solvent, ko, since there is no certainty that the value measured on pure solvent is exactly the same as it is in solution. I n the case of our data the uncertainty was amplified by the fact that ko comprises about 27y0 of total conductance for AgCl solutions, and that the AgBPh4 solutions had to be filtered. The second largest source of error, common to all conductometric work, is sorption on the electrode surfaces, which causes initially observed conductance to decrease with time. I n the case of very dilute solutions, such as studied here, several equilibrations with fresh volumes of the solution may be required before a constant reading is obtained. This difficulty can be bypassed, with some sacrifice in accuracy, by taking the immediate (highest) conductance value as the correct one.
A major deterrent against the proposed method may appear to be the possible lack of the necessary A0 data in the solvents of interest. Fortunately, the volume of conductance data for nonaqueous solvents has been growing steadily and is by now quite formidable for such media as ethanol, methanol, nitrobenzene, acetonitrile, and chlorinated hydrocarbons. I n these solvents, the no’s of many electrolytes can be calculated by the Kohlrausch law. Furthermore, in other solvents, AO’S can be estimated approximately with the aid of Walden’s rule. LITERATURE CITED
Standards on Petroleum Products and Lubricants,’’ 38th ed., Vol. 1, p. 296-306, Am. SOC.Testing Materiag, Philadelphia, Pa., 1961.
(1) “ASThI
( 2 ) Fuoss, R. M., Accascina, F., “Electro-
lytic Conductance,” pp. 195, 230, Interscience, New York, 1959. (3) Fuoss, R. hL, Shedlovsky, T., J . Am. Chem. SOC.71, 1496 (1949). (4) Popovych, O., ANAL.CHEM.36, 878 (1964). (5) Popovych, O., J . Phys. Chem. 6 6 , 915 (1962). (6) Popovych, O., unpublished work. (7) Wirth, H. E., J . Phys. Chem. 6 5 , 1441 (1961). ORESTPOPOVYCH
Department of Chemistry Brooklyn College of the City University of New York Brooklyn 10, ?J. Y. RECEIVED for review September 13, 1965. Accepted October 21, 1965. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
An i n situ Spectrophotometric Method for Observing the Infrared Spectra of Species at the Electrode Surface du ring EIectrolysis SIR: The exact mechanism of a complex electrode process (coupled with chemical reactions) or the nature of an intermediate activated state of a species during electron transfer cannot be deduced unambiguously from electrochemical measurements alone (1, 18). At a minimum, a second independent but simultaneous measurement of the system properties is necessary to sort out the true mechanism or transition state from the alternate possibilities. The technique of carrying out an electrolysis in an Electron Spin Resonance (ESR) cavity (16) has been employed successfully to detect and identify free radial products and intermediates in organic electrode reactions ( I , 16). Transparent conducting glass electrodes (CGE) have been employed to determine the visible absorption spectra of certain dyes during electrolysis (14, 1 5 ) (this technique measures only the spectra of species generated in reasonable quantity which diffuses into the bulk of the electrolysis solution). Neither of the above techniques is sufficiently sensitive to measure the spectra of species present a t the electrode interface alone. One optical technique, ellipsometry, has been employed to measure in situ the nature of oxide films on platinum during electrolysis (17). However, it is doubtful that this technique could detect the slight variations of spectra that might be expected for
intermediates, species perturbed in the electric field of the double layer, or adsorbed species on the electrode surface, as the light path in this case is directed through the bulk of the electrolysis solution. Thus, the extensive absorption of the light by the similar reactants and products in the bulk would obscure the spectra of the species present in small quantity a t the interface itself. As the nature of the species a t the interface is of major interest, it is of extreme importance to develop an in situ optical technique which is capable of measuring the spectra of the first monolayer (or first few layers) a t the interface without interference from similar species present in the bulk solution. The properties of Frustrated Multiple Internal Reflectance (FMIR) which was developed simultaneously with single Attenuated Total Reflectance (ATR) spectroscopy (4, 7-10) make it an ideal technique to adapt to the study of the spectra of species a t the electrode surface during actual electrolysis as it was designed to study surface effects. I n this technique, light from the spectrophotometei is focused to enter a specially designed crystal or plate (made of quartz, NaC1, Ge, etc. depending on the spectral range desired) a t an angle. If the crystal or cell is of higher refractive index than the gas or liquid media in contact with it, and this incident angle of the beam a t the interface is slightly larger than the
critical angle, total reflectance of the beam will be attained, provided that the incident beam frequency is in a region where the second (lower refractor index) media is nonadsorbing ( K = 0) (4). When K z 0, however, total reflection of the beam is not attained and, if a frequency scan is applied as the incident beam, a plot of the intensity of resulting exit beam from the cell us. wavelength yields a spectrum which closely resembles the transmission spectrum of the second medium (7, 8 ) . Thus, during the process of reflection, the beam actually penetrates a short distance into the second medium. The depth of this penetration is a function of the incident angle as well as the wavelength of the light. In the visible region of the spectrum, the effective path length of each reflection is several angstroms deep ( 7 ) and in the infrared region, the spectra of monolayers can be observed (19, 20) with instrumentation capable of about 50 to 100 reflections (9,229. It should be noted a t this point that the spectrum of the species of interest can be obscured by the absorption of the solvent and supporting electrolyte if they adsorb in the same region. Thus, if the FMIR plate is also made to be an electrode, the spectra of species a t the electrode surface during electrolysis can be determined directly. This communication reports the preliminary results obtained for the surface VOL. 38, NO. 1, JANUARY 1966
1 19
RESULTS AND DISCUSSION
HOLES FOR FILLING, Nc FLUSH AND REFERENCE ELECTRODE
7TT/A
GERMANIUM
P
L
T
I ELECTRODE A R Y'
COMPkRTMENT,
LARGE BACK PIECE
SMALL FRONT PIECE LIQUID FILLER HOCES
TYPICAL LIGHT PATH
Figure 1. Details of Teflon trolysis cell-FMIR plate holder
elec-
infrared spectra of two organic molecules under electrolysis at a germanium FXIR electrode. The reported results were reproducible. EXPERIMENTAL
The F M I R spectra under electrolysis were obtained by means of a modified Wilks Model 12A Double Beam Internal Reflection Attachment, (Wilks Scientific Corp., 140 Water Street, South Norwalk, Conn.) on a PerkinElmer Infracord Model 137 IR spectrophotometer. The details of the actual Ge-sample holder which fits into the Model 12.4 attachment are shown in Figure 1. This Ge-sample holder was constructed from Teflon and served both as the electrolysis solution holder and Ge internal reflectance plateelectrode holder. The Ge internal reflectance plate-electrode was made by the Recticon Corp., (22 Summit Grove Ave., Bryn Mawr, Pa.) from n-type semiconducting Ge which had a specific resistance of less than 1.0 ohm-em. The dimensions of Ge plate were 50 X 20 x 1 mm. thick [same dimensions as the Standard Wilks model 3001C Ge (high resistance) plates]. These plate dimensions give approximately 50 to 100 reflections of the incident light (29). The conducting Ge plate can also serve as an electrode. There has been considerable work reported by other investigators on the electrode properties of Ge (6, 11). Controlled potential (three electrode) electrolysis of the sample solutions, employing the Ge-plate as the working electrode, were carried out in the conventional manner (S) employing either a Duffers Model 600 Potentiostat (Duffers Associates, Troy, N. Y.), or a potentiostat constructed from Philbrick Model USA-3 and P-2 operational amplifiers using the circuits of DeFord (2). 120
ANALYTICAL CHEMISTRY
-4marked change in the FMIR-IR spectrum of a saturated 8-quinolinol solution in dimethyl formamide (D.M.F.) contains 0.1M LiC104.3Hz0as supporting electrolyte was observed on electrolysis a t -1.8 volts us. S.C.E. as shown in Figure 2. At this potential the 8-quinolinol is presumably reduced to dihydro-8-quinolinol (21). Specifically, the absorption band a t 1048 em.-' [probably one of the in-plane hydrogen wag bands ( I S ) but, as yet, definitely not assigned] disappears rapidly during a brief electrolysis, This spectrum change is not observed in the bulk electrolyzed solution obtained using conventional transmission IR spectrum with NaCl solution cells. Thus, the FMIR-electrolysis technique permits the spectrum of this electrolysis product (or intermediate) a t the electrode surface to be determined, although it cannot be seen by conventional techniques as it is produced in small quantity at the surface only in this case during the brief electrolysis. A well characterized free radical process ( 5 ) was also investigated to better understand the nature of the type spectra observed under electrolysis. The system chosen was the electroreduction of a stable tetramethylbenzidine free radical to tetramethylbenzidine (in DMF-O.1M LiClOa solution). The free radical was prepared chemically as described in the literature (15). /cH3
CH3'
As the reduction proceeded, a new peak, presumably corresponding to formation of tetramethylbenzidine, appeared a t 1170 em.-' and grew in intensity as the electrolysis proceeded. After extensive electrolysis at the Ge plate, the solution was observed to have turned from a green color, which is the reported color of the radical ( 5 ) , to a brown color. Studies of the effect of potential on the F M I R spectra of several solvents (HzO, CHBCN, C a i j , DMF, CHBOH, etc.) were also made. KO apparent changes in the spectra were observed over the entire range of potentials where the solvents did not undergo electrolysis. I t was hoped that changes in the spectra indicating changes in orientation or adsorption of the solvent molecules a t the interface might be observed on potential variation. The spectra obtained agreed in all cases with published F M I R or ATR spectra of these solvents (12). The fact that certain bands for each solvent are substantially reduced or
t
'20 .30 w
0
5
i
.40-
m
1200 I( WAVENUMBER CM-1
0
Figure 2. Infrared spectra of 8 q&olinol (saturated solution in dimethylformamide-0.1 M LiC104- 3Hz0) A.
Before electrolysis E. After brief electrolysis at S.C.E.
- 1.8
volts vs.
in some cases eliminated in the F M I R spectra as compared to transmission spectra indicates that only a thin film of molecules at or very near the F M I R plate-electrode surface is being observed (19). Distinct changes in the F h l I R spectra of the solvent molecules are $ 4 volts u s . SCE.
+ le-
b
observed when the applied potentials are sufficiently large to result in electrolysis of the solvent. For example, most of the vibrational motion bands of acetonitrile (containing 0.1M LiC104) were observed to shift slightly and change intensity when breakdown occurred. CONCLUSIONS
Although complete understanding of the spectra described above has not yet been achieved, the results do indicate that the technique can be employed to obtain in situ I R spectra of species a t or near the electrode surface during actual electrolysis. More exhaustive and comprehensive investigations are in progress and will be reported in detail in the near future. Also, the nature of the bonding formed on adsorpt,ion at Ge and other electrode surfaces is being studied. Also, corrosion of the Ge surface during electrolysis can be a problem in some
cases, This problem will be discussed a t length in a future report. Investigations on the feasibility of using conducting glass as well as other visible range transparent conducting materials for FMIR plate-electrodes is also in progIt would be of considerable ress, interest to obtain the charge transfer spectra of metal complexes as well as other species in the electrical double layer on electrolysis. The results of this work will be published at a later date. ACKNOWLEDGMENT
The authors are endebted to Paul A. Wilks, Jr., of the Wilks Scientific Corp., South Norwalk, Conn., for his interest in this project and for the donation of the Wilks Model 12A Double Beam Internal Reflectance attachment for this project. We also thank T. Kulyana, University of California at Riverside, and R. A. Osteryoung, North American Aviation Science Center, Thousand Oaks, Calif., for their helpful discussion about this work; and Jeffery L. Huntington for some preliminary research on the electrochemical behavior of germanium.
LITERATURE CITED
(1) Adams, R. X., Symposium on Electrode Reaction Mechanisms, Division of Analytical chemistry, 145th Meeting, ACS, New York, Sept. 1963. Abstract p. 7B. (2) DeFord, D. D., Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., kpril 1958. (3) Delahay, P., New Instrumental Methods in Electrochemistry,” Interscience Publishers. New York. 1954. (’$) Fahrenfort, J., Spectrochim.’ Acta 17, 698 (1961). (,5 ) Galus, Z., Adams, A. N., J . Am. Chem. SOC.84, 2061 (1962): (6) Gerischer, H., in “Advances in Electrochemistry and Electrochemical Eneineering.” P. Delahav. ed.. Vol. I. pp.- 139-252, Interscience,’ New Yorkj 1961
( 7 ) Hansen, W. N., ANAL. CHEM. 35, 765 (1963). (8) Harrick, N. J., Ibid., 36, 188 (1964). (9) Harrick. N. J.. Ann. N . Y . Acad. h i . 101, 928 (1963). (10) Harrick, N. J., J . Phys. Chem. 64, 1110 (1960). (11) Holmes, P. S., “Electrochemistry of Semi-Conductors,” Academic Press, New York, 1962. (12) Kaflatsky, B., Keller, R. B., Inst. ATews, Perkin-Elmer Corp., Norwalk, Conn., 15, No. 2, l(1964). (13) Katritzky, A. R., Jones, R. A., J . Chem. SOC.1960, p. 2942.
(14) Kuwana, T., University of California, Riverside, private communication, 1965. (15) Kuwana, T., Darlington, R. K., Leedy, D. W., ANAL.CHEW 36, 2023 (1964). L. H., Geske, D. H., J . Chem.
vision of Analytical Chemistry, ’145th Meeting, ACS, New York, Sept. 1963. Abstract p. 8B. (19) Sharpe, L. H., Inst. News, PerkinElmer Corp., Korwalk, Conn., 15, No. 4, 1 (1965). (20) Sharpe, L. H., Proc. Chem. SOC. (London)1961, p. 461. (21) Stock, J. T., J . Chem. SOC.1949, 586. (22) Wilks, P. A., Jr., S. Norwalk, Conn., private communication, 1964. HARRYB. MARK,JR. B. STANLEY PONS Department of Chemistry The University of Michigan Ann Arbor, Mich. RECEIVEDfor review October 18, 1965. Accepted November 9, 1965. Research supported by a grant from the U. S. Army Research Office, Durham, Contract No. DA-3 1-124-ARO-D-284.
~~
Determination of Molybdenum in Ferrous Alloys by Atomic Absorption Spectrometry SIR: Following upon the work of David (1, 2 ) , who examined various factors affecting the determination of molybdenum by atomic absorption spectrometry, the suggested analytical procedure was applied to the estimation of molybdenum in several types of ferrous alloy. Errors of more than the indicated figure of 10% were often obtained, with the majority of results tending to be low and it appeared that the recommended addition of 2000 p.p.m. aluminum and 4 ml. concentrated HNOa to all solutions to control interferences was not always effective. The differences in results could probably be ascribed to differences in optical arrangement and flame conditions between the Perkin-Elmer Model 303 equipment used in the present work and the original apparatus of David (1). I n addition, however, preliminary experiments indicated that the molybdenum absorption was extremely sensitive to small changes in solution composition and i t was probable that these solutions formed a complex interfering
ion system of the type described by Firman (3), in which the magnitude of an interference by one ion upon another, and also the further effects produced by the presence of additional ions could not safely be predicted for all concentration levels from trials at one level. Moreover, the method of successive additions, normally a reliable device for overcoming interferences due to variable composition, also failed in a number of the analyses. This determination has therefore been re-examined, and the use of ammonium chloride as a new and more general interference suppressing agent for molybdenum is suggested. EXPERIMENTAL
Apparatus and Operating Conditions. These are given in Table I. T h e experimental conditions are essentially those recommended previously (1) except that there is a case for adopting the line Mo 3798.3 A. instead of the more sensitive Mo 3132.6 A. The same beam energy can be achieved at a lower
gain control setting and hence the null meter noise is reduced. The sensitivity is still adequate for the analysis and there is a small improvement in precision of absorption readings; interference effects are similar at both wavelengths.
Table 1.
Experimental Conditions
Perkin-Elmer Model 303 spectrophotometer Wavelength, 3798.3 A. Range, ultraviolet Slit, 3 (0.3 mm.) Lamp current, 25 ma. (or nearest steady value) Air Supply, 25 p.s.i. Flowmeter. 5.5 Acetylene Supply, 8 p s i . Flowmeter, 9.0 Burner height adjusted for maximum absorption Sample uptake, 3 ml./min. Scale expansion, 1 Sensitivity, 1.3 p.p.m. Mo/lyo absorption
VOL. 38, NO. 1 , JANUARY 1966
121
