JAMES N. BUTLER
2312
The Zero-Charge Potential of Indium Amalgams in Perchloric Acid
by James N. Butler Tyw Laboratories, Inc., Wdtham, Massachusetts 0.9164 (Received February 2, 1966)
The potential of zero charge for amalgam electrodes containing up to 64 mole % indium has been measured in perchloric acid solutions of concentration from 0.01 to 1.0 M at 25". The results indicate that the specific adsorption of perchlorate ion is smaller on the amalgams than on mercury and goes through a minimum at approximately 5 mole % indium. The temperature coefficient of the zero-charge potential for mercury and indium amalgams in 0.1 M HC104 was measured in a cell without liquid junctions. Using thermodynamic data together with some nonthermodynamic values, temperature coefficients of the Galvani potential difference between metal and electrode were calculated. These values were compared with those calculated from data in the literature; the comparison confirms the observation that specific adsorption of perchlorate ion on the amalgams is probably smaller than on mercury. The increase of double-layer capacity with indium concentration is thus probably due to changes in the structure of the dipolar layers at the interface, and not to specific adsorption of ions.
Introduction Accurate measurements of the potential of zero charge are a vital prerequisite for theoretical analysis of electrical double-layer capacities. The zero-charge potential of a liquid metal electrode can be measured most precisely by observing the potential at which no charging current flows when the area of the electrode is changed. However, this method is restricted, not only by the condition that the electrode be a liquid metal, but also by the requirement that the electrode be ideally polarized. No charge-transfer reactions can have an appreciable rate in the potential range under consideration, or else the zero potential obtained is not a thermodynamic property of the interface, but instead depends on the nature of the charge-transfer reaction. These requirements greatly restrict the number of systems on which accurate measurements of zerocharge potential can be made. For mercury in aqueous solutions, a wide range of data has been but little work has been done using other electrode materials. There is good reason for this: virtually all liquid alloys of mercury and gallium with other metals undergo dissolution reactions, making the measurement of zero-charge potentials much less accurate than on mercury. 9,10 The Journal of Physical Chembtry
Indium amalgams, however, provide a good approximation to the ideal polarized electrode over a wide range of potentials, including the zero-charge potential. This system has the further advantage of wide composition range; liquid alloys containing up to 70% indium in mercury can be prepared a t room temperature. I n a previous publication" we presented some preliminary measurements of the zero-charge potential of indium amalgams in 0.1 M HC104 a t 25O, but no measurements giving the variation of zero-charge (1) D.C. Grahame, R. P. Larsen, and M. A. Poth, J. Am. Chem. SOC., 71, 2978 (1949). (2) D.C. Grahame, E. M. Coffin, J. I. Cummings, and M. A. Poth, ibid., 74, 1207 (1952). (3) R. Parsons, Proc. Intern. Congr. Surface Actiuity, Ind, 3 , 38 (1957). (4) D. C. Grahame and R. Parsons, J . Am. Chem. Soc., 83, 1291 (1961). (5) R. Parsons and F. G. R. Zobel, J. EZectroanaZ. Chem., 9 , 333 (1965). (6) R. Payne, Thesis, Imperial College, London, 1962; G. J. Hills and R. Payne, Trans. Faraday Soc., 61, 316,326 (1965). (7) R. Payne, J . EZectroamZ. Chem., 7 , 343 (1964); J. Chem. Phya., 42,3371 (1965); J . Phya. Chem., 69,4113 (1965). (8) R. Payne, ibid., 70, 204 (1966). (9) A. N.Frumkin and F. J, Cirves, ibid., 34, 74 (1930). (10) J. N.Butler, J. EZectroanaZ. Chem., 9 , 149 (1965). (11) J. N.Butler and A. C. Makrides, Trans. Faraday SOC.,60,1664 (1964).
2313
ZERO-CHARGE POTENTIAL OF INDIUM ~ A L G A M SIN PERCHLORIC ACID
potential with electrolyte composition or temperature were reported. From the variation of the zero-charge potential with the composition of the solution, qualitative information about the specific adsorption of anions can be obtained,3*8a12*13 and such measurements are the logical antecedent to measurements of capacity14 and interfacial tensionJ6 from which quantitative information about adsorption can be obtained. The variation of the zero-charge potential with temperature, although it is often difficult to interpret theoretically, is again an essential prerequisite to any studies of the temperature dependence of adsorption phenomena by means of capacity measurements. One of the questions which arose in our previous studies of the electrical double layer on indium amalg a m was ~ ~ whether ~ the increase in capacity with increasing indium concentration was due to specific adsorption or to changes in the dipolar structure of the interface. The study presented here was undertaken primarily to answer this question. By analogy with m e r ~ u r y indium , ~ ~ ~ ~amalgams ~ might be expected to show relatively little specific adsorption of fluoride ions. However, preliminary experiments with sodium fluoride solutions showed that the indium amalgams underwent reversible dissolution at potentials where the surface charge was still negative. This happened because the high stability of fluoride ion complexes“’ with In+3 in aqueous solutions can shift the reversible potential of the In/In+3 electrode as much as 200 mv more negative than perchlorate solutions. The streaming electrode method is thus unsatisfactory for determining the zero-charge potentials of indium amalgams in fluoride solutions. Perchlorate ion forms only very weak complexes with indium18 and shows only moderate specific adsorption on mer~ury,~J’ so perchlorate solutions were the next choice after fluoride solutions. In this paper we report measurements of the zerocharge potentials of indium amalgams in solutions of HC1O4 as a function of temperature and composition, and interpret these in terms of the specific adsorption of perchlorate ions in the electrical double layer.
Experimental Section The electrolyte was prepared from triple-distilled water, using reagent grade perchloric acid (J. T. Baker), and was purified by preelectrolysis with platinum electrodes for 16 hr. The solution was saturated with purified hydrogen throughout preelectrolysis and measurement. The cell was constructed entirely of and Teflon’ ‘leaned with chromic-su1furic and rinsed in triple-distilled water. A platinized-
platinum hydrogen reference electrode in the same solution, separated from the main compartment by a glass frit, was used for all of the measurements. Indium amalgams were prepared from triple-distilled mercury (Doe and Ingalls) and 99.999% pure indium (American Smelting and Refining Co.) by mixing weighed quantities of the two materials under an argon atmosphere. The indium dissolved in the mercury to give a homogeneous liquid within a few minutes. Most of the values for zero-charge potentials reported here were measured by the streaming electrode method: method V described by Grahame.l The amalgam flowed from a reservoir through a capillary into the solution, and its potential was measured with respect to a reversible hydrogen electrode in the same solution. Since measurements of electrode impedancel* show that no reversible electron-transfer reactions take place under these conditions, the streaming electrode potential should be the potential of zero charge. However, an irreversible electrode reaction could shift the streaming electrode potential and yet have no effect on the ac impedance of a dropping electrode, but such a reaction should be detectable as a dependence of streaming electrode potential on the rate of flow of amalgam. We observed no such dependence. As an additional test, direct measurements of surface charge were made using a dropping electrode. As we have shown p r e v i o ~ s l ya, ~plot ~ of It/’, where I is the instantaneous current flowing at time t from the birth of the drop, under potentiostatic conditions, can be extrapolated to zero time to give a quantity A proportional to the surface charge density q
A = 2/3(36~)’/’q(m/p)”3 where m is the mass flow rate of amalgam through the caDillary and p is the density of the amalgam. Potentiostatic current-time curves were measured for dropping electrodes at several potentials either side of the streaming electrode potential, extrapolated to (12) 0.A. Esin and B. F. Markov, Acta Physicochim. U R S S , 10, 353 (1939). (13) P. Delahay, “Double Layer and Electrode Kinetics,” Interscience Publishers, Inc., Nbw York, N. Y.,1965,Chapter 4. (14) J. N.Butler, M. L. Meehan, and A. C . Makrides, J . Electroanal. Chem., 9, 237 (1965). (15) J. N.Butler, J . Phys. Chem., 69, 3817 (1965). (16) D.C. Grahame, J. Am. Chem. Soc., 76,4819 (1954). (17) D. C. Grahame, M. A. Poth, and J. I. Cummings, a%id., 74, 4422 (1952). (18) N.Sunden, Svensk Kem. Tidskr., 66, 50 (1954). (19) J. N. Butler and M. L. Meehan, J . Phys. Chem., 69, 4051 (1965).
Volume ‘YO,Number 7 July 1966
JAMESN. BUTLER
2314
zero time as described,lg and the intercepts plotted as a function of potential. The point at which the interpolation curve crossed the axis was taken to be the potential of zero charge. Although this method gives somewhat less precise ( h 2 mv) values of zero-charge potential than does the streaming electrode, any interference from reversible or irreversible chargetransfer reactions is immediately apparent as a nonzero slope for the curve It’1aus. t or { l a ; this method thus serves as an additional check on the accuracy of the streaming electrode potentials.
Table I: Zero-Charge Potentials of Indium Amalgams in HClOa a t 25” Mole % In
0 5.00 10.00 20.00 39.83 63.02
Results 0 0 0.015 0.138 1.23 5.00 5.97 10.00 10.02 20.34 39.89 63.02
E.’’
axn
1.OO
M HClO4 (a* 0 0.0707 0.196 0.717 3.83 13.46
= 0.823)
-0.273 -0.375 -0.407 -0.462 -0.561 -0.637
0.100 M HClO4 (a* = 0.0803) 0 -0.160 0 -0.160* 0.00015 -0.167* 0.0014 -0.202* 0.0134 -0.255* - 0.297* 0.0707 0.0900 -0.301 0.196 -0.334* 0.197 -0.327 0.743 -0.388 3.84 -0.483 13.46 -0.553
Zero-charge potentials of mercury and indium amalgams in perchloric acid solutions at 25” are given in Table I. We have also listed values for the activity of indium in the amalgam20and of HClO4 in the electrolyte. 21 In Table I, we have given a value of -0.160 v vs. a reversible hydrogen electrode for the zero-charge potential of mercury in 0.1 M HC1O4. This agrees with Payne’s precise valuezzof -0.162, and with Grahame’s values2 for 0.1 M KC104 and NaC104, both of which correspond to -0.162. This value is more positive 0.0100 M HClOa (a* = 0.00903) than our previously p u b l i ~ h e d ~values ~ J ~ of -0.165 0 0 -0.086 5.97 0.0900 -0.241 and -0.170, and the value of -0.183 (0.493 vs. sce) 10.02 0.197 -0.267 ~ the maximum of the obtained by Hansen, et U Z . , ~ from 20.34 0.743 -0.321 electrocapillary curve for mercury in 0.1 M HC104. 39.89 3.84 -0.400 Figure 1 shows the dependence of the zero-charge 63.02 13.46 -0.468 potential (us. a fixed reference electrode) on the cona The zero-charge potential E . is given in volts us. a reversible centration of indium in the amalgam and perchloric hydrogen electrode in the same solution. To convert to the acid in the electrolyte. The differences for tenfold saturated calomel electrode scale (Figure l), add -0.250 to the change in perchloric acid concentration are smaller 1.0 M values, -0.310 to the 0.1 M values, and -0.365 to the 0.01 M values. Measurements were by the streaming electrode (20 to 30 mv) than the differences observed for mercury except those marked with an asterisk (*), which were (30 to 50 mv). The values obtained p r e v i o ~ s l y l ~ ~ method ’~ measured by the direct surface charge method. for indium amalgams in 0.1 M HC1O4 agree within experimental error with the present measurements. The probable error of the present data is approxiture coefficient of the zero-charge potential of mercury mately 2 mv, a considerable improvement over errors in KCI, which were quoted in a recent review,27appear of up to 10 mv in the older data. to be incorrect.z4 These values were not in fact measZero-charge potentials in 0.1 M HClO4 at various ured by Lee and Tai but were calculated from some temperatures are listed in Table 11. Very few data exist with which our results on the temperature co(20) J. N. Butler, J . Phys. Chem., 68, 1828 (1964). efficient (Table 111) can be compared. Randles and (21) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Whiteleyz4 measured the temperature coefficient of 2nd ed, Butterworth and Co. Ltd., London, 1959, pp 491-501. the zero-charge potential for mercury in three cells (22) R. Payne, private communication. without liquid junctions. The temperature coef(23) R. S. Hansen, D. J. Kelsh, and D. H. Grantham, J . Phys. Chem., ficients given by Grahame25for mercury in KCl and 67, 2316 (1963). (24) J. E. B. Randles and K. 9. Whiteley, Trans. Faraday Soc., 5 2 , NaF were measured in cells with the reference elec1509 (1956). trode held at constant temperature. Only for KC1 are (25) D. C. Grahame, J . Am. Chem. Soc., 79, 2093 (1957). sufficient data available so that these results may be 8 , 60 (1941). (26) F. H. Lee and Y. K. Tai, J . Chinese Chem. SOC., compared with ours. (27) B. E. Conway, “Modern Aspects of Electrochemistry,” Vol. 1, The values given by Lee and TaiZ6for the temperaButterworth and Co. Ltd., London, 1954, pp 54-61. The Journal of Physical Chemistry
ZERO-CHARGE POTENTIAL OF INDIUM AMALGAMS IN PERCHLORIC ACID
(”-) b In ai A A
2315
=
?[1
+ 2(%),1
(1)
where E, is the potential with respect to an external, fixed reference electrode, ai is the mean activity of the 1: 1 electrolyte, p is the surface charge on the electrode, p- is the charge in solution contributed by anions, R is the gas constant, T is the absolute temperature, and F is the Faraday constant. If there is no specific adsorption of ions at the electrode, the charge in solution is entirely in the diffuse double layer, and the Gouy-Chapman theory may be used to calculate the dependence of q- on q. A t the potential of zero charge, this theory gives simply
DIRECT SURFACE CHARG STREAMING ELECTRODE
(2) 0.01
AI
ACTIVITY OF
LO INDIUM
10
Figure 1. Zero-charge potentials of indium amalgam in HClOd solutions of various concentrations, with respect to the saturated calomel electrode (see Table I )
Table 11: Zero-Charge Potentials in 0.1 M HC104 a t Various Temperatures Electrode material
Temp,
OC
BEa
9.8 19.4 25.0 26.5 36.7 50.0 55.0 59.0 65.0
-0.1596 -0.1600 -0.1598 -0.1586 -0.1590 -0.1597 -0.1605 -0.1614 -0.1620
20% I n
25.0 59.0
-0.388 -0.394
40% I n
25.0 44.5 56.5 59.0
-0.483 -0.485 -0.488 -0.488
a E, is in volts vs. a reversible hydrogen electrode in the same solution. All values were measured by the streaming electrode method.
values quoted in a textbookzs for the “absolute emf of the calomel electrode.” Unfortunately, no reference is given to the original measurements.
Composition Dependence For an ideally polarized electrode, the shift of zerocharge potential with electrolyte composition is given by the equations>l3
which says that E, is independent of a+. I n the presence of specific adsorption, the coefficient (bq-/bq) is different from - 1/2, a more negative value indicating stronger specific adsorption, and may often be independent of electrolyte concentration and surface charge. Studies of the variation of zero-charge potential and surface charge of mercury in KI, KBr, KC1, and HC1 have s h o ~ n ~that J ~ the , ~ potential ~ at constant charge is closely approximated by a linear function of electrolyte activity, and that the slope of these lines is essentially independent of the surface charge. In the case of nitrate and perchlorate,'^* which are more weakly adsorbed, this dependence is more complicated, but qualitatively similar. This means that a measurement of the dependence of zerocharge potential on electrolyte composition can give considerable qualitative information about the extent of specific adsorption of ions, without the extensive measurements required for a detailed study of capacity and interfacial tension. It cannot, of course, give the detailed form of the adsorption isotherms. We calculated the coefficients (bq-/bp) from our data in the following manner. From the graphs of Figure 1, values of the zero-charge potential were interpolated at round values of indium concentration, and a plot of these interpolated values was made as a function of log a*. This plot was linear within experimental error over a 100-fold variation in concentration. In Figure 2 are plotted the values of (bq-ldq) calculated using eq l . The vertical bars indicate the approximate error in the values of the coefficient calculated from the measured slopes, and the line is a smooth function which approximates the calculated values. (28) J. Reilly and W. N. Rae, “Physico-Chemical Methods,” 2nd ed, D. Van Nostrand Co., Inc., New York, N. Y . , 1933,p 731.
Volume 70, Number 7 July 1966
JAMESN. BUTLER
2316
1
1 I
I
I Ill1
0.7" i O
ACTIVITY OF INDIUM
Figure 2. The coefficient -( dp- /dq) calculated from the data in Figure 1, using eq 1. A larger value of this coefficient is a qualitative indication of increased specific adsorption of perchlorate ions.
A value for the coefficient -(bq/bq) greater than indicates that specific adsorption of the anion occurs, and from Figure 2 it is apparent that some specific adsorption of perchlorate ions occurs on indium amalgams, but less than on mercury. The adsorption appears to pass through a minimum at approximately 5 mole % indium (UI, = 0.07), and on the most concentrated amalgams is nearly as large as on mercury. In our previous studies14of the double-layer capacity of indium amalgams in 0.1 M HC1O4,we observed that at constant surface charge the capacity increases with increasing amalgam concentration. The results of the present study have shown that the adsorption of perchlorate ions is smaller on indium amalgams of all concentrations than on mercury. Specific adsorption of clod- ions is therefore unlikely to be responsible for the large increase in double-layer capacity with indium concentration. The other possible effect, a change in the dipolar structure of the amalgamaqueous interface with composition, is more likely to be responsible for the increased capcity. The lower specific adsorption of perchlorate ion and higher double-layer capacity on indium amalgams parallels the results for gallium. Frumkin, et aZ.,2g observed that the adsorption of perchlorate, as well as other anions, is less on gallium than on mercury. This, together with the higher double-layer capacity, was attributed to a stronger bond between water and gallium than between water and mercury. A similar explanation could be invoked for the indium amalgams, l/z
The Journal of Physical Chemistry
although data on adsorption of water vapor are not available to support this hypothesis. In addition to changes in the electronic dipolar layer in the metal and the water dipole orientation in the electrolyte, the increased capacity of indium amalgams with respect to mercury could also be attributed to the indium-mercury dipoles in the interface. Our studies of interfacial tension15 showed that the surface deficiency of indium went through a maximum a t approximately 20 mole % indium (urn = 0.7), and that the surface deficiency itself was potential dependent. This implies that shifts in composition of the metallic side of the interface could contribute significantly to the capacity measured by an ac impedance bridge, and that this contribution would be most important in the region from 5 to 20% indium. This effect may be responsible, at least in part, for the maximum in capacity a t the zero-charge potential observed between l and 20% indium.14 The minimum in the specific adsorption of perchlorate ions a t approximately 5 to 10% indium (Figure 2) could result from the opposition of two tendencies. The decrease in specific adsorption at low indium concentrations may result from changes in the orientation of water dipoles in the same way that specific adsorption is lower on gallium than on mercury.2g At higher indium concentrations, however, a larger fraction of the metal surface consists of indium atoms, and chemical bonding of the perchlorate ions to indium atoms may become important. To test this hypothesis effectively, measurements should be made of the complete adsorption isotherms for a number of anions having different effects on the structure of water and different affinities for indium and mercury.
Temperature Dependence The temperature coefficient of the zero-charge potential in a cell without liquid junctions can be expressed as a combination of entropy terms, most of which are independently measurable. Three terms, however, occur in inseparable combination: the absolute entropy of the hydrogen ion (or any other single ion), the entropy of electrons in the metal, and the entropy associated with the specifically adsorbed ions and the dipolar layers at the metal-electrolyte boundary. Since the entropy of the hydrogen ion is approximately known, and is in any case independent of the nature of the metal or electrolyte being studied, and the entropy of electrons in the metal is small, the way in which the temperature coefficient of the zero~~
~~
(29) A. N. Frumkin, N. Polianovskaya, N. Grigorev, and I. Bagotskaya, Electrochim. Acta, 10, 793 (1965).
ZERO-CHARGE POTENTIAL OF INDIUM AMALGAMS IN PERCHLORIC ACID
~
~~~~
~
2317
~~
Table I11 : Temperature Coefficient of Zero-Charge Potential Measured in Cells without Liquid Junctions Ideal polarized electrode
Ref eleotrode
Electrolyte
0 . 1 M HClO4 0 . 1 M HClOa 0.1 M HClOi 1.OMKC1 0.1 M KC1 0 . 1 M KC1 0.1 M KC1 0 . 1 M KOH 0 . 1 MKOH 0 . 1 M KzSO4 0 . 1 M &SO4
PtlHz\H+
Hgl HgzC1z1C1-
Exptl dEz/dT, mv/deg
Calod" d(Ah)/dT, mv/deg
Ref
-0.05 f 0.01 -0.18 f 0.05 -,0.15 f 0.03 $0.07
0.68 0.55 0.58 0.72 0.63 0.67 0.66 0.57 0.55 0.56 0.51
This work This work This work 2, recalcd 2, recalcd 24 24, recalcd 24 24, recalcd 24 24, recalcd
-0.184
HglHgOlOHHg/HgzSOaISOa*-
-$3.153 -0.153 $0.548 $0.548 $0.292 $0.292
" See Discussion. charge potential varies can give some information about the structure of the double layer a t the metal-electrolyte interface. Unfortunately, measurements of the temperature coefficient of zero-charge potential sufficiently accurate to make even qualitative observations are extremely difficult to obtain. We have alredy ment,ioned the studies on mercury by Randles and Whitele~,~* and this discussion of our measurements on indium amalgams follows theirs rather closely. For our cell PtslH21H+,C104-jIn, HglPt
(3)
where the amalgam-electrolyte interface is assumed to be a t the zero-charge potential and ideally polarized, we may make the usual analysis of electrochemical e q u i l i b r i ~ m t'o ' ~ ~obtain ~ ~ the cell potential E, in terms of the chemical potentials of the component,s
where A& is the Galvani potential difference between ' p ~ + ' are the the amalgam and electrolyte, p ~ ~and standard chemical potentials of Ht(g) and H+(aq), respectively, and pe&m is the chemical potential of electrons in the amalgam. Taking the temperature derivative of eq 4,we obtain
Seam - R In a H +
- RT-
b In Y H + dT
The absolute entropy of hydrogen gas30 is SH~'= 32.211 cal/mole deg. The activity coefficient21 of 0.1 M HC1o4 is 0.803, which gives R In a H + = -5.01
cal/mole deg. The partial molar enthalpy of dilutions1 for 0.1 M HC1 is 202 cal/mole, which gives a reasonable estimate of +0.34 cal/mole deg for the last term of eq 5. The other terms are not so well known, but can be estimated. The entropy of the conduction , ~ ~than 0.1 cal/mole electrons in metals is s ~ E L I Iless deg, and was assumed to be 0 for these calculations. The absolute entropy of the hydrogen ion, as estimated from measurements of the entropy of transfer in reis approximately SEI+'= -5.5 cal/ versible ce11s,24,27 mole deg. Making these approximations, eq 5 yields the result d & #-)
-dEz + 0.73 mv/deg dT
(6)
Our experimental results for dE,/dT gave the values for [d(A&)/dT] shown in Table 111, and are compared there with those obtained by other workers for the interface between mercury and various electrolytes. Since Randles and W h i t e I ~ ydid ~ ~not give the details of the values they used for the activity coefficients and partial molar enthalpies, we repeated their calculations. We have also calculated [tl(A+,)/dT] from Grahame's data.2 In our calculations of tlhese data, the absolute entropies of Hg, Hg2C12, HgO, and HgSO4 were taken from Latimer,30 activity Coefficients were taken from Robinson and Stokes,21pa:rtial molar enthalpies of dilution were taken from Harned and Owen,a1the entropy of electrons in the metallic phase was assumed to be zero, and the absolute entropies of the various ions (30) W. M. Latimer, "Oxidation Potentials," 2nd ed, PrenticeHall, Inc., Englewood Cliffs, N.J,,1952,pp 30, 176. (31) H. 9. Harned and B. B. Owen, "Physical Chemistry of Electrolyte Solutions," 3rd ed, Reinhold Publishers, Inc., New York,
N.Y.,1958,pp 709-710.
Volume 70, Number 7 Julg 1066
2318
were calculated from Latimer's assuming that the absolute entropy of H+ was -5.5 cal/mole deg. As can be seen from Table 111, in no case did the results agree exactly with the values obtained by Randles and Whiteley, but the difference was significant only in the case of Hg in KzS04. Although the probable error in these values is large, certain trends do exist which may be related to the specific adsorption of anions, and to the structure of the dipole layer at the interface. Because the specific adsorption of an anion at the interface is not only exothermic, but also probably involves a loss in entropy, the extent, of specific adsorption is probably smaller at higher temperatures, and hence the temperature coefficient of the potential drop across the interface will be larger in the presence of specific adsorption than if no specific adsorption occurs. Looking at the values for KC1 measured by Grahame,2 we see that the coefficient [d(A+.)/dT] is larger for 1.0 M KC1 than for 0.1 M KC1, which confirms this notion since the specific adsorption of chloride ion is much more extensive in the concentrated solution.17ra2Similarly, comparing the values obtained by Randles and Whiteley for KC1, KOH, and KzSO4, we see that the temperature coefficient in KCl, where specific adsorption is moderate, is larger than in KOH or KzS04, where specific adsorption is probably sma11.6Jp17
The Journal of Physical Chemistry
JAMES N. BUTLER
Turning now to our results for indium amalgams in perchloric acid we find that the temperature coefficient [dAt#Js/dT]on mercury is comparable to the values obtained for KC1, indicating moderate specific adsorption. The indium amalgams show smaller values for the temperature coefficient, which is consistent with our observation that - (dq-lbq) is smaller on the amalgams than on mercury. Unfortunately, this result does not allow us to separate unambiguously specific adsorption from dipolar effects. Because of the larger entropy of the amalgam interface, due to its more complex dipolar structure, we expect that the coefficient [dAt#J,/dT) would probably be smaller on the amalgams, even if there was no change in the degree of specific adsorption.
Acknowledgments. This work was supported by the U. S. Office of Naval Research, Materials Sciences Division. The author wishes to thank Mrs. Mary L. Meehan for her assistance with the experimental work, Dr. Richard Payne for data in advance of publication, and Dr. A. C. Makrides for his helpful criticism and discussion.
(32) J. R. Sams, C. W. Lees, and D. C . Grahame, J . Phys. C h e m 63, 2032 (1959).
