S. GILMAN
2098
Studies of Anion Adsorption on Platinum by the Multipulse Potentiodynamic (M.p.p.) Method, I.
Kinetics of Chloride and Phosphate Adsorption
and Associated Charge at Constant Potential'
by S. Gilman General Electric Research Laboratory, Schenectady, New York
(Received January 19, 196.4)
A multipulse potentiodynamic (m.p.p.) method has been developed which allows the rapid determination of the extent of anion adsorption as a function of time a t constant potential. The rapid kinetics of chloride and phosphate adsorption leads to diffusion control under our experimental conditions. An oxidative current is found to 00w during the rapid adsorption of chloride from solution a t 0.7 v. This current may correspond to a change in valence state of the surface.
Introduction Adsorption of anions on platinum has previously been studied by a variety of nonvoltammetric techniques. These methods, most commonly applied to platinized platinum, include: (1) observation of the effect of anion adsorption on hydrogen deposition2ia; (2) nieasurernent of the electrode potential shift resulting on addition of the adsorbable ion to a solution of sulfuric or perchloric acids4-e; (3) measurement of the pH or conductivity change of a relatively small volume of liquid, caused by displacement of hydrogen ions from the ionic double layer by anions'; (4) direct determination by quantitative analysis of anions lost from solution*; and (5) direct determination of anion surface concentration by means of radioactive tracer~.~-l' The results reported for any particular anion vary with surface preparation, pH, potential, etc. One fairly clear-cut conclusion is that the halide ions specifically adsorb, with the extent of adsorption increasing as one descends down the series to the higher atomic weights. Perchlorate ion is assumed to adsorb very slightly or not a t all. Recently, Breiterl3 made measurements of the capacity of a smooth platinum wire in perchlorichydrochloric acid solutions under the conditions of periodically varying potential. This method served to bring the electrode to a more reproducible surface state than generally accomplished in the nonvoltamThe Journal of Physical Chemistry
metric measurements of his predecessors. One disadvantage of the method is that it suffers from the inability to assign quantitative interpretation to the measured capacities. The purpose of this study was to develop methods for the study of ion adsorption along the same lines as previously applied to Cold-16 and "oxygen adsorp(1) This work was made possible by the support of the Advanced Research Projects Agency (Order No. 247) through the U. 8. Army Engineer Research and Development Laboratories under contract No. DA-44-009-ENG-4909. (2) (a) B. Ershler, Acta Physicochim. U R S S , 7, 327 (1937); (b) Zh. Fzz. Khim., 13, 1092 (1939). (3) B. Ershler, G. Deborin, and A. Frumkin, Acta Physicochim. URSS, 8, 565 (1938). (4) A. Obrucheva, Zh. Fio. Khim., 32, 2155 (1958). (5) A. Obrucheva, Dokl. Akad. Nauk S S S R , 141, 1463 (1961). (6) A. Obrucheva, ibid., 142, 859 (1962). (7) A. Shlygin, A. Frumkin, and V. Medvedovsky, Acta Physicochim. U R S S , 4, 911 (1936). (8) I. Kolthoff and T. Kameda, J. A m . Chem. SOC.,51, 2888 (1929). (9) N. Balaschova, Electrochim. Acta, 7, 559 (1962). (10) N. Balaschova, 2. p h y s i k . Chem. (Leipzig), A207, 340 (1957). (11) K. Schwabe, Electrochim. Acta, 6, 223 (1962). (12) A. N. Frumkin, "Advances in Electrochemistry and Electrochemical Engineering," Vol. 3, John Wiley and Sons, Inc., New York, N. Y., 1963, Chapter 5. (13) M.W. Breiter, Electrochim. Acta, 8 , 925 (1963). (14) S. Gilman, J . Phys. Chem., 66,2657 (1962). (15) S. Gilman, ibid., 67, 78 (1963). (16) S. Gilman, ibid., 67, 1898 (1963).
ANIOTADSORPTION O N PLATIKUM BY MULTIPCLSE POTEXTIODYXAJZIC METHOD
tion.”l7 Such a “multipulse potentiodynaniic” approach has the advantage, when successfully applied, of providing simple initial boundary conditions and allowing for quantitative interpretation of data. TWO anions of different mature were chosen for this study. Chloride is highly adsorbed and is an oxidizable ion. Phosphate is relatively slightly adsorbed and is nonoxidizable. The study resulted in development of the desired quantitative methods for measurement of anion adsorption. ‘These methods permit the measureinen t of kinetics of adsorption and desorption, and determination of equilibrium surface coverages as a function of potential. Part I of this series will deal niainly with the developnieiit of methods and with the study of adsorption kinetics. Part I1 will deal mainly with deterinination of equilibrium surface coverages. The solutions studied were 1 N in perchloric acid and relatively dilute in hydrochloric or phosphoric acid.
2099
L
0.4 v C
Sweep Speedv
D
.
I.@
Experimental Equipment and Chemicals. The electronic equipment, glassware, arid electrodes have been described previ0us1y.l~ The working electrode was a length of C.P. platinum wire of 0.064 geometric area. The saturation hydrogen coverage, QSH, determined as previously described15 was 0.272 mcoul~nib/cni.~, suggesting a roughness factor of approxiinately 1.3. Q’H was not found to vary appreciably during the ineasurenients, suggesting little or no etching of the surface in the dilute solutions used under pulsed conditions. The working electrode was treated with hot chromic acid cleaning solution for several minutes and thoroughly washed in distilled water before each day of experimentation. Solutions were made with permanganate-treated, triply distilled water and A4.R.grade perchloric, hydrochloric, and phosphoric acids. All measurements were made in a bath therinostated to 30’. All potentials are referred to a reversible hydrogen electrode in 1 N perchloric acid. Graphical areas were measured using an Ott polar planimeter. Procedure. The potential functions employed are diagramed in Fig. 1. The procedure during each step of each sequence i s suniinarized in Table I along with the significance of the procedure. All HC1 solutions were made using a stock solution of 1 N perchloric acid.
Results and Discussion I . Establishment of a Reproducible Surface CondiThe general requirements of a surface pretreatment are : (1) removal of previouslg adsorbed species, including impurities originally dissolved in the electrolyte; ( 2 ) protection (blocking) of the surface against the readsorption of inaterial while the solution tion.
I
I.0V
1.2v
n I l lfv. D
I
,
r
l
r
L
(fl TIME
-
U.lV
E
*
Figure 1. Potential-time sequences applied to the working electrode.
near the surface is beiug brought into concentration equilibrium with the bulk; (3) rapid removal of the “surface protection” of (a),so that adsorption inay commence froin essentially zero time. These rcquireiiieiits are met for a solution of 1 N perchloric acid of ordinary purity16 by means of sequeiice I of Table I. The criteria previously employed for a reproducibly clean surface are constancy of (1) saturation hydrogeii coverage, Q s ~ ~ , 1 5 ,(127) “oxygen adsorption,” Q 0 , 1 5 , 1 7 and (3) differential electrode capacity.” According to these criteria the surface is reproducibly clean (to within a few per cent of fractioiial surface) for 10 nisec. < T , < 10 sec. in an uizstirred solution without prcelectrolysis, and for longer periods of time employing pre-electrolysis with an auxiliary electrode. It is not possible to use sequence I of Table I for the study of (17) S.Gilman, Electrochim. Acta, 9, 1025 (1964).
Volume 68, Sumber 8
August, 1964
S. GILMAN
2100
Table I : Procedures Followed during Potential Sequences Sequence no.
I
I1
Fig. no.
la
lb
Step no., refers t o Fig.
Procedure
Purpose
A
(1) Bubble argon through solution with paddle stirring for 15 sec.
(1) To remove previously adsorbed species from the electrode surface and to deposit passive “oxygen” film which hinders readsorption
B
(2) Bubbling and stirring continued for 0.5 min. Solution allowed to become quiescent for 1.5 additional min.
(2) To retain passive film deposited during step A, while sweeping away rejected impurities and molecular oxygen generated in step A. Final quiescence of solution limits mass transport to linear diffusion ( a t small values of time)
C
(3) The solution is unstirred. chosen of duration T ,
Step C is
(3) The passive film is largely reduced during the first few milliseconds, exposing a reproducibly clean surface
D
( 4 ) Apply linear cathodic pulse of sweep speed v. hfeasure current-time trace
( 4 ) T o measure the charge corresponding to hydrogen deposition for any time, T ,
A
(1) Bubble argon through solution with paddle stirring for 2 sec.
(1) To remove previously adsorbed species from the surface (other than large quantities of anions), and to deposit “passive oxygen” on the surface which protects surface against readsorption
B
(2) Continue bubbling and stirring for 15 sec.
(2) T o retain passive film while moleciilar oxygen and other impurities from step ( A ) are swept away and diluted. The potential is too low for discharge of chloride ions
C
(3) Bubble and stir for additional 0.5 min. Allow solution t o become quiescent for 1.5 min.
(3) The passive film is largely reduced during the first few msec. when U is chosen less than 0 4 v. At sufficiently low values of U (less than -0.06 v.) anioris are desorbed and swept away while stirring. Also deposited hydrogen serves to block the surface against adsorption. Allowing the solution to become quiescent limits the rate of mass transport to ordinary diffusion
D
( 4 ) Step D is of 10-see. duration
(4) AdRorbed hydrogen is removed within the first few msec. Dissolved hydrogen is depleted to some small calculable level
I11
IC
E
( 5 ) Apply cathodic linear sweep of speed v = 30 v./sec., and measure resulting current-time trace
( 5 ) T o measure the charge corresponding t o hydrogen deposition under these conditions of electrode pretreatment
A
(1) Same as II(1)
( 1 ) Same as 11( 1)
B
(2) Same as II(2)
(2) Same as II(2)
C
(3) Bubbling and stirring continued for 0 . 5 min. Allow solution to become quiescent for 1.5 min.
(3) Passive oxygen film is reduced and adsorbed chloride is desorbed. A monolayer of adsorbed hydro-
(4)Step D is of IO-sec. duration
(4) The concentration of dissolved hydrogen adjacent
D
gen helps blork the surface against the adsorption of impurities. The potential is too low for the adsorption of the anions under study to the surface is gradually reduced toward a value corresponding to equilibrium with 0.01 atm. of hydrogen. This makes the hydrogen oxidation current negligible in the next step
E
( 5 ) Apply linear anodic sweep of speed v, and measure ciirrent-time trace
The Journal of Phusical Chemistrz
( 5 ) QL?,O and hence the amount of anion adsorbed may be measured from the traces obtained
2101
ANIOXADSORPTION ON PLATIKUM BY NULTIPULSE POTENTIODYSAMIC METHOD
Table I
(Continued)
Sequence no.
Fig. no.
Step no., refers t o Fig.
IV
Id
A-D
(1)-(4) Same as for sequence I11
(1)-(4) Same as for sequence I11
E
( 5 ) Potential step of duration TE
( 5 ) To allow anions to adsorb a t potential U under well-defined conditions
F
(6) Apply anodic linear sweep of speed ZJ, and measurle resulting current--time trace
(6) The current-time traces may be interpreted t o
(1)-(3) Same as for sequence I11
(1)-(3) Same as for sequence I11
D
(4) Solution umstirred. duration
(4) To adsorb anions corresponding to the maximum surface coverage for 0.8 v.
E
(5) Step E applied for time interval T E
V
le
A-C
Purpose
Procedure
Step of 10-sec.
reveal the extent of adsorption a t potential U during preceding step
(5) To test possible oxidation or repulsion of the anions adsorbed a t 0.8 v.
VI
If
F
(6) Step F applied for 10 ms’ec.
(6) To sufficiently reduce “oxygen” deposited in the previous step so that no correction need be made for “oxygen” in step G
G
( 7 ) Apply step G with u = 1000 v./sec. hfeasure the resulting current-time trace
( 7 ) To determine if step E has exerted an effect on the trace obtained during step G
(1)-(4) Same as 111, steps 1-4
(1)-(4) Same as 111, steps 1-4
(5) Step E applied and left “on” indefinitely. The resulting current-time trace is recorded in the presence and in the absence of dissolved chloride
(5) To measure the flow of current which accompanies adsorption of chloride a t 0.7 v.
A-D E
anion adsorption since considerable surface coverage with the anion will result during step C due to the positive potential involved. Therefore, a modified schenie had to be developed. In sequence I1 of Table I, a brief treatnient a t 1.8 Y. serves to clean oxidizable impurities off the surface while minimizing the extent of chloride discharge. Step B serves to transport away and dilute any oxygen or chlorine (in the presence of HCI) produced during step A. The potential of step B is too low for additional release of either oxygen or chlorine. In step C, the potential is chosen so that eCl- (fractional surface coverage with specifically adsorbed chloride) is zero and so that adsorbed atomic hydrogen serves as a protective film against impurity adsorption while the solution is vigorously stirred for equilization of solution concentrations To determine the efficacy of steps A-C of sequence TI, the potential was next brought to 0.4 v. in step D. At this potential, adsorbed hydrogen is immediately renioved from the surface, and adjacent dissolved hydrogen is gradually depleted. In step E, a cathodic linear potential sweep was applied and the ch:uge corresponding to hydrogen deposition &E was measured in the usual way.15 The values of &H obtained in this manner for different
values of U in 1 N perchloric acid (absence of Cl-) are compared with Q p H (obtained by ineans of sequence 1 T, = 10 nisec.) in Table 11. It is seen that QH has a value within 2% of Q’H if U 0.02 v., and decreases sharply above this potciitial. The larger values of OH obtained are ascribed to the action of the (iiearly) coniplete hydrogen layer and perhaps the negative potential in preventing adsorption of impurities froni solution. It should be noted that when potential U of sequence I1 is taken as 0.4 v., we have a sequence identical with I with T , = 130 sec. with intensive stirring during the
10 sec., with no stirring, under these conditions. If “oxygen adsorption” is measured by substituting an anodic linear sweep for the negative sweep of step E, no variations of Qo (area under the “oxygen adsorption” curve) is observed for L7 6 0.02 v. While pre-electrolysis was found to increase the period of constancy of Q S ~ l 5by sequence I, Table I, it was actually found to have the reverse effect when sequence I1 was used. This is probably due to conversion of original impurities to impurities which have a greater tendency to adsorb a t potentials below 0.4 v. (step C of sequence 11). Hence, pre-elecbrolysis was not further employed in this study. I I . Chloride Adsorption and Discharge during a n Anodic Linear Potential-Time Sweep. Sequence I11 of Table I was used in this study. (Steps A, €3, and C were adjusted to the potentials and duration which led to a reproducible surface state in section I. Step D was introduced to minimize the concentration of dissolved hydrogen during the next step.) The concentration of HC1 in 1 N perchloric acid, was varied from 0 to 0.1 N for any value of the sweep speed, v. A concentration sequence for D = 40 v., sec. appears in Fig. 2. Traces for 0.1 N HC1 and v = 4 and 40 v./sec. appear in Fig. 3. In Fig. 2 for zero HCI concentration, we obtain the usual current-potential trace for platinum in 1 N perchloric acid. l8 Excluding double-layer charging, the currents (and area in millicoulombs) lying to the left of 0.4 v. are due to oxidation of adsorbed hydrogen and hydrogen dissolved in the solution adjacent to the electrode. The currents (and area) to the right of 0.8 v. are due to “oxygen adsorption” (and finally, oxygen evolution) on the surface. As the concentration of HC1 is increased, both the hydrogen and oxygen regions are affected. These ill be discussed separately. A . Hydrogen Region. The effect of increasing C1concentration on the hydrogen region is to make the curve shift towards the left and to increase the size of the maxima. Such qualitative effects have already been observed in experiments employing periodic potential-time sweeps13 and in the measurement of charging curves. l 2 The hydrogen maxima are generally regarded as an expression of surface heterogeneity.12 The shifts in the hydrogen maxima with the adsorption of anions are generally regarded as due to changes in the heat of adsorption of hydrogm.12 The largest effects upon the hydrogen region in Fig. 2 are observed for 0.01 N HC1. The equilibrium surface concentration of adsorbed chloride depends only on the potential at fixed concentration and the rate is determined by mass t r a n ~ p o r t . ’ ~Further the dependence of The Journal of Physical Chemistry
0 N HCI 10-3 10-2 10-1
I.0
2.0
U, VOLTS Figure 2. Anodic current-potential traces obtained for HC1 solutions in 1 .V perchloric acid using sequence 111, Table I ; sweep speed, v = 40 v./sec.
1
inn
TRACE I-v=4v/sec., s - I TRACE 2-v =40u/sec., s- 5
U, VOLTS Figure 3. Anodic current-potential traces obtained for 0.1 N HC1 in 1 N perchloric acid, using sequence 111, Table I.
surface coverage on concentration of the solution is ~ 1 i g h t . l ~Hence, the significance of traces 3 and 4 is that approximately equilibrium surface coverage is achieved a t these concentrations (0.01 and 0.1 N HC1) whereas it is not for trace 1 (0.001 N HC1). At equilibrium, OCl- = 0 at U = 0.06 v.,19 which is the potential a t which these anodic sweeps were initiated. An important question is whether the total amount of hydrogen deposited on the surface a t U = 0.06 v. and reiiioved during the sweep is affccted by chloride adsorption. To investigate this possibility, the anodic traces for v = 40 and 400 v./sec. were ~
(18) F. G. Will and C. A. Knorr, 2. Elektrochem., 64, 258 (1960). (19) S. Gilman, part I1 of this series, J . Phys. Chem, 6 8 , 2112 (1964).
ANIONADSORPTION ON PLATINUM BY MULTIPCLSE POTENTIODYNAMIC METHOD
Table 111: Charge,
Passed in the Region 0.06 0.7 v . of the Current-Potential Curve (see Fig. 2) 71,
&E’,
6
U
6
v./sec.,
step E , sequence 111,
Table I
40.0 40.0 40.0 400 400 400 400
QH’,
Concn. HCl, N
0 0.0100 0,100
0 0,001.00 0.0100 0,100
mcoulombJ cm.8
0 , 3 2 4 Av. = 0.325, 0,326 ‘ a v . deviation = A0.3Yo 0.324 0.312 Av. = 0.311, 0.304 av. deviation = &1.2% 0.312 0.316
integrated from 0.06 to 0.7 v. The upper potential was chosen large so as to include any possible overlap of hydrogen into the “double-layer region.” Values of total charge Q H ’ are ;summarized in ‘Table 111. From the table, we see that, QH‘ remains constant to within an average deviation of 0.3% at 40 v./sec., and to within an average deviation of 1.2% at 400 v./sec. The quantity Q H ’ includes charge corresponding to the oxidation of hydrogen deposited at 0.06 v. and to the charging of the ionic double layer.12,1z1It appears that Q H ’ also includes some charge corresponding to a change in valence of the surface (see ElectionVI). This latter quantity can contribute a maximum of approxi‘mately 8% to Q H ’ and must be compensated for by combined decreases in the electrode capacitance and in the amount of hydrogen adsorbed at 0.06 v. for QH’ to reniain constant. Hence, the amount of hydrogen deposited at 0.06 v. must stay constant to within 8% as the concentration of dissolved chloride increases. B . “Oxygen Adsorption” Region. The iiiaxiiiiuiii values of Bcl- obtained for potentials above 0.8 v. are approximately the equilibrium values for 0.8 v. l 9 Smaller values of ecl- are determined by the rate of mass t r a n ~ p o r t . ‘ ~Hence, for traces 3 and 4 of Fig. 2 we might judge that the similarity of the traces in the oxygen adsorption region are due to maxiiiiuiii values of & - . Trace 2 differs from traces 3 and 4 because of the smaller Bcl- values obtained. We see that for trace 2, total oxidation current up to 1.6 v. is noticeably decreased, compared with trace 1 (no chloride). Retardation of platinum surface oxidation by chloride was previously observed by Ershler.2u Traces 1 and 2 merge in the high-current region where oxygen is evolved. Ipor traces 3 and 4, we see that although the surface oxidation current is even more drastically reduced below approxiiiiately 1.3 v., additional current is passed at higher potentials. Traces 1 and 2 of Pig. 3 reveal that this additional current reaches a maximuiii
2103
-
a t -1.5 v. for v = 4 v./sec., and obviously corresponds to the evolution of molecular chlorine (E, = 1.36).21 From v = 4 v./sec. (trace 1, Fig. 3) to v = 40 v./sec. (trace 2, Fig. 3)) the peak current rises only as the square root of 8 rather than 10, as predicted for linear diffusion under conditions of continuously varying potential, 22 Also the calculated peak currents are only 0.6 of the calculated maximum value, indicating partial kinetic control of the chloride discharge reaction under these conditions. In summary, we see that the effect of dissolved chloride ion on the anodic current potential curves described in sections A and B above is: (1) chloride adsorbs in the hydrogen region and affects the shape of the curve, but not the total amount of hydrogeii deposited (to within 8y0); (2) chloride adsorbs on the surface and decreases the aniount of “oxygen” deposited, &,; (3) chloride is oxidized to chlorine and provides additional charge, QCl,, ab potentials above 1.3 v. The scheme employed in the reiiiaiiider of this study is to increase the sweep speed v, relative to the concentration of HC1 so that QCl2 becomes negligible compared with AQo. Then AQo is used as a measure of Bel-. III. Measurevaent of Fractional Surface Coverage with Chloride I o n through Measurement of AQ,‘. We saw in section IIB that during an anodic sweep, adsorbed chloride tended to cause a decrease in surface oxidation charge, AQ,, and to provide additional charge Qcl, due to chlorine evolution. The experinieiits described above are further complicated by the fact that adsorption occurs over the eiitire range of poteiitials covered by the potential sweep rather than at fixed potential. Hence, iuodificatioii of the procedure is required in order to obtain more quantitative inforination on chloride adsorption. This is accomplished by means of sequence IV of Table I, using a solution of N HC1 in 1 N perchloric acid. In step C of sequence IV, the surface is completely bare of C1-. This is established by noting that the same current-time trace (for higher values of v ) may be obtained during step F both in the presence and absence of dissolved chloride (see also ref. 19). During step D, dissolved hydrogen is depleted so that the hydrogen oxidation current will be negligible during step I?. In step E, the potential is raised to I: = 0.7 v., and adsorption of chloride is allowed to occur at coiistaiit potential either for 10 sec. (resulting in the equilibriuni value of Ocl- for this potential) or for 10 msec. (ecl- = -0). (20) B. Ershler, Z h . F i z . Khim., 14, 357 (1040) (21) N. A. Lange, “Handbook of Chemistry,:’ 10th Ed., 3IcGrawHill Book Co., New York, N. Y ~1081. , (22) P. Delahny, “New Iiistruinental AIethods in Electrochemistry,”
Interscience Publishers, Inc., New York, N. Y.,1954.
2104
S. GILMAN
Finally sweep F is applied and the current-time trace recorded. If v is sufficiently large, additional adsorption or discharge of chloride during the sweep will be negligible compared with the charge equivalent of Ocl for U = 0.7 v. To determine a niinimuin suitable range for v, let us assume semi-infinite linear diffusion of C1- to the surface. Then the rate of transport of C1to the surface, expressed as a current, I ~ Iwill - , beza I C I - = nFADcl-”zCcl-/a‘/2 t ‘/z (1) and the corresponding charge QCl- is Qcl-
=
$d Icl- dt = 2nFADcl-’/2C~~-t’/’/n’/a (2)
where n = number of electrons involved in the process following transport, F = Faraday constant, A = electrode area in c n 2 = 0.064, Dcl- = diffusion constant of C1- = 2.03 X cm.2/sec. (25’ value CCI- = concentration of C1- in m ~ l e s / c m . ~t , = diffusion time in sec. Choosing Ccl- = mole/ ~ m . t ~=, TF (duration of sweep F, sequence IV), and n = 1 (chloride discharge reaction), then the maximum charge due to chlorine evolution during sweep F, QCI, is, QCI,=
Qcl- =
+ Ago” +
AQo
100
0
E H ’
50
v = 90v /sec.
0.05T~‘/‘mcoulomb/cm.2 (3)
Assuming that Q’E = 0.3 mcoulomb/cm.2 is equivalent to a (hypothetical) full monolayer of chloride ions, we might attempt to limit Qc1, to 0.003 mcoulomb/ cin2,and correspondingly limit T Fto 4 msec. Figure 4 shows the results obtained for v = 9, 90, and 900 v./sec. The top trace for each value of v is obtained for T E = 10 msec. (virtually no adsorption of C1- at 0.7 v.) and the bottom trace for T E = 10 sec. The hatched area between traces will be called AQo’. Then
Ago’ =
E
0
QCI,
+ QcI,’ +
AQmp
I
I
IO00
5 00
(4) 1.0
where AQcap = the difference in capacitive charge included under the upper and lower traces of Fig. 4; AQo = dirnuiiition of Q0 caused by chloride adsorbed a t 0.7 Table IV : Frequency Dependence of AQo’ AQo‘, U,
mooulomb/
?=./sea
om.2
1800 900 180 90 36 18 9
The Journal of Physical Chemistry
0.228 0,226 0 I222 0.224 0.224 0.238 0.238
TF S
2.0
m sec.
Figure 4. “Oxygen adsorption” traces obtained in the presence of N HC1 in 1 iV perchloric acid, using sequence I V , Table I. The upper traces were obtained for an adsorption time, T E , of 10 msec. The lower traces were obtained for T E = 10 sec. The hatched areas are values of AQo‘.
v . ; A&,” - additional diniunition of Qo caused by chloride adsorbed during sweep F; Qcl, = charge ~~
(23) H. A. Laitinen and I. M.Kolthoff, J . Am. Chem. Soc., 61, 3344 (1939). (24) R. Parsons, “Handbook of Electrochemical Constants,” Butterworth and Co. Ltd., London, 1959.
ANIONADSORPTIONOK PLATINUM
2105
B Y n/IULTIPULSE POTENTIODYKAMIC METHOD
corresponding to discharge of chloride transported to the electrode during sweep F; QcI,’ = charge corresponding to discharge during sweep F of chloride adsorbed at U = 0.7 v. It will be demonstrated below that QcI,’ = 0. Therefore, as v is increased AQ,’ ought to be expected to approach AQ, AQcap = AQ, since all the remaining terms depend on mass transport. Table I V summarizes the results for a wide range of values of the sweep speed, v. At v = 180 v./sec., the duration of the sweep is approximately 4 msec., which eq. 3 suggested for virtual elimination of transport considerations. However, we see that AQ,’ remains constant to within 1% average deviation down to v = 36 v./sec. ( T F = *30 msec.) and does not increase greatly down to v = 9 v./sec. This large range of constancy (where A&,’ -, AQ,) is likely due in part to two factors. (1) Adsorption during step F occurs during both the sweep corresponding to T E = 10 msec. and 10 sec. Since AQ,’ is given by the digerenee in area under these sweeps, the adsorptions tend to cancel each other. (2) The partial surface oxidation which occurs during the early part of sweep F tends to repress further adsorption of chloride during the remainder of the sweep and hence effectively reduces T F . The results suggest that AQ, is a quantity which may be precisely measured over a large range of values of v. I V . Kinetics of Adsorption of Chloride I o n from Solwtion at Constant Potential. In section 111, it was shown that adsorption of chloride ion at 01.7 v. caused a decrease in “oxygen adsorption” as measured by application of a subsequent anodic potential-time sweep of sufficient speed v. By means of sequence IV of Table I it is possible to measure AQ, as a function of adsorption time, T Eat potential U . A solution of N HCI in 1 N perchloric acid was chosen since a t this concentration of chloride adsorption is virtually complete within 100 msec., allowing us t o assume the conditions of semiinfinite linear diffusion even for our wire electrode. A sweep speed v = 1000 v./sec. was chosen. The 1-msec. duration of this sweep provides an effective maximuin uncertainty of less than 1 msec. on the time scale, and becomes negligible (because of dependence on the square root of time) after the first few msec. A sequence of current-time traces for various values of T E appear in Fig. 5 . AQ, for any value of T E is taken as the area enclosed by the trace for that value of T E ,by the trace for T E = 0 (measured in absence of chloride ion), and the dashed line. A plot of AQ, us. TEi” appears in Fig. 6 where excellent linearity is apparent. If we assume that AQ, is determined by mass transport, \hen AQO = Qci(5)
+
5- i; .,loo0
I
I.o
,m
T,
2.0
sec
Figure 5. Sequential “oxygen adsorption” traces for 10-3 N HC1 in 1 iV perchloric acid a t U = 0.7 v.; traces obtained by means of sequence IV, Table I. AQo for any value of TE is obtained by measurement of the area bounded by trace 1, the dashed line, and the trace corresponding to T E .
I
/
a 4
3 E
0
A 0 0 FOR ADSORPTION OF CHLORIDE FROM IO-’ N HCI, IN HCIOI SOUTION
U
A00 FOR ADSORPTION OF PHOSPHATE FRMl 10-‘M H, PO,, IN HClO, SOLUTION AT U . 0 . 8 ~
0”
AT U:07v
TE ”1
, $11:
Figure 6. Analysis of the charge-time data obtained in the adsorption of chloride and phosphate from solution a t constant potential.
where Q C I - has been defined by eq. 2. Taking “t” of eq. 2 equal to T E and n = 2 (one equivalent of chloride pfkveiits the “adsorption” of two equivalents of “oxygen”) we obtain a linear plot of AQ, us. Ti/=which exactly fits the experimental points of Fig. 6 up to high values of AQ,. The results establish the measurement of AQ, as a precise and linear measure of the amount of chloride ions adsorbed. This is further supported by the constancy of A&, with varying sweep speed (section 111). Since the adsorption is diffusion controlled even iii the several msec. range, the rate constant for the adsorption may not be obtained. It may merely be said to exceed the usual limitation of the multipulse potentiodynamic niethod,16 Le., 1 cni./sec. AQo reaches a Volume 68, Number’8 August, 1964
S. GILMAN
2106
plateau value of 0.267 mcouloinb/cm.2, equivalent to the adsorption of 0.134 nicoulomb/cni.2 of chloride ion. Since the results presented above demonstrate a simple relationship between the aniount of chloride adsorbed and the “adsorbed oxygen” film, it is in principle possible to determine the structure of the chloride adlayer froin a knowledge of the “oxide” stoichiometry, and the extent of a full chloride monolayer. The latter inforination iiiay not be obtained since “oxygen adsorption” limits chloride adsorption in the high potential range.lQ The “oxide” stoichiometry is still a matter of controversy. V . T h e Anodic Current Associated with Chloride Adsorption. The idea that chloride ion is specifically adsorbed on platinum, implicit in the work of previous investigations,’-’’ is distinct from the suggestion recently made by Peters and LinganeZ5”that a platinum chloride filii1 is formed on the surface in hydrochloric acid solutions. The latter concept calls for the flow of oxidative charge (1 faraday of electricity/equivalent of chloride “adsorbed”). The structure of the two different species obtained in the absence and presence of an oxidative process inay be generalized by
soc1sm+cl-
(A) (B)
where So and S m f are platinum surface sites with the indicated valences. If S is taken as one-half a platinum atom and in is taken as one, we would have PtClz stoichiometry, but not necessarily PtC1, structure. The direct test of whether structure A or B results upon “adsorption”2jb of chloride ions is to measure the flow of oxidative current during the adsorption, since only structure B requires the flow of noncapacitive current. Peters and LiiiganeZ5& have reported that they do obtain chroiiopoteiitiogralms indicating a Faradaic process between 0.55 and 0.85 v. (against a hydrogen electrode). The arrcst is obtained only after employing a particular pretreatment sequence. On the other hand, BreiterIY did not observe a wave corresponding to platinum chloride formation, upon application of a continuous triangular potential-time sweep to his electrode. In this work, as in Breiter’sI3 no discrete platinum chloride wave was observed (see Fig. 2) under the condition of nieasurenient of chloride ion adsorption. Hence, if the surface does undergo valence change during or after the adsorption step at potentials lower than 0.8 v. and during the application of a linear anodic potential-time sweep, the corresponding current must be sinall and largely compensated for by a diniunition of the charge stored on the surface as hydrogen and/or in The Journal of Physical Chemistry
the ionic double layer as capacitive charge. To eliniinate such compensating effects and to seek out a possible small oxidative current during adsorption of chloride ions, sequence VI of Table I was employed. This sequence employs our knowledge of the rate of chloride ion adsorption and of the maximum surface coverage with chloride ion a t 0.7 v., to measure the small oxidative current which flows during the adsorption. At the end of step D of sequence VI of Table I, the concentration of dissolved inolecular hydrogen near the electrode surface corresponds to equilibrium with 0.01 atm. of hydrogen (Sernst equation). Assuming applicability of Henry’s law, the effective concentration of hydrogen, CHz,is one-hundredth of the value a t 1 atni. or 7.6 X m o l e / ~ m . ~In . ~ step ~ E, the electrode potential is raised to 0.7 v. and in the presence of chloride ion, five processes are possible : (1) oxidation of previously adsorbed hydrogen; (2) oxidation of dissolved molecular hydrogen; (3) adsorption of chloride ions accompanied by electron transfer ; (4) dissolution of platinum; ( 5 ) discharge of the ionic double layer. Steps 2 and 3 are diffusion controlled. The current corresponding to step 2 is given by an equation similar to eq. 1. Taking C E ~= 7.6 X mole/cm.a, D H = ~ 4.1 X low5cm.2/sec.,26and n = 2, we obtained for the diffusion current of molecular hydrogen
IHp
=
0.0053T~-”~ ma./cmSz
(6)
At the beginning of step E, the current which flows in the first 0.6 nisec. both in the presence and absence of chloride ion is orders of magnitude larger than predicted by eq. 6 and hence corresponds mainly to the oxidation of previously adsorbed hydrogen. The area under the curve up to T E = 1msec. both in the presence and absence of dissolved chloride has the value 0.31 incouloinb/cni.2, which is close to the value obtained for Q H in section IIA when 2, = 400 v./sec. (Table 11). In the absence of dissolved chloride, the current in the I-100-msec. range approaches the value given by eq. 6 and therefore corresponds entirely to the oxidation of niolecular hydrogen. In the presence of dissolved HC1, electrode process 3-5 are possible in addition to (1) and (2) and we do indeed observe a “hump” in the current-time trace over a range of €IC1 concentrations (25) (a) D. G. Peters and J. J. Lingane, J . Electroanal. Chem., 4, 193 (1962). (b) Adsorption in quotation marks is taken to signify the possibility of a buildup of a surface excess of ions with the passage
of Faradaic current, so that a surface chloride is formed. The quotation marks will be dropped henceforth except for emphasis and the term will be used t o signify a surface excess of anions with or without the passage of Faradaic current. (26) G. Tammann and V. Jessen, 2. anorg. allgem. Chem., 179, 125 (1929).
ANIONADSORPTION ON
2107
PLATINUM BY h/IULTIPVLSE POTEKTIODYNAIMIC :\/lETI-IOD
(Fig. 7). The differences in current, A I , between traces 1 and 2 of Fig. 7a-d are plotted in Fig. 8 against T-I”. If “n” is taken as in eq. 1, the resulting value of I C I -is the current which would flow if two equivalents of chloride ion “adsorbed” resulted in the flow of 1faraday of oxidative charge. A plot of I C -I us. T-‘/’ appears in Fig. 8. It is observed that A I tends to approach Icl- especially a t the higher concentrations. I n the paragraphs below, we will present arguments to support the conclusion that A I is a gross oxidative current corresponding to a change in valence of the surface platinum atoms (electrode process 3). The sign and magnitude of the capacitive current which might flow during the adsorption of chloride ions a t 0.7 v. must be estimated in order to gauge its contribution to AT. The total charge &, stored in the ionic double layer may be expressed in terms of the differential capacity C, (7)
c-
C: 2 x
1 0 - 3 ~HCI
;iL’
0.5
G
N HCI
0.5
2
~ 0~ 3 ~ L 1HCI0 - 3 L~
C : ~ x I O - ~HCI N
,
IO
5
2.5
IO
T,
50
, in sec
Figure 7 . Transient current-time traces obtained a t 0.7 v. in the presence (upper traces) and in the absence (lower traces) of dissolved chloride. Sequence VI, Table I, was employed.
where C, is the differential capacity for any fixed value of 8 ~ 1 and is a function of U , U o is the value of U at the zero point of charge and depends 019 8cl-. Then
- 10’ --- Zrio-’
NHCl
IC,- ASSUMING
- - 3r10-’
n:
(EQUATION I I
QxIO-’
20
o
IO-’
n
2110-~
A
SrIO-’
NHCl
A I FROM
FIG r p
-id
v 4x10-’
where I , is the capaciiive current whtch flows a t constant potential due to adsorption of chloride ions. Experimental plots of C, os. U are available13,17 for Bel- = 0. Similar capacitance-potentia1 plots for constant & I - > 0 are neither available nor easily derived. However, a p1otI3 of capacitance us. potential where 8 ~ 1 -increases with potential reveals that the differential capacitance drops with increase of ecl- a t any value of U > -0.4 v. Hence, IC must be negative, and the experimental values of A I obtained from Fig. 7 must be gross oxidative currents. It may be the negative capacitive current which is responsible for the smaller (than diffusion-controlled) values of A I at short times. This would tend to give the current-tiime traces of Fig. 7 the appearance of a nzaximum a t short time. The total negative charge released by the double layer upon adsorption of the equilibrium aniount of chloride ions is given by
where “a” and “b” are for corresponding zero points of charge for the two different surface states and are assumed approximately equal. If we cholose the values of capacity reported by Breiter13for zero and N HCI
Figure 8. Analysis of the anodic current measured during adsorption of chloride ion a t 0.7 v.
solutions in 1 N HC104, integration of the difference in capacities to a potential of 0.7 v. should give us a minimum value of AQ, [since the values of Bel- in Breiter’s experiments do not correspond to the (larger) equilibrium value for 0.7 v.]. The value of AQc so obtained is 0.016 mcoulomb/cm.2. The integration of the charge between traces 1 and 2 of Fig. 7a-d results iii a value of 0.027 mcoulomb/cm.2 = J A I d t . The sum of the minimum estimated value of AQc and J AIdt is 0.04 mcoulomb/cm. z. This figure approaches the value of charge which would be passed if one electron were passed through the external circuit for each two chloride ions adsorbed, or Q,/4 = 0.06 mcoulonib/cm.2. Volume 68, Number 8
August, 19FQ
S.GILMAN
2108
Hence, it is likely that the capacitive current can account for soiiie of the deviations of the experimental points of Fig, 8 froin the linear diffusion plot of Icl-. We have thus far established that during the adsorption of chloride ioii at 0.7 v., an oxidative current flows corresponding approximately to 0.5 faraday/equivalent of chloride ion adsorbed. This flow of current must correspond to either electrode process 3 or 4. If dissolutioii of platinum (process 4) did occur, we would not expect the retardation of oxygen adsorption to follow h e a r diffusion laws precisely as observed in section IV. That dissolution is not a general problem when measureiiient is made in dilute HCl solution is iiot surprising. ErshlerZ7ineasured dissolution rates which did not exceed a few tenths of a milliampere on a 30 cin.2 electrode in an HC1 solution as concentrated as 0.1 N . Having ruled out the other possibilities, we are led to the coiiclusioii that the current which flows at 0.7 v. during the adsorption of chloride ions corresponds to forination of less than a iiioiiolayer of material with an average stoichiometry of SC12- where the platinum surface site “8” has an effective valeiice of + l . In a sense, this resembles adsorption of chloride as a coniplex ion. One may next inquire whether soine such electron transfer occurs over the entire range of chloride adsorption, or siiiiply ensues at soine potential less than 0.7 v., as a reaction between previously adsorbed chloride and the zerovalent surface. Since the total amount of chloride “adsorbed” depends on the potential,lg studies a t niuch lower potentials would involve even smaller passage of oxidative charge and becoine increasingly inore difficult to follom. Studies of the oxidative current at potentials larger than 0.7 v., would involve correction for surface oxidation and again would become more difficult. Such extensions of this work must likely await improved knowledge of the necessary doublelayer charging correction. V I . Further Consideration of the “Platznuna Chloride” W a v e . As already indicated, Peters and Liiig a i i ~obtaiiwd ~ ~ ~ a distinct cliroiiopoteiitiograi~i for platinum in 1 N HCI in the potential range 0.55-0.85 (against reversible hydrogen) which they ascribed to the formation of PtCl2. (No actual nieasuremeiit of the ratio of chlorine to platinum was made however.) I n this work, evidence was also found for a valence change hi the surface platinum atoms a t 0.7 v., which signifies that a t least a t this potential electron transfer accoinpanies ion adsorption. However, no distinct “chloride wave” appears in the voltainmetric traces of Pig. 2 or in similar experiments by B r e i t ~ r , although ’~ the “adsorption” of chloride is clearly detected in both cases. To help resolve the anomaly, sequence IV of T h e JoiirnaZ
of
Phwsical Chemistry
Table I was employed with v = 4 v./sec.; the conceiitration of HC1 was 0.1 N . To duplicate the coiiditioiis of Peters and Iliiigaiie,26aU was chosen at 0.246 v. (potential of the saturated caloinel electrode). When the electrode was held at 0.246 v. for T E = 10 sec., the trace obtained (trace 1 of Fig. 9) was similar to traces 3 and 4 of Fig. 2 , exhibiting no discernible “chloride wave.” When T E was chosen as 15 niiii. with the solution stirred with a paddle stirrer, a wave (trace 2 of Fig. 9) did appear beginning a t -0.45 v. (maximum a t 0.75 v.) and must correspond to the Peters and Lingane wave. The surface state of the recently pretreated electrode of trace 1 is usually referred to as “active” and that of trace 2 as L1inactive.JJ1z A qualitative explanation of the two diff ereiit voltammetric behaviors may be made on that basis.
U. VOLTS Figure 9. Anodic current-potential traces obtained for 0.1 A‘ HC1 in 1 N HClOa solution using sequence I\-, Table I, v = 4 v./sec. The initial potential, U = 0.246 v. Trace 1, U held for T E = 10 sec.; trace 2, U held for T E = 15 min.
(1) I n trace 1 of Fig. 8 the current nieasured in the potential range between approximately 0.3 and 0.95 v. corresponds to a slight amount of hydrogen atom discharge, double-layer charging, and electron transfer involving chloride ions. These charges are all sinall (compared with a nionolayer of hydrogen), and merge, resulting in an almost constant current, and no obvious “wave.” ( 2 ) In trace 2 of Fig. 9, the “inactive” electrode has no hydrogen adsorbed on its surface (note the appearance of a sinall hydrogen peak a t 0.25 v. in trace 1, and its absence in trace 2). Further, the capacitance of the “inactive” electrode is quite small” as is verified by observing that the current which flows between 0.25 and 0.45 v. of trace 2 is extremely small. Therefore, (27) B. Ershler, Acta Physicochim. U R S S , 19, 139 (1944)
2109
ANIONADSORPTIOX O N PLATINUM EIY MULTIPULSE POTENTIODYNAMIC METHOD
no appreciable current flows until the “chloride wave” commences at 0.45 v. While adsorption of chloride coniniences from 0.1125 v. on the “active” electrode,lgit is possible that no adsorption of chloride occurs on the “inactive” electrode below 0.45 v., with or without the passage of charge. Gilman15g17has presented evidence that the several manifestations of platinuni surface “inactivity” (reduction in capacitance, decrease in hydrogen adsorption, higher overvoltage for oxygen “adsorption”) niay be correlated with adsorption of impurities froiii the solution during the first 1000 sec. following electrode pretreatment. Further work would be required to test the same hypothesis on the system involving chloride ions. V I I . Nonoxidizability of Preadsorbed Chloride at Elevated Potentials. By nieans of sequence V, Table I, chloride was adsorbed a t 0.8 v. from a M solution of HC1 in 1 N perchloric acid (step D). The potential was then brought to 1.6 v. for a period of time, TE. The “adsorbed oxygen” 011 the surface was next reduced by means of a 10-mseo. pulse at 0.4 v. (step F). When sweep G (v = 1000 v./sec.) was applied, the same trace was obtained for TE = 0 or 10 sec., and the coiistant value of AQo ’was 0.19 mcoulc~mb/cnr.2. This result suggests no iioticeable oxidation of the preadsorbed chloride ions for TE:= 1 sec., subject to the following possibilities for readsorption of chloride ion during step F: (1) dissolved chloride may be adsorbed from the l o r 4 II// HCI solution; (2) chlorine gas produced by dlscharge of dissolved chloride ions during step E may be reduced back to dissolved chloride ions during step F and readsorbed; (3) chlorine gas generated by discharge of adsorbed chloride ions during step E may be reduced back to dissolved chloride ions during step F and readsorbed. VVe may immediately discard complications froin possibilities (1) and ( 2 ) by noting that the total contribution of dissolved chloride and chlorine froiii these possibilities niay not exceed that of equiv.11. of HCl (the initial HCl concentration). From eq. 5 and 2 , such a concentration of HC1 can contribute only 0.01 nicoulon~b/cm.2to &,, which is small compared to the observed value of AQo retained after step E. &’or possibility (3), if all of the adsorbed chloride ions were to bc discharged early during step E, the resulting chlorine would tend to diffuse away during the remainder of the 10-sec. step. During the 10iiisec. duratioii of step li‘, only a portion of the chlorine would have sufficient time to diffuse back to the surface where it could be ineduced and readsorbed. The observed coiistaiicy of AQ, for T E = 0 arid 10 sec., argues against any such rapid discharge of adsorbed chloride ions during step E. This confirms the conclusion (al-
ready proposed in section 111) that no appreciable oxidation of previously adsorbed chloride ions occurs during a fast linear anodic sweep (which is always chosen of duration much smaller than 1 sec.). When a very small amount of chloride ions was preadsorbed froin a N HC1 solution, some removal of the chloride a t 1.8 v. was observed after 15 sec., suggesting some very small rate of removal for sniall values of chloride surface coverage. It is possible that this effect might involve an extremely slow rate of platinum dissolution, rather than simple discharge of adsorbed chloride ions. The tendency of an adsorbed halide ion to resist discharge as the halogen might in general be ascribed to stabilization by the very forces that bind it to the surface. The evidence for actual change in valence of the surface a t a potential of 0.7 v. (presented in section V) suggests that a t potentials this great, or greater, a chloride-like species is formed on the surface. Hence, it is somewhat intuitive that simple oxidation of the chloride would no longer be possible. Since “adsorbed” chloride is found not to discharge a t potentials above the value of E , for the chloride-chlorine couple (1.36 v.), the generation of molecular chlorine may not proceed through the mechanism
8
+ C1-
+ e-
(dissolved) +SCL-
2SC12-
--+
X
+ Clz + e-
(“adsorption”)
(discharge)
(I)
but may proceed by either of two alternative paths
XC12- (“adsorbed” chloride ions) 2C1- (dissolved) --+ XClz-
+
+ Clz + 2e-
(11) (discharge of dissolved chloride ions on “adsorbed” chloride) or
S
+ 2C1-
(dissolved) +X
+ Clz + 2e-
(111)
(discharge of dissolved chloride on a “vacant” site) Arbitrary reaction orders where chosen for paths I1 and 111. It is well known12that the adsorbability of iodide ions on platiiiuin is even greater than that of chloride ions. Osteryoung, et a1.,28failed to detect the presence of adsorbed iodide on a platinum electrode by oxadation of iodide during application of an aiiodic linear sweep. This result need not be taken, as they ~uggests,~g as evidence that iodide does not adsorb. An alternative explanation which does not contradict the previous (28) R. A. Osteryoung, G. Lauer, and F. C . Anson, J . Electrochem. Soc., 110, 926 (1963).
Volume 88, n’umber 8
August. 1984
21 10
S. GILMAN
literatureI2 is that the iodide which was already adsorbed before application of each anodic pulse could not be reiiioved during the anodic sweep as is the case for chloride ions. A reduction in the ainouiit of oxygen “adsorbed” a t higher potentials than used in their experiiiieiits would be anticipated on the basis of experience with the chloride system. V I I I . Kinetzcs of Adsorption of Phosphoric Acid f r o m Perchloric Acid Solution. Experiments similar to those described for chloride ion in section IV were perforined for a solution of lop4 M phosphoric acid in 1 N perchloric acid. b’ of sequence IV was chosen at 0.8 v. Typical traces appear in Fig. 10. The maxiniui-ii value of AQo, produced by phosphoric acid adsorption is approximately an order of magnitude less than that produced by chloride at 0.7 v., hence extreiiie reproducibility of the “oxygen adsorption” trace is required. This was accomplished by recalibration of thc oscilloscope before each individual measureineiit and by iiieasuriiig the reduction in oxygen adsorption for each subsequent t h e relative to the value obtained for TE = 10 insec. Resulting values of AQo are plotted against T E ~ in ” Fig. 6.
750
c
N
TRACE I - TE- IO m sec TRACE 2- TE = 50 m sec T R A C E 3-TE: IO sec
L 0.5 1.0 T,
,m
sec
Figure 10. Sequential “oxygen adsorption” traces for 10-4 M phosphoric acid in 1 A: perchloric acid at G = 0.8 v. Traces were obtained by use of sequence IT’, Table I.
The plot is linear as was the case for chloride adsorption and agaiii suggests diffusion control. For phosphoric acid, we can assuine that it is undissociated acid which is transported to the electrode because of the strong acid coiiditioiis, and that conversion to the ionic form occurs either during or just preceding the adsorption step. We iiiay then write an expression for AQo siinilar to eq. 5 The Journal of Physical Chemistrg
AQo QH,PO~
= QH,PO,
(10)
may be expressed by an equation similar to eq. 2 QH,PO~
=
~ ~ F A D H , P O , ~ / ~ Cr~L ,I a~ ~(1,1)T E ~ / ~ /
where, as before, n is the number of equivalents of “oxygen” retarded by the adsorption of 1 mole of ions. We have no previous knowledge of whether the ion adsorbs in the dihydrogeii phosphate, nioiiohydrogeii phosphate, or phosphate form. Let us assuine that it is the valence of the anion which determines the surface bond and hence the extent of retardation of oxygen “adsorption.” Then since n has the value 2 for the singly-charged chloride ioii, the corresponding value of n for the three different phosphate species might be n = 2, 4 , and 6, respectively. I-Ieiice, if we knew the diffusion constant for undissociated phosphoric acid under these conditions, we could deterniiiie n from the slope of the plot of Fig. 10. Unfortunately, the required diffusion constant is not known. However, the slope of the plot corresponds exactly to the value obtained using the diffusion constant for chloride ion and n = 2 . To get the same result for n = 4 we would have to assume a diff usioii constant for phosphoric acid l,’4 that for chloride ion, which seenis unlikely. Hence, the available evidence suggests that phosphoric acid adsorbs as the dihydrogen phosphate ion. We have already seen that the gross anodic current which flows during the adsorption of chloride ion is equivalent to less than one-half the calculated diffusion current. Hence, the total charge passed is less than A.&,/4. For the very sniall aniounts of phosphate ioii adsorbed and the resulting sniall values of AQo measured, it would, at this time, be extremely difficult to measure a possible oxidative current.
Summary (1) A pretreatment potential sequence has been devised for use in solutions containing adsorbable anioiis. The pretreatment results in an extreinely reproducible electrode surface with zero initial surface concentration of the anion. ( 2 ) Adsorption of chloride or phosphate ions causes a decrease iii “oxygen adsorption,” AQo, during ai1 aiiodic linear potential-time swcep. AQo is constant over a wide range of sweep speeds and is directly proportional to the aiizouiit of anions adsorbed. (29) After submission of this paper for publication, Osteryoung and Anson presented iodide radiotracer data at a Gordon Conference (February, 1964). These data lead them to conclude that iodide is indeed adsorbed during the application of an anodic linear sweep, but is not “electroactive.” This seems to confirm the similarity of t,he chloride and iodide systems.
ANIONADSORPTION OK PLATINUM BY MULTIPULSE POTENTIODYNAMIC METHOD
(3) The adsorption kinetics of chloride and phosphate ions is so rapid as to be diffusion controlled even in the millisecond range of observation. (4) The addition of chloride ion to perchloric acid does not perceptibly alter the am.ount of hydrogen which may be deposited on the surface a t 0.06 v. ( 5 ) Adsorbed chloride ion is resistant to discharge as molecular chlorine at potentials well above the reversible chloride-chlorine potential both during the application of an anodic linear sweep, and when held at constant high potential. Phosphate ion is also not desorbed a t high potentials.
2111
(6) The oxidative current which flows during the adsorption of chloride ion at 0.7 v. appears to correspond to the formation of a surface complex ion. It is such stabilization of the anion which may possibly best explain the retardation of oxygen “adsorption” and of anion discharge.
Acknowledgment. The author is pleased to acknowledge the helpful assistance of R. W. Kopp and J. T. Adamchick. Thanks are extended to Drs. D. Vermilyea and M. W. Breiter for helpful discussions.
Volume 68, Number 8
August, 1964
