COMMUNICATIONS TO T H E EDITOR THE THEORY OF THE STICK ANTIMONY ELECTRODE
The stick antimony electrode is a type of the metal-metal oxide electrode, the antimony oxide being present as an adsorbed. film on the surface of the metal. This oxide is formed by air oxidation on the metal surface after polishing. The reaction of this adsorbed film with the hydrogen-ions in the solution sets up a potential, according to the reaction (Roberts and Fenwick: J. Am. Chem. SOC.60, 2125 (1928)): Sbz03
+ 6Hf + 6(-)
= 2Sb0 f
3Hz0
The potential is found by the Nernst equation:
Developing the equation: E
RT
= E, - -In
6F
RT (H+)6= E, - -In (H+) = E, F
+ 0.05915 pH
This potential is identical with that developed from the solution of the metal forming its ions in a solution saturated with the metallic oxide. However, it is not necessary for the solution being measured to be saturated with the insoluble oxide for the metal-metal oxide electrode to function correctly. Tartar and McClain (J. Am. Chem. SOC.63, 3201 (1931)) have stated that the potential of a mettal electrode is due “to an adsorbed ionic film extending from the electrode a measurable distance out into the solution.” Since this is so, an adsorbed oxide film on a metal electrode will furnish saturation of that portion of the solution from which the potential is derived. From this it would seem that the electrode reaction takes place a t the interface of the metal with its oxide. I n preliminary work with several stick metal electrodes, as antimony, bismuth, tungsten and tantalum, the potential obtained seemed to be dependent on the nature, thickness and stability of the oxide film on the metal. The oxide should be a definite compound, not a mixture of oxides, and of specific crystalline form (Roberts and Fenwick: J. Am. Chem. SOC.60, 2125 (1928)). The thickness of the film should be such that it will allow penetration of the solution to the metal. A thick film of the oxide on a metallic stick tends to slip off, as a glove from the finger. This supports the contention that the electrode reaction takes place a t the interface of a metal with its oxide. If the oxide film dissolves, ions will be formed in the 821
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COMMUNICATIONS TO THE BDITOR
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 Sbz03 on 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:
