gurevich and P l e s k o ~the , ~ authors of the ref 1 conclude: ”We now have five estimates of the absolute value of p H + / H 2 which lie close together. Each of these estimates has been obtained by a different method, and, if compromised by uncertainties involving the potentials of dipole layers, then at least by different dipole layers. The variation among these values give some sense of the magnitudes of these dipole potentials.” (Reference 1, p 4212.) The reader may gain an impression that, first, the of the determination of &++/HI are independent and, second, that somehow or another they all make use of the estimates of the dipole interphase potential drops. But this is not the case! First of all, one and the same method is used in ref 2-4. The method is essentially based on the consideration of the thermodynamic cycle including the stages of the vaporization of the electrode metal, ionization of its atom, hydration of the resulting metal ion, and back transfer of the electron to the metal. It was pointed out in ref 5 on the basis of Frumkin and Damaskin’s work6 (see also ref 3) that this path, correct as it is, is unnecessarily complicated concerning the computations. Indeed, the hydration energy of the metal ion is, in its turn, determined by using a similar cycle, so that eventually the metal vaporization and ionization energies drop out. Therefore the quantity sought boils down to the difference (taken with the opposite sign) between the electronic work function of the metal and the contact potential difference in the metal-solution system (measured at the standard hydrogen electrode potential): FH+/HI
= 6 M - ea&
(2)
Actually, it is the choice of somewhat different values of &, and A;$ (and not different estimates of dipole layer potentials) that underlies the slight differences between the values of wH+/H2 determined in ref 2, 4, and 5 , and the significantly different value in ref 3 is based on the contact potential difference measured in the above work, which differs by 0.25 V from that found by Randles’ and used in the c a l ~ u l a t i o n s .(Recently ~ ~ ~ ~ ~ repeated measurements8s9seem to confirm the correctness of the contact potential difference value obtained in ref 7 with an accuracy of 1 ) Reiss, H.; Heller, A. J . Phys. Chem. 1985, 89, 4207. 2) Lohmann, F. Z . Naturforsch. 1967, A22, 843. 3) Gomer, R.; Tryson, G. J. Chem. Phys. 1977, 66, 4413. 4) Trasatti, S. Pure Appl. Chem. 1986, 58, 955. 5) Gurevich, Yu. Ya.; Pleskov, Yu, V. Elektrokhimiya 1982, 18, 1477. 6) Frumkin, A.; Damaskin, B. J . Electroanal. Chem. 1975, 66, 150. 7) Randles, J. E. B. Trans. Faraday SOC.1956, 52, 1573. 8) Farrell, J. R.; McTigue, P. J . Elecrroanal. Chem. 1982, 139, 37. 9) Antropov, L.I.; Gerasimenko, M. A,; Khirkh-Yalan, I. F.; Shkolnaya, 2. Elektrokhimiya 1984, 20, 1357.
0022-3654/87/2091-1691$01.50/0
f0.02 V. This proves the reliability of the value of pH+/H2 obtained in ref 2, 4, and 5 . ) The m e t h ~ d ~ is -a~rigorous, modelless approach; it does not require dipole interphase potential drops to be separately estimated (and therefore does not involve any fundamental inaccuracies associated with such estimation) since the quantities contained in eq 2 all are real energies. In other words, these are defined and determined without dividing the total energy into the bulk (“chemical”) and the surface (electrical) components; they implicitly include the corresponding dipole drops (as, e.g., the electronic work function of a metal includes the surface potential of the metal). Essentially, this method needs no further refinement though later, as more accurate experimental data on metal work function and the contact potential difference metal-to-solution have been obtained, it will yield a more precise value of pH+lH2. Here one may argue that practically work function is not a thermodynamic quantity (although per definition it certainly is) since it is either measured under nonequilibrium conditions or a model theory is somehow used in its determination.’O With due reservation for this point we may say that the absolute potential is a measurable quantity in the sense the work function is. At any rate, even if we introduce an (probably small, especially for a metal with the rather simple shape of the Fermi surface like mercury) inaccuracy into the thermodynamic meaning of the absolute potential ~ b t a i n e d , the ~ - ~rest of the determination has been done in a purely thermodynamic way by using the directly measured contact potential difference, instead of explicitly estimating interfacial potential drops. (In the m e t h ~ d these ~ - ~ are implicitly accounted for by comparative measurement, e.g., of the metal work function in vacuum and in the presence of water vapor,” etc. In doing so, the dipole (e.g., adsorption) layers are not disregarded, but “packed up” into the directly measurable quantities in order to avoid exthermodynamic steps.) Contrary to the aforementioned m e t h ~ d , ~the - ~method’ is a model and basically an approximate method. It is based not on the measurement of the contact potential difference metal-tosolution (Le., the thermodynamic quantity), but on the estimation of the potential drops arising at the interfaces in circuit 1 when it is made up of the constituting phases. Only part of these potential drops is subject to direct measurement (as, e.g., that in the space charge region of the InP phase). The other parts, namely, the interphase drops proper, are, in principle, not measurable; nor can they be calculated in terms of thermodynamics. In particular the potential drop at the Pt(H2)/HC104 solution interface cannot be found by measuring the zero (full) charge potential of platinum; the latter only takes account of the ionic component associated with Hf and the part of the true dipole component of the interphase potential that is associated with H chemisorption, but it does not account for the part of the dipole component arising, first, due to a change in the initial surface potentials of platinum and solution upon bringing them into contact and, second, due to orientated adsorption of the solvent molecules. (These contributions do not vanish at the zero-charge potential, as was thoroughly discussed in the literature, see, e.g., ref 12). Although the authors of ref 1 claim the error due to these dipole potentials is small, still a vagueness remains. Surely an exthermodynamic approach, like ref 1, is welcomed since it provides more insight into the interfaces involved. E.g., using the exact value of the absolute p ~ t e n t i a l ,one ~ , ~can estimate (10) Reiss, H.J. Phys. Chem. 1985, 89, 3783. (1 1) Burshtein, R. Kh.; Shurmovskaya, N. A,; Kalish, T. V.; Larin, L. A. Elektrokhimiya 1977, 13, 799. (12) Frumkin, A. N . Porentsialy nulevogo raryada (Zero-Charge Potentials; Nauka; Moscow, 1979.
0 1987 American Chemical Society
1692 The Journal of Physical Chemistry, Vol. 91, No. 6 , 1987
the dipole potential drops at the interfaces. The experimental data of ref 1 show that such drops at the interfaces p-InP/Pt(H,) and Pt(H,)/aq HC104 solution are roughly the same in absolute value-a conclusion by no means trivial. It should be specially mentioned, however, that in ref 1 the absolute potential is determined from a point near the solution surface not in the vapor phase as in ref 2-5 but in the vacuum beyond the limits of the vapor phase. It seems to us that this circumstance does not introduce any significant difference in the value of P H t / H 2 to be determined. Indeed, it was shown that the values of the contact potential difference mercury-aqueous solution* and the work function of mercury" used in the most accurate version of the determination5 do not depend on the
Comments composition of the surrounding atmosphere (Le., air, nitrogen gas, and water vapor); but still the above point should be taken into account in a numerical comparison of the results. Registry No. Hz,1333-74-0;Pt, 7440-06-4; InP, 22398-80-7; HC104, 7601 -90-3.
A . N . Frumkin Institute of
Yu. V. Pleskov
Electrochemistry Academy of Sciences of the USSR MOSCOW V-71. USSR Received: June 17, 1986: In Final Form: September 18, 1986
