COMMUNICATIONS TO THE EDITOR
1338
halogenated organic molecules.6 The use of mercury in such studies introduces considerable experimental difficulties because it reacts directly with halogens,8 hence our interest in organic sensitizers. We have had some success in sensitizing the decomposition of chloroform and methylene chloride with benzene irradiated with 2537 A. The irradiation times, however, are inordinately long (72 hr with chloroform and 96 hr with methylene chloride), and complications are introduced because the benzene itself is attacked by the radicals produced. We have recently explored the possibility of using hexafluorobenzene excited with 2537 A as a photosensitizer in the decomposition of dichloromethane and have obtained most encouraging result^.^ As for benzene sensitization, the primary chemical process is a carbon-hydrogen bond cleavage, but irradiation times can be reduced to less than 5 hr, and there is no evidence for any chemical participation by hexafluorobenzene in the resulting radical reaction. Both the photolytic and sensitized (with benzene Or hexafluorobenzene) decompositions of dichloromethane proceed analogously to the deconlpositions of chloroform.6 Thus, the photolysis with 1849-A radiation can be explained by the sequence CHzClz
C1
+ h~
CHzCl
+ C1
+ CH2Clz +HC1 + CHCl2
The main organic products are CZH4CI2, CHCIz-CH2C1, and CzHZCI4 which arise from combinations of the two organic radicals. The benzene-sensitized decomposition appears to proceed via a carbon-hydrogen bond break, followed by a sequence of reactions analogous to those which have been considered in the sensitized decomposition of chloroform, the major organic product being CZH2Cl4. The hexafluorobenzene-photosensititized reaction also proceeds through a carbon-hydrogen bond split, but in this case there are no product molecules derivable from the sensitizer. A mechanism which adequately explains the observed products is
+ CHzClz +C ~ F +B CHClz + H H + CHzClz +Hz + CHClz H + CHzClz +HC1 + CHzC1
C6Fe*
2CHC12 CH&1
C2HZC14
+ H +CHSC1
where C&’6* is the excited sensitizing species. It is not possible at this time to identify- which excited state transfers the energy’ at the University of Texas is currently investigating hexafluorobenzene The Journal of Physical Chemistry
and informs uslo that a t 2537 A the fluorescent quantum yield is about This suggests that almost all excited (singlet) molecules of hexafluorobenzene convert the absorbed energy to some other state which does not fluoresce. Obviously, one possibility is that this other state is the triplet and that this is the energytransfer agent. The advantages of using C6F6 as a sensitizer are threefold. (a) The excited state of C6F6 has a much larger reaction cross section than does C6H6, so that reaction times are much shorter. I n addition, the extinction coefficient at 2537 A of C6F6is approximately twice that for C6H6. (b) C6F6 is not decomposed at wavelengths greater than 2300 A. (c) C6F6 is not significantly athacked by radicals, or H atoms, near room temperature under these conditions. We hope in the near future to publish a more detailed account of the photolysis and sensitized decomposition of CHzC12. (8) C . R. Masson and E. W. R. Steacie, J . Chern. Phys., 19, 183 (1951); H. E.Gunning, Can. J. Chem., 36,89 (1958). (9) A. K. Basak and G. P. Semeluk, unpublished results. (10) D. Phillips, private communication. CHEMISTRY DEP.4RTMENT
UNIVERSITYOF NEWBRUNSWICK FREDERICTON, NEWBRUNSITICK, CANADA
A. K. BASAK G. P. SEUELUK I. UNGER
RECEIVED FEBRU.4RY 15, 1966
Quadrivalent Arsenic
Sir: Electrochemical evidence has been obtained in support of the existence of the arsenic(1V) oxidation state. The basis for this identification comes from the study of the irreversible voltammograms of millimolar As(II1) and As(V) solutions in 1.0 111 perchloric acid. The foot of the anodic wave corresponding to the reduction of As(III), when analyzed on a Tafel plot, shows a distinct and reproducible inflection (see Figure 1). This effect is characteristic of electrode reactions involving a sequence of two consecutive oneelectron-transfer steps’
+e As(V) + e
As(II1)
4As(1V)
As(1V)
4
The over-all reaction is As(II1) . . 4As(V)
+ 2e
(1) p. Delahay, “Double Layer and Electrode Kinetics,” Interscience Publishers, Inc., New York, N. Y.,pp 178-180.
COMMUNICATIONS TO THE EDITOR
1339
I
wave having the larger exchange current. This was the case under the experimental conditions. 111. The last criterion involves the agreement between the symmetry factors. The equation characteristic of the mechanism requires that the symmetry factor determined from that linear portion of the Tafel plot occurring at low currents be greater than 1.0 but less than 2.0 and the linear portion occurring at larger currents be greater than 0 but less than 1.0. A series of ten determinations yielded the following symmetry factors for the linear portion at higher and at lower currents, 1.3 0.1 and 0.6 f 0.1, respectively. The electrochemical experiments described herein when taken together with the kinetic evidence obtained from measurements made of the reaction in homogeneous media5s6provide a convincing argument for the existence of the As(1V) intermediate.
-5.1
- 5.3 - 5.5 - -5.7 c)
e 2
-5.9
M
s -6.1
-6.3
-6.5
-6.7
1/ /
I
1
0.70 0.75 0.80 Potential of indicator electrode (volts) us. saturated silver-silver reference electrode.
I
0.85 chloride
Figure 1. A typical Tafel plot of the foot of the anodic voltammetric wave of 10-8 11.17 As(II1) in 1.0 M perchloric acid. The symmetry factors (1 - p), calculated in the usual manner: slope = n(1 - p)F/2.3RT, of each of the linear portions are 1.2 and 0.6, respectively. n, the number of electrons transferred in the rate-determining step, is taken to be unity.
Kinetic equations describing this mechanism in the form of current-voltage curves obtainable at microelectrodes were derived by Vetter,2in terms of exchange currents, and Catherino and J ~ r d a nin , ~ternis of specific rate constants. Hurd4 studied the properties of Vetter’s equation via the application of computer techniques and established a set of criteria for identifying such a mechanism operationally. Listed below are the experimental observations and a brief discussion of the manner jn which they satisfy the established criteria. I. The most important characteristic of the mechanism is illustrated in Figure 1, which shows two discrete linear portions which occur below the limiting current, 11. The second criterion involves the relationship between the exchange currents of the oxidation and the reduction processes. Since As(V) was not electroreducible anywhere in the range of potentials accessible for study, the ratio of the hypothetical exchange currents obtainable from the anodic waves to that from the reduction wave approaches infinity. The properties of the equation require that the wave having the two linear logarithmic portions must occur on that
Acknowledgment. This investigation was supported in part by a grant from the Horace H. Rackham School of Graduate Studies of the University of Michigan. The author also acknowledges the facilities made available by the Computing Center at the University. (2) K. J. Vetter, 2.Naturforsch., 7a, 328 (1952); 8a, 823 (1953). (3) H. A. Catherino and J. Jordan, Talanta, 11, 159 (1964). (4) R. M. Hurd, J . Electrochem. Soc., 109, 327 (1962). (5) R. Woods, I. M. Kolthoff, and E. J. Rleehan, J . Am. Chem. Soc., 86, 1698 (1964). (6) M. Daniels, J . Phys. Chem., 6 6 , 1473 (1962). DEPARTMENT OF CHEMISTRY THE UNIVERSITY OF MICHIGAX
HER‘RY A. CATHERINO
ANN ARBOR,NICHIGAN RECEIVED FEBRUARY 18, 1966
Measurement of Contact Angle between Thin Film and Bulk of Same Liquid
Sir: The surface tension of a liquid at equilibrium is generally considered to be uniform and its surface smooth as a consequence. However, as the thickness of a liquid layer decreases, intermolecular forces come into action and give rise to a “disjoining” pressure which should correspond to a change in surface tension and to the existence of a contact angle between a thin filni and the bulk of liquid. Alternately, one may consider that intermolecular forces pull molecules out of the region between the film and the bulk until the resultant forces become normal to the surface, when the proper contact angle is established. A qualitative observation of such a contact angle for a multimolecular film supported by a solid was Volume 70, Number +$
April 1966
