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solution which will affect the potential being measured. It would seem that any stable metal which could have an adsorbed film of its compound insoluble in the solution might function as a measure of the hydrogen-ion concentration of that solution. L. R. PARKS. H. C. BEARD. Pond Chemical Laboratory, The Pennsylvania State College, State College, Pennsylvania. THE THEORETICAL LIMITATIONS OF THE STICK ANTIMONY ELECTRODE
I n a recent paper (Parks and Beard: J. Am. Chem. SOC.64, 856 (1932)) the authors have shown that stick antimony electrodes in unstirred buffer solutions in contact with air may be calibrated to give a curve which agrees with that of the hydrogen electrode as to slope, thus sat,isfying the Nernst equation from the pH value of 1.60 to that of 7.87. I n a highly acid solution, pH lower than 1.60, the potentials obtained with the stick antimony electrode are not a true measure of the pH of the solution, because the adsorbed film of Sbz03on the metal electrode has dissolved and antimonyl ions have been formed according to the reacttion SbzOs
+ 2H+ = 2SbO+ + Hz0
Antimonyl ions with hydrogen-ions in equilibrium with the antimony metal and water develop a potential (Schuhmann: J. Am. Chem. SOC.46, 52 (1924)) which may be found by the Nernst equation:
Thus the concentration of the antimonyl ion is a factor in the potential developed and the electrode does not correctly measure the hydrogen-ion concentration of highly acid solutions. I n alkaline solutions, above pH 10.53, there is also a deviation of the values obtained by the stick antimony electrode away from the theoretical values. The deviation in this range is due to the antimonite ions (SbOz-) in the solution formed by the dissolving of the adsorbed film of antimony oxide on the metal stick by hydroxyl ions according to the reaction Sbz03
+ 20"
= 2Sb02'
+ Hz0
These antimonite ions with water in equilibrium with the metal and hydroxyl ions give a potential (Grube and Schweigardt: Z. Elektrochem. 29, 257 (1923)) which may be found by the Nernst equation:
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Thus in extreme alkaline solutions the potential will be that denoted by the Nernst equation involving the SbO2- ion, and does not give a value that is a measure of the hydrogen-ion concentration of the solution. ,The deviation of the stick antimony electrode values between pH 7.87 and 10.53 away from the theoretica1,values is due t o a differential effect of the Sbz03and the Sb02- ions. If the Sbt03 alone were present with increasing pH (from 7.87) the curve would continue in a straight line from its lower values and give higher potentials. If, on the other hand, the SbOz- ion were the only antimony ion present with decreasing pH (from 10.53) lower potentials would be obtained according to the Nernst equation which has (SbOz-) as a factor. Grube and Schweigardt (Z. Elektrochem. 29, 257 (1923)) state that the SbO-2 ion readily absorbs oxygen from the air. This excess oxygen in the solution in contact with the metal would aid in the formation of Sb203 and so retard the deviation of the values in this range. Thus in extreme acid and alkaline solutions where there is a limited amount of antimony oxide, as in the case of an adsorbed film on a metallic stick, the potential obtained by the stick antimony electrode is not a true measure of the hydrogen-ion concentration of the solution due to the formation of oxy-antimony ions in the solution. Although the stick antimony electrode is very convenient as a means of measuring the hydrogen-ion concentration of a solution, it is theoretically correct only in the pH range from 1.60 to 7.87. In alkaline and highly acid solutions the electrode can be used only if it has been previously calibrated.
L. R. PARKS. H. C. BEARD. Pond Chemical Laboratory, The Pennsylvania State College, State College, Pennsylvania. THE ROLE OF SURFACE ELECTRONS IN ADSORPTION AND CATALYSIS
A theory that the adsorption of gases by metals and the catalysis of reactions following this adsorption is due to some electrical or mechanical effect of the surface electrons of the metal was advanced by Pisarzhevskii (Ukrain. Khem. Zhur. 1, 1-18 (1925)) in 1925. More recently this idea has been taken up again by Nyrop (Chemistry & Industry 60,752-5 (1931) and Phys. Rev. 39, 967-76 (1932)), who gives it a more theoretical and mathematical treatment. A quotation from the abstract (C. A. 22, 2512 (1928)) of the Pisarzhevskii paper will serve to give the nature of his theory: “Catalytic activity of metals is due t o freely moving electrons. Adsorption of gases by metals is the result of electrostatic forces between gaseous mols. and a film of free electrons on the surface of metals. ImTHE JOURNAL OF PHYBICAL CHEMISTRY, YOL. XXXVII, N O .
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