ACKNOWLEDGMENT
‘I’lic s u l y o r t of thc Office of Ordnnncc Rcscarch, U. S. ,4rmy, under Contract DAk-04-495-ORD-149 is gratefully acknowledged. Extensive corrcsgondence between t h e author and James ,J. Ling a m contributed t o the interpretation of results, but responsibility for the conclusions rcwlied rests with the author. LITERATURE CITED
(1) Anson, E’. C., J . Am. Chem. SOC.81, 1554 (1959).
(2) Anson, F. C., Ph.D. thesis, IIarvarci
University, 195’7. (3) Anson, F. C., Lingane, J. J., J . Am. Chem. Soc. 79, 1015 (195.7). (4) Baker. B., MacNevin. W.,Ibid.. ~, 75, 1476 (1953). (.5\ Butler. J. A. V.. “Electrical Phenomena‘atInterfac&,’) Chap. IX, Macmillan, New York, 1951. (6) Davis, D. G., Talanta 3,335 (1960). (. 7,) Hammett. L. P., J . Am. Chem. SOC. 4 6 , 7 (i924j. (8) Hickling, A., Wilson, W.,J . Electrochem. SOC.98,429 (1951). (9) Kolthoff, I. hf., Nightingale, E. R., Anal. Chim. Acta 17,329 (1957). \-I
(10) L:iitincn, H. A., Enke, C. G., J . Electrochem. Soc. 107, 773 i 1 9 f i O l (11) Lingane, J. J., “L1ect;oa;alytical Chemistry,” 2nd ed., Chap. XXII, Interscience. S e w York, 1958. (12) Lingane, ‘J. J., J . Electround. Chem. 1. 379 f 196O’i. (13j I b i d , in press. (14) Whiteker, R. A., Davidson, N., J . A m . Chem. SOC.75,308 (1953). RECEIVEDfor review January 10, 1961. Accepted April 11, 1961. Contribution 2641 from the Gates and Crellin Laboratories of Chemistry.
Exchange Current De nsities f o r Fe (I I)- Fe(III) SoI utio ns in Sulfuric Acid and Perchloric Acid FRED C. ANSON California institute of Technology, Pasadena, Calif.
b Exchange current densities for the Fe(ll)-Fe(1ll) couple have been evaluated galvanostatically in sohtions of sulfuric and perchloric acids a t a platinum electrode. The results indicate that the heterogeneous rate constant for this couple is independent of p H between p H 0 and p H 1 and is decreased b y complexation with SUIfate. Perchloric acid solutions of Fe(ll) and Fe(l1l) display an anomalous concentration dependence of the exchange current density that may b e due to contributions to the measured exchange currents from Fe(l1l) or Fe(ll) adsorbed on the electrode.
E
current clensitics a t platinum electrodes for solutions of Fe(I1) and Fe(II1) in a few supporting electrolytes have been measured by a number of experimenters (5,6,8,1b, 14). The agreement among the experiments has not been satisfying, and the discrcpancies have usually been accepted as a n inevitable result of the use of solid electrodcs n i t h their difficult-toreproduce surfaces. hloreorer, several authors were concerned primarily with thc development of the technique employed and were interested only incidentally in the electrode kinetics of the Fe(I1)-Fe(II1) couple. I n short, the data of various workers are not readily comparable, and the electrode kinetic parameters for the Fe(I1)Fe(II1) couple cannot be regarded as well establishcd. Experiments in these laboratories have indicated t h a t exchange current densities a t platinum electrodes for the Fe(I1)-Fe(II1) couple can be reproduced to 1 5 to 10% if sufficient care is paid to solution purity and, especially, to the state of the electrode surface XCHANGE
with respect to the presence or absence of platinum oxides and of freshly formed, finely divided platinum metal (1). In this report heterogeneous rate constants for Fe(I1)-Fe(II1) solutions in sulfuric and perchloric acids have been evaluated with a galvanostatic technique similar to that of Berzins and Delahag (3). EXPERIMENTAL
X constant current in the form of a step function was passed betxeen platinum working arid auxiliary electrodes in Fe(I1)-Fe(II1) solutions. The potential of the working electrode with respect to a third, nonpolarized platinum .reference electrode was observed as a function of time on a Tektronix Model 536 cathode ray oscilloscope (CRO) equipped with a Type D differential preamplifier having a maximum sensitivity of 1 mv. per em. and a Type T time base generator. The CRO was triggered by the voltage being observed. The point at which the trace crossed the central, graduated voltage axis on the CRO screen was observed visually and recorded. This voltage consisted of the overpotential, the ohmic potential drop, and the potential difference due t o concentration polarization corresponding to the particular current density and time for each experiment. The recorded voltage was corrected for ohmic and concentration polarization contributions, and the exchange current density corresponding to the prevailing current density and Fe(I1) and Fe(II1) concentrations mas calculated from the corrected voltage. Apparatus a n d Procedure. T h e constant current source consisted of 300 to 400 volts dropped across a bank of resistors in series with t h e cell. T h e voltage drop across t h e cell never exceeded 2 volts. T h e voltage was supplied b y a regulated
variable power supply (General Radio Co., Cambridge, Mass.). T h e current passed either through a d u m m y resistance t h a t was matched t o the cell resistance or through the cell, depending on the position of a mercury reliiy (Western Electric Co., Type 275B). T o eliminate switching transients associated with toggle switches, the relay was activated by charging a capacitor through a resistor selected so that several hundred milliseconds elapsed between the time the toggle switch was thrown and the mercury relay was activated. The s~$itching transients resulting from the mercury relay decayed in less than 3 psec. T o reduce transients associated with the power supply, a procedure used by Mattsson and Bockris (10) was employed: A resistance of 25,000 to 50,000 ohms was placed parallel to the cell and the dummy resistance in such a way t h a t the current could pass through this resistor even during the instant when the moving relay pole was not in contact n i t h either fived contact. The presence of the resistor caused the current to be somewhat less constant as the voltage drop across the cell varied; but all measurements were completed in a few milliseconds before any significant change in the current had occurred. The voltage and time scales of the CRO were calibrated and checked regularly. The calibrations remained constant within =t3% throughout the course of these experiments (12 months). Chronopotentiometric transition times of 0.2- to 1-second duration were measured with the CRO for use in calculating concentration polarization contributions to the measured overvoltages. The accuracy of the measurements of 7 with the CRO was only about +57,, but this was adequate, inasmuch as concentration polarizaVOL. 33, NO. 7, JUNE 1961
939
r :tniouiitccl to more t1i:in 25% of thc mtasurcd ovcrvoltngc ant1 was usually less than 10yc.
for pl:111c working electrodes, both arrangemrnts were arrived a t in an nttcmipt to satisfy the same critvrix. 1 o assure uniform currcmt density in tlic. prescnt rsperiincnts it was nccessnry to include significant ohmic drop (10 to 207, of the t o t d O l ~ S C l t i d diffrrence) iii sonic of t h e measurcd voltagcs. I n those instancrs where the ohmic drop was significailt, its magnitude n'as determined by closely obwrving the very initial portion of the t r w c on the scwon of the CRO. The time constant for rstahlishmcnt of the ohmic potential drop resulting from the current step is so much sniallcr than the time constants for the other processw that cause the potentiid to change that a small dark scgmc.nt corrcsponding to thc ohmic drop appcars at the start of each trace. If the ohmic voltage drop measured in this way is plott,ed against the current, a straight line through the origin results, with a slope that gives the effective resistance between the reference and the working electrode. This method for measuring resistances in three-electrode cells has been used successfully by Mattsson and Uockris ( I O ) and llchlullen and Hackerman ( 9 ) . r 7
'1'11~.wll W:LS a 150-m1. bc:ikcr fitted ii.itli :I polyctliylciie W J ) in n-hic>hthe clwtrotl(~s:ciid Iiiti,ogen inlet tube ivcrc mountcd. l'he polyethylene cap !vas turncd out on a 1:ttlie from a cllindc*r of p~lyttliylc~nc sto('k having a dinineter of 10 rni. IIolcs \wrc drilled to accept the soft glass tubes in which the electrodes were sealed. The auxiliary electrode v a s a coil of 0.030-inch platinum wire sc~tledin soft glass. ilbout 3 cm. of n-ire was exposed beneath the seal. T h e working rlcctrode was a piece of 0.020-incli 1)l:ttinuni wire s e n l d in soft glass to 1e:ivc ail exposed area of 0.096 sq. c m . l'hc size of the working electrode was kept' as small as nxs ronsistent with the maintenance of uniform current density to minimize the currents used and, thereby, the ohmic contributions to measured voltages. The reference electrode was a piece of 0.020-incli platinum wire contained in a 4-nini. glass tube that was open a t both ends. The lower 6 mm. of the tube was bent at a right angle to the vertical and the opening constricted to a diameter of about 1 mm. The platinum wire was inserted in the tube so that the lower end of the wire was located about half way between the constricted opening and the right-angle bend. The reference electrode was mounted in the cell so that the constricted opening was about 0.5 mm. from the surface of the working electrode. The test solution rose inside t,he tube and covered the platinum wire for about 6 em. The electrode in the tube remained a t the equilibrium potential of the solution, so that the potential of the working electrode was measured with respect t o this equilibrium potential. The glass tubing surrounding the reference electrode was used to reduce the ohmic contribution to the measured electrode potentials. At unshirlded reference electrodes t'hc ohmic potential differences were two- to threefold larger.
The optimum relative position for the working and reference electrodes is that in which the ohmic pickup is the smallest while the current density at the working electrode remains uniform over the whole electrode surface. These two desiderata are partially antagonistic, because the ohmic drop is smallest when the two electrodes are very clos2 together, while the current density is no longer uniform over the surface of the working electrode if the reference electrode and its shield are brought very close to the working electrode. The arrangement described above proved to be the best compromise. The problem of the positioning of reference electrodes has been investigated in considerable detail by Pionklli, Bianchi, and Aletti (11). Although the electrode arrangement used in this study with a cylindrical working electrode differs somewhat from that recommended by Piontelli 940
ANALYTICAL CHEMISTRY
Chemicals. Solutions were prepared from triply distilled water, the second distillation being from alkaline permanganate. Reagent grade perchloric acid was usc.d without further purification. No difference in behavior resulted if the perchloric acid was pre-electrolyzed with a large-gage platinum electrode for 20 hours prior to use. Reagent grade sulfuric acid could not be used nithout a prior treatment. Solutions of Fe(I1) and Fe(II1) in 1F H2S04 prepared directly from 18F acid and examined immediately gave valucs for the heterogeneous rate constant that were erratic and always 2 to 10 times larger than the lower and much more reproducible values obtained mith the same solution after i t had aged for several hours. If the reagent grade 18F sulfuric acid was heated to fuming before being used to prepare iron solutions, the loner, reproducible values for k, aere obtained immediately. This effect was observed nitli all batches of sulfuric acid tested. It appears to be associated with the action of the unfumed acid on the surface of the platinum working electrode because the high, erratic values for k, were obtained if the electrode n a s alloxed to stand in 1F H2S04prepared from unfumed acid before being used to measure k, in 1F HzS04 solutions prepared from fumed acid. Pre-electrolysis of 12F H2S04failed t o eliminate the effect in solutions subsequently prepared. Solutions prepared from sodium hydrogen sulfate
and perchloric acid did not exhibit this anomalous bv1i:tvior. In all the cxperinicnts in which HzSOl nas usrd the solutions were prepared from fumed H2S04 or were allowed to age a suficicnt time to ensure that the equilibrium value for k , was obtained. Stock solutions of Fe(C10J3 and Fe(C104)2 in 1 to 2F HCIO, were prepared and analyzed as described previously ( I ) . Initial mcasurements w r e performed in solutions freed of oxygen with prepurified nitrogen. Later experiments shon-ed that the presence of oxygen made no difference in the experimental results a t the concentration levels involved; subsequent expcrinicnts were performed in air-saturated solutions. Experiments m r e carried out a t room teniperature that, remained n-ithin 2 O C. of 25" c. Electrode Pretreatment. A pretreatment' procedure for the working electrode was necessary to achieve reproducibility. T h e exchange current density usually decayed with time, unless the electrode was treated periodically to restore t h e surface t o its initial condition. The layer of platinum oxide on the surface of platinum electrodes that have been oxidized and the layer of platinized platinum that results when a n oxidized electrode is reduced n.ere shown previously (1) to exert marked effects on the reversibility of reactions occurring a t electrodes. Platinum oxide on the electrode causes oxidations to be less reversible ( I ) and reductions to be either more (2) or less ( I ) reversible. Freshly formed plat,inum on the electrode appears to increase reversibility for both oxidations and reductions, but as the fresh platinum surface ages the reversibility decreases (1).
I n accord with these earlier observations the largest and most reproducible values for k, were obtained \\.hen the electrode had on its surface a fresh layer of finely divided platinum produced by recent oxidation and reduction. The values of k, decrease as the platinized surface ages, but the decrease of k, with time is sufficiently s l o so ~ that i t is usually not necessary to subject the electrode to the oxidization and reduction treatment more often than every 45 to 60 minutes to achieve reproducibility. I n the experiments with sulfuric acid solutions of Fe(1I) and Fe(II1) i t was convenient to treat the electrode b y oxidizing it in the test solution and then allowing the Fe(I1) in the solution t o produce the layer of finely divided platinum by chemical reduction of the oxide film. I n the experiments with perchloric acid solutions of Fe(I1) and Fe(II1) i t was necessary to reduce the
tion to 7 due to conwiltration polarizat,ion. This contribution is evaluated by performing chronopotentioniPtric experiments on the test solution to measure the cat'hodic and anodic transition times. The product of current and square root of the transition times is calculated and substituted into Equation 3. The exchange current density, &, can then be calculated and k , evaluated from Equation 2 . Although a single measurement of 7 is in principle sufficient to ?valuate e 4 6 8 k,, a more trustworthy procedure is to measure 7 for se\.eral values of the GONG., rnoles/cm3 K io5 current density in a series of solutions t h a t contain equal concentrations of oxiFigure 1 . Exchange current densities vs. concentrations for dant and reductant but have varying solutions containing equal concentrations of Fe(1l) and Fe(ll1) total concentrations. A plot of the Supporting electrolytes averaged values of & in each solution 1. 1F HrSOd against the common concentration of 2. 1F NaHSOa 3. 1F NaHSOd-lF NaiSO4 oxidant and reductant is a st,raight line with a slope of k, nF. Three such plots of & us. C for Fe(I1)Fe(II1) solutions in sulfate-supporting 1 oxide film electrocheniically because electrolytes are shown in Figure 1. Fe(1I) reduces the oxide much more The linearity of the plots and the fact slowly in HC104. One of these prothat the lines pass through the origin cedures was applied to the electrode show that the Fe(I1)-Fe(II1) couple in just prior to each experiment. this medium is behaving in accordance where q , the observed overvoltage, is with Equation 1. the difference between the equilibrium RESULTS AND DISCUSSION The values for k, resulting from the potential of the electrode under conplots in Figure 1 are 5.3 x lop3, ditions of no net current flow and the 2.7 X and 1.4 x 10-3 cm. see.-' If the notation of Berzins and Delahay electrode's potential a t time t; & is the (5) is adopted, the equation relating for 1F H2S04, 1F h'aHS04, and 1F exchange current density; i is the curthe exchange current density, io, and ?r'aHS04 - 1F Sa2S04supporting elecrent density passing through the electhe Concentrations of oxidant and retrolytes, respectively. The value for ( i ~ l ' ~ and ~ ) ( i ~ ~ are ' ~ ~ ) trode; and ductant, Coand CR,is: k, in IF H2S04 is in good agreement with the current-independent products of the value 5.0 X IO+ cm. see.-' obcurrent and square root of transition tained recently by Wiznen and Smit time measured chronopotentiometrically (14) by a different bechnique. Gerischer for cathodic and anodic currents, rewhere k, is the concentration-indeobtained a value for k, in 1F H$04 of spectively. Equation 3 is written for pendent heterogeneous rate constant, 3 X em. see.-* in 1950 ( 5 ) , but the case where any ohmic contributions a is the transfer coefficient, and n and F the technique used to measure k , a t q have already been subtracted. to have their usual significance. The that time involved much larger conThe second term in the bracket in object of the experiments is to evaluate tributions to the measured overvoltEquation 3 corresponds to the contribuk, and C Y ; a convenient method for
,
evaluating k, independently of CY is t o use solutions in which Co = CR = C. For such solutions k , is given by I; = a
io
CnF
so that k, may be calculated immediately from a measurement of the exchange current density. The exchange current density is evaluated as described in the experimental section b y observing with a CRO the potential-time curve that results when a current step is passed through the electrode in a solution containing equal concentrations of Fe(I1) and Fe(II1). If contributions t o the potential-time curve due to the double layer are neglected [a very good approximation for the Fe(I1)-Fe(II1) couple in HB04 and HC1041 and overvoltages are restricted to less than about 7 mv., t h e equation for the potential-time curve in the case of a cathodic current ster, is
i
lot/
0
I 2
I
1 6
4
GONG., rnoles/crn?x
I 8
to5
Figure 2. Exchange current densities vs. concentration for solutions containing equal concentrations of Fe(l1) and and Fe(lll) perchlorates in 1 F HCIOd VOL. 33, NO. 7, JUNE 1961
941
ages from concentration polarization with a corrcsponding decrease in the accuracy n i t h wliich k , could be determined. The fact tliat k, decreases as the p H and concentration of sulfate ion increase may be due to increasing complexation of Fe+3 by sulfate. One can calculate from the Fe(II1)-sulfate complex-dissociation constants determined by Whiteker and Davidson (2 3) t h a t t h e relative amounts of Fe+3, FeS04+, and Fe(S04)2- present in the three solutions of Figure 1 are 7%-55%38%, 1%-29%-70%, and
