THEKINETICS OF
THE
show that higher concentrations of ammonium persulfate were required to obtain comparable decreases for G(Ce1IX)in 0.4 M sulfuric acid. This is consistent with negligible hydrolysis of ammonium persulfate 1 determined in 0.4 M sulfuric and k14/k13 = 9 in 4.0 M sulfuric acid. The hydrated electron reacts with the S208*- anion according to reaction 16 with an absolute rate conX 10’0 J4-l sec-’. Since the highest ~ t a n t ~ ~ 3of3 1.06 2
*
eaq-
+ S2OS2-+Sod2- + SO4-
(16)
ammonium persulfate concentration of 0.5 M is approximately equal to the hydrogen ion concentration in 0.4 M sulfuric acid, reaction 16 should compete significantly with reaction 3. The dependence of G(CelI1) on ammonium persulfate concentration would then be given by the equation G(CeI’I) = G(CelI1)O
2479
TUNGSTEN-OXYGEN-BROMINE REACTION
- ~ G -H 2Ge,,-
-+
There are too many unknowns in eq I11 to ob-
tain a unique solution by the method of least squares from the limited data shown in Figure 3. Assuming GH = 0.6,33values for G(CelI1)O,Ge,,-, and k~e/(kg[H+]) were obtained as a function of k13/(kg[02]). The best fit was obtained for k ~ ~ / ( k ~ [=o ~2.8 ] )with G(CexTx)O = 2.33 0.02, Gesq- = 4.16 0.08, and k16/(h[H+]) = 2.6 0.2. Although the data adhere well to eq 11, the good fit is only fortuitous since the value of GH Ge,,is much higher than the “standard of 3.65 for 0.4 M sulfuric acid solutions. We have not established why eq I11 is not valid, but assume that it is due to an effect of the high ammonium persulfate concentrations on G values of earliest detectable intermediates.
*
+
(31) J. K.Thomas, S. Gordon, and E. J. Hart, J . Phys. Chem., 68, 1524 (1964). (32) W.Roebke, M. Renz, and A. Henglein, Int. J . Radiat. Phys. Chem., 1, 39 (1969). (33) J. T . Allan and G. Scholes, Nature, 187, 218 (1960). (34) A. 0.Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” D. Van Nostrand Company, Inc., Princeton, N. J., 1961.
The Kinetics of the Tungsten-Oxygen-Bromine Reaction by E. G.Zubler Lamp Research Laboratory, General Electric Company, Nela Park, Cleveland,Ohio 44112
(Received January $6, 19YO)
A microbalance-flow system technique has been used to investigate the kinetics of the reaction of tungsten with Torr oxygen and 0.3-2.7 Torr bromine in the range 600-950’. The reaction was zero order with to respect to bromine and first order with respect to oxygen. The function of the bromine was to remove the oxidation product by the formation of volatile WOzBrzwhich permitted the oxidation to proceed as it would at higher temperatures where the oxide is volatile. An empiricalrate equation with an apparent activation energy of 31.O kcal/mol was obtained and compared with similar equations for tungsten oxidation at higher tempera-
tures.
Introduction The use of iodine and bromine transport cycles in incandescent has contributed to the current interest in high-temperature tungsten-oxygen-halogen chemistry. Recent work on the W-O-14-* system has provided important thermochemical data and an insight into the transport m e c h a n i ~ m . ~A more complete understanding of the transport chemistry requires kinetic data which are not available for either system. Extensive kinetic data are available for the
tungsten-oxygen reaction1’J-20but lacking for the tungsten-halogen reactions. Even with these data, addi(1) E.G. Zubler and F. A. Mosby, IlZ. Eng., 54,734 (1959). (2) G. R . T’Jampens and M. H. A. van de Weijer, Philips Tech. Rev., 27, 165 (1966). (3) F. A. Mosby, L. J. Schupp, G. G. Steiner, and E. G. Zubler, Ill. Eng., 62, 198 (1967). (4) J. Tillack, P. Eckerlin, and J. H . Dettingmeijer, Angezu. Chem. 78, 451 (1966). (5) J. H. Dettingmeijer and B. Meinders, 2. Anorg. AZlg. Chem., 357, 1 (1968). The Journal of Physical Chemistry, 7’01. 74, No. 1.2, 19YO
E. G.ZUBLER
2480
d iL
Absorption Cell
Bromine Reservoir
5
nalyzer
Figure 1. Microbalance-flow system: VI, VZ,O-ring stopcocks; Val metal needle valve; Vd, high-vacuum stopcock.
tivity procedures for predicting tungsten removal rates can be grossly inaccurate as shown by Rosner and Allendorf for the tungsten-oxygen-chlorine system.21 The present investigation was undertaken to elucidate the kinetics of the reaction of tungsten with to Torr oxygen and lo-' to 10' Torr bromine in the range 600-1000". This was based on some preliminary work by Dr. L. J. Schupp of this laboratory which suggested that the reaction of tungsten with oxygen and excess bromine was kinetically similar to the tungsten-oxygen reaction at temperatures above 1000" where the oxides are volatile. Experimental Section The apparatus, shown schematically in Figure 1, was based on a Cahn R H microbalance with a Pyrexquartz-stainless steel flow system at atmospheric pres sure. Hoke needle valves, high-vacuum stopcocks and glass flow meters were used to regulate and direct the gas flow. Metal diaphragm regulators were used on the cylinders containing the Nz (99.9985% purity) carrier gas 56 ppm Oz which were mixed to attain and the Nz the desired Oz level measured by a solid electrolyte sensor.22 I n addition, a small amount (5-109.1,) of highpurity He could be added to prevent condensation of the Nz if the gas were passed through a liquid Nz trap. With the pure Nz or X2 He, the minimum 0 2 level after appropriate flushing was 0.5 ppm (4 X Torr) and was unaffected by passing the gas over Cu filings at 600". Normally, the gas flow was 224 cm3/min through the balance and 110 cm3/min through the Brz reservoir. This minimized back diffusion of Brz(g) into the balance. The reagent grade Brz was distilled through magnesium perchlorate into a glass-vacuum system. After several condensing (- 196")-purnping-thawing cycles, it was distilled into a temporary reservoir on a manifold. The flow system reservoir was attached to
+
+
The Journal of Physical Chemistry, Vola74, No. 12, 1OYO
this manifold by Viton O-ring connectors and baked out. The Brz was then distilled into the reservoir which was filled with an atmosphere of Nz and returned to the flow system. When the flow of Nz (+He) through the Br2 reservoir was passed through a liquid Nz trap and into the Oz sensor, there was no increase in the residual 02 level. The Brz pressure was determined spectrophotometricallyz3 to 0.1 Torr in a 10cm absorption cell. The pressure was variable from 0.3 to 2.7 Torr by a combination of the reservoir bath temperature (-20 to 20") and the by-pass valve. In some preliminary runs, bromine corrosion of the balance mechanism was encountered and precluded runs at higher bromine pressures. The Brz pressure was insensitive to flow but was a function of the liquid Brzlevel. The pure tungsten was a 1.02 X 1.93 cm strip cut from 1-mil stock (General Electric-a typical analysis is availablez4). A small hole was punched at one end for the quartz fiber suspension of the microbalance. Initially the weight of the sample was 99.216 mg and the geometric surface area was 3.93 om2. After chemical cleaning, the tungsten was suspended in the reaction chamber and heated to 1000" for several hours in N a containing 0.5 ppm Oz. No attempt was made to reduce the carbon13 and oxygen levels by high-temperature (>2000") treatment. In a typical run, the gas flow and Oz level were adjusted and allowed to stabilize. The reaction chamber was rapidly brought to temperature and 13rz was admitted to the gas stream which passed through the absorption cell. An immediate, sharp decrease in (6) J. Tillack, Z . Anorg. Allg. Chem., 357, 11 (1968). (7) H. Schafer, D. Giegling, and K. Rinke, ibid.,357, 25 (1968). (8) S. K. Gupta, J. Phys. Chem., 73,4086 (1969). (9) A. Rabenau, Angew. Chem., 79,43 (1967). (10) I. Langmuir, J . Amer. Chem. Soc., 35,105 (1913). (11) R . A. Perkins, W. L. Price, and D. D. Crooks, Proceedings of the Joint AIME-Air Force Materials Laboratory Symposium, Technical Document Report No. ML-TDR-64-162, 1962, p 126. (12) R . W. Bartlett, Trans. A I M E , 230, 1097 (1964). (13) J. A. Becker, E . J. Becker, and R . G. Brandes, J. Appl. Phys., 32,411 (1961). (14) R. J. Ackermann and E . G. Rauh, J . Phys. Chem., 67, 2596 (1963). (15) J. B. Beckowita-Mattuck, A. Btichler, J. L. Engelke, and S. N. Goldstein, J . Chem. Phys., 39,2722 (1963). (16) Yu. G. Ptushinskii and B. A. Chuikov, Surface Sci., 6, 42 (1967). (17) P. 0. Schissel and 0. C. Trulson, J. Chem. Phys., 43, 737 (1965). (18) B. McCarroll, ibid.,46,863 (1967). (19) J. H. Singleton, ibid.,45,2819 (1966) ; 47,73 (1967). (20) D. E. Rosner and H. D. Allendorf, J.EZectrochem. Soc., 114,305 (1987). (21) D. E. Rosner and H. D . Allendorf, A I A A J.,5 , 1489 (1967). (22) S. S. Lawrence, H. 8. Spacil, and D. Schroeder, Automatika (Sept 1969). (23) A. A. Passchier, J. D. Christian, and N. W. Gregory, J. Phys. Chem., 71,937 (1967). (24) E . C. Sutherland and W. D. Klopp, NASA T N D-1310, Feb 1963.
THEKINETICSOF THE TUNGSTEN-OXYGEN-BROMINE REACTION weight was observed. This was more pronounced at the higher temperatures and Oz levels and was due to the removal of the oxide layer formed in bringing the sample to temperature. This was followed by a linear decrease in weight with time. The output signal from the Cahn microbalance was displayed on a 1-mV strip chart recorder with a chart speed of 6 min/hr. The balance controls were generally adjusted to give 1. mg full scale on the recorder. Under these conditions, the noise level was about 0.015 mg. For high-reaction rates, 2 or 4 mg full scale settings were used. Generally, the weight loss was observed for 1 hr. Thirty minutes was adequate €or the higher rates, and 2 hr was necessary for the lowest rates. Near the end of the run, the partial pressure of Brz was determined spectrophotometrically. Finally, the tungsten weight loss was converted to mol/(cm2 sec).
Results and Discussion Rate measurements above mol/(cm2 sec) were made at 50" intervals from 600 to 950' with Oz at 4.7 X to 4.3 X Torr and Brz at 0.3 to 2.7 Torr. The weight loss of tungsten was linear with time at all temperatures and gas compositions investigated with one exception which will be discussed. The geometric surface area of the tungsten was used in the rate calculations. During the principal series of runs, the tungsten sample weight decreased from about 96 to 65 mg. Microscopic examination showed a slightly roughened surface containing a number of etch pits and holes about 0.04 mm in diameter. Some were at a 45" angle to the surface. The reaction rate was independent of total gas flow in the range 250-500 cm3/min at all partial pressures of Oz. Below about 250 cm3/min, the residual Ozpressure increased with decreasing flow and produced a corresponding increase in reaction rate. All runs reported here were made at 334 cm3/min or higher where the rate was independent of flow at all O2 pressures. A reddish brown wall deposit of W02Brz (by X-ray diffraction) was observed downstream of the reaction zone. At the highest temperatures and O2 pressures, a less volatile yellow deposit assumed to be WOa was observed nearer the reaction zone. The quantities collected were insufficient for chemical analysis. There was no evidence for the purple-black WOBr4 which has been obtained with excess bromine below 400°.26 Recently, an absorption and emission band at 998 cm-l has been attributed to gaseous WOBr4 over a WOs-LiBr mixture above 500" .26 The appearance of the band coincided with a pale brown sublimate in a cold zone. The reaction rate was independent of Brz pressure in the range 0.3-2.7 Torr a t different temperatures and OZ pressures as shown in Figure 2. Most runs were made with a Br2 pressure of about 1 Torr. Dur-
2481
Po2 :7.6~16~ Torr 9OOOC X
I
J
Figure 2. Reaction rate dependence on bromine pressure. I
I\
I
I
I
I
lO4/T('Kl
Figure 3. Temperature dependence of reaction rate at various oxygen pressures (Torr): 0, 4.3 X lo-*; 0 , 6.8 X A, 2.3 X lo-'; V, 1.8 X l P a ; 0, 1.5 X lo-'; A, 7.0 X 0,6.8 X IOws; X, 4.7 X 10-4.
ing a series of runs, the Brz(l) level in the reservoir gradually decreased with a corresponding decrease in the Br2 pressure from about 1.2 to 0.8 Torr with no indication of change in the reaction rate a t a given oxygen pressure. The fraction of Brz dissociated as calculated from JANAF dataz7varied from 0.008 to 0.03 a t 600" and 0.3 to 0.7 a t Q O O O . While the reaction rate was independent of Brz pressure, it was dependent on the O2pressure as shown in the Arrhenius plot in Figure 3. The rate measurements a t a given O2 pressure were normally obtained Torr, in 1 day. The data a t PO% = 6.8 X however, were obtained on different days (indicated by V and 0 ) and are indicative of the reproducibility of the system. (25) R. Colton and I. B. Tomkins, Aust. J.Chem., 21, 1975 (1968). (26) B. G. Ward and F. E. Stafford, Inorg. Chem., 7, 2569 (1968). (27) "JANAF Thermochemical Tables," Dow Chemical Co., Midland, Mich., 1967. The Journal of Physical Chemistry, Vol. rd9 N o . I d , 1970
2482
E.G.ZUBLER
t
1
/ 162
16'
Po2,Torr
\*j
Figure 4. Reaction rate dependence on oxygen pressure at 800".
At each O2 pressure the data are reasonably well represented by a straight line although a slight curvature is apparent and expected. Under these experimental conditions where the tungsten and reactive gas are a t the same temperature, the 02 impingement rate which is a function of temperature should be considered. Impingement rates calculated from the term which Hertz-Knudsen equation introduce a T accounts for the apparent curvature. Over the limited temperature range investigated, this correction is less than 20% which is within the accuracy of the final rate equation. For convenience in comparing the final rate equation with those in the literature, this minor refinement was not introduced. A plot of the rate data a t 800" against 0 2 pressure in Figure 4 indicates a linear 02 pressure dependence. The data at the lowest (residual) 0 2 pressures are less reliable but included. I n view of the experimental technique of measuring the O2 pressure at room temperature, the residual O2 pressure during a run may well be slightly above that, indicated due to the evolution of gases from the system a t the elevated temperature. This * complication should be restricted, however, to the residual O2 pressures. For the determination of the activation energy, the ratio B of the rate of tungsten removal, mol/(cm2 sec), to the rate of 0 2 arrival, mol/(cm2 sec), was used as shown in Figure 5. This convenient reactivity parameter brings all the data except that at the residual 02 pressures onto a straight line. A least-squares treatment gives an activation energy, E,, of 31.0 2.2 kcal/mol. Combining this activation energy with the O2 pressure dependence in Figure 4, the following empirical rate equation was obtained R(mol/(cm2 sec)) = 4.05 X 10-2P02e-a1~000'RT The experimental data indicate that the tungsten removal rate a t 600-950" by loF4 to lo-* Torr 0 2 and 0.3-2.7 Torr Br2 is a linear function of the 0 2 and independent of the Br2. From the Arrhenius plots, the apparent activation energy which may contain the The Journal of Physical Chemistry, Vol. 74, N o . 1.2, 1970
0
80
8.5
9.5
9.0
10.0
105
110
104/T I 'K 1
Figure 5 . Temperature dependence of the reactivity parameter e (reaction rate/Oz impingement rate), for various oxygen pressures (Torr): 0 , 4.3 X lo-*; 0, 6.8 X A, 2.3 X V, 1.8 X 0, 1.5 X low2;A, 7.0 X V, 6.8 X IOb3; X , 4.7 X 10-4.
heat of adsorption of 02 is 31.0 kcal/mol. The role of the BrP is to remove the surface oxidation product by the formation of volatile W02Br2which condenses on the cold walls outside the reaction zone. This relatively fast reaction prevents the formation of an oxide surface layer and permits the reaction to proceed as it would a t slightly higher temperatures (>1000") where the oxide is volatile. In this temperature range, 1000-1500", and with comparable or lower O2 pressures, there is substantial evidence14-17 for the formation of a tungsten oxide on the surface. In addition, Schi~sel'~ has identified (WO& and (WO& as the major gaseous species under these conditions. WO,(g) and W02(g) become dominant above about 1500 and 1700", respectively. If the W O2 Br2 reaction at 600-950" is kinetiO2 reaction a t 1000-1500", cally the same as the W a comparison of the extrapolated rate data should be revealing, Several investigators have determined similar rate equations for the pressure and temperature ranges of interest. After converting to the same units, mol/(cm2 sec), these rate equations with the oxygen pressure in Torr are compared in Table I. I n the Arrhenius plot based on these rate equations in Figure 6, the current work compares favorably with the extrapolated data of the previous investigations at lower total pressures. This agreement suggests that the N2 carrier gas in these experiments does not influence the reaction kinetics. During this investigation, the reaction rate was independent of bromine with (Br2)/(02) ratios in the range 25-2500. At slightly lower (Brz)/(Oz) ratios,
+ +
+
2483
THEKINETICS OF THE TUNGSTEN-OXYGEN-BROMINE REACTION Table I : Comparison of Empirical Rate Equations for Tungsten Oxidation Ref
Worker
Rate, mol/cmZ sec (Pol, Torr)
Temp range, O C
Poa range,
Torr
Langmuir 10 2.32 X 10-2Poae-2'~000/RT 927-1500 10-4-10-2 Becker, etal. 13 2.32 X 927-1327 10-7-10-6 10-ZPo2e-%,400/R?' Perkins, 11 0 . 2 1 6 P 0 ~ ~ ~ 1300-1760 ~ e - ~ ~ 10-8-3 ~ ~ ~ ~ ~ ~ ~ et al. Bartlett 12 0 . 1 9 1 P 0 ~ ~ ~ 1320-1600 ~ ~ e - ~ 10-8-1 ~ ~ ~ ~ ~ ~ ~ ~ Singleton 19 2.4 X 1000-1200 lo-' 10-2Poz0.?e-32,800/RT Zubler 4.05 X 600-950 10-4-10-2 10-2Po,e -31,000/RT
e.g., 10, there was some indication that the rate of formation of W02Brzwas insufficient to prevent the formation of an oxide surface layer. While Br2 will react with W02 forming volatile WO2BrZ, there is evidence28 that it will not react with W03. During a Torr and (Br2)/ run at 600" with PO%= 4.3 X (02)= 10, the initial tungsten removal rate was normal but then slowly decreased. Subsequent runs with (Br2)/(02) = 25 gave abnormally low but linear weight losses. When the tungsten was then taken to 1000" in pure N2, a weight loss was observed with the simultaneous appearance of a yellow wall deposit assumed to be WOa. This suggests that the initial oxide product on the surface was WOz (or WO,-,) which in the presence of a sufficient excess of Brz (or Br) was removed by the rapid formation of W02Br2. Below some critical Brz level, there was competition from the reaction W02 1/202 + W03. As the W03 was formed and remained on the surface, the rate of tungsten removal decreased. After a short time a t 1000° where the WOs was volatile, the tungsten removal as W02Brz was again predictable. Previous mechanistic work13-18 on tungsten oxidation has established two distinct binding states or layers of oxygen. Schissel and Trulsen" have proposed a two-layer oxygen atom model while McCarro1118has evidence that the first layer is atomic oxygen. I n addition, the reaction of tungsten with chlorine and oxygen at 10-6-10-4 Torr has been investigated a t 1200-2400°K.21~29 McKinley29 has identified WO2Clz, WOz, and W03 as the dominant gaseous species desorbing a t the lower temperatures. With chlorine:oxygen ratios in the range 1-10, the desorption of WOzClz was first order in chlorine and oxygen. Assuming a two-layer model and that the first layer was atomic oxygen, the presence of simple oxides and absence of simple chlorides suggest that the second layer contains both chlorine and oxygen with the oxygen more extensively adsorbed.
+
Figure 6. Comparison of rate data at
Torr oxygen.
The present work supplies no evidence for the twolayer model. The zero-order dependence on the excess bromine and first-order dependence on oxygen do indicate that while the surface was saturated with respect to bromine, there was no interference with the adsorption of oxygen. Either the oxygen and bromine were adsorbed on different kinds of sites or the oxygen successfully competed with the excess bromine for the same sites. McKinleyze found that oxygen was more strongly adsorbed than chlorine on tungsten. He also observed a rapid decrease from first- to zero-order dependence above some total chlorine oxygen presTorr). This transition was temperasure (-5 X ture dependent, and as the temperature was increased, the first-order dependence was followed to higher total pressures. While the reaction product was not monitored here, the condensate outside the reaction zone was predominantly W02Brz with no evidence for WO13r4. The simplest possible mechanism, therefore, would involve the direct combination of WOz and Brz on the surface. A similar proposal was offered by McKinley29because no oxychlorides other than W02Clzwere observed.
+
Conclusion The reaction rate of tungsten a t 600-950" with 10-4-10-2 Torr oxygen and 0.3-2.7 Torr bromine was zero order in bromine and first order in oxygen. The bromine removed the oxidation product by the formation of volatile W02Br2 a t a sufficient rate to permit the oxidation to proceed as it would a t higher (28) L.J. Schupp, unpublished results. (29) J. D. McKinley, Reactiv. Solids, Proc. I n t . Sump., 345 (1969). The Journal of Physical Chemistry, Vol. 74, No. 18, 1070
SEIHUNCHANGAND WILLIAM H. WADE
2484 temperatures where the oxide itself is volatile. An empirical rate equation compares favorably with similar rate equations in the literature for tungsten oxidation a t comparable pressures but higher temperatures. The correlation with the oxidation data a t very low total pressures indicates that the Nz carrier gas a t atmospheric pressures does not affect the reaction kinetics. The oxygen either successfully competes with the excess bromine for adsorption sites or is
adsorbed on different kinds of sites than the bromine. The reaction mechanism, which is independent of the dissociative state of the gaseous bromine, may simply involve the direct combination of WOZ and Br2 on the surface. Acknowledgements. Drs. L. V. McCarty, S. I(. Gupta, and R. J. Campbell participated in many helpful discussions and critically reviewed the manuscript .
The Kinetics of Interaction of Oxygen with Evaporated Iron Films
by Seihun Chang and William H. Wade Department of Chemistry, The University of Texas at Austin, Austin, Texas 78711 (Received May 19, 1970)
The fast interaction of oxygen with iron films has been investigated with a quartz crystal microbalance in the pressure range 2 X lo-' to 5 X Torr a t 24'. A relatively rapid formation of the equivalent of 4 oxide (FeO) layers was observed which, subsequently, was followed by a much slower growth up to 10 layers. The process kinetically approaches first order with respect to oxygen pressure in the initial stages and decreases to fractional orders as incorporation proceeds. This can be interpreted in terms of a surface regeneration by incorporation of adatoms into the interior at rates comparable to the adsorption process. The Elovich direct logarithmic rate law does not hold for the rapid chemisorption since it is observed to be autocatalytic.
Introduction There have been several reports on the kinetics of oxygen chemisorption on iron films.'-* They show several inconsistencies but there appears to be general agreement that an initial fast process is followed by a slow one. The fast process takes place so rapidly that, to the present time, it has not been followable using conventional gravimetric or volumetric methods, and all the extant rate data for oxygen chemisorption on iron pertain to the slow process. Recently Pignocco and P e l l i s ~ i e r reported ~~~ LEED data for the first stages of the interaction of oxygen with oriented iron crystal faces. Their study was mainly concerned with structural changes during oxygen adsorption coupled with rather qualitative studies of adsorption kinetics. The quartz crystal microbalance (QCM) introduced by Sauerbreya is a new tool for investigating surface processes. This instrument has several advantages over conventional gravimetric measuring devices : (1) The sensitivity of the QCM is sufficient to permit measurement of changes in surface mass of 10-lO g/cm2. (2) The response time of a QCM to changes in mass of the active surface is on the order of microseconds, enabling one to follow fast adsorption processes. (3) The frequency shift of the crystal due to factors other than mass change can be minimized or corrected. (4) The Journal of Physical Chemistry, Vol. 74, No. 12, 1970
Buoyancy and wall effects can be completely eliminated. Previously a quartz crystal microbalance was used successfully by Slutsky and Wade' for measuring adsorption isotherms of hexane on a quartz single-crystal face. Later, using the same technique Wade and Allen* followed fast adsorption kinetics for oxygen on evaporated aluminum films.
Experimental Section The vacuum system and circuits used in this work have been described in a previous paper.8 The quartz crystals used in this study were 10-mHz, AT plates furnished by Scientific Electronic Products Corp. of Loveland, Colo. The quartz crystal was mounted in the vacuum system and covered by an aluminum hous(1) J. Bagg and F. C . Tompkins, Trans. Faraday SOC.,51, 1071 (1955) (2) M.A. H.Lanyon and B. M . W. Trapnell, Proc. Roy. SOC.,Ser. A ., 227, 387 (1955). (3) M.W. Roberts, Trans. Faraday SOC.,57,99 (1961). (4) A.J. Pignocco and G . E. Pellissier, J. EZectrochen.SOC.,112, 1188 (1965). (5) A. J. Pignocco and G . E. Pellissier, Surface Sci., 7, 261 (1967). (6) G.Sauerbrey, Z . Phys., 155,206(1959). (7) L.J. Slutsky and W. H. Wade, J . Chem. Phys., 36, 2688 (1962). (8) W. H.Wade and R . C. Allen, J . Coll. Interface Sci., 27, 722 (1968).
.
