Fisher Award Symposium
Some of the papers presented in the Fisher Award Symposium Honoring Herbert A. Laitinen, Division of Analytical Chemistry, 139th Meeting, American Chemical Society, St. Louis, Mo., March 1961.
EIectroanaIyticaI Chemistry of Surface Monolayers H. A.
LAITINEN
Noyes Chemical laborafory, Universify of Illinois, Urbana, 111.
b Three electrochemical approaches to the analysis of a surface for foreign materials in monolayer or lesser quantities are described. The first approach, coulometry, has proved to b e useful in determining the extent of oxidation of the surfaces of platinum and gold, and in cotrelating the fraction of monolayer oxidation with the differential double layer capacity. The second approach, differential double layer capacity measurements, is the most general, because it responds to all substances which change the dielectric properties of the metalsolution interface, and because it does not require an electrode reaction. It is limited to regions of potential relatively far removed from oxidationreduction or adsorption-desorption potentials. The third approach, chronopotentiometry, is limited to substances undergoing reduction or oxidation without kinetic complications. By a suitable extrapolation technique, reaction of the adsorbed phase is distinguished from that of the diffusing species.
I
of the expanding scope of the field of analytical chemistry is the recent application of electroanalytical techniques to the analysis of surfaces for foreign substances in amounts corresponding t o a monolayer or less. The purpose of the present paper is to compare three approaches to this problem, showing their applicability and limitations. LLUSTRATIVE
COULOMETRY O F SURFACE MONOLAYERS
To apply coulometric techniques to surface monolayers, it is necessary for the monolayer to undergo a single quantitative electrode process at 100% current efficiency, and for the measurements to be sufficiently sensitive and accurate for the purpose a t hand. The method will be illustrated by the determination of the extent of surface 1458
ANALYTICAL CHEMISTRY
oxidation of platinum (7, 12) and gold (4). Let us first consider in principle the alternative methods of constant current and constant potential as applied to the anodic and cathodic reactions. Neither method can be applied on the anodic cycle because a mixed electrode process occurs. This is illustrated by the schematic curve in Figure 1, showing coulombs of anodic or cathodic current passed as a function of electrode potential for platinum. If a constant anodic current is passed, a mixed electrode process must occur, because the first step in the oxidation of the surface involves the formation of a monolayer of hydroxyl radicals, which are also involved in the first step of oxygen evolution (12). The oxygen overpotential develops relatively slowly, because the hydroxyl radical layer is gradually transformed into a n oxygen atom layer, and the overpotential a t constant current, correspondingly, increases gradually. The total area under the anodic charging curve is considerably larger than that corresponding to the surface oxide. If, instead of a constant anodic current, a fixed anodic potential is applied, the steady state is reached more rapidly, but again a mixed process occurs. Following a relatively rapid surface, a slow and more deep-seated oxidation takes place both on platinum (Figure 2 ) and on gold (Figure 3). Turning now to the cathodic cycle, it is found that the surface oxidation accomplished in the first 30 seconds or so of anodic reaction a t constant potential is readily reduced a t constant current (Figure 1). The current efficiency has been studied as a function of applied cathodic current density with the results shown in Figure 4 for gold ( 4 ) . Over a wide range of current densities, the same coulombic charge is found to within a few per cent, but a t very low current density, the apparent result is low. If an interrupted cathodic electrolysis is carried out,
using on and off cycles of higher current density, the efficiency drops below that observed a t constant current. R e deduce from this observation that a side reaction occurs a t very low current densities or during the off cycle, involving the nonfaradaic loss of oxygen, perhaps as molecular oxygen formed from active intermediates formed as a step in the cathodic reduction cycle. If a constant potential cathodic process is used, a n interesting hysteresis phenomenon is observed, in which no reduction a t all occurs until a critical region of potential is reached (Figures 5 and 6). At sufficiently negative potentials, a reduction occurs a t a rate indicative of a first-order reaction with respect to surface oxidation. The first-order rate constant is greatly dependent on potential in this region (12). For this reason as well as for convenience of integration of the current-time curve the constant current cathodic method is preferred for the coulometric determination. The hysteresis curves of Figures 5 and 6 were actually determined by this method. As a n estimate of the necessary sensitivity of coulometric measurements, a simple calculation shows that surface oxidation comesponding to one oxygen atom per surface gold atom on a 111 crystal face corresponds to 450 wcoulombs per sq. em. of surface. If 30 seconds is taken as a convenient transition time, an applied current density of 15 pa. em.-* is needed. The lower limits of applied current density are imposed by the fact that it should be large compared with the average steady state residual current density, and also by a side reaction that appears to interfere, a t least in the case of gold, a t very low current densities (Figure 4). T o relate the extent of surface oxidation to the amount of surface metal, it is necessary to have some independent measure of the true surface area. Spe-
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- F O R DENSITIES LOW CURRENT
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HC ANODIC
COULOMBS
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CATHODIC COULOMBS (QUAL ITAT IVE I
(QUALITATIVE)
cifically, we wish to "count" surface metal atoms, so that we can determine the number of surface hydroxyl radicals or oxygen atoms per surface metal atom. For this purpose, a gas adsorption method involving a gas molecule o r atom comparable in size to the metal atoms ivould seem to be ideal. Through the cooperation of Professor Norman Hackerman of the University of Texas, a n estimate of the roughness factor of a 7 x 7 cm. piece of unused bright platinum foil was estimated through a scaleddown BET measurement of surface area using krypton gas. This method gave a roughness factor of 1.12, with an estimated uncertainty of 10%. Examination of the suriace by electron microscopy, using a plastic replica technique followed by shadow casting revealed that the surface structure consisted of flat grains up to 1 micron across, with some evidence of small vertical discontinuities at the grain boundaries. I t may be remarked here that a method of greater "resolving
power" than krypton adsorption may have yielded a somewhat larger roughness factor, but that the factors of two to three which are often applied to convert geometric area to true area are probably much too large, a t least for mirrorbright foils. Using the roughness factor of 1.12 it is possible to conclude that a surface oxidation corresponding to about two oxygen atoms per surface platinum atom is reached a t very anodic potentials] where the rate of oxygen evolution is relatively large. A continuous and essentially linear increase in surface oxidation with increasing anodic potential is observed. With gold, on the other hand, the extent of surface oxidation soon reaches a value corresponding to AuZ03, and beyond that region a deepseated oxidation to gold(II1) oxide occurs (Figure 7 ) . The difference in behavior between gold and platinum is to be ascribed to the tendency for gold(II1) oxide to undergo ionic con-
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ANODIZATION,
I20
duction, so that a thick layer of oxide can form. Another significant difference in behavior between platinum and gold is that platinum has a pronounced tendency to absorb hydrogen upon strong cathodization, whereas gold does not. Therefore, it is necessary to interrupt the cathodic cycle on platinum before hydrogen evolution occurs (note that there is a n appreciable apparent "underpotential" for h,-drogen evolution in a n inert atmosphere because of the low activity of hydrogen in solution). Otherwise, the metal becomes contaminated with hydrogen which is slow to diffuse out. This diffusing hydrogen, besides undergoing anodic oxidation, also reduces oxide from the surface. A platinum electrode may be conditioned against the presence of surface oxide and absorbed hydrogen by holding it a t a potential of about +0.5 volt us. S.C.E. in 1M perchloric acid for a period of hours or days. The coulometric method is limited to cases in which neither mass transport nor chemical reaction can act to produce more of the electrochemically active surface layer during the measurement. Although the method can no doubt be extended to monolayer films
[Oal) and
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OF
100
80
Figure 2. Effect of time of anodization of platinum a t constant potential on amount of surface oxide
ti* C ,
Oxidation of adsorbed hydrogen Anodic charging of double layer Oa,, Oa,, Oxidation of surface plus oxygen evolution on clean surface oxidized (Os,) O., Oxygen evolution reaction Oo,Cathodic reduction of oxide C , Cathodic charging of double layer He, Formation of adsorbed hydrogen
60
TIYE
Figure 1. Schematic representation of coulometry of platinum surface a t constant current
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C U R R E N T D E N S I T Y , M I C R O A M P E R E S PER C M 2
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Figure 4. Effect of current density on current efficiency of cathodic cycle for gold
Figure 3. Effect of time of anodizationof gold at constant potential on amount of surface oxide
Circles correspond to uninterrupted current; triangles to double current interrupted for equal intervals, either 10 or 30 sec.
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VOL. 33, NO. 1 1 , OCTOBER 1961
1459
POTENTIAL
OF
ANODIZATION
Figure 5. Hysteresis curves for platinum, amount of surface oxide as function of potential of anodization
other than oxides, its applicability is inherently rather limited. The anodic dissolution of fractional monolayers of deposited metals deserves special study. MEASUREMENT OF DIFFERENTIAL LAYER CAPACITY
DOUBLE
In principle, the electrical capacitance of the double layer is a sensitive index of the surface state of a conductor. If the effective dielectric constant, the effective thickness of the double layer, or the area is altered by a chemical or physical change, a corresponding change occurs in the double layer capacity. To complicate matters the capacitance is in general a function of the electrode potential, so that it is necessary to measure the differential capacity of the double layer defined by Cd I = dQ/dE or the rate of change of surface charge with potential at 3ome fixed potential. From the measurement viewpoint, this imposes the restriction that the capacitance must be measured with a voltage change of the order of a few millivolts. Otherwise, the measurement yields an average value over a range of potentials.
Another complication is that the occurrence of a n electrode reaction introduces an additional apparent capacitance, the “pseudocapacity” if the reaction shon-s a n appreciable reversible character a t the potential in question. The pseudocaparity in general is frequency-dependcnt if a t least one coniponent of the reaction enters the solution phase, and disappears in the limit a t infinite frequency, so that a high frequency measurement is generally a more reliable measure of the true double layer capacity t’han a low frequency one. By high frequency is meant hew 10 or x t most 100 kilocycles per second, far below the frequencies :it which ionic relaxation effects are encourittvd. Even in the : h e n c e of known electrode reactions, a frequeiicy dispersion of capacitance is sollietimes encountered, for reasons there are still obscure in part. With a mercury electrode arranged symmetrically to avoid nonuniform alternating current distribution, the measured capacitance values are very nearly frequency-independent over a range from a fen cycles per second to several kilocycles per second. A small dispersion is introducrd b!. shielding, for
example with the capillary of a conventional dropping mercury electrode ( I O ) . With solid electrodes, frequency dispersion is always observed, either because of unsuspected electrode reactions because of the effects of nonuniform surface (active sites), or roughness. At relatively large negative or positive potentials, an adsorptiondesorption equilibrium is often observed. If this equilibrium is rapid, a peak very similar to a pseudocapacity peak is observed. Such a peak (sometimes called a tensammetric peak) is likelvise frequency dependent, but it differs in that it does not correspond to a faradaic process. The fundamental characteristics of several methods of measuring differential double layer capacit,y will now be considered. First, the bridge method, which has been extensively used by Grahame ( I O ) and many others, is capable of high prevision, but it tends to be tedious. At each potential, a series of measurements is made a t various frrq u e n c k , and, if necessary, a n extra1)olation is made to infinite frequency. A sinusoidal input signal of a few millivolts is applied to an impedance bridge incorporating the cell as one of its arms, and the off-balanee signal is amplified. Oscilloscopic recording of off-balance signal against time is convenient for measuring a changing rapacitance as with a dropping mercury ctlcctrode. An off-balance bridge can be used to record slowly changing capacitances, recording an amplified and rectified off-balance signal on a stripchart recorder (24). Second, a voltage step method, as used by Enke (7, 1.2) for the study of platinum ran be used for diffprential
4 Figure 6. Hysteresis curves for gold, amount of surface oxide as function of potential of anodization
I
12
E N T I A L C)F A N O D I Z A T I O N , VOLTS
1460
ANALYTICAL CHEMISTRY
L 16 20 P O T E N T I A L OF A N O D I Z A T I O N , VOLTS
4
Figure 7. Effect of strong anodization on amount of surface oxide
065
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125
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Figure 8. Hysteresis curves for platinum, double layer capacity as function of potential of anodization
capacity measurements provided that the voltage step is of the order of a few millivolts. A potentiostat and rapid rise-time switch to apply the voltage step, and a sensitive oscilloscope with direct current input permit the display of the charging current as a function of time. Translation to a semilogarithmic plot permits an evaluation of resistance from the intercept and capacitance from the slope (16). Alternatively, an electronir integration of the changing curve permits the direct display of capacitance as a step function on a second channel of a dual trace oscilloscope ( 7 ) . This method is particularly convenient when a n instantaneous value of capacity starting from a potentiostatically controlled potential is needed, as when the capacitance shows hysteresis behavior dependent on the past history of the electrode. It is inconvenient from the viewpoint of finding frequency dispersion effects, because these become evident only from the curyature of a semilogarithmic plot. Third, a current step method, as used by Chao (4) for the study of gold is available. I n principle, it is identical to the square wave method of Brodd and Hackerman (3) except that a single cycle is used. The potentialtime trace for a pure capacitor in series with a resistor consists of an instantaneous potential change determined by the iR drop followed by a linear trace, the slope of which is inversely proportional to capacitance. By keeping the total potential change during charging to a few millivolts, a dif, ferential capacity can be measured. This method has the advantage of showing frequency dispersion effects as nonlinear charging traces. Still another method for differential capacity measurement is to measure the charging current during a linear voltage sweep; using either a small or a large total potential change. The instantaneous current a t any potential is a direct measure of charging current (19). This method is convenient for rapid display but it is less straightforward for cases involving hysteresis effects and frequency-dependent ca'
pacitance. I t will not be considered further in this paper. Turning now to the interpretation of differential double layer capacity measurements in terms of surface composition, let us consider first the case of platinum, in' which a n absolute comparison is possible between the extent of surface oxidation and the capacitance. It is interesting that in this case a hysteresis effect is observed in double layer capacity just as in surface oxidation (Figure 8). Whichever branch of the hysteresis curve the points are derived from, it turns out that there is a single-valued functional relationship between surface oxidation and double layer capacity. This is shown in Figure 9. Below 0.25 millicoulomb cm.-* of surface oxidation, the capacity rises
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04 SURFACE
0'5
0 6
OXIDE
Figure 9. Double layer capacity as function of amount of surface oxide
linearly with surface Oxidation. Since 0.25 millicoulomb corresponds to about 1 electron and therefore 1 hydroxyl radical per surface platinum atom, this portion of the curve is interpreted by considering that the double layer may be regarded as two cypes of parallel capacitors, each with a characteristic differential capacity per unit area. I n one type, adsorbed water molecules constitute the innermost region of the liquid phase whereas in the other, chemisorbed hydroxyl radicals (which correspond to water molecules lacking a hydrogen atom) do. I t is significant that the differential capacitance does not increase with further surface oxidation beyond 1 electron per surface platinum atom. Neither does it depend markedly on perchloric acid concentration in the range 0.01 to 1M. These observations suggest that a large fraction of the potential gradient near the surface occurs in the first dipolar layer a t a platinum surface, once each surface platinum atom is associated with an oxidizing species, whether hydroxyl radical or oxygen atom. The surface condition might formally be written Pt+ O+ H + and Pt+20-2, in which the positive charge on platinum is to be regarded as a measure of charge density in the metal phase. K i t h increasing surface oxidation beyond 1 electron, it appears that an increasing
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I65
185
Solid curve, increasing anodic polarization Dotted curve, decreasing anodic polarization
fraction of the total potential drop occurs in the first molecular layer, and that the diffuse part of the double layer makes relatively little contribution to the measured capacity; hence the insensitivity to electrolyte concentration. In agreement with the findings of Bockris and coworkers (1, Z), we a b tribute the potentiddetermhling step of the first stage of ox4dation of platr inum or gold to the reaction .hl
MILLICOULOUBS OF
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Figure 10. Double layer capacity of gold as function of potential
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+ HzO
M OH
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where M OH represents chemisorbed hydroxyl radicals. With gold, the double layer capacity does not increase with the extent of surface oxidation, but appears to remain essentially constant (4) (Figure IO). This apparently anomalous finding becomes rational when it is considered that in the region of potentials immediately preceding the surface oxidation step, the capacitance was rapidly decreasing with increasing positive potential. Actually this also occulg with platinum, but a t a more negative potential, so that a shallow minimum occurs before oxidation begins. With gold, then, a similar minimum would presumably have occurred had not the surface oxidation intervened, so that with respect to the hypothetical unoxidized surface, an increase of double layer capacity with oxidation occurred with gold, just as with platinum. Turning now to the determination of fractional monolayers of organic adsorbates from double layer capacity measurements, we find the situation less straightforward because of the lack of a direct calibration. If we plot the change in differential double layer capacity at constant potential against concentration of adsorbate, we obtain a plot strongly reminiscent of a Langmuir isotherm. Such a plot is shown for *butyric acid in Figurr 11 (13).
The simplest interpretation, which has been applied by several workcn (5, 11, 14, 18, 19) is to assume a parallel capacitor model of the double layer for fractional monolayer adsorption, such that VOL. 33, NO. 1 1 , OCTOBER 1961
1461
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Figure 1 1.
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+ (1
400
Adsorption isotherm of butyric acid
- e) c:~.
(2)
where e is the fraction covered Crk = differential double layer capacity of saturated monolayer C:.l = differential double layer capacity of clean surface C d l = measured double layer capacity. This interpretation is equivalent to saying that, even on a molecular scale, it is proper to regard the covered frsction to be represented by a capacitor of lower dielectric constant in parallel with a capacitor unaffected by the adsorbate. A somewhat more detailed model is represented by the assumption that only the inner region of the double layer is affected by adsorbate, and that the outer or diffuse region of the double layer is unaffected. This corresponds to a parallel capacitor combination similar to the one described above in series with a capacitor representing the diffuse part of the double layer. Since the diffuse double layer becomes relatively more important a t lower electrolyte concentrations, the effect of this correction likewise becomes more important in dilute electrolytes. This is illustrated by Figure 12 showing Langmuir-type isotherms caIculated with and without corrections for the diffuse part of the double layer, for palmitic acid in various concentrations of sodium perchlorate. The situation is complicated by the presence of surfaceactive contaminants in the supporting electrolyte, the effect of which first had to be subtracted out by an extrapolation procedure (16). The noticeable difference between the uncorrected isotherms and the corrected ones is brought out more clearly by the inverse plots of Figure 13. 1462
I
100 200 300 CONCENTRATION OF BUTYRIC ACID, mM
0
it is a t present not possible to evaluate separately the possible nonlinearity of the capacity change with coverage and the possible invalidity of the Langmuir isotherm. I t is remarkable, however, that these two effects, if they are large, seem to cancel each other in many cases. Another complication is the possible existence of multilayer adsorption processes. Multilayer formation has been previously postulated by several workers (6, 8, 9, 20, 21), as evidenced, for example, in the case of a saturated solution of n-octanoic acid, by means of surface tension measurements by Frumkin et al. (Q),and for n-octyl alcohol at concentrations above half-saturation, by means of double layer capacity measurements by Melik-Gaikazyan (11). An island-like structure of multilayers has been proposed by Frumkin (8). It is interesting in this connection to examine the inverse isotherm of nnonanoic acid (Figure 14), in which concentration is plotted on a relative scale taking a saturated solution in 0 . W sodium perchlorate as unity. At concentrations below two thirds of saturation, the Langmuir model holds well, and gives an extrapolated value of C X of 3.8 pf. crneFafor a saturated monolayer. The experimental results a t high concentrations fall far below this value (1.76 pf. cm.-* for a saturated solution), thus suggesting the formation of a multilayer. It is clearly necessary for considerable more research to be done before an unambiguous interpretation of double layer capacity measurements in terms of surface composition can be made.
ANALYTICAL CHEMISTRY
There is, however, still a fundamental question as to the validity of Equation 2, the parallel capacitor model, even for the inner region of the double layer. Parsons (22) has maintained that if a Langmuir-type monolayer adsorption actually occurs, the measured doublelayer capacity should be a quadratic rather than a linear function of surface coverage. He suggests that the apparent validity of the Langmuirisotherm for intermediate functional coverages (14) might equally well be represented by a logarithmic isotherm. In the absence of independent calibration data I O
0 1
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s-\
Adsorption isotherms of palmitic acid
y
purpose, consider first a hypothetical model in which the surface excess concentration, r, is first reduced to zero and then the diffusing species is electrolyzed in the usual way. If T. and T d are the partial transition times due to the adsorbed phase and the diffusing species, respectively, we write the following expression for the total transition time T :
1
1
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2 2
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or
At any concentration C, a plot of I r against 1/I should yield a straight Figure 13.
A possible approach, which should be valid for reducible adsorbates in the absence of kinetic complications is chronopotentiometry, which will be presented next. CHRONOPOTENTIOMETRY OF SOLUTE
line, of intercept nF and slope n2F2nDC2/ Equation 4 was first proposed by Lorenz (17). Now, if the hypothetical model is incorrect in that the removal of part of the surface excess causes a gradual shift in adsorption equilibrium, then the second term of Equation 4 would be affected, but the intercept which corresponds to an infinite current density, would nevertheless give the correct result for the surface excess concentration, in the absence of kinetic complications. To demonstrate that an adsorbed monolayer should make an appreciable contribution to the .total transition time, some calculated plots of Equation 4.
Inverse isotherms of palmitic acid
ADSORBABLE
Suppose that, a t a given solution concentration C moles liter-1, an equilibrium surface concentration I' moles cm1.-2 of solute is present in the adsorbed state. We suppose further than both the adsorbed and solution phases undergo electrode reaction a t constant current to give a transition time without necessarily showing a n intermediate break due to the adsorbed monolayer. We shall show that, in principle, it should be possible to determine adsorption isotherms by a suitable interpretation of transition times measured at various current densities. It is assumed that the electrode reaction is not complicated by slow kinetic steps and that diffusion is the only rate process involved.
Reinmuth (23) has recently discussed the effect of adsorption on chronopotentiometric transition times, making various assumptions as to the relative ease of reduction of the adsorbed or solution species, and the existence of a n adsorption equilibrium a t the surface during the reduction process. We are interested here in measuring a total transition time due to adsorbed and solution phases, and extrapolating out the diffusion contribution. For this
C=LATED
n z 2 , D:5
X
CURVES
126M2E'-
/3;5 26 T7lM-i A Z O 0 3 C M 2
MOLEC AREA 50 x 10-6 C M ~ IN P A R E Y T H E S f S
c(mLJ)
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/
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b 4 h Figure 14.
h
*/3
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Inverse isotherm of n-nonanoic acid
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X iG-3(CM2AM$p') I5
25
Figure 1 5. Calculated curves for chronopotentiometric transition times of surface active substance VOL. 33, NO. 1 1 , OCTOBER 1961
1463
4 are shown in Figure 15. For the calculation it is assumed that a substance of molecular area 50 X cm.? undergoes a 2-electron reduction. A diffusion coefficient of 5 x cm.2 sec.-l and an electrode area of 3 x lo-' cmS2are taken. It is further assunied that a monolayer adsorption equilibrium, obeying a Langmuir isotherm, with half coverage a t a soiution concentration of 1.9 x l0-dXj is valid. Applied current densities are such as to bring the total transition times into a convenient range below 1 second. I t is evident that, in principle, this method is capable of determining the surface excess in amounts corresponding to fractional monolayers. Experiments are now under way to test the validity of this approach. It is hoped that this method may serve not only as a method of determining adsorption isotherms, aid of proving the existence of multilayers, but also as a method of calibration for the double layer capacity rwthod.
LITERATURE CITED
(1) Bockris, J. O'M., Huq, A . K. M. S., Proc. Roy. SOC.(London) A237, 277 (1956). (2) Bockris, J. O'hI., Oldfield, L. F., Trans. Faraday SOC.55,249 (1955). (3) Brodd, It. J., Hackerman, N., J . Electrochem. SOC.104, 704 (1957). (4) Chno, ?If. S., Ph.D. thesis, University of Illinois. 1961. (5) DelahaG, P., Trachtenberg, I., J. Am. Chem. SOC.79, 2355 (1957); 80, 2094 (1958). (6) Eda, K., J . Chem. SOC.Japan, Pure Chem. Sect. 80, 343, 461, 708 (1959); 81,689,875 (1960). (7) Enke, C. G., Ph.D. thesis, University of Illinois, 1959. (8) Frumkin, A. N., Nova Acta Leopoldina 1 9 , a (1957). (9) Frumkin, -4.K.,Gorodetskaya, A . Chugunov, P., Acta Physicochim. U.R.S.S. 1. 12 11934'1. (10) Graham;, D'. C., 'J. d i n . Chem. SOC. 68,301 (1946). (11) Hansen, R. S., Minturn, R. A., Hickson, D. A., J . Phys. Chern. 60, 1185 1956). (12) Laitinen, H. A,, Enke, C. G., J . Electrochem. SOC.107, 773 (1960). (13) Laitinen, H. A., Morinaga, K., ARL Technical Kote 60-129, U. S. Air
Force, Wright-Patterson .4ir Forre Base, Ohio. (14) Laitinen, H. A., Mosier, B., J A m
Chem. SOC.80, 2363 (1958). (15) Laitinen, H. A,, Roe, D. K., Coll. Czechosloa. Chem. Cornmuns. 25. 3065 (1960). (16) Laitinen, H. -4., Scarr, R. F., WADD Technical Note 60-104, U. S. Air Force,
Kright-Patterson Ai1 Force Base, Ohio,
July 1960. 171 Lorenz, W.,2. Elektrochem. 59, 730 (1955). 18) Lorenz, W.,blockel,, F.., Ibid.,, 60,. ' 507,939 (i95sj. (19) Los, J. M., Tompkins, C. K., Can. J . Chem. 37, 315 (1959). (20) Loveland, J. W.,Elving, P. J., J . Phys. Chem. 56, 935, 941, 945 (1952). (21) Melik-Gaikazyan, V. I., J . Phys. Chem. (U.S. S . R.) 26,1184 (1952). (22) Parsons, R., Trans. Faraday SOC. 55,999 (1959'1. (23) Reinmuth, W.H., ANAL. CHEM.33. 324! (1961). \ - - - - ,
(24) Scarr, R. F., Ph.D. thesis, University of 1[Ilinois, 1960.
RECEIVED for review May 1, 1961 Accepted June 23, 1961. Division of Analytical Chemistry, Fisher Bward Symposium Honoring H A. Laitinen, 139th Meeting, .ICs, St Louis, No., March 1961
Thermodynamic Constants of Complex Ion Formation between Mercury(l1) and Three Alkylamines D. K. ROE,' D. B. MASSON,2and C. J. NYMAN Deparfment of Chemisfry, Washington State University, Pullman, Wash. Formation constants of the ions Hg(en)22+, Hg(pn)Z2+, and Hg(dien)22+ have been measured polarographically as a function of temperature. The enthalpy and entropy of formation of these ions are calculated from the temperature coefficient of the free energy data, and comparisons are made with the Zn(ll) and Cd(ll) analogs.
C
REACTIONS involving the first transition metal series have been the center of many studies, concurrent with interest in the corresponding entropy changes of such reactions and in testing the predictions of the crystal field theory. Values of the thermodynamic constants, AF, AH, and AS, of the formation of alkylamine complexes of cadmium(I1) and zinc(I1) are available (3-6, 7, 16), but only equilibrium constants (2, 8, 11, IS, 16) have been found in the literature for the corresponding reactions involv"ATE
Present address, Shell Development Co., Emeryville, Calif. Present address, Department of Metallurgy, Washington State University, Pullman, Wash. 1
1464
ANALYTICAL CHEMISTRY
ing the third member oi periodic Group IIb, mercury. Data are reported here which, when combined with previous polarographic results a t 25" C. ( 8 ) , permit the calculation of the enthalpy and entropy changes accompanying the formation of the bis(alky1amine) complexes of mercury(I1) and ethylenediamine, 1,2propanediamine, and diethylenetriamine; the notations en, pn, and dien, respectively, mill be used for these alkylamines. The present measurements were made with solutions at 10" and 40" C., again using the polarographic method. EXPERIMENTAL
Anodic polarograms of the dissolution of mercury in the presence of aqueous alkylamine solutions were measured point by point. -4Sargent Nodel XXI Polarograph recorded the current a t a set applied voltage, and the potential of the dropping mercury electrode was measured relative to a saturated calomel reference electrode. Corrections were subsequently made for the cell iR drop. The polarographic cell contained a supporting electrolyte of 0.1V KS03 and was thermostated to within =l=0.05" C. Purified nitrogen ~ m s bubbled
through the cell to remove dissolved oxygen. Individual solutions of the respective alkylamines were prepared from standardized stock solutions, which were made up from distilled chemicals. TREATMENT OF EXPERIMENTAL DATA
In the presence of increasing concentrations of alkylamines, the anodic current-voltage curve of mercury is shifted in the negative direction. The relation between the current and the voltage is obtained from the Xernst equation, assuming that the electrode reaction is ideal, or nearly so, at low current densities, In the present case, successive coordination ions are formed, so that the potential a t any given current is a rather complex function of the chelate concentration (6). The general form of the equation is
where FJX)
=
1
+ K? [ Y ]+ K ; [Y12+ K," [ Y ] 3+ . . . .
(2)
The roncentration of the complexing
