Surface Solubility and Reaction Inhibition in Lead Bromide and Iodide Adsorbed on Mercury Electrodes H a r v e y B. H e r m a n , ' R . L. M c N e e l y , 2 P. S ~ r a n a C. , ~ M. Elliott, a n d R o y c e W . M u r r a y The Wiiliam R . Kenan. J r . . Laboratories of Chemistry, University of North Carolina, Chapel Hill, N . C . 27574
Adsorptions of Pb(ll) on mercury from aqueous iodide or bromide solutions plotted against [ X - ] exhibit sharp increases from modest to very high surface excesses. The solution condition producing the discontinuity for each halide is describable by [ P b 2 + ] [ X - l 2 = constant. This effect, not observed in fluoride solutions, is inconsistent with simple anion induced adsorption and is interpreted as a two-dimensional precipitation forming the surface phase PbX2. Chronopotentiometric measurements of the inhibiting properties of adsorbed PbX2 on H g ( l l ) reduction and Hg(O), Cd(O), and Zn(0) oxidation are presented to show that the inhibition is the same for electrodeward or solutionward passage through the adsorbed layer. The sudden appearance of the inhibition with increasing lead concentration is independent of the reaction being inhibited and appears to originate in a structural or crystal-perfecting change in the PbX2 surface phase.
Data on the adsorption, on mercury electrodes, of white metal cations from aqueous solutions of simple anionic ligands is now fairly extensive. We have available surface excess data for the adsorptions of Cd(I1) (I, 2), Hg(I1) ( 3 ) , and Pb(I1) ( 3 4) from iodide medium; Cd(I1) (2), Hg(I1) ( 3 ) , and Pb(I1) ( 3 ) from bromide medium; Pb(I1) ( 5 , 6) from chloride medium, Cd(I1) ( 7 ) , Hg(I1) (3, 8 ) , Pb(I1) (3, 4 ) , In(II1) (9). and Zn(I1) (6, 10) from thiocyanate medium, Cd(I1) ( 2 1 ) from thiosulfate medium, and Cd(I1) (12) and Zn(I1) (12) from azide medium. Accumulation of these data has been materially aided by development of the double potential step chronocoulometric technique (13) as used with minicomputer control and data acquisition (141, the current approach of choice. In 1968, Anson and Barclay ( 2 ) advanced a model for Present address, D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of N o r t h C a r o l i n a , Greensboro, S C. Present address. D e p a r t m e n t of C h e m i s t r y . U n i v e r s i t y of T e n nessee, Chattanooga, T e n n Present address. V e r o n a D i v i s i o n , B a y c h e m Corporation. Charleston. S.C
G W. O'Dom and R. W. Murray. A n a l Chem 39, 51 (1967) F. C. Anson and D. J. Barclay, Anai Chem.. 40, 1791 (1968) R. W. Murray and D. J. Gross, Anal Chem . 38, 392 (1966). J. H . Christie and F. C. Anson, Calif. Inst. Tech.. Pasadena, Calif., unpublished data, 1965. M . Sluyters-Rehbach, J. S. M. C. Breukel, K. A. Gijsbertsen, C. A. Wijnhorst. and J. H . Sluyters, J . Eiectroanai Chem 38, 17 (1972) D. J . Barclay and F. C. Anson, J . Electroanal C h e m . 28, 71 ( 1970). F. C. Anson, J. H . Christie, and R. A. Osteryoung. J . Electroanal Chem . 13. 343 (1967) F. C. Anson and D. A. Payne. J . Electroanal Chem.. 1 3 , 35 (1967). G W O'Dom and R. W. Murray, J Eiectroanai. Cbem.. 16. 327 (1968) R. A. Osteryoung and J. H . Christie, J . Phys. C h e m . 71, 1348 (19671. D. J Barclay and F. C. Anson, J . Electrochem Soc.. 116. 438 (1969) 2 . Kowalski and F. C. Anson, J Eiectrochem S o c . 116, 1208 (19691. J. H Christie. R . A. Osteryoung, and F. C. Anson. J . Electroanai Chem., 13. 236 (1967). G. Lauer. R. Abel. and F C. Anson, Anai. Chem.. 39, 765 (1967).
1258
the white metal cation adsorptions. In this anion-induced adsorption model, a complex of the metal cation, generally the neutral form ( e . g . , CdIZ), becomes bound to one or more anionic ligands already adsorbed on the electrode, thereby increasing its coordination number. The metal complex adsorption is induced and primarily controlled by the quantity of already adsorbed ligands-e.g., these act as adsorption sites. This model appears to be experimentally well justified and is now generally accepted for most white metal cation adsorptions. The work reported here was begun several years ago with the suspicion that the particular cases of Pb(I1) adsorption from iodide and bromide media were not adequately explained by the anion-induced adsorption model. These adsorptions, first detected in experiments with dropping mercury electrodes (15-18), had been quantitatively characterized using chronocoulometric (3, 4) and chronopotentiometric ( 3 ) experiments over a fairly limited range of solution conditions. Their distinctive features were twofold. First, the lead surface excesses, 8.2 x 10-IO and 9.1 x mole/cm2, measured in 1M iodide and 1M bromide containing 0.2mM and 0.5mM Pb(II), respectively, appear to be limiting coverages and are considerably larger than any of the other metal adsorptions. A geometrical estimation (3) of the limiting monolayer coverage of iodide and bromide ions, 9 x and 11 x mole/cm2, shows, in fact, that the Pb(I1) halide adsorption nears that imposed solely by the size of the halide ion. An adsorbed layer of close-packed X-Pb-X units, with the Pb-X bond axis perpendicular to the mercury surface, inferred from this, finds support from other reports (18, 19) which also suggest a 2 : l halide:Pb(II) surface stoichiometry. Second, and in apparent reflection of the tight packing which must exist in the adsorbed Pb(I1) halide layers, the lead adsorption is capable of severely inhibiting the electrode reactions of other species (20, 21). A particularly interesting feature of the inhibition of Hg(I1) reduction by adsorbed Pb(II) bromide was a discontinuous dependency on solution [Pb(II)]; alteration from a non-inhibited Hg(I1) reduction to a totally inhibited one occurred over a very narrow [Pb(II)]interval. The anion-induced adsorption model does not lead one to expect either of the above observations. Further. a recent report ( 5 ) on adsorption of a relative, Pb(I1) chloride, presented surface excess and capacitance data which are also a t odds with the anion-induced adsorption picture.
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 9 . A U G U S T 1974
V . Klemencic and I . Filipovic, Croat. Chem Acta. 31, 29 (1959): Chem Abstr.. 54, 9557a (1960). G . C. Barker, "Faradaic Rectification.' in Transactions of Symposium on Electrode Processes, E. Yeager, Ed., Wiley. New York, N.Y., 1961, p 325 Y. S Srinivasan and A. K . Sundaram, Aust. J . C h e m , 1 5 . 729 (1962). G. C. Barker and J A. Bolzan, Fresenrus Z Anal Chem . 216. 215 (1966). I . Filipovic, M . Tkalcec, B Mayer, and I . Piljac, Croat Chem. Acta. 41, 145 (1969). D. J. Gross and R . W. Murray, Anal Chem , 38, 405 (1966). R. W Murrayand R. L. McNeely, Anal. Chem.. 39. 1661 (1967).
Part I of this report will deal with further double potential step chronocoulometric measurements of Pb(1I) surface excess ( r p b ) in iodide and bromide solutions. We should note here that suggestions have appeared in the literature that surface solubility (19, 22) or formation of a two-dimensional crystal phase (3, 21) might be important. Description of an overall adsorption isotherm was, thus, an experimental goal. Part I1 of this report will further characterize the surface-insulating properties of the adsorbed Pb(I1) layers through experiments on inhibition of mercury reduction and of Cd(0) and Zn(0) amalgam oxidations.
RESULTS F O R LEAD ADSORPTION
LEAD SURFACE EXCESS MEASUREMENTSPART I: EXPERIMENTAL Cells a n d Chemicals. The hanging mercury drop electrode (HMDE) was a microburet type (Brinkmann Instruments) and a typical area was 0.04 cm.2 Solutions were prepared from water distilled, passed through carbon and ion exchange columns, and redistilled. Solutions contained recrystallized KNOs in concentrations giving ~r= 1 with the [KX] under study, and HC104 to p H -13. Iodide solutions were prepared under nitrogen to avoid air oxidation. The beaker cell was thermostated to 25 "C. All potentials are reported L'S. SCE. Technique a n d Apparatus. Measurement of the Pb(I1) surface excess, I', on a HMDE equilibrated for 1-2 minutes with a wellstirred solution containing Pb(I1) and halide at a fixed potential (E,,,,,) was accomplished with the double potential step chronocoulometric technique (7, 23, 1 4 ) . In this technique, the working electrode potential is stepped from E,,,,, to a potential on the plateau of the Pb(I1) reduction wave, Efillal,for a time T , and then stepped back to Ei,,,t and charge-time measurements continued for an equal period of time. The pertinent theory for the forward and reverse charge-time transients is ( 1 3 )
Q(t
= 7) -
Q(t
>
As an aside, we should note a curious artifact observed during test experiments with Pb(I1) in K N 0 3 (and K F ) . If the potential step is imposed on the HMDE immediately after extrusion into an unstirred solution, apparent negative r of up to 5 /.LCcm can be measured. The error disappears if the HMDE rests in stirred solution for a few minutes before applying the potential step, and was traced to a constant cathodic current background component appearing after T (absent for t < 0) in the former experiment. The faradaic background artifact is most pronounced for the most highly purified electrolytes and for high metal concentration (10mM); it is apparently suppressed by adsorption of a few traces of impurity during the normal stirred pre-equilibration.
7)=
where ( 3 ~ 1 ,is the difference in double layer charge between E,,,,, and Efl,,al,0 = ( t - 7 ) 1 ' 2 il 2 - tl *, Qc = 2nFA Cob ( D O T / 7)' 2 , and QO and a1 are constants having values near zero and unity, respectively, which result from the accurate linearization (13) of a more complex rigorous equation to the simpler form of Equation 2 . Equations 1 and 2 show that I' can be obtained from the difference in the zero time intercepts Qf and Q r of plots of Q ( t < T ) us. t1 and Q(t = T ) - Q ( t > 7 ) u s . 8 , respectively, using the relation
+
(3) The double potential step chronocoulometric experiment was implemented using a computer-controlled data acquisition system consisting of a laboratory terminal remote to a Raytheon Model 706 computer. This system generates the potential step waveforms and other experimental switching functions, acquires data (12-bit ADC) a t a maximum 33-KHz rate, performs immediate off-line least-squares computations of slopes and intercepts according to Equations 1 and 2 , and displays TTY results a t the remote terminal. Hardware and software details are described elsewhere (23, 2 4 ) . An evaluation was conducted at an early implementation stage (and periodically since) of the computer-remote terminal-control potentiostat-cell ensemble using Cd(I1) and Pb(I1) in 1M K N 0 3 as model, non-adsorbing examples. Data for zero time intercept uncertainty in these reactions, given in Table I, compare favorably with that of reference 14, and the apparent adsorptions show that real adsorptions of 2-3 X 10-12 mole/cm2 could be detected with the system. ( 2 2 ) A . M . Bond and G . Hefter,J. Eiectroanai Chem . 42, 1 (1973). (23) T. H . Ridgway. P h . D . Thesis, University of North Carolina, Chapel Hill, N.C., 1970. (24) W . S. Woodward, T . H. Ridgway, and C. N. Reilley, A n a / . Chern.. 46, 1151 (1974)
Of principal interest in the lead adsorptions from bromide and iodide solutions were the surface excesses a t much lower halide concentrations than those employed earlier ( 3 ) .Results for double potential step chronocoulometric lead surface excesses at low halide concentrations are shown in Tables I1 and 111. The dependency of surface excess on the equilibrating potential (E,,,,,) was also examined; the narrowness of the potential window between mercury oxidation and the lead wave severely limited this exploration in iodide solutions. The surface excess data are averages of a t least triplicate runs and the tables also give values for diffusion coefficients, D , obtained from Equation 1. We consider the constancy of D values as useful measures of overall experimental consistency. Isotherms prepared from the data of Tables I1 and I11 are shown as Figures 1 and 2 . At low concentrations in both halides, the r p b increases as El,,{, is made more positive, as would be anticipated in anion-induced adsorption. As halide concentration is increased, however, a rather precipitous increase in r f > b occurs. The halide concentration a t which this discontinuity in the adsorption appears is a function of both Elllitand [Pb2+],moving to lower halide concentration as E,,,,, becomes more positive or as [Pb2+] increases. The adsorption rises to surface excess values slightly lower than those measured earlier in 1M halide (3). Figure 3 shows the adsorption isotherm for bromide medium with closely spaced data in the region of the discontinuity. These data demonstrate that the discontinuity is much sharper than could be discerned from the coarse data spacing of Figures 1 and 2 and, in fact, occurs over a 1-270 change in bromide concentration. Sluyters-Rehbach e t al. ( 5 ) have recently described chronocoulometric surface excess data for lead adsorption from chloride medium which parallel our results in bromide and iodide except that the isotherm discontinuity was displayed in terms of a potential dependency a t a single [Clkl]. In all three halides. the surface excess of lead a t the top of the isotherm discontinuity far exceeds the surface excess of the halide which would exist in the absence of lead. In lOOmM bromide alone, for example, 17f3r is approximately 3.4 X and 2.4 X mole/cm2 a t -0.2 and -0.3 volts, respectively ( 2 ) ,less than the plateau r p b by a factor of about 3. As noted by Sluyters-Rehbach e t al. (4, this circumstance is inconsistent with the anion-induced model ( 2 ) , a t least for adsorptions occurring a t or beyond the r P b discontinuity. We believe that the key to at least an incipient understanding of this new type of adsorption lies in the dependency of the lead adsorption on halide concentration and in the manner in which this is affected by lead concentration. We wish to adopt the premise suggested earlier (20) that the large surface excesses for lead reflect the formation of a two-dimensional structurally ordered and dense Dhase a t the electrode-solution interface. This phase ap-
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974
1259
Table I. Double Potential Step Chronocoulometry on Model Reactions in 1.OM KNOl E stepa
Metal
0.4mM Cd O.1mM Pb 0 . 4 m M Pb
-0.4 -0.3 -0.3 -0.3 -0.2 -0.1
to - 0 . 9
12.31 15.12 14.95 14.58 17.85 19.90
to - 0 . 9 to - 0 . 9 to - 0 . 9 to - 0 . 9 to -0.9
' 100 data points each for forward and back steps; T
i 0.33 i 0.06
i f i f
-0.19 -0.02 -0.01 -0.07 0.15 -0.06
0.12 0.18 0.14 0.28
= 100 msec for Pb, 20 msec for Cd.
No. of expts
D X 106, cm2 sec-1
2 F r , PC cm-2
Q D L ~ ,PC cm-2
i 0.15
7.92 9.75 9.14 8.97 9.13 8.91
0.06 0.04 0.06 0.08 i 0.07 f f f f
+ f f f f f
0.13 0.14 0.06 0.04 0.07 0.34
8 3 4 4 3 5
Difference in double layer charge between Einit and Eyinai.
Table 11. Double Potential Step Chronocoulometric Surface Excess for Lead Adsorption from Aqueous Bromide" rpb
[Pbp-I, mM
0.1 1.0
10
[Br-I, mM E l n l t =
1.0 100 1.0 5.0 50 70 90 100 120 200 1000 10 30 40 50
-0 3
0.014 0.66 i 0.055 0.078 1.33 i 2.00 f 2.65 i 2.78 i 9.25 i 9.20 i 8.75 i 1.13 2.60 i 5.65 i 8.18 +
X
1030,
-0.25
molelcm2
-0
2
-0
15
-0
1
D X 106 cm*/sec
... 0.01
0.72 i 0.01
f 0.04
i 0.08
1.69 2.48 9.29 8.90 8.95 8.83 0.99 7.88 7.42 7.64
i 0.43
3 . 8 2 i 0.49 6.80 i 0.24 8.59 f 0 . 3 3
0.03 0.05 0.03 0.01 0.16 0.17 0.13
1.53 2.38 2.98 9.10 9.08 9.07
0.50 0.70 0.45
Hg electrode, KNO? electrolyte t o
@ =
0.82 f 0.01
=t
0.04
f 0.02 i 0.03 =t 0 . 0 1
i 0.06
1.0, Eilnni = -0.9
V,
T
=t
41 =t
i i
1.85 2.71 8.27 8.60
0.04 0.13 0.07 0.12 0.04
f i f f
i 0.41 i 0.27 i 0.36
0.03 0.06 0.08 0.04
1.90 2.70 7.97 8.14
i f i f
0.08 0.05 0.16 0.03
6.84 i 0.53 7 . 1 8 i 0.27
8.24 8.24 8.30 7.93 8.29 8.22 8.13 8.44 7.52 8.48 8.49 8.76 8.68
f i i i i i i i i i i i i
0.13 0.10 0.11 0.17 0.34 0.36 0.57 0.21 0.15 0.23 0.18 0.13 0.18
= 100 msec, 200 data points.
Table 111. Double Potential Step Chronocoulometric Surface Excess for Lead Adsorption from Aqueous Iodide" P p b X 10'0, mole/cm*
Pb2-1, mM
0.1
0.4 1.0
a
11-1, m M Elnit =
1.0 2.0 3.5 4.0 4.25 4.5 5.0 10 0.5 1.0 5.0 1.0 1.5 2.0 3.0 4.0
-0.35
0 . 5 3 i 0.01 1.10 i 0.02 1 . 7 0 =t 0 . 0 4
8.32 i 0.08 8.32 i 0.04 1.18 i 0 . 0 1
7.68 i 0.06
-0.25
-0.3
0.59 1.01 1.40 1.37 4.41 7.24 8.77 7.84 0.58 1.00 7.15 1.89 7.42 7.13 7.19 7.19
0.03 0.02 0.01 0.06 0.86 0.03 i 0.07 i 0.09 i 0.04 =t 0 . 0 1 i 0.01 i 0.05 i 0.07 i 0.07 i 0.09 i 0.03 i i i i i i
-0.2
0.61 0.68 f 0.02
0.87 f 0.01
1 . 2 8 i 0.16 1.13 f 0.04
D X 108, cm2/sec
9.00 8.62 8.62 8.34
f i i f
0.26 0.35 0.23 0.18
8.04 9.77 7.88 9.19 9.00
i i f i i
0.81 0.94 1.05 0.16 0.16
8.27 8.05 8.15 8.13 8.43
f 0.12 i: 0 . 2 5
f 0.11 i 0.18 i 0.20
Hg electrode, KNOa electrolyte to 1 = 1.0, Eilnnl= -0.9V, T = 100 msec, 200 data points.
pears suddenly, a t the adsorption isotherm discontinuity, via a surface precipitation. The surface crystal resulting is a two-dimensional entity, in equilibrium with the solution, and does not increase in depth as would be the case for a crystal in a solution of constituent ions in concentrations exceeding Kqp. (In no case are the solutions of T a bles I1 and I11 saturated with respect to conventional precipitation of PbXz. Also r p b on the isotherm plateau does not change when the equilibration time is increased from 2 to 10 minutes.) If the surface precipitation premise has merit, then one must inspect description of its occurrence using a surface 1260
solubility product. Using the halide concentration at the midpoint of the isotherm discontinuities of Figures 1-3, Table IV shows the results for surface K s u . Considering the appreciable uncertainty in the critical halide concentrations, the constancy of the four sets of K,, values over an order-of-magnitude change in [Pbz+] is striking. We believe that these results confirm formation of a surface phase of composition PbXz. Sluyters-Rehbach e t al. ( 5 ) detected a drop in the differential double layer capacitance as potential was made more positive than the isotherm discontinuity for Pb(I1) in chloride. Figure 4 shows data for Q D I , , obtained from the
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 9 , A U G U S T 1974
lOmM Pb
IO
I
I
I
I
I
I
I
I
I
I
I
20
x)
40
50
60
70
80
90
I00
110
120
Figure 1. Surface excess for lead adsorption from bromide ValuesofE,,,t
are -0.3 ( O ) ,-0.25 ( O )-, 0.2(0), -0.15(1)
IO c
0 ImMPb IrnMPa
4 r
70
90
80
I00 Br'
1-
Figure 3.
,mM
Figure 2. Surface excess for lead adsorption from Iodide Value of E,,,,(
IS
I I0
,mM
Surface excess for lead adsorption from bromide
1 OmM Pb Values of E I n l tare - 0 2 5 ( 0 ) , - 0 2(x)
-0 3 [Br-: , m M IO0
Table IV. Lead and Halide Concentrations Inducing Surface Precipitation Pbl, mM
Bromide
Iodide a
[X-I, mM
1
112
10 1 1 10 1 1 10
40
0.1 1
996 98
E,,,t
Surface, KqPa
-0.3
1.2 x 10-5 1.6 x 10-5 0.98 X 0.96 x 10-5 1 . 2 x 10-5 0.79 x 10-5 0.69 x 10-5 0 . 4 8 x 10-5 1.9 x 10-9 1 . 7x 10-9
-0.25
35
8gh
-0.2
83
22 4.35 1.3
-0.3
200
KSp= [ P b * + ][X-12. * F r o m Figure 3; other data are from Figures 1 and 10
2.
double potential step experiment (see Equations 1 and 2) for the solutions of Tables I1 and 111. The data in the presence of Pb(I1) adsorption are of modest (-20%) accuracy, owing to the difficulty of measuring QDI. when r is very large, and so some modest alteration in the double layer charge (qm) at El,,ltcannot be ruled out. The extensive desorption of halide found by Sluyters-Rehbach for the chloride case does not, however, seem to occur within the range of the chronocoulometric data for bromide and iodide. In view of the massive value of r P b , this is a surprising result of which we are sufficiently skeptical to avoid drawing conclusions.
Figure 4.
Values of ODL (differenceof charge at € i n i t and -0.9 and bromide (- - - - - - -) zero ( e ) ,0.1 mM(o), 1 mM ( m ) . Arrows represent adsorp-
volt) in iodide (-)
[ P b ( l l ) ] is tion isotherm break points
The conventional solubility products for PbBrz and PbIz are 3.9 x 10-5 and 7.1 x respectively ( 2 5 ) .By comparison to Table IV, the solubility of PbXz a t the electrode-solution interface is seen to be decreased by a factor (25) T. Bjerrum, G. Schwarzenback, and L. Sillen, "Stabiiity Constants." The Chemical Society, London. 1958
A N A L Y T I C A L C H E M I S T R Y . V O L . 46, NO. 9. A U G U S T 1974
1261
24
tors. There is no evidence, for instance, that the crystal “structure” of adsorbed PbXz, and corresponding lattice energy, is even remotely similar to that of ordinary solid PbXz. The situation cries for “more data.” We have accordingly considered other cases of sparingly soluble metal salts in a search for repetitions of isotherms such as found for the lead halides. We have determined that the Tl(1) halides (Cl-, Br-) also exhibit isotherm discontinuities conforming to solubility products. A description of the properties of the T1X surface crystals forms the basis of another report (26).
A
INHIBITION STUDIES-PART 11: EXPERIMENTAL I
l
l
1
I
I
I
I
I
I
I
I
I
I
I
I
0 IO 20 30 40 50 60 70 80 90 100110 120130140 CURRENT,pA
Figure 5.
Chronopotentiornetric constant for 1.25rnM
1.OM NaBr in the presence of [ P b ( l l ) ] = zero ( A ) , ( B ) , 0 . 1 7 5 rnM ( C ) , 0.187 rnM ( D ) . 0.20 rnM (E)
H g ( I I)
in
0.15 rnM
Table V. Double Potential Step Chronocoulometry of Lead in Aqueous Fluoridea rpb X
[F-I, mM Elnlt =
0 10 20 30 40 80b
-0
1
-0.11 0.84 4.62 0.95 0.47 1.39
lo’?, mole
cm2
-0 2
-0.87 -0.73 -0.69 0.10 -1.04
...
-0
3
2.22 4.33 1.32 0.78 3.8 2.9
Hg electrode, 1 M KNOB,Eilnni= - 0.9 V, 7 = 100 msec, 200 data points, [Pb*+]= l.OmM, no replicate measurements. Visible precipitation in solution. a
*
of 2 to 4 ~ The . origin of this solubility decrease is a fundamental question to which we have no definitive answers a t present. A factor which could conceivably play a role is the steep potential gradient at the electrode-solution interface. Another factor, classical in theoretical considerations of solubility. is the relation of a crystal’s surface energy to its solubility. A reduction of surface energy yields a more stable crystal and a reduced solubility. A two-dimensional crystal has a large surface energy in that crystal surface area is maximized, but one half of the PbXz crystal surface contacts the mercury electrode. If the energy of the contacting surface is lowered through specific bonding interactions between mercury metal and the halide ions, a connection would exist between surface solubility and the specific surface adsorption properties of the halide ion involved. The fluoride ion is known t o not exhibit specific adsorption at a mercury surface, and it happens that the solubility of PbFz is of the same magnitude as that of PbBr2. If formation of a surface crystal bears no relation to anion adsorbability a t all, then PbF2 might exhibit behavior analogous to that of PbBrp and PbI2. We have conducted chronocoulometric measurements of rPbover a range of [F-] encompassing the onset of precipitation of PbFz from bulk solution. The results, shown in Table V, indicate no significant lead adsorption, and clearly no adsorption discontinuity of the type found for the other halides. We stress that while the existence of a lowered surface solubility seems clear, our discussion of its origin is speculation and may omit other unrecognized important fac1262
Apparatus and Chemicals. Chronopotentiograms were obtained using operational amplifier circuits of conventional design, mercury(I1) reduction and mercury(0) oxidation inhibition measurements were carried out in a thermostated beaker cell; those on amalgam oxidations used an ambient-temperature Teflon cell designed for closed-system exchange of cell solution without disturbing the HMDE. Electrolyte solutions were 1.OM in reagent grade NaI or NaBr and 0.01M in HC101, for which Pb(I1) solubility is 1.4 and 12mM, respectively. Procedure. Inhibition of Hg(I1) reduction was characterized using cathodic chronopotentiometric transition times a t a HMDE equilibrated for a few minutes with a Hg(I1)-Pb(I1)-halide solution. Hg(0) oxidation inhibition was detected from the potentialtime response to an anodic current step a t a HMDE pre-equilibrated with a Pb(I1)-halide solution. In the latter experiment, subsequent current-reversal reduction of the electrogenerated Hg(I1) provided a secondary and equivalent test for Hg(I1) reduction inhibition. Inhibition of Cd(0) or Zn(0) amalgam oxidation by adsorbed Pb(I1) halide was studied by anodic chronopotentiometry a t a Cd(0)-Pb(0) or Zn(0)-Pb(0) mixed amalgam HMDE in metalfree halide solution. The amalgam HMDE was prepared by sequential constant current reduction from a solution of Pb(II), thorough flushing of the cell with degassed metal-free halide solution, introduction of and constant current reduction from a solution of the test metal ion, final flushing of t h e cell with degassed metal-free halide solution. and a brief burst of cathodic current in the final halide solution to remove any trace metal oxide contamination. Detection of oxidation inhibition was through the Pb(0) transition time, theory for which is given in a later section. MERCURY INHIBITION
Cathodic. In 0.1M NaI or NaBr solutions containing Hg(I1) and 0.5mM Pb(II), the normal cathodic chronopotentiometric wave for Hg(I1) is obliterated; Hg(I1) reduction does not proceed until potentials are attained which result in reduction of the interfering adsorbed Pb(I1) halide. Only a small fraction of the adsorbed lead need be reduced to remove the inhibition. These observations were reported as Figures 4,5, and 9 of Reference 20. Results of more detailed experiments in bromide medium are given in Figure 5. In the absence of all extra-diffusional effects, the i 7 I * parameter for Hg(I1) should be independent of applied current. When Pb(I1) is absent, the Hg(I1) i7I value increases gradually with applied current owing to the adsorption of a Hg(I1) bromide complex ( 3 ) . Addition of Pb(1I) in concentrations up to 0.175mM (curves B and C) results only in some attenuation of rHe.For [Pb(II)] 1 0.187mM, however, curves D and E indicate an abrupt, severe inhibition of Hg(I1) reduction. A t high applied current, the normal Hg(I1) wave vanishes altogether (totally inhibited condition). A t lower currents, some Hg(I1) reduction proceeds a t the normal potential, but its transition time is attenuated, and the balance of the Hg(I1) wave is shifted to the lead reduction
(26) C. M . Elliott and R. W . Murray, J Amer. Chem. SOC.. 96, 3321 (1974).
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 6 . NO. 9. A U G U S T 1 9 7 4
i
-I 2
T I M E , SEC
Figure 6. Chronopotentiometric oxidation of Hg(0) in presence of indicated rnM concentration of P b ( l l j . Arrow represents cur-
rent reversal
TIME.
potential. The data in the low current region extrapolate to the normal diffusion controlled irl value. The current dependency shown in Figure 5 is diagnostic of the adsorbed Pb(I1) bromide layer introducing a slow kinetic step in the Hg(I1) electron transfer. The rate-determining step is. presumably, the penetration of the Pb(I1) bromide layer by the reducible Hg(I1) species. Further, the abrupt onset of this inhibition must reflect some discontinuous change in the character of the adsorbed Pb(I1) bromide, and since r p b a t these [Pb(II)] is already on the adsorption isotherm plateau ( 3 ) , it would appear that the change is an internal alteration in the adsorbed film’s structure. The kinetics of establishing this change are fairly slow; a long stirred pre-equilibration (5-10 minutes) was required for reproducible results on the descending irl plots (Curves D and E ) . In the previous report (20), Hg(I1) reduction inhibition in 1.OM NaI medium was complete a t the lowest [Pb(II)] tested (0.05mM). Additional experiments have shown that the total inhibition vanishes between 0.010 and 005mM Pb(I1). The r [ > h again is already near saturation a t [Pb(II)] below the inhibiting condition. Measurement of r I ’ b proved elementary for 0.005mM Pb(I1) in 1.OM NaI; chronopotentiograms have the appearance of Curve 2, Figure 2 of Reference 3 a t all currents tested. and the transition time is nearly totally dominated by 2 F r P b ( i . e . , virtually a coulometric experiment). Table VI gives products for i~ for the Pb(1Ij wave. A plot of ir us. l / i (almost horizontal) yields a t l / i = 0 , a t rl’h = 7.4 x 10-lO mole/cm2. Anodic. Application of an anodic current step to a HMDE pre-equilibrated with Pb(I1) iodide or bromide solution produces a positive-going potential-time response (Figure 6) which, like the Hg(I1) reduction wave, may or may not reflect an inhibition effect depending on [Pb(II)]. The lower [Pb(II)] have no effect on the gently rounded anodic and current-reversal cathodic responses. At the higher [Pb(II)], the Hg(0) oxidation exhibits an anodic shift and alteration in wave shape, and current reversal reveals the coincident presence of Hg(I1) reduction inhibition. The concentration intervals over which the inhibiting transition occurs are in both iodide and bromide media. the same as observed in the foregoing Hg(I1) reduction experiments, an important observation with respect to interpretation of the anodic mixed amalgam chronopotentiometry discussed next. AMALGAM OXIDATION INHIBITION The inhibiting effects of adsorbed Pb(I1) halide on either oxidation or reduction of metals less noble than lead cannot be explored directly since the adsorbed film would not exist at the more negative electrode potential of the teat metal. Given a mixed amalgam HMDE of lead and the test metal in a metal- and lead-free halide solution,
SEC
Figure 7. Anodic chronopotentiometry of C d ( O ) / P b ( O ) H M D E in 1M Nal Curve A 8 OmM
mixed amalgam
[Cd(O)] = [ P b ( O ) ] = 2 OmM Curve B [Cd(O)] = [Pb(O)] =
Table VI. Chronopotentiometry of 0.0050mM Lead(I1) i n 1.OM Iodidea i, P A
i ~ fiA ,
2.00 3.00 4.00 5.00 7.00 10.00 15.0 20.0 30.0 40.0 a
sec
4.46 4.22 4.34 4.31 4.34 4.27 4.14 4.24 4.11 4.12
Electrode area = 0.290 cm?; 5-minute pre-equilibration,
an indirect assessment is possible through the diffusional mathematics of the second wave of a two-wave chronopotentiometric oxidation of the mixed amalgam. The test metals examined were cadmium and zinc. An illustrative anodic chronopotentiogram of a Cd-Pb mixed amalgam HMDE in iodide medium is shown in Figure 7 ; chronopotentiograms in bromide solution and of Zn-Pb amalgam in iodide solution have similar appearance (27). The first anodic wave is for Cd(0) oxidation; its transition time, rrn (in the absence of intermetallic compound formation with (Pb(0)) should be unaffected by the presence of Pb(0) and yield constant i ~ 2 ., The ~ following Pb(0) oxidation wave is split into two transition time segments: T,, an adsorption prewave during which adsorbed PbX2 is formed and T ~ the , remainder of the Pb(0) oxidation during which diffusing Pb(I1) is formed. The prewave is essentially a coulometric step; i~~ is a constant for a lead amalgam (Table IV in Reference 3). The overall lead wave, T~ + rS represents a Pb(0) diffusion-controlled reaction which, in the absence of Cd(O), would yield a constant i ( ~ +, T , ) o ~ product. In the presence of Cd(0). the overT~ is increased by the conall lead transition time T~ tinuing residual diffusion of Cd(0) from the interior of the amalgam HMDE. Whether or not this residual diffusion enhancement actually occurs or not provides a criterion for whether Cd(0) oxidation becomes inhibited upon formation of the absorbed Pb(I1) halide during T,. Quantitative description of the residual diffusion enhancement of T~ + T~ is simple a t two extremes. In one, oxidation of Cd(0) is not hindered by the adsorbed lead.
+
(27) R . L. McNeely, Ph.D. Thesis. University of North Carolina, Chapel Hill. 1969.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9 , A U G U S T 1974
1263
a
4
1.2
--$I
0
P
4 0.8
4 0.6
COpa mM I
Figure 8. Inhibition
parameters for the oxidation of mixed amal-
gam HMDE ( 0 )Cd(O)’Pb(O) Nal ( A ) Zn(O)/Pb(O)/Nal ( 0 )Cd(0) Pb(0) NaBr
and the residual diffusion effect is given by ordinary twocomponent chronopotentiometric theory (28).
where Dpb and Copt, are the diffusion coefficient and concentration of Pb(0) in the HMDE, respectively. Inasmuch as Equation 4 contains a correction for the presence of Cd(0) and, thus, also represents the chronopotentiometric parameter for Pb(0) alone. i ( ~ , T , ) o ~ 2, we can define
a =
L[(r,,
+
7, l(T,
+ + TY’- ( T , ) ~ ” I / c ~ ~ ~ ~
+
TJ”l/‘/Copll
(5)
where N = 1.00 for the case of no inhibition. If inhibition occurs during the lead wave, then Equation 4 overcorrects for Cd(0) and cy < 1.00. Equation 5 is normalized for C o P b and current to facilitate comparison of lead transition time data in mixed and pure lead amalgams a t differing i and C’pb. In the second extreme, Cd(0) oxidation proceeds normally during T~ but becomes totally inhibited a t T a and . this case, Equation 4 is rewritten does not enhance T ~ In so as to correct only the period T , for residual diffusion and, following the definition of Equation 5, we write
in which p = 1.00 for complete inhibition of Cd(0) oxida. inhibition leads t o /3 > 1.00. tion during T ~ Incomplete The two extremes discussed above are found in the data of Figure 7 . At the lower (2mM) concentrations of Cd(0) and Pb(O), T~ is substantially enhanced by cadmium residual diffusion. At the higher (8mM) concentration, the T~ enhancement vanishes, but T, and T, are substantially unaffected. The final potential hold-up in both cases is the oxidation of electrode mercury. We note that the Cd(0) oxidation inhibition a t the higher concentration is accompanied by an onset of Hg(0) oxidation inhibition. Results. The anodic chronopotentiometric oxidations of pure Cd(O), Zn(O), and Pb(0) amalgam HMDE were diffusionally normal, as anticipated. The current-independent chronopotentiometric constants for Cd and Zn, i ~ , l 2,ACa = 573 and 610 A secl cm mole-1: respectively, were not affected by the presence of Pb(0) in their amalgams. The chromopotentiometric constant for Pb(0) was i(’rn + T ~ ) O ’ 2/ACopb = 517 A Secl cm mo1eC1. (28) C. N Reilley, G. W. Everett, and R. J. Johns, A n a / . Chem.. 27, 483 (1955).
1264
Mixed amalgam chronopotentiometric oxidations were conducted for the Cd(O)/Pb(O)/iodide, Zn(O)/Pb(O)/iodide, and Cd(O)/Pb(O)/bromide systems, varying [Pb(O)] and maintaining [test metal] = [Pb(O)] and [Pb(O)]/current = constant. Under these conditions, a transition of qualitative behavior such as illustrated in Figure 7 could be readily detected in the raw potential-time data. The transition time data, cast in terms of the quantitative definitions of Equations 5 and 6, are summarized in Figure 8. These results show that for all three mixed amalgam systems the two anticipated extremes of behavior are found: non-inhibition ( N = 1) at low [Pb(O)] and total inhibition after T, ( p = 1) a t high [Pb(O)]. For the two cadmium amalgams, the transition from one extreme to the other is moderately sharp, although much less so as compared to the mercury redox inhibition transitions. In the zinc amalgam, some degree of inhibition persists a t lower [Pb(O)], but its transition to complete inhibition does coincide with the Cd(O)/Pb(O)/iodide case a t approximately 6.5mM. As was the case with mercury redox. the inhibition transition occurs a t lower lead concentrations in iodide medium as compared to bromide. The results in iodide medium are unfortunately shadowed by the 1.4mM Pb(I1) solubility in 1M NaI. These solutions supersaturate readily, however, and we conclude from the absence of evidence for precipitation during the anodic experiment on or near the electrode that this was not a deleterious factor. ( i and ~ ~wave shape are independent of [Pb(II)];@ is unity a t high p b ( I I ) ] ) . Throughout the experiments of Figure 8, the anodic chronopotentiograms were allowed after T~ to proceed to the oxidation of electrode mercury. Hg(0) inhibition took place coincident with the non-inhibition inhibition transition of Figure 8 in all three amalgam systems. becoming apparent a t approximately 6-7mM Pb(0) in iodide solution and about l l m M in bromide solution. Thus, in iodide medium, three different electrochemical oxidations, those of Cd(O), Zn(O), and Hg(O), become abruptly inhibited by adsorbed Pb(I1) iodide under identical conditions of forming the adsorbed layer. A similar result with two different oxidations, Cd(0) and Hg(O), is obtained for adsorbed Pb(1I) bromide. This demonstrates that the inhibition process is initiated by a change in the physical character of the adsorbed lead layer, itself not a change in the chemical or electrochemical property of the inhibited reaction. For example, the adsorption of Hg(I1) and Cd(I1) halide complexes themselves appears t o play no important role in the inhibition. The coincidence of the inhibition transition for amalgam Hg(0) oxidation with that of the test metals Cd(0) and Zn(0) assumes a further significance in reconciling the results of the mixed amalgam oxidations with those for inhibition a t electrodes pre-equilibrated with Hg(I1)/ Pb(II)/iodide, Pb(II)/iodide, Hg(II)/Pb(II)/bromide. and Pb(II)/bromide solutions. The inhibiting concentration levels of Pb(II) in the two sets of experiments are quite different for both halides. The key to this apparent contradiction lies a t least partially in the fact that, in the amalgam oxidations, the adsorbed lead halide layers are being made “on the fly,” and are very unlikely to represent any close approach to adsorption equilibrium. Second, the actual solution [Pb(II)] a t the electrode surface at the time ( T , ) of completion of the adsorbed layer will be much lower than the initial [Pb(O)] or the solution [Pb(II)] a t time T ~ LVhichever . of these two factors is dominant, the same general conditions must exist at the electrode/solution interface in both pre-equilibrated and amalgam “on-the-fly” experiments to produce the sudden inhibition of Hg(0) oxidation.
A N A L Y T I C A L C H E M I S T R Y , V O L . 46. NO. 9 , A U G U S T 1974
-
In part I of this report, we described chronocoulometric measurements of lead iodide and bromide adsorptions in which discontinuities in adsorption isotherms were related to surface solubility properties of the lead halide salts. While it is tempting to try to relate those abrupt changes in rIlbto the abrupt occurrences of inhibition observed in Part 11, we believe this would be an error. We regard the two sets of observations as representing two different transitions for the adsorbed lead species. It is unfortunately not possible, for solubility (of Hg(I1) halides) reasons, to conduct the inhibition tests in solutions of composition yielding adsorption isotherm discontinuities. We do know, however, from data in 1.OM I- and Br- obtained in this and a preceding report ( 3 ) that no large change in r p b occurs over the inhibition transition interval for Hg(I1) reduction, and that r p h is furthermore close in value to the limiting plateau r p b of adsorption isotherm in Figures 1 and 2. The inhibition results appear to tell us that a t least bits and traces of further lead halide adsorption can occur beyond formation of the two-dimensional PbX2 crystal a t the adsorption isotherm, which is to say that the initially
formed surface phase must not represent a “perfect crystal.” The imperfections may be small boundary openings between patches of surface crystal, intruding adsorbed halide, or possibilities for structural reorganizations which can be eliminated or accomplished, by small amounts of additional lead halide. Whatever the reason, the rather readily penetrable surface crystal formed a t the adsorption isotherm discontinuity is transformed into a surface phase exceedingly impenetrable from either the solution or electrode side by species other than the Pb(I1) ion itself. In the case of Pb(II), actual penetration of intact surface phase is, of course, not required since Pb(I1) can pass the surface lattice by simple exchange. Received for review December 26, 1973. Accepted March 28, 1974. Assistance a t various phases of this study was obtained through the following: Air Force Office of Scientific Research under Grant AFOSR-69-1625; Materials Research Center, UNC, under Grant GH-33632, National Science Foundation; and Grant GP-38633X, National Science Foundation.
Radioelectrochemistry-A Review Helen P. Raaen Technicai Pubiications Department ’ Information Division Oak Ridge Nafionai Laboratory Oak Rdge Tenn 37830
This article reviews the conjoint use of radioisotopes with electrochemistry. The major subjects considered are: electrode behavior, electric-double-layer structure, mechanisms and kinetics of processes that occur at electrodes, electrochemical behavior of certain substances, radioelectrochemical analysis, and radiation electrochemistry. A few uses of radionuclides in microcoulometry and in the study of the direct conversion of ionizing radiation to electrical energy are included. The works selected for citation describe studies of conducting surfaces in fluid systems and the charge-transfer reactions involved; thus, they include corrosion as related to the behavior of electrodes but exclude semiconduction. Finally, those areas of electrochemistry are suggested which offer considerable possibility for exploration with radionuclides.
Electrochemistry has been defined as the application of electrostatics to ions in fluid systems and to conducting surfaces in contact with such solutions and the chargetransfer reactions across the surfaces ( I ) . The ions may be simple or complex. The fluid systems may or may not be liquid at room temperature. The conducting surfaces may be liquid, solid, or gaseous. Keen interest exists in such topics as fuel cells. new electrical-energy sources, electrochemical preparation and treatment of metals for new technologies, and the protection of metals from corrosion (2). Also, many biological mechanisms are recognized to be electrochemical. The increasing need for electricity will inevitably result in a new kind of electrochemistry ( 3 ) in which stud?; of the properties of systems gives way to study ot the mechanisms of processes.
__ Re\iew written in the Isotopes Information Center
This article is a special review which, although not appropriate for periodic review, makes a contribution by integrating. evaluating, or correlating past research. Such reviews are not regular features in the technical section: however, they are not arbitrarily excluded just because they deal with new evaluations of past data rather than presentation of new data. Radioelectrochemistry is of arbitrary definition; herein, it is taken to mean the conjoint use of electrochemistry and radionuclides (either naturally occurring or induced). Although as early as 1929 radioelectrochemistry was used indirectly by Heyrovsky when he deposited radium at the dropping-mercury cathode ( 4 ) , most of the papers in the field have appeared since about 1960. They reflect the trend in electrochemistry study away from systems properties to process mechanisms. The complexity of electrochemical processes necessitates that they be studied with methods that are specific, sensitive, and accurate. Radionuclides provide these characteristics. They also permit rapid measurements on samples of low concentration or small volume. The use of tracers is uniquely suited to studies of electrochemical mechanisms because single atoms or molecules can be labeled. Radionuclides may be used as sources of radiation as well as tracers, or sometimes as both simultaneously. They offer versatility of positioning; they may be external to the system under study-for example, as the radiation source-or a part of any phase (solid, liquid, gaseous) of the system. Their use permits either discontinuous or continuous measurements. Parallel studies made with and without radionuclides often furnish information that neither study can give separately. Although use of radionuclides in electrochemical studies has been extensive, surveys on electrochemistry ( e . g . , refs. 3, 5, and 6) mention it only scatteredly. A purpose of this review is to show the significance and potential of radioelectrochemistry.
ANALYTICAL CHEMISTRY. VOL. 46, NO. 9 , AUGUST 1974
1265
