F. M. Kimmerle ond H. Menard Universit; de Sherbrooke Sherbrooke, Quebec
I
The Streaming Potentid Iletfrode A senior undergraduate electrochemistry experiment
The number of experiments dealing with adsorption a t interfaces found in the undergraduate curriculum is extremely limited and usually confined to the measurement of surface tension by the capillary rise, the platinum ring, or the Langmuir trough technique. All of these measurements normally refer to the air-solution interface, and normally serve to calculate the projected surface area of the ahsorbed (organic) species. Measurements of the adsorption of ions and molecules a t an electrified interface (e.g., mercury/solntion) might he carried out in a 3-hr laboratory period by the drop time technique hut requires considerable apparatus and dexterity ( 1 ) . The technique described here gives far less information than the complete electrocapillary curves (surface tension versus potential), hut requires only a very high impedance potentiometer and minimal glass blowing skills. It consists of measuring the potential of an ideally polarized electrode (a mercury jet) with respect to a suitable connter electrode as a function of electrolyte composition and/ or temperature. The technique, albeit in a more cumhersome form was used by Grahame et al. (2): the results have also been interpreted by Randles and Whiteley (3), Paik, Anderson, and Eyring (4), and more recently by Barradas and Sedlak (5). The mathematical treatment follows that given in standard texts and should encourage the student to review the thermodynamics of the electrical double layer (6, 7) as well as the Stockholm sign convention (8).
An). arrumulatad chdrrt on rhe mercury, in the absence uf far. adalr reacrwns, ia diibipatrd by the jet: and its potential uith respect to a reference electrode reaches a stable plateau termed the potential of zero charge or E,,,,. The initial and final variations of
Experimental The apparatus employed is shown in Figure 1. It consists of a mercury column about 1-1.5 m long, joined to a small bulb, 2 ml volume, by a Teflon stopcock. A hand-drawn capillary plunges intd the solution contained in a tbermastated vessel. Provisions are made for deoxygenating by means of prepurified nitrogen and for the insertion of one or two reference electrodes. The stopcock is opened to admit some mercury into the bulb and then closed at time t = 0. Hence, the jet of mercury isminx from the capillary is forced out with decreasing pressure as the mercury bulb empties for about 10-20 s (according to the capillary size and mercury head). The capillary used consists of a Pyrex 0.5-m i.d. tubing handdrawn to a n internal diameter of less than 0.05 mm. An eaoeri-
V Figure 1. Streaming potential cell. A ) SCE. 6 )specific ion or mercurous salt electrode. C) fitted glass tube tor deoxygenating, D ) stopcock, E ) partially filled Hg reservoir. Total volume -50 ml.
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Journal of Chemical Education
on the hydrogen scale by (arbitrarily) attributing part of the emf to the counter electrode (CE). When using an internal reference electrode, or specific anion electrode
where 4-0 refers to its standard potential and a refers to the activity otthe monovalent anion. When u d n g a saturated calomel electrode (SCF:)
10
5 Figure 2. Variation of emf
versus
time. t
where ( E d represents the liquid junction potential, and
15sec
= 0 corresponds to
4 s c ~the potential of the SCE on the normal hydrogen
the instant
when stopcock Disclosed.
the potential difference, Figure 2, have not been explained adequately. They may be due to traces of electro-reducible species (e.g., molecular oxygen) but do not affect the reproducibility of the results. Use of the partially filled mercury bulb serves rather to produce a continuous pressure change instead of painstakingly recording the potential at various fixed mercury column heights as suggested originally. The reproducibility of the data (+0.2 mV) seems to be limited rather by the precision of the potentiometer and the stability of the counter electrodes, and with care can be increased by a factor of ten. We used a Tacussel Aires 1WO potentiometer with saturated calomel electrodes (SCE) prepared according to Ref. (8) but found that Orion specific ion electrodes and Keithley 630 or Orion potentiometers also gave satisfactory results. At least three readings should be taken for any given experimental conditions with care being taken that thermal equilibrium of the solution and the electrodes has been attained. Among the many experiences possible we suggest that the effect of electm-reducible impurities (e.g., oxygen), the effect of electrolyte composition, and the effect of temperature he investigated, in increasing order of experimental difficulty. The latter normally involves variations of only a few millivolts and for greatest accuracy, the solutions should be discarded after each temperature change in order to avoid the possibility of contamination by mercury salts. Theory
Free Energy of Adsorption By definition, a t an ideal polarized electrode (6, 7, 9) no transfer of charge occurs across the electrode/electrolyte interface, and this ideal electrode is approximated by a mercury electrode plunging into a solution containing no species which can be oxidized or reduced over the potential range considered. Variations of its potential with respect to a suitable (ideal nonpolarized) electrode reflect changes in the structure of the electrical douhle layer. The thermodynamic interpretation of the cell
has been given by Grahame (10). The free energy change for the "reaction"
where "a" refers to species adsorhed on the surface, "aq" refers to species in solution, and r + is the transference number of the adsorbate in the double layer, is given by
AG
=
-FE,,
= FE,,,-
(2)
where Em=..- ~ ---. . . ~ indicates that the emf was measured with respect to a n electrode reversible to the anion in solution. Since the main concern here is with the transport of the ions a t the electrode/electrolyte interface it ii useful to calculate the streaming potential of the mercury jet 4,, ~
scale. The free energy change for the absorption of a monovalent cation a t the mercury surface is thus given by
the entropy by
and the enthalpy by
Calculations of the entropv necessitate reliable values of the temperature coefficiek of the half-cell potentials when using an internal reference electrode. Since
both the variation of the activity coefficient and of the standard electrode potential must he known to a high degree of accuracy. A different technique employed by Grahame and Paik et al. uses nonthermodynamic arguments but can he expected to give more reliable results in the hands of a novice. They proposed measurements of the potential of the mercury jet with respect to a constant temperature (25.0°C) SCE. Thus
Since the temperature coefficient of the liquid junction potential may he approximated by ELIT and normally constitutes less than 1% of the value of ~E,,,s'f:/i,T negligible error is introduced. Esin Markov Effect The variation of the E n , , with electrolyte composition gives a simple indication of the relative importance of the interactions a t the electrode/electrolyte interface. The potential varies according to
where fi refers to the chemical potential of the uni-univalent electrolyte, T, refers to the relative surface excess of the anion or cation, and q, is the charge on the mercury surface. If the cation or anion populate indifferently the interfacial region, the slope of the curve En,,, versus RT in a, equals -0.5. T h e E,,, shifts towards increasingly negative potentials for predominant anion adsorption while for predominant cation adsorption the mercury surface must he rendered more positive in order to conserve both electroneutrality and q~ 0. When both ions are strongly adsorbed the
--
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-
W
-
e.300%
d L.-. 8 %
- 2 0 0 - -3 -I
-2
-3
0
.o
-I
log ( a * )
log a Figure 3. Variation of emf versus time of daaxygenatian of test solution. Nitrogen flow rate (rnllmin) .400; 0 200: A 100.
potential shift may be reversed. Shifts of the ED,,in the presence of organic molecules such as phenols or n-amyl alcohol depend on the dipole moment or the nature of the r electron overlap, and for symetrical molecules, such as pyrazine ( 1 1 ) depend also on the preferential orientation of the water dipole being displaced. Results and Discussion In Figure 3 we note that traces of oxygen give rise to stable but low readings of the E,,,,. In the presence of other electro-reducible species such as P b + + , the mercury jet will also act very much like a redox electrode. Its steady state potential is determined by the reversible couole Pb++IPb(He) and the rate a t which charge . is dissipated by the jdt. -' Firmre 4 comDares the values of the En,, a t 25'C for two sim$e potassi"m salts obtained by various techniques. The measurements of the streaming potentials are by far the more precise and an excellent agreement is reached with Grahame's results. Good agreement with electrocap-
-5
-2
Figure 5 Erin Markav plot ED,versus lag a , (reference electrodes: Orion specific ion electrodes) a) MerNCi, b) EtaNCI. c ) Pr,NCi. d) B u N C i , e) EtaNF, f) E t N B r , g) Et4NI.
illary results (inherent precision 1-5 mV) is also obtained except a t the lowest concentrations. When the En,,- is recorded with respect to a specific Orion electrode the potential scale is of course shifted by the difference between the standard potential of the silver/silver halide and mercury/mercurous halide electrode. (45.5 mV for C1- and 67.9 mV for Br-). If the E,,,SCE is recorded, eqns. (4) and (5) can be used to compute ED,,- by using the Henderson equation to estimate EL. The Esin Markov plots, Figure 5, illustrate the relative
I
2
4
6
8
10
12
14
16
minutes Figure 4. Potential of zero charge. KCI: E p r Grahames values (151: KBr: EPacSCE 0 O w m a t h a n and Perries (161. A Lawrence and Parsons ( 1 7 ) . A Grahame (151. -this work.
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I Journal of Chemical Education
Figure 6. Variation of E&CE 0.1 M. A EtaNBr 0.1 M.
with temperature. 0 KC1
0.1 M.
.
KBr
Analysis of Streaming Potential Data in 0.1 N Aqueous Solutions at 25 f 0.05% .3E,,PE/ OSCE.
m-"
E~~~~~~ (Vl
0.2881
-0.4634
-0.5078
-0.2200
-0.2138
aT DQplr/DT ADHE ATS% AH"= (mV/Kl (keal/mole) 0.68. 0.671 5.07 4.65 9.72 n 67.1
0.1371 0.1370 0.1393 0.1391
-0.5336
-0.4274
-0.2902
-0.2905
0.845
n
0.84 "."..d
6.70
6.0
12.7
-0,4890
-0,3852
-0.2461
-0.2461
0.610
0.601
5.68
5.17
10.8
bSCE CL'
KC1
0.2434
0.0446
KBr
0.2434
-0.1063
Et CI- > B r > I indicating that with increasing anion size both anions and cations are subject to specific attractive forces. Finally, for the tet;abutylamkonium salts we note that the done >0.5 at low concentrations and -0 a t higher concentrations indicating predominant control by the anion as the concentration increases and monolayer coverage is reached. Similar plots for various detergents or other minor constituents a t fixed electrolvte concentrations distinrmish clearly between anionic, nonionic, or cationic surfac'tants according to whether aE,,,.la lop c < 0, = 0 or > 0. A thennodynamic evaluation of adsorption is possible from the data plotted in Figure 6. At room temperature internal reference electrodes gave satisfactory results. Accalculated from a SCE cording to the table values of and Orion Specific Ion electrodes agree remarkably well and agree also with the values quoted in the literature (4). During a laboratory period i t was impossible to obtain time invariant readings of E,,,,- a t other than 25°C when using an internal mercurous halide reference electrode. This phenomenon is not unexpected since mercurous salt electrodes are notoriously sluggish in attaining thermal equilibrium (8). Although the response of the silver halide electrodes was found to be somewhat faster, the parameters necessary to calculate the temperature coefficient according to eqn. (9) are not always known with sufficient precision to warrant their use. Conversion of values of aE,,,.-/aT obtained with a silver halide electrode to those quoted in the literature w.r.t. mercurous halide electrodes is nevertheless possible as illustrated for a 0.1 N KC1 solution
+,,
Values taken from R e t (4)
Calculations of LGH, follow from eqn. (6) and ASH= and AHw. from eqns. (7) and (8). Although a detailed interpretation of these parameters is beyond the scope of this article some general comments are in order. With increasing specific adsorption of the anion (KBr versus KC1) we note not only the increasingly negative value of the ED,, such as illustrated in Figure 5 hut also a larger temDerature coefficient. For the EtlNBr solution the cation also experiences non-electrostatic interactions a t the electrode and its relative surface excess exceeds that found for equivalent KBr solutions. Both the entropy and enthalpy values indicate that i t is more difficult to transfer the tetraalkylammonium cation from the double layer region to the hulk phase. These phenomena have been discussed in terms of current views on the structure-making properties of the ions involved (5). A systematic study of a series of cations would involve the cooperative effort of several groups of students hut is quite feasible with the apparatus and the technique described. Summary
A relativelv s i m ~ l ea p ~ a r a t u sfor the measurement of the streaming potential bI E,,,, and hence the elucidation of the structure of the electrode/electrolyte interface is described. Three different experiments illustrating the properties of an ideal polarized electrode, the Esin Markov effect, and the entropy of adsorption suitable for a senior undergraduate physical chemistry laboratory are suggested. Acknowledgment
The financial support of the National Research Council of Canada is gratefully acknowledged. Literature Clted Ill Menard, H.,andKimmede,F.M.. J Elertmonol. Cham.. 47,375119731. 121 Grshame. D. C., Coffin, C. M.. Cummins. J. I.. and Path. M. A . Cham. Soc.. 74.1207
11952).
J
Amsr.
(31 Rand1rn.J.E.B., and White1y.K. S., Trans. Famdoy Soe., 52.15(18119561. 141 Paik. W-K.,Andcrsen,T.A..and Eyrine, H . J Phvr C h m , 71.1891 11967). 151 Banadas,R.G..and Sedlsk. J.M.. Ebc6oehim. Act.. 17.67 119721 16) Conway, B. E.. "Theory and P~inciplesof Electrode Pmeeases." Ronald Press. Near York, 1865. 17) Deiahay, P.. "Double Lsyer and Elsetmde Kinetics." Wiiey-Intsracience, New
1965. J. G.. and Janz. G. J.. "Rcforcneo Electrodes." Academic ,ac, ."".. 191 Conway, B. E.,andSalomon,M., J. CHEM. EDUC.,44,554119651 1101 Grshama.D.C..J. Chem. Phys., 16.1117 119481. York,
181 Ives. D.
P~ess.Now
L . "
in excellent agreement with the value 0.153 mV/K determined directly by Randels and Whitely (3). The temperature coefficients listed in the table were nevertheless obtained by using the more convenient constant temperature SCE as outlined above. For highest accuracy this external SCE may be thermostated a t 25°C.
I l l ) Conway. 8.E.. Dhar. H.P.. snd Gottedold. S.. J Coiloid. Infedor. Sei.. 43. 303 (1471) ,....,. I121 Gibson. G., and Sudworth. J. L.. "Speeine Energ% of Galvanic Reactions and ReistedThemodynsmieDafa,"Chapmanand Hail, London. 1973. I131 "Handbookof ChemistryandPhysics," 51at Ed., TheChemicslRubberCo..1970. 1141 Lindenbaum. S..andBoyd, G.E..J Phys Chem.. 68911 11964). (151 Grshame, D. C.. Coffin. E. M., and Cummings, J. C., Tmhn. Rep. No. 2 to
O.N.R..1950.
1181 Devansthan,M. A.V..sndPenies.P..Tmns. Fernday Soe., SO. 1246 (19541. 1171 Lawrence, J.,sndParnon%R..J Eleclraa~l.C h m . 16, 19311968).
'Calculated from entropy values given in Ref. (121.
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