Chronopotentiometric and Chronocoulometric Measurements of Adsorption of Lead and Mercury(l1) at Mercury Electrodes ROYCE W. MURRAY and DONALD J. GROSS Department o f Chemisfry, University o f North Carolina, Chapel Hill, N. C.
b The adsorptions on mercury electrodes of lead from iodide, bromide, thiocyanate, and chloride media and of mercury(l1) from iodide, bromide, thiocyanate, and thiourea media have been detected and measured chronopotentiometrically. The lead surface excesses were also measured chronocoulometrically with results in general agreement with the chronopotentiometric values. The sequence iodide bromide thiocyanate chloride describes both the tendency for lead adsorption to occur and the electrode potential separation between the adsorbed and solution lead reductions. N o wave splitting i s observed in the mercury adsorptions, and the chronopotentiometric data follow the equilibrium adsorption model of Lorenz. Mercury(l1) adsorptions from thiocyanate and iodide media are experimentally demonstrated to occur as HgX4-2 complexes; calculated saturation coverages are in reasonable agreement with observed coverages. Comparisons of calculated coverages for various lead iodide and bromide species with experimental coverages suggest an adsorption with 1 / 1 or 1 /2 lead/halide ratio.
>
>
F
>
and double layer capacitance data obtained by Barker ( 5 ) have provided qualitative evidence for the adsorption of lead(I1) on mercury electrodes from I F chloride, bioniide, and iodide supporting electrolytes. I n the case of iodide, this deduction has since been confirmed in a polarographic invedgation by Srinivasan and Sundarani (28), who noted a post-wve following the main, reversible lead wave in 1.OF S a 1 for lead concentrations larger than 0.21nX. The post-wave was interpreted in the classical BrdiEka (4) sense as indicating adsorption of lead a t the electrode Furface; it was apparently overlooked in an earlier polarographic study (1.4). Polarographic investigations in the other halide media, bromide ( I S , 31)) and chloride (18) have, on the other hand, produced no indication of adsorption waves, nor has polarography in thiocyanate medium ( 1 2 ) . ARADAIC RECTIFICATION
392
ANALYTICAL CHEMISTRY
27515
The electrochemical behavior of the mercury(O)/mercury(II) couple can be studied in the presence of high concentrations of ligands forming stable mercury(I1) complexes. Thiocyanate, iodide, bromide, and thiourea are examples of such ligands. Several polarographic investigations of the anodic properties of mercury(0) in the presence of these ligands have been described (9, 15, 20, 25-25, 27, 2 9 ) ; fewer reports can be found concerning the cathodic reductions of the corresponding mercury(I1) complexes (16, 29). I n all instances in which the ligands were present in substantial concentrations, single, diffusion-controlled, fairly reversible polarographic waves were observed. Chronopotentionietric exainination of the reductions of mercury(I1) and lead from the above media has revealed an increase in the chronopotentiometric constant, i+, of their reduction waves with increasing applied current. This behavior is diagnostic of adsorption of electroactive species a t the mercury electrode surface. Quantitative information on the extent of the adsorption is accessible by analysis of the current-transition time variations. ;Inother technique, the potential-stepintegral method (6, 7 ) [also recently termed chronocouloinetry ( 2 )] is also capable of providing surface excess values for adsorbed electroactive inaterials. lysing the chronopotentiometric and chronocoulonietric techniques, an investigation of the lead and niercury(I1) adsorptions from the several media has been conducted in the interest of obtaining detailed inforniation on surface excess values and the nature of the adsorptions. Coincident with this purpose has been that of a coniparison of surface excess data obtained with the two techniques and several variations thereof. EXPERIMENTAL
Reagent grade chemicals 11ere used throughout and were standardized by conventional methods where necessary. Lead and mercury(I1) were used as the nitrate salts. -111 supporting electrolytes n-ere chronopotentiometrically
"clean" in the potential region of interest. A small inipurit~y wave a t -0.45 volt L I S . S.C.E. appeared in thiocyanate solutions unless precautions were taken to avoid contact with atmospheric oxygen. I t could be restricted to insignificant proportions by preparing .;olutions in the cell under nitrogen. S a 1 solutions were similarly prepared to minimize air oxidation of iodide. Supporting clectrolyte solutions contained 0.011: HCIO, and a sinal1 aiiiounb of HSOI (pH -3) in the lead and niercury(I1) experiments:, respectively, to avoid metal ion hydrolj A concentration of 0 . W HCIOI way required in thiourea solutions for adequate stability of the ligand. Water was deionized and distilled from alkaline pernianganate. The water-jackcted beaker cell was mounted on a rubber cushioned platform for freedom from extraneous vibrations. All exlierinients were conducted a t 25' C. unless otherwise noted. I3ot,h hanging mercury drop ("DE) and inercury pool electrodes were employed in the chronopotentiometric experiments; measurement> in overlapping transition time regions were in excellent' agreenient. Only the HlIDE was used in the chronocoulonietric mcasurenients. The H M D E consisted of a l't xire sealed in soft glass (blunt end), polished flush, etched with aqua regia, and plated with electrodeposited mercury. Mercury drops were transferred to it from a standard D.M.E. assem1)ly with a scoop made of Teflon. Electrode areas were calculated from the D,AI.E. characteristics with correction for the P t wire tip area. The mercury pool electrode consi>ted of a cylindrical Teflon c u p supported by a hollow glass "J-tube" through which electrical connection !vas made to the pool with a sealed-in I't wire. The effective pool area [larger than the cylinder crosssectional area (5)]was measured chronopotentiometrically with cadmium, zinc, lead, thallium, and iodate (under conditions of constant i~''*) test ions. Using diffusion coefficient values of 7.2, 7.2, 9.75, 20.0, and 10.9 X cm.z/second, areas of 1.81, 1.77. 1.73, 1.70, and 1.66 c m 2 , respectively, were obtained, giving an average pool area of 1.73 f 0.04 c n 2 Present address, U. S. Army Nuclear Defense Laboratory, Edgewood Arsenal, Md.
The instrument was a n operational amplifier type, based on a Philbrick Rrscnrches Model K7--110 manifold ant1 utilizing conventional passive circuitry. A separate follower circuit employing I'hilbrick K 2 - X and K2-Q amplifiers (8) was employed for potential monitoring in the usual manner. Chronocouloiiietric experiments with lead and all chronopot entiometric esperinicnts were conduc ted with threeelectrode circuitry, for which the instrument tinie constant was less than 1 iiisec'ond. The initial norking electrode potential in chronol,otcntionietric es1)eriments on lead was pre-set, using a previously described circuit (21), a t in the chronothe m n e potential as Ea,8it coulometric mea.surements. Chronopotentiometric current-revewal experimc>nt s 11ti 1i z c d f or 3 ut omat i c c urrf nt polarity .n.itrhiiig a 1-oltage croiying ami)lificr (1S485.1 tliotlri) henbing the sum of a calibratcd voltage sweep and a fixed reference voltage and driving with its outl)ut a Clare 11odel HGS-1017 -wetted rday. Depending on red rcs1)on.e time, a Sargmt RIodel SK rccordcr, a Sanborn Model 151-10OA single-clianncl rccorder with LIodel 150-1800 preaniplifier, or a Tektronix llodel 564 storage oscilloscopz with C-12 camera \vas employed for recording 11urposcs. I n each experimental system, the equilibration time required for attainment of adsorlition equilibrium was ascertained by obhcrvation of chronopot en tic gram^ as a function of the ~ a i t i n g pciiotl. 1 k c l ) t for lead in bromide medium, nliich required about 2 minutcs of equilibration, ad.-orption equililirium \vas attaincd w r y rapidly in all ,-vstems, a few seconds of stirring or siml;ly the stirring provided by drop transfer when using the H N D E being adequate. The required stirring period was followed by a 2-minute unstirred wait before application of current or potential. THEORY
Chronopotentiometry. Reactant adsorption in chronopotentiometry is detected by a n increase in the product i&? with increasing applied current. Four chronopotentionietric adsorption situations have been described; these can be denoted as AR,SR, S d R , S R , d R , and equilibrium cases ( 2 2 ) . Their mathematical relations are:
AR,SR (19)
SAR (19)
i7 =
is =
nFdr
nFAr
+ D r ( n4iF d C ) 2
+ nFAC(?rDr)'/2 2
nFA & SR,AR (1, 26) --
i
(so
+
rd)uTCCOS
[:: 5 :,I - 2(TaTd)'/2 ___
(3)
,l/Zr + 2D W '
Equilibrium (19) +I2
where p = esp (C2Drp2)erfc (CD'/?+?/I?), r represents the adsorbed reactant surface excess in mole/cm.*, C is the solution reactant concentration in iii~le/lcni.~, r is a total transition time, T~ and ~d are adsorption and diffusioncontrolled transition times, respectively, and the other symbols carry their usual electrochemical connotation. The first three eases assume mathematically t h a t , after adsorption equilibrium has been attained and the current applied, the adsorbed ( d R ) and solution ( S R )reactants undergo electron transfer as essentially independent species. The d R , S R case assumes coiiiplete depletion of d R (at time 7,) prior to a diffusion controlled reaction of SR. Ideally, but not necessarily, this case displays two transition times corresponding to constant i r , and i ( r - r a ) l / *products, where T is the total transition time. When a ra is not directly observed, inspection for this case involves a plot of iT os. l/i according to Equation 1. The SR,AR case is the reverse of this; S R is depleted first in a diffusion controlled manner and, when a distinct transition time is observed for this step, yields a constant ~ T and P a constant r calculated from Equation 3 with the two individual transition time values, rd and r - T d = ra. When a rd is not observed, a computer solution to Equation 3 can be sought (30) using the total r , or a constant-ramp current chronopotentiometric comparison may be employed as an operational test ( 2 2 ) . The S A R case approximates a situation where A R and S R react concurrently and assumes a fixed division of the applied current between the two such that they are simultaneously exhausted. Although irregularities in the potential-time curve are conceivable in this case, only a single meaningful s is espected, and according to Equation 2 a plot of i r xs. ~ 1 1 2 should be linear if this case is present. The equilibrium case assumes a rapid equilibration between A R and S R according to a linear adsorption isotherm; the species actually undergoing electron transfer is unknown inasmuch as the adsorption equilibrium shifts during the smooth potentialtime curve to produce a simultaneous depletion a t a single observed transition time. An iterative graphical allproach is taken for coniparison of data us. 1/i plot \Till to Equation 4. X be linear at long T (# is small there), providing a D value from its slope. Then the entire left-hand side of Equation 4 is replotted us. l/i for a selected value of r (using a master plot of 6 us.
its argument to obtain a
for each
7).
1better estimate of D is obtained from the new plot, and Equation 4 replotted again, iterating r until it is ascertained whether a linear plot can be obtained over the entire r range giving an intercept consistent with the chosen values of D and I?. For the most accurate extraction of surface excess from chronopotentiometric data, it is necessary to ascertain to which of the above cases an experimental system best' conforms. This is ordinarily based on a "best fit" comparison of experimental data to the several equations according to the criteria noted above. The current-transition time characteristics of the theoretical models are not widely different, and sonie previous chronopotentiometric adsorption studies have produced reservations concerning the feasibility of distinguishing between the different cases (27, 30). This experience has to a certain extent been confirmed in the present and other studies. I n many of the lead and mercury(I1) adsorptions examined, however, a clear conclusion \vas possible. Because the experimental conditions are important in facilitating the choice of adsorption case, it is pertinent to comment here on the several factors influential in this respect. Range of r mearured. The range of r investigated should be as large as practical, particularly when only a total T can be measured, so as to better detect any trends, curvature, etc., in the examination. Short r values provide a useful maximization of the adsorption contribution; the equally important long r values provide a better estimate of the unknown value of the diffusion coefficient. Constancy of D over a range of solution concentrations provides an auxiliary criterion for the validity of any given case. Concentration of SR. The use of low solution concentrations is beneficial toward magnifying an adsorption effect on r if the slope of the absorption isotherm is such that halving of the solution concentration halves (or less than halves) the surface excess, The reason for this can be roughly explained by the observation that halving the solution concentration lowers the diffusional part of 7 by a factor of 1/4 but lowers the adsorption part only by or less. The advantage of low R O ~ U tion concentrations will diminish when working on very steep adsorption isotherms. Solution temperature. Lowering the solution temperature below the usual 25" C. value reduces the diffusional contribution to r (simply by lowering the value of D ) and also carries the beneficial possibility that the value of r may be enlarged. Chronocoulometric Method. In this method, a potential step is applied t o the electrode of magnitude VOL. 38,
NO. 3,
MARCH 1966
393
-0.1 0.5
0 TIME ISEC.)
I2
Figure 2. Chronopotentiograms for lead in 1.OF Nal f 0.01F HCIOa
w
U
Current = 0.764 ma./cm.z Curve 7 . 0.790mM Pb Curve 2. 0.0993mM Pb, volt on potential oxis
U f,
V
-0.45
-0.55
-0.50
-0.60
E vs S . C . E . ( V O L T S )
.
Figure 1 Potential-sweep chronoamperograms for 0.495mM lead in 1 .OF Nal 0.01 F HCIOa
+
Potential scale shown is for Curve 2. HMDE area = 0.0267 Curve 1. Scan rate = 0.02 v./rec. Curve 2. Scan rate = 0.005 v./sec.
sufficient to produce zero surface concentration of both A R and SR a t t > 0. The equation (6, 7 ) representing the integral of the resulting currenttime curve is
where Qc represents the double layer charge passed for the potential step. -4plot of Q vs. t1l2 provides in its positive intercept a measure of the adsorption, after correction for Q e . This technique was proposed for adsorption studies by Christie, et al. (6, 7 ) . It has the significant advantage that a single experiment (plus a blank to measure QJ provides a complete data set for adsorption measurement, whereas chronopotentiometry requires a series of measurements a t different currents. I n addition, inasmuch as the adsorbed layer is, presumably, instantaneously reduced, no choice of adsorption case is required for r evaluation. This feature is simultaneously a disadvantage, as the order of reaction of adsorbed and solution species is often of interest in itself. Such ipformation is readily obtained from chronocoulometry (by stepping to various potentials) only in AR,SR and SR,AR cases involving a clear potential separation of the A R and SR waves. The chronocoulometric technique
394
ANALYTICAL CHEMISTRY
places a stringent demand on the response characteristics of the potentialcontrolling circuitry employed; submillisecond response is desired. A lag t, in the response time, with consequent delay in reducing the surface concentration to zero, amounts to a time delay before accumulation of the integral Q which is not reflected in the origin of the t 1 / 2 axis. This effect is evidenced in a downward curvature of the Q - t1/2 plot at short times and negative errors in I?. If t, can be assumed to be a well-defined time, then replacement of t1/2 with (t - t,)1/2 in Equation 5 ( I O ) can compensate for this effect. A potential-determined desorption of A R during the delay time t, can provide an additional source of negative error; a theoretical approximation can be also be used to partially compensate for this effect (IO). The instrumentation used for the present chronocoulometric experiments with lead was adequate to avoid such time delay effects. RESULTS AND DISCUSSION FOR LEAD ADSORPTION
Iodide Medium. The polarography of lead was briefly examined in 1.OF KaI 0.01F HC104, and the postwave and electrocapillary characteristics reported (68) were confirmed. Potential-sweep chronoamperograms also consist of two waves, as shown in Figure 1. The first rounded peak is
+
displaced
-0.1
doubled by a four-fold increase in scan rate as expected for a diffusion controlled process; the second, sharp peak is more than doubled and appears to be adsorption-controlled. The qualitative appearance of a chronopotentiogram in this medium is a function of the lead concentration (Figure 2) and the applied current. Chronopotentiograms having the shape of Curve 1 are characteristic of high lead concentrations or low currents. h decrease in concentration or increase in current ultimately causes disappearance of the segment ' T ~ (Curve 2). Pronounced adsorption of lead is indicated a t all concentrations by a substantial increase in with increasing current, as exemplified by the data of Table I, column 4. By inference from the polarographic and potential-sweep chronoamperometric results and the behavior of the time 7 d relative to the time T ~ the , time rd is expected to respresent a diffusional depletion of the solution species, and the time 'T. marks exhaustion of the adsorbed species, according to the SR,AR adsorption case. Table I gives values of hdl/* and r S R , A R calculated from chronopotentiograms having the appearance of Curve 1 in Figure 2. The trend observed in i ~ ~ob-~ / ~ , tained for all rational graphical measures of ' T d , shows that the cathodic chronopotentiometric adherence of the lead iodide system to SR,AR behavior is only approximate. Less variation is noted in r S R . A R ; the average, 9.3 X 10-10 mole/cm.2, is probably a fair estimate of the extent of the lead adsorption. Tatwawadi and Bard (SO) have suggested that high currents might produce transition times sufficiently short as to be entirely dominated by the adsorption contribution. I n view of the ability to shorten T~ to the point of oblivion, this approach was investigated. At moderately high currents, with both the constant and ramp (i = P'T) current techniques, a limiting, current-independent
charge could be attained, as shown in Table 11. The average riim. value, 10.5 X 1 O - I o nio1e:Icni.2, is in reasonable value, conagreement with the rsR,aR sidering some probable charging connumber. The tribution to the rlLm. diffusional part of T in Table I1 is small; for example, for i = 400 pa., taking L) = 1.0 X 10-5 cm.*,’second, the calculated diffusion-only T is 0.32 nisecond as compared to the measured total T = 13.6nisecond. Comparison of the chronopotentiometric. d a t a to the AR,SR and SAR adsorption cases is also pert’inent; Figure 3 shows typical plots according to Equations 1 and 2. The AR,SR plots are curved under all conditions, eliminating this case from further cond e r a t i o n . 1‘he linear SdR plots show t h s t thib niodel is a fair approximation of the adsorption behavior, a result not surprising in view of the imperfect adherence t o the S R , d R case noted above. T S A R and DslR data are given in Table 111. It is apparent that a liniiting surface excess has been reached even a t the lowest concentration tested. dome typical current-reversal curves taken at, long transition times are shown in Figurc 4. Reversal prior to T , ~produces a single back wave; a secmxl, more cathodic reosidation \vave, T ~ ’ , occurs for rcverqal after r d . In either event, the total T, = T ~ ’ rd’, measured from the time of reversal, is ‘ / 3 the time of cathodic current application, inasmuch as it is controlled by diffusion of lead in the amalgam
+
Iihase. Thc appearance of the anodic waves of Figure 4 suggests that the replenish-
ment of the adsorbed lead layer (time T ~ ’ ) and the production of diffusing
Table 1.
Chronopotentiornetric Data for 0.495rnM Lead in 1.OF Nal HCI04 at a Mercury Pool Electrode
+ 0.01F rSR,AR
i, ma. 2.000 1.000
Td,
0.23 0.84 3.30 12.4
0.250 0.125 a 7
=
Td
$-
ma. sec.l’* 1.160
T . ,
0,070
0.500
ir1‘2 a
sec. 0.265 0.K 1.32 3.28 8.9
set.
0.915
0.735 0.642 0.576
x 10’0, mole/cm.*
iTd”*,
ma. sec.*i* 0.530
8.84
0,480
9. 10 8.86 (5.02)c
0.45Sb 0.455b 0.440b
10.5
Ta.
Average diffusion coefficient calculated from the Sand equation for these values is 9.55 x 10-6 cm.*/sec. T d - T~ = 0.02, and experimental error in these times makes this value questionable.
+
Table II. Surface Excess Concentrations for Lead(l1) in 1 .OF Nal 0.01 F HCI04 from Limiting Chronopotentiometric Constants Obtained at High Currents
[Pb]
=
Constant current
0.495mM; HMDE, area 2,
Pa.
800 400 p, pa./sec.
x
Ramp current
10-4 5.00
2.00 1.00
solution species (time T ~ ’ )are more distinctly isolated from one another in the anodic reaction than are the corresponding reductions in the cathodic process. If the current is reversed at the cathodic transition time, r , the ~ ’ be a constant if product i ~ should T ~ represents ’ formation of an adsorbed layer only. Reversal exactly a t r is difficult to accomplish with precision, however, and recourse was made to anodic cqronopotentionietry a t a hanging lead-amalgam drop electrode. This
7,
=
0.0268 cm.2
msec. 6.6 13.6
msec. 14.8 23.5 33.0
-
-0.8
mole/cm.* 10.6 10.7 10.5
electrode was prepared by plating a known amount of lead into a HMDE, transferring this amalgam drop electrode, after intermediate rinsing, to
Table 111. Surface Excess and Diffusion Coefficient Data Obtained from the SAR Plots of Figure 38
[Pb] mU 0.0993 0.198
5 SEC.
U
5. -0.7
w: $
rlim. x 1010,
rcoul. 5.48 5.52 5.45
T,
-0.9
0
10.5
2
0.789
5
mole/cm.* 10.2
5.28 5.45
0.396
c v)
rlrm. x 1010,
i r , pcoul.
DSARx io5 r S A R x lo“, crnZ/sec. mole/cm.2 1.16 8.6 1.20 8.5 1.19
9.0
10 -5
10-10
1.20 8.6 Av. 1.19 X Av. 8 . 7 X
I 1
-0.6
ln
>
w -0.5 -0.4
5.04
3.06
3.01
I
I
Figure 3. Comparison of chronopotentiometric transition time data for lead in 1.OF Nal 0.01F HClO4 to the AR,SR (A), TIME and SAR (8)adsorption cases Figure 4. Current reversal chronopotentiograms for [Pbl HMDE area 0.495mM lead in 1 .OF Nal f 0.01 F HCI04 at the mercury Curves 1,5. 0.0263 cm.* 0.789mM pool electrode Curves 2,6. 0.396 0.0262
+
Curves 3,7. Curves 4,8.
0.1 98 0.0993
0.0260 0.0261
Numbers below curves a r e the ratio of time of cathodic current application to the reverse transition time, T~
VOL. 38, NO. 3, MARCH 1966
395
-
t
Ii
-0.8
v)
2 -0.7 v)
-0.5
I
I
T I ME
Figure 5. Current reversal chronopotentiograms at the hanging lead amalgam drop electrode ((Pb)ag] = 10.0mM; 1.OF NaI
+ 0.01F HCIO,;
another cell containing only iodide supporting electrolyte, and stripping off any oxides formed during transfer with a short burst of cathodic current. Resultant anodic chronopotentioqrams are shown in Figure 5. Table IV shows that the product ita’ is, indeed, quite constant. The surface excess, romol, calculated from the average ita’, is 8.7 i 0.4 x mole,’cm.*, in good agreement with the previous figures. The following amalgam electrode experiments serve to further characterize the adsorption wave T ~ ’ : reversal ’ a of the current before or a t T ~ produces single cathodic wave 7,’’ of identical length (Figure 5); ratios of 0.97, 1.02, and 1.00 were measured using reversals a t different times and currents. If a 1.00-ga. anodic current (which produces t o ’ = 3.95 seconds) is applied for 3.00 seconds, interrupted for 1 minute (without stirring) and then reapplied, 0.90 second is required to reach T ~ ’ . Interruption for periods longer than several minutes could produce Lome loss of the adsorbed film ( to’ lengthened). An experiment in which an attempt was
Table IV. Chronopotentiometric Transition Time Data for r a ’ at the Hanging Lead-Amalgam Drop Electrode
i, pa. 1.00
2.00 2.00 3.00 4.00 5.00 5.00 10.00 20.00
T,,’,
secSa i i a t ,pcoul.
3.95 1.90 1.96 1.28 0.96 0.75 0.76 0.38 0.18
3.95 3.80 3.92 3.84 3.84 3.75 3.80 3.80 3.60
a In no case do these times approach those producing lead concentrations in excess of the measured -1mM solubility of lead in IF iodide; for example, i ~ ’ ‘ 2 t ~ t a 1 = ~ 1 . X 2 amp. sec.1’2for a 1mM amalgam.
396
ANALYTICAL CHEMISTRY
0.175 ma./cm.2
3 2 4 6 8 IO 12 14 t”‘ Figure 6. 1.OF Nal
5
(MSEC?)
plots for +Chronocoulometric 0.01F HCIO4 according to
Potential step is from
- 0.45 to - 0.70 volt VI. ~[Pbl, m M
Curve I Curve 2 Curve 3
made t o transfer the adsorbed layer formed on a HMDE in a lead iodide solution, with intermediate rinsing, to another cell containing only iodide for cathodic current experiments met with failure. The presence of 0.5m.U lead in the solution contacting the amalgam electrode causes the complete disappearance of to’,inasmuch as the adsorbed layer is already present before anodic current application. These experiments and the results of Table IV show that the wave 7.’ is governed solely by the production of an adsorbed film and that this adsorbed film is tenaciously retained a t the electrode surface. They also demonstrate that the sloping part of the cathodic lead wave just before t is governed by the potential-surface excess properties of the adsorbed layer, as this feature of the wave also appears in T ~ ’ . The reason for this peculiar potential-time behavior is not understood. The second anodic wave observed in Figure 5 is presumed to give rise to diffusing (solution) lead only. For example, in anodic-cathodic current reversals at the amalgam drop electrode the ratio td’/tdt’ = 3.14 & 0.15 for 7 reversals a t different currents, with no discernible trends. (These data may be affected, however, by the problems of measuring tdt‘.) If the anodic current is interrupted during td’ for 10 minutes (unstirred) and then reversed, no td” is present on the reverse wave.
0.396 0.198
0.0
lead in Equation
S.C.E. WMDE area, cm.*
0.0270 0.0262 0.0265
The most likely esplanation for the improved definition of the anodic wave to’ as compared to the cathodic td lies in the fact that it represents the conclusion of a purely adsorption-controlled process, intuitively expected (by analogy with, for example, chronopotentiometry of metal oside films and in thin layers of solution) to display a more clearly defined transition time. d n intriguing, but unproved, alternative is that a small overvoltage exists in the diffusion controlled waves T~ and td’ owing to the presence of an imperfectly penetrable adsorbed film on the electrode surface during those times. The chronocoulometric method was also applied to the lead iodide adsorption system. X potential step of -0.45 t o -0.70 volt us. S.C.E. is adequate to produce the necessary diffusion-controlled condition. Plots of Q us. t 1 I 2 (Figure 6) are linear with slope proportional to lead concentration (Table V, column 2 ) . Extraction of the lead surface excess from the intercepts of Figure 6 requires measurement of Qc; a n estimate of 26.4 pcoul./cm.* is provided by the blank Curve 3. This Qe, however, will be in error if lead adsorption appreciably alters the double layer capacitance at the initial potential (-0.45 volt). -4 convenient approach accounting for this possible effect lies in measurement of the difference in the (double layer) charge required to create D.N.E. drops of known size a t the initial (-0.45) and final (-0.70)
4.4
-
4.2
-
T
2
5
4.0-
p 0
3.8
+ a
n
3.6 3.4t
, , ,
-0.1
,
, , 1
-0.2 -0.3 -0.4 -0.5 -0.6 E vs S.C.E. (VOLTS 1
Figure 7. Effect of lead on electrocapillary curves in 1.OF NaBr 0.01 F HCIO, Curve 1. No lead
+
Curve 2. Curve 3. Curve 4.
0.0993mM Pb 0.495mM Pb 0.984mM Pb
potentials, as suggested by dnson ( 2 ) . 1Ieasurernent of D.M.E. drop charges provides, in the absence of lead, values of +24.9 pcoul./cm.2 and -3.2 pcoul./ cm.2 a t the two potentials, respectively, or a Qe of 28.1 pcoul.jcm.2, in good agreement with the directly measured Qc. I n the presence of 0.21nM lead, however, the D.1I.E. charging value a t -0.45 volt changes to f12.0 proul./ (Essentially the same value is obtained also for 0.4mJI lead.) ;\laking the reasonable assumption that the double layer capacity a t -0.70 volt is not appreciably affected by the dilute lead amalgam, this provides an improved Qcof 15.2 pcoul.jcni.2 r,, data obtained using this correction are given in Table V. ( I t should be noted that use of the Qc of Curve 3, Figure 6, would cause a -10% error in I'cc.)
Comparison of the average surface excess results r S ~ . A =R 9.3, riim= 10.5, rsaR= 8.7, ramal = 8.7, and r,, = 8.2 X 10-lo mole/cm.2 reveals a reasonably general and satisfying agreement between the model-dependent chronopotentiometric values and the latter two more direct measures. A similar degree of consiatency in the values of D from the several methods can alqo be noted. It can be concluded that the chronopotentiometric method, even in model-dependent forms, and chionocoulometiy eyhibit similar degrees of capability for the measurement of surface excess in strongly adsorbing systems. Bromide Medium. At low concentrations of lead in 1.OF S a B r 0.01F HC104, in accordance with a previous report (13), a polarogram displays a sharp mayimum rvhich can be easily suppressed to yield a single, fairly reversible wave with El/Q = -0.45 volt us. S.C.E. At 2mJI lead (which does not exceed the solubility of lead in this medium), a very small inflection can be observed in the rising portion of the polarogram. Lead adsorption is more obvious in electrocapillary curves taken in this medium, Figure 7 , which bear a strong resemblance to those reported for the lead iodide case (25). A potential-sweep chronoamperogram, Figure 8, also gives evidence of an adsorbed layer through the sharp current maximum superimpoqed on a rounded peak of more normal appearance. .I chronopotentiogram of < l m V lead in bromide medium is split into two segments (Figure 9). Xt higher lead concentrations, a sloping segment precedes the flat segment shown, and the curve is similar in appearance to curves
Table V.
Surface Excess and Diffusion Coefficient Data Obtained from the Plots of Figure 6
D,,x 105, recx
[Pb], m X 0.198 0.396
1010,
cm.Z/sec.
mole/cm.z
1.02
8.25 8.10
1.06
in iodide medium a t high lead conincentrations. The value of i+ creases rvith increasing current at all concentrations tested. The reductions of the adsorbed and solution species are less clearly isolated from one another than was the case for iodide; no degree of constancy could be discerned for various types of currenttime products for any of the wave d coniparison segments observed. of data taken in the range 7 = 0.091.2 second to the dR,SR and SdR adsorption cases by plots of Equations 1 and 2 produced an acceptable linearity in both cases for all concentrations tested. The surface escess and diffusion coefficient data taken from these plots are given in Table VI. The lead bromide system was also examined with chronocoulometry. Plots of Q 'L's. t1'2 for a potential step of -0.30 to -0.70 volt were similar to those in Figure 6 and displayed excellent linearity and large positive intercepts. The background correction was determined as before; a blank chronocoulometric charging curve and the difference of D.1I.E. drop charges a t -0.30 (in the absence and presence of lead) and -0.70 volt gave Qc = 24.1, 28.7, and 28.0 pcoul./cm.2, respectively. ;Idsorbed lead has a rather minor effect on the double layer charge in this case. Uqe of the latter Qc value to correct the intercepts of the Q - t1'2 plots results in the recvalues of Table VI. The generally reasonable correspondence of the chronocoulometric results to the S L R data suggests that the S A R case is probably the more appropriate representation of
+
t I-
z W
U U 3
-0.7-
V
50
-
-0.6
2 -0.5 W'
-0.4
-
w -0.3
-
0' In
- 0 -35
-0.40 -0.45 E v s S.C.E. (VOLTS)
-0.50
Figure 8. Potential-sweep chronoarnperograms for 0.496mM lead in 1 .OF NaBr 0.01 F HClOa
+
Potential scale shown is for Curve 2. HMDE area = 0 . 0 2 7 6 cm. Curve 1. Scan rate = 0.02 v./sec. Curve 2. Scan rate = 0.005 v./sec.
4
I TIME
Figure 9. Chronopotentiogram of 0.495mM lead in 1.OF NaBr 0.01F HCIOI at a mercury pool electrode
+
i = 125 pa.
VOL. 38,
NO. 3 ,
MARCH 1 9 6 6
397
4.4
s
-06-
5
-
0' -055
-
j-040
v,
-
ur-03-
w
-0.1 -0.2 -0.3 -0.4
-0.5
-
0 4 SEC.
)r
-
I(
1.0
-
SEC.
-0.6 -0.7
E v s S.C.E. ( V O L T S ) -,-
-
Figure 10. Effect of lead on electrocapillary curves in 1 .OF NoSCN 0.01 F H C l Q
a
.
-
+
Curve I . Curve 2. Curve 3.
No lead
0.984mM Pb 1.95mM Pb (some precipitation noticeable)
the chronopotentiometric behavior. Saturation coverage has apparently been reached a t the higher lead concentrations; the diminution of r a t the lower concentrations shows that the adsorption properties of lead bromide are somewhat weaker than those of lead iodide. This observation is qualitatively verified by comparison of the efficiencies with which the lead iodide and bromide adsorption layers can retard electrode reactions of other species (11). Thiocyanate Medium. A polarogram of lead in 1.OP KaSCN 0.01F HClO, consists of a single, welldefined wave (Eliz = -0.42 volt us. S.C.E.) exhibiting a maximum only at
+
high lead concentrations. A maximum also appears in 0 . W NaSCN (14). The electrocapillary curve is depressed by lead at potentials anodic of the lead wave, indicating lead adsorption from this medium (Figure 10). At 25" C., both chronopotentiograms (Figure 11, Curves 1 and 2) and potential-sweep chronoamperograms of lead in 1F thiocyanate consist of single waves with no discontinuities observable a t any concentration or current. Chronopotentiometric i d 2 values increase with increasing applied current, but to a much smaller extent than observed for the bromide and iodide cases. Figure 12 compares the chrono-
+
Table VI.
Surface Excess and Diffusion Coefficient Data for Lead in 1 .OF NaBr 0.01 F HClOa Obtained from Chronopotentiornetric AR,SR and SAR Plots and Chronocoulometric Plots
[Pb], mM
0.0993 0.198 0.495 0,789 Table VII.
DAR,SR X lo5,
rAR,SR
X
lolo,
DSAR
X 105,
rSAR
X
lolo,
cm.2/sec. mole/cm.2 cm.I/sec. mole/cm.2 2.88 8.78 0.72 8.24 1.68 10.8 0.78 9.51 1.51 12.1 1.02 9.50 ... ... ... ...
lolo,
mole/cm.z
...
8.25 9.05 9.11
+
D4
x 106, rZ5x lolo, moles/cm.z x 108, cm.*/sec. -0.2 v. -0.3 v. cm.Z/sec. 0.095 ... ... 4.25 ... 0.198 6.76 3.80 0.80 0.98 0.298 ... ... ... 3.92 0.396 6.50 1.52 1.69 3.92 0,594" 7.35 2.29 1.91 3.69" 7.84 0 . 790a 2.24 2.62 ... 0,990" 8.53 2.41 2.80 ... 1.95. 8.06 3.68 4.13 Supersaturated solutions; see text.
[Pbl, mM
e
...
0.89 1.00 1.04
rCcX
Chronocoulornetric Results for Lead Surface Excess and Diffusion Coefficient at 25" and 4 " C. in 1.OF NaSCN 0.01F HClOd Dzj
398
D,, X lo5, cm.2/sec.
ANALYTICAL CHEMISTRY
r4 X
lolo,
-0.2 v. 0.89
...
4.83 5.04
moles/cm.2 -0.3 V. 1.38 3.15 5.35 5.45 5.05 ...
...
potentiometric data to the AR,SR and S,lR cases (Curves 1 and 2). A slight curvature is present in the
