30 90 *..e..
*--
60
Figure 1.
I
eo 100 TIME,MIN.
120
83 9b
I
140
180
d
Desorption of monolayer at 1.65 volts
0’
20
40
ek
80
IbO
,io
d
51ME.MIN.
Figure
fatty acids has been used by many authors to estimate the roughness factor (12, IS). If such an estimate i s valid the roughness factor of the present gold electrode must be close to unity. This result is not altogether improbable, in view of the fact that the roughness factor of bright platinum foil was evaluated at 1.12 by the BET method using krypton adsorption (8). The fact that palmitic acid could be removed by simple toluene estraction from either the oxidized or the reduced surface could mean that salt formation probably did not take place between the acid and the oxidized gold surface. Bomden and Moore (3) found no evidence of chemical reaction between stearic acid and gold. Double Layer Capacity Measurements. The double layer capacity was observed by transferring the electrode into a clean, quiescent solution of 1M perchloric acid and making measurements a t intervals. .4t the conclusion of each experiment, the palmitic acid remaining on the electrode was determined by scintillation counting. I n Figure 1, results are shown for electrodes standing a t an oxidizing potential (1.65 volts in 131 HC104), and similarly in Figure 2 a t the reducing potential (0.65 volt). I n both figures, curve I represents a monolayer which had been coated on an oxidized surface (kept at 1.65 volts in 0.01;1.1 HCIOJ, and curve IJ, a monolayer which had been coated on a reduced surface (kept a t 0.45 volt). Curve I11 represents a clean electrode to serve as a comparison. The percentage figure a t the end of each curve represents the amount of palmitic acid left on the surface at the end of the experiment. Curves I and I1 in each case started with the same amount of adsorbed acid on the surface, as had been shown in the earlier counting experiments. It was also known that adsorbed palmitic acid lowers the double layer capacity, and that the change in capacity would serve as a n indication of the change of coverage. The gradual change of capacity of a clean surface is due to its gradual contamination by traces of 1838
e
ANALYTICAL CHEMISTRY
2 . Desorption of monolayer
surface active impurities. The contamination is more pronounced at low potentials than at high potestials, where desorption occurs because of the excessively high positive charge density. Both the capacily measurements and the final radioactive count show that in an oxidizing environment, the reduced surface had lost more acid than the oxidized one. Conversely, in a reducing environment, the oxidized surface had lost more acid than the reduced one. The larger loss in each case was no doubt due to the disrupting effect of oxidizing a reduced surface or reducing an oxidized one. The monolayer produced when reducing conditions were maintained throughout is the most stable that can be obtained, and it is significantly more stable on gold than on mercury under similar conditions (9). ACKNOWLEDGMENT
The authors gratefully acknowledge the help of R. F. Nystrom and George Wolf in radiochemical measurements. LITERATURE CITED
(1) Adam, N. K., “T$ Physics and Chemistry of Siirfaces, 3rd edition, p.
413, Oxford University Press, London, 194i. (2) Blodgett, K. R., J . Ana. Chem. Sac. 57, 1007 (1935). (3) Born-den, F. P., Moore, A. C., Trans. Faraday SOC.47, 900 (1951). (4) Brodd, R. J., Hackerman, IC., J.Electroehem. Soc. 104, 704 (1957). (5) Chao, M, S., Ph.D. thesis, University of Illinois, 1961.
(6) Enke, C. G., Ph.D. thesis, University of Illinois, 1959. ( 7 ) Laitinen, H. A., Chao, M. S., J . Electrochem. SOC.108, 726 (1961). (8) Laitinen, H. A., Enke, C. G., Ibid.. 107, 773 (1960). (9) Laitinen, H. A., Morinaga, K., Rept. ARL-TN-60-129 to Aeronautical Researrh Laboratories, U. s. Air Force. Wright-Patterson Air Force Rase Ohio. (10) Langmuir, I., Shaefer, V. J., j . Am. Ghem. SOC.58, 284 (1936). (11) Laug, E. P., IND.ENC. CHEnf., ANAL.ED. 6 , 111 (1934). (12) Orr, C., Jr., Dallavalle, J. M., “Fine Particle Measurement,” p. 207, Maemillan. New York, 1959.
at 0.65 volt
(13) Radlein, G., Honrath, H., Z. EEektrochem. 63, 397 (1959).
REC&WEDfor review August 16, 1961. Accepted October 4, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. Work supported by grants from the National Science Foundation (grant 6-24201 and the U. S. Air Force [contract AF 33(616)-54461.
Correction Chrsnopotentiometry
with
Very recent experiments in the author’s laboratory have shown that the chronopotentiograms described [Anson, F. C., A I ~ A LCHEM. . 33, 1498 (1961); J . Am. Chem. SOC.83, 2387 (1961) ] were erroneously identified as arising from iron(I1) and iron(II1) adsorbed uniformly on platinum electrodes. dlthough the observed chronopotentiograms are quite reproducible and are in excellent agreement with the theoretical wave equation for true adsorption chronopotentiograms, the apparent adsorption disappeared when the platinum wire electrodes were removed from the glass tubing in which they were sealed and the experiments repeated. It was also discovered that no apparent adsorption was observed with electrodes sealed in glass when the exposed wire but not the wire-glass seal was immersed in the electrolyte. Experiments in progress to ascertain the actual source of the chronopotentiograms indicate that they arise from reactant that is contained in tiny invisible cracks on the inner wail of the glass tubing in which the electrodes ard sealed, Additional experiments are in progress to clarify more fully the nature and origin of the chronopotentiograms.
