and then the adsorbed C3pecies. This will not necessarily hold for irreversible electrode reactions, where the adsorbed molecule may react more rapidly than the solution species. When only the product of an electrode reaction is adsorbed, polarographic pre-waves are obtained, m c e the ret ersible reduction of a substance to the adsorbed state requires less energy than reduction to the species in solution 1:4). From polarographic studies, Brdi6ka first assumed that only the product of the electroreduction of riboflavin-i.e., leuco-riboflavin-was adsorbed. More recent ax. polarographic studies (6) present evidence for the adsorption of both riboflavin and leuco-riboflavin. Our studies support this idea; recent studies using current reversal chronopotentiometry (10) also indicate adsorption of leuco-riboflavin. The appearance of a pre-wat e in this system provides evidence that the leucoriboflavin is adsorbed more strongly than the riboflavin itself. Under these conditions the electrode reaction may proceed by the reduetion of the diffusing riboflavin to form an adsorbed layer of the leucoform. -4s this adsorbed layer Of lellco-riboflavin is formed, the adsorbed riboflavin is
desorbed and then reduced. If this mechanism is correct, the behavior of the system during chronopotentiometry is intermediate between models 2 and 3. No pre-wave was obtained on the chronopotentiometric reduction waves. We have no explanation for this effect. The electrochemical reduction of riboflavm is complicated by the formation of an intermediate free radical (semiquinone), and is being investigated further. The chronopotentiometric nieasurement of adsorption has the advantage of being independent of the reversibility or irreversibility of the electrode reaction. Since several other possible effects can cause increases of with decreasing 7, particularly when solid electrodes are employed (a),interpretation of such increases as due solely to adsorption should be made with caution, and preferably independent methods of measuring adsorption should also be used. LITERATURE CITED
( I ) Anson, F. C., ANAL.CHEM.33, 1123 (1961). ( 2 ) Bard, A. J., Ibid.1 35, 340 (1963). (3) Ibid., 33,11(1961). (4) BrdiEka, R., Collectzon Czech. Chem. Commun. 12, 522 (1947).
( 5 ) Breyer, B., Biegler, T., Ibzd., 25,3348 (lg60). ( 6 ) Brezina, M., Zunian, P., “Polarog-
raphy in &Iedi&e, Biochemistry, and Pharmacy,” pp. 389-94, Interscience, New York, 1958. (’) ‘Orbusier> p.r Gierst, L . ~ Chim. Acta 15, 254 (1956). (8) Craxford, s. R., M ~ K H. ~ A, ~ ,c., J. Phys. Chern. 39, 545 (1935). (9) Herman, H., Bard, A. J., ANAL. CHEM.35,1121 (1963). Herman, H., Tatwawadi, s. v., Bard, A. J., Ibid., 35, 2210 (1963). (11) Kolthoff, I.,, M., Lingane, J. J., “Polarography, pp. 256, 844, Interscience P”’ew 1952. (12) Laithen, H. A., ANAL. CHEM.33, 1458 ( 1961). (13) Landsberg, R., Nitache, R., Geisaler, W., Z. Physik. Chem. Leipzzg 222, 54 (1963)* (14) Lorenz, W., 2. Elektrochem. 59, 730 (1955). (15) Lorenz, W., Mdhlberg, H., Zbid. 59, 736 (1955); Z. physik. Chem. Frankfurt 17, 129 (1958)* (16) Munson, R. A., J . Electroanal. Chem. 5,292 (1963). (17) Parsons, R., “Modern Aspects of Electrochemistry,” J. O’M. Bockris, ed., pp. 128-34, Butterworths, London, 1954. (18) Reinmuth, W. H., ANAL.CHEM.33, 322 (1961). RECEIVEDfor review July 19, 1963. Acce ted October 30, 1963. Presented at
the louthwest Regional Meeting, A.C.S. December 1962. Research supported by the Robert A. Welch Foundation.
A Chro nc)pote ntio metric Stu dy of Adsorption H. A. LAITINEN and L. M. CHAMBERS Noyes Chemical Iaboriitory, University of Illinois, Urbana, 111.
b The chronopotentiometric method is a sensitive qualitathe indication of adsorption. Quantitative determination of surface excess 3f Alizarin Red S failed because of the current required for the desorption of reduction product. Using tris(ethy1enediamine) cobalt(ll1) and (11) ions, three equations, representing different physical models for the electrode reaction of the solution phase and the surfa1:e excess, were tested. All indicated a small amount of adsorption, but no choice could b e made among the models. Several limitations on the quantitative study of adsorption by chronopotentiometry are pointed out.
resent an appreciable portion of the entire electrolysis process (10). Of the many possible theoretical models that might be set up to describe the simultaneous or sequential reduction or oxidation of adsorbed and diffusing phases, three are particularly amenable to experimental investigation. The first model (10, 16, 91) assumes the additivity of transition times,
C
IT. = n F r (2) where I? is the surface concentration of adsorbate in moles per square centimeter. By adding r 0 to T d , as given by the Sand equation for linear diffusion (ad), we obtain (nFC)*TD (3) I T = nFr 41
has been used by only a fev workers for the investigation of adsorFtion of reactants (1-4, 10, 16, 17, 21). When short transition times are used, it is particularly important to establish the presence or absence of an adsorbed phase, because even a fractional monolayer of adsorbed resctant can repHRONOPOTENTIOMETRY
7-
= 7,
+
Td
(1)
as would be observed if the adsorbed phase were electrolyzed first in a time r 0 , followed by the electrolysis of the diffusing species to bring its surface concentration to zero in time T d . If I is the applied current density, then
+
~
A plot of I T us. 111 should be linear, according to this model, with an intercept
nFI’ which can be used to determine surface excess (10) and a slope proportional to C2D. The second model (4, 16, 21) assumes a simultaneous reaction of adsorbed and diffusing phases in such a way that a constant fraction of the applied current goes to each phase during the entire electrolysis. Thus I = I,
+ Id
(4)
and
According to this model, a plot of IT us. 71’2 should be linear, with an intercept of nFr and a slope proportional to
CD1I2. The third model (16,167 requires that the adsorbed layer be electrolyzed a t the 1 Present address, Ivorydale Technical Center, The Procter and Gamble Co., Cincinnati 17, Ohio.
VOL. 36,
NO.
1, JANUARY 1964
5
end of the transition time. The equation describing the process is
(6)
Therefore, a plot of (17)1/2 us. I-"* should be linear, with an intercept of (nFr)'jZ and a slope proportional to CD1'2. Equation 6 is an approximation given by Lorenz (IS) to avoid mathematical difficulties. A more rigorous although less convenient equation is available for this model (21). This investigation compares the three models experimentally for several adsorbates to determine which is the best description of the system. To enable such a comparison with a clean, reproducible surface, and to permit establishment of adsorption equilibrium prior to the measurement, a hanging mercury drop electrode was used. This required that spherical diffusion, as described by Mamantov and Delahay (It?), be taken into account. By comparing the equations for linear and spherical diffusion for an electrode area sq. cm., it was found that of 3 X the two equations gave transition times agreeing within 3% if the transition time was kept below 100 mseconds. By oscilloscopic techniques, measurements of adequate accuracy in the range of 2 to 100 mseconds could be achieved. A'lathematically, it is evident that if the electrode process fits one of these physical models, the plots corresponding to the other two models must yield curves rather than straight lines. Trial calculations, assuming the first model to be valid, indeed showed curvature for the plots of the second and third models. The amount of curvature, however, depended upon the relative surface and solution concentrations. If a surface concentration of 8.3 X mole per sq. em. (corresponding to a monolayer of 20-sq. -4.molecular area) is assumed at a solution concentration of 10-*M, a barely detectable curvature would be expected at transition times in the proper experimental range. A more pronounced curvature would be expected at lower solution concentrations, but of course the surface concentration would also decrease, and the relative experimental error would therefore increase. We conclude from these trial calculations (and the experiments bear out this conclusion) that curvature cannot be expected to be a useful experimental criterion of choice among these three models. From the analytical viewpoint, it is important to assess the probable magnitude of error in determining surface concentrations from plots of Equation 3, 5, or 6, if the model is not known. As might be intuitively expected, r as determined from the intercept of the 6
ANALYTICAL CHEMISTRY
best straight line decreases in the order Equation 3 > Equation 5 > Equation 6. For this reason, the safest procedure is to use Equation 5, which yields an intermediate value of I?. The magnitude of the deviation increases rapidly with increasing solution concentration. For example, if the intercept of Equation 5 were taken to be correct and to correspond to 4.15 X 10-'0 mole per sq. cm. at a solution concentration of 10-4~4, the intercepts of Equations 3 and 6 corresponded, respectively, to 4.28 and mole per sq. cm. How4.14 X ever, if the same surface excess were reached at a solution concentration of 10-3M, the corresponding intercepts of Equations 3 and 6 were 5.03 and 3.99 X mole per sq. cm., respectively. EXPERIMENTAL
Apparatus. The constant current source used is described elsewhere (11, 82). It consisted essentially of a 270-volt battery pack in series with suitable variable resistors to deliver the desired current. The switching system was arranged so that the oscilloscope would be triggered an instant before the electrolysis current was switched through the cell. The current was measured during the electrolysis as the I R drop across a 1000-ohm standard resistor in series with the electrolysis cell by means of a Rubicon Model 2700 potentiometer. A Tektronix Type 502 oscilloscope with an attached Tektronix Type C-12 camera equipped with a Polaroid back was used to display and record the potential-time curves. Type 47-3000 speed Polaroid film was used. A mercury drop, transferred from a dropping mercury capillary, by a glass spoon, to the tip of a mercury-plated platinum electrode, served as the hanging mercury drop indicator electrode. The support for the mercury drop was the cross-sectional area of a piece of No. 30 €3. and S. gauge platinum wire which was sealed in the end of a piece of 4-mm. 0.d. soft glass tubing that had been drawn out to about 1 mm. a t the tip to minimize shielding. The platinum-glass seal was ground flat, polished with fine emery paper, and examined for cracks under a 50X microscope. In order to permit the mercury drop to adhere to the platinum surface. it was first cleaned and then plated with mercury according to specifications similar to those given by ROSS,DeMars, and Shain (23). A saturated sodium chloride electrode (S.S.C.E.) with a porous Vycor probe was used as the reference electrode. The electrolysis cell was essentially a Pyrex 71/60 standard-taper joint with the outer member equipped with appropriate standard taper joints to receive the electrodes, etc., and the inner member a flat-bottomed vessel of approximately 100-ml. volume to contain the mercury pool counterelectrode and the electrolysis solution. Provisions were also made for thermostating the cell solution and for keeping the electrolysis solution free from the
atmosphere during experiments and while adding solutions from outside the cell. Purified nitrogen, used to deaerate solutions, was kept flowing above the solutions during the experiment. The port for the extension of the glass spoon was fitted with a mercury seal to prevent atmospheric oxygen from entering the cell while at the same time permitting the necessary mobility to catch and hang the mercury drop. Chemicals. All chemicals used were reagent grade except where indicated otherwise and solutions were prepared by direct weighing of chemicals except where noted differently. T h e Alizarin Red S (biological grade) was further purified by twice extracting with absolute methanol in a Soxhlet extractor and was found to be 98.5% pure as analyzed by a procedure prescribed by Ford (8). The Alizarin Red S solutions were buffered at p H 8 with a Clark and Lubs (9) phosphatetype buffer. The preparation and purification of bromocamphor, chlorocamphor, and sodium fluoride are described by Sherman (26). Tris(ethy1enediamine) cobalt(II1) chloride was prepared from a procedure by Work (87). Cobalt(I1) perchlorate and tris(ethy1enediamine) cobalt (111) perchlorate solutions were prepared by passing a solution of the appropriate chloride salt through a 2- X 20-em. column of Dowex ZX8 anionic exchange resin in the perchlorate form. The cobalt concentration in the resulting solution was determined polarographically using a calibration curve prepared from standard solutions of [Co(en)s]Clr prepared by direct weighing of the pure salt. Ethylenediamine (Eastman Organic Chemical, 98y0 pure) was purified by distillation from an all-borosilicate glass system and standardized by a potentiometric acid-base titration. Sodium perchlorate was prepared by neutralizing a solution of previously ignited sodium carbonate with perchloric acid that had been purified by boiling with 10% nitric acid until the nitric acid was expelled. Water used for preparation of solutions and rinsing glassware was distilled from alkaline permanganate in a block tin still. Kitrogen for deaeration was purified by passing tank Nz through a copper on earth type of oxygen purifier (19) and washing towers containing 1JI NaOH, concentrated H&04, and the supporting electrolyte used in the electrolysis cell. The glassware mas frequently cleaned by rinsing with hot 2 : 1 concentration perchloric-nitric acids, followed by a distilled water rinse. Procedure. Solutions were pipetted into the cell already containing the mercury pool and Teflon-covered magnetic stirring bar. After the solution mas d e x m t i d , a mercury drop was transferred from the dropping mercury cnpiliary to the mercury-plated platinum support electrode by means of the glass spoon. The solution wa9 stirred if necessary to attain adsorption equilibrium and allowed to stand quietly for at least 1 minute before applying the electrolysis current. A new mercury drop was used for each electrolysis. The drop time was meas-
z
Figure 1. Potential-time curves for 0.744mM Alizarin Red S in 0.1M KCI at pH 8 Applied current, p a .
At t
1. 102.61 20 msecond cm.-' 2. 192.4; 10 msecond cm.-' 3. 312.7; 5 msecond cm.-' = 0,E $0.20 volt VJ. S.C.E.
ured before and after each experiment and the drop area wa,3 calculated from knowledge of the drop time and the capillary mass flow rate. Duplicate potential-time curves were recorded for each current density used over the range that gave trancition times from 2 to 100 mseconds. All transition times were measured by the r / 4 procedure described by Delahay and Berzins ( 7 ) . RESULTS AND DISCUSSION
measurements of ARS on mercury, the dye is desorbed a t about -0.8 volt us. S.C.E. When the dye is desorbed, the differential double layer capacity is increased severalfold. It is, therefore, necessary to supply charge in an amount equal to the difference betweenIthat held by the uncovered and covered surface a t the desorption potential. The effect of desorption on the transition time is enhanced by the fact that only a fraction of the current is available for supplying the excess charge because the reduction process is still going on. A simple trial calculation reveals that the effect of desorption is of the right order of magnitude to account for the experimental results. The desorption process occurs over a range of potentials, and, like the adsorption process, is not instantaneous. The rate of change of potential with time is therefore determined in part by the kinetics of desorption. Both the slopes and intercepts of the plots would be expected to be in error. The effect of adsorption on the conventional chronopotentiometric I P / C us. I plot is shown in Figure 5 for the
4 6 a i / I x 10-2 (CM?AMP.")
lo
Figure 2. /T vs. 1 / I for Alizarin Red in 0.1M KCI at pH 8
I 12
S
rnM Alizarin Red S
1. 2. 3. 4. 5. 6.
2.77 1.83 0.744 0.436 0.300 0.149
ARS case. While this type of plot is not useful for determining surface excess, it may be used to detect adsorption complications, since a purely diffusioncontrolled process gives straight lines horizontal to the I axis. It can be readily shown that the limiting slopes of the curves of Figure 5 a t I = 0 should be proportional to r/c if Equation 3 is valid. Thus it is reasonable that the limiting slope should increase with decreasing concentration if the usual shape of adsorption isotherm (Langmuir or Freundlich) is obeyed. Tris(ethy1enediamine) Complexes of Cobalt. The tris(ethy1enediamine)cobalt(I1) and -cobalt(III) ion system is polarographicaIly reversible in excess ethylenediamine (en) (12) and the species are preferentially adsorbed on mercury (14). The chronopotentiometric method offers a technique which can be used to study the relative amounts of the two species being adsorbed in separate solutions or in a mixture of the two. The [ C o ( e n ) ~ ] +complex, ~ as the
Sodium alizarin sulfonate, Alizarin Red S (ARS), was the first compound to be studied by the prcposed technique, since it was known to be adsorbed on mercury (IS) and give a smooth polarographic wave. Solutions of ARS ranging from 0.149 to 2.77rnM in 0.131 KCl a t pH 8 were studied. A typical potential-time curve is shown in Figure 1. Up to 10 minute:; of stirring was required for adsorption equilibrium to be reached. From the shapes of the curves it can be seen that the twoelectron reduction of XRS is probably not the only process that occurs during electrolysis. The transition times measured were too large considering the diffusion process only. This W R evident ~ from the 1 7 us. 1/1 (Figure 2), IT us. TL'Z (Figure 3), or ( 1 ~ )us. ~'~ I-1t2 Table I. Comparison of Interpretations for Alizarin Red S in 0.1M KCI at pH 8 (Figure 4 ) plots, whii,h were straight Equation 6 Equation 3 Equation 5 lines of finite intercepts but with slopes D X lo6 r x 1010 D X 106 r X 1010 D X 106 r X 10'" which were much too large to be acmoles cm.-2 cm.2sec.-l moles cm.+ cm.asec.-l moles cm.? cm.2sec.-l C, m31 counted for solely by a diffusion process 5.17 6.4 3.93 5.44 8 3 2.77 7.10 (Table I). This effect #as attributed to 2.99 5.55 4.66 6.9 9.9 6 56 1.83 a large charging process which occurred 2.84 12.0 4.15 22.4 21.6 5.73 0.744 as a result of the desor2tion of the ARS 4.10 9.14 4.66 18.2 35.8 0.436 5.50 a t a potential before the end of the 15.5 18.9 4.12 55.5 3.85 0 300 5 08 3.57 5.84 3.60 18.6 3.96 49.3 0.149 transition time. According to Mosier's (do) differential double layer capacity VOL. 36, NO. 1,
JANUARY 1964
7
'
I
I
0 0
I
I
I
I
2
1
1-k x - ' Figure 4. ( / T ) ' / %vs. KCI a t pH 8
/-'/z
3
( 0 4 % AMFJ2)
for Alizarin Red
S in 0.1M
m M Alizarin Red S 1. 2.77
Figure 3. a t pH 8
2. 3. 4. 5. 6.
IT vs. r1/z for Alizarin Red S in 0.1M KCI 1 OM Alizarin Red S 1. 2.77 2. 1.83 3. 0.744 4. 0.300 5. 0.149
chloride salt in 0.1M en and 0.1M NaC104, was the first system of this series to be studied. It was necessary to stir the solution for approximately 2 minutes to attain adsorption equilibrium. The open circuit potential for the system a t equilibrium was about -0.18 volt us. S.C.E. [Adsorption of the complex occurs a t this potential according to electrocapillary curves (14).] A slight surface excess was detected by using the experimental plots for the models. Since chloride ion was present in a variable quantity in the above mixture and since it is surface active, it was desirable 'to perform experiments with no chloride present and with chloride in excess. The former was performed by using the perchlorate salt of the [Co(en)a]+a complex. Only a minute amount of adsorption, if any a t all, was detected even a t bulk concentration 6.2mM in [ C o ( e n ) ~ ] + ~showing , that
chloride ion played a role in the apparent adsorption of [ C ~ ( e n ) a ] +in~the previous experiment. Solutions of [Co(en),]+s in 0.1M KC1 and 0.1M en were studied to determine the effect of excess chloride ion on the adsorption of the cobalt(II1) complex on mercury. Adsorption equilibrium took place within less than 1 minute for these solutions and much larger amounts of adsorbed complex were found in excess KC1 than with stoichiometric amounts of chloride or in the absence of chloride. Figures 6, 7, and 8 are plots of the three modeis previously discussed. The data are summarized in Table 11. The slopes yielded reasonable diffusion coefficients from all three plots, while the surface excess concentration, r, decreased in the order Equation 3 > Equation 5 > Equation 6. Although it is not possible to make a clear choice of models from these results, only Equations 5 and 6 yielded intercepts which sho\ved a
1.83 0.744 0.436 0.300 0.149
monotonic change with solution concentration. The adsorption of t h e [ C ~ ( e n ) ~ ion ]+~ was studied in both the absence and presence of chloride ion in 0.1M NaClOd and 0.1M en. Little or no direct evidence of adsorption of the cobaltous complex could be obtained from any of the plots. The open circuit potential of the system was about -0.52 volt us. S.C.E., which is in the potential range for adsorption to take place (14). Apparent equilibrium was obtained -4 possible almost instantaneously. explanation of the above results is discussed below. Equimolar mixtures of the cobaltous
u
4'
:I
. ! !
Table II.
Comparison of interpretations for [ Co(en),] Cis in 0.1 M KCI and 0.1 M en Equation 3 Equation 5 Equation 6 X 10'0 D X 108 r x 1010 D x 108 r X 1Olo D X lo8 moles cm.-* cm.2 set.-* moles cm.-S cm.2 see.-' moles cm.+ em.* sec.-l C, m M 0.10 3.8 4.1 3.9 0.71 8.00 1.34 0.10 3.7 4.1 4.0" 0.61 5.46 1.17O 0.09 3.8 1.21 0.62" 4.6 4.6" 0.40 0.16 3.9 5.3 0.31 5.2 0.588 0.54
0
8
Least squarefl plot via digital computer.
ANALYTICAL CHEMISTRY
CO
8
4
12
I x IC3(AMD.CMTZ)
Figure 5. /T'/Z/C vs. I for Alizarin Red S in 0.1M KCI at pH 8 rnM Alizarin Red S 1. 0.545 2. 3.
1.00 1.38
Figure 6. KCI
IT vs. 1 /I for [Co(en)a]Cla in 1M en and 0.1M m M [Co(en),] CIz
1. 2.
8.00 5.46
3. 1.21 4. 0.588
and cobaltic complexes were studied by measuring the anodic transition time for the former and the cathodic transition time for the latter, using a separate hanging mercury drcp for each. The chloride salts of the niixture in 0.1X en and both 0.1M N,tC104 and 0 . l X KCl were investigated. Cathodic chronopotentiograms shon ed the following effects upon the adsorption of cobalt(II1) in the presence of the cobalt
Figure
1. 2.
8.00 5.46
3. 1.21 5. 0.580
for [Co(en)l]Cla in 0.1M en and
1. 2.
8.00 5.46
3. 1.21 0.588
4.
(11) complex. Under a given set of conditions, the adsorption of [Co(en),]+S was decreased by the presence of [Co(en),l+? As with [Co(en)a]+salone,
vs. /-'Iafor [Co(en)3lCIa in 0.1M en m M [Ca(en)s] CIS
TI/*
mM [Co(en)a] CIS
-
Figure 8. ( / ~ ) ' / a and 0.1M KCI
7. IT vs.
0.1M KCI
more adsorption was found in the presence of excess KCI than in NaC104. Anodic chronopotentiograms gave 1 7 us. l/I plots with zero and sometimes negative intercepts, indicating a complication not taken into account in the derivations of Equation 3, 5, or 6. Such a complication could be a slow preceding chemical step, which qualitatively would decrease the IT intercept, and in extreme cases render the plots nonlinear. It is instructive to examine the "classical" plots of I s ~ ' * / Cus. I , which have a negative slope for cases involving kinetic complications but no
I
Figure
I
9. / T ' / ~ / Cvs. I for [ C o ( e n ) ~ ] +ions ~ and [ C ~ ( e n ) a ] +ions ~ in
0.1M en and 0.1M KCI
mM [Co(en)al Cln in equimolar [Co(en)a]Cla 1. 6.66 2. 4.29 m M [Co(en)s] Clz in equimolar [Co(en)s] C12
3. 6.66 4.
4.29
VOL. 36, NO. 1, JANUARY 1964
0
9
adsorption (6) and a positive slope with possible nonlinearity when adsorption is involved. For the equimolar mixtures of cobalt(I1) and (111) complexes, the slopes are negative for the oxidation process and positive for the reduction process (Figure 9). If the adsorption of cobalt(I1) complex is ignored, and a value is assumed for the equilibrium constant, K , of a preceding chemical step, a limiting value may be estimated for the forward rate constant, k,, of the preceding chemical equilibrium from the following equation, given by Delahay (6) Z T ~ / ~= / CnF(rD)1/2/2 1n”a/2CK(k, f
kb)”*
From the slope of the lines in Figure 9 for the oxidation process, K ( k , -k k b ) y 2 s 100. According to Delahay, a value of this quantity smaller than 500 should show up as a detectable kinetic effect on the plot. If the equilibrium constant, K , is taken to be 10-2, kf = 10Bsec.-l; if K = IO+, kf = IO7 see.-’, etc., showing that even a relatively rapid prior chemical step could cause the observed kinetic effect. The true k , may be much smaller than indicated by these estimates because of the influence of adsorption on the measured slope. Two possible sources of a slow preceding chemical step have been considered. The ethylenediamine groups on [ C ~ ( e n ) , ] +are ~ known to be labile and stepwise equilibrium constants with free ethylenediamine in solution are known ( 5 ) . By a simple calculation it can be shown that over 99% of the total cobalt(I1) is present as the fully coordinated complex under the prevailing experimental conditions, showing that the formation of the tris(ethylenediamine) cobalt (11) ion from lower complexes could not account for the kinetic effect. Another possible kinetic step could be the formation of a n inner orbital complex from the predominent outer orbital complex to form the species undergoing electron exchange : kl -e [Co (en),] +’.s [Co (en)*]+IC [Co (en),] +I kb +e
outer
inner
inner
Halocamphors in 0.1M NaF and The bromocamphor system gave well defined transition times and reasonable ZT us. l/I plots with positive intercepts. Adsorption equilibrium was reached very fast (less than 1 minute) a t all concentrations without stirring. A plot of the apparent surface concentration, r us. bulk concentration gave a linear isotherm which did not level off even at ImM, corresponding to a surface concentration of 2.7 X mole per sq. cm. Saturation of the electrode surface was 25% Methanol.
10
ANALYTICAL CHEMISTRY
reached at less than 1mM according to Sherman (25), who made differential double layer capacity measurements at a dropping mercury electrode. The open circuit potential at which the system was allowed to equilibrate was found to be about 200 mv. positive to the S.S.C.E. potential (0.436 volt us. the standard hydrogen electrode) and was probably controlled by a minute amount of mercurous ion resulting from reaction of mercury with a trace of dissolved oxygen. According to differential double layer capacity measurements for this system (25) adsorption does not occur a t potentials more positive than -0.1 volt us. the S.S.C.E.potential. The apparently linear adsorption isotherm presumably arises from a simultaneous adsorption and reduction occurring after the application of the electrolysis current. Under these conditions, adsorption equilibrium could not possibly be reached. An attempt was made to obtain a true adsorption isotherm of bromocamphor in the chosen electrolyte. Tliib was done by applying a potential, sufficient t o hold the hanging mercury drop electrode at a potential of -0.1 volt us. S.S.C.E., between the mercury pool counterelectrode and the hanging mercury drop electrode by means of a Sargent Model XV polarograph. When this wm done, a small amount of faradaic current flowed during equilibration as a result of reduction of bromocamphor. This current flow reduced the electrode surface concentration a sufficient amount to decrease the transition time significantly. Even though the half-wave potential for the reduction of bromocamphor in 0.1Jf N a F and 25% methanol is about -0.45 volt us. S.S.C.E., a minute amount of current flows already a t -0.10 volt us. S.S.C.E. Also, since the time necessary for equilibration and the solution to become quiet after transfer of a mercury drop was of the order of 1 minute, even a small amount of current flowing for that length of time would cause a detectable decrease in surface concentration of bromocamphor. This effect was shown by a decrease in transition time with increasing equilibration time. The study of bromocamphor mas, therefore, pursued no further. Data concerning this study are in the appendix of the thesis of the junior author. Our inability to obtain meaningful results from the bromocamphor study may have resulted from a heretofore not discussed limitation of this method for determining adsorption of electroactive species. In order that adsorption can take place, the electrode potential during equilibration must be in the range where the species of interest is adsorbed on the electrode surface. This potential may or may not be the
open circuit potential for the system. The open circuit potential is governed by the electroactive species present and by the relative reversibility of the possible electrode reactions. In the case of Alizarin Red S, the Ellzpotential is in the range where it is adsorbed. Therefore, the open circuit potential is well defined, since a minute amount of the reduced form is always present. This is not the case for bromocamphor in 0.1Jf KaF. The bromocamphor reduction on mercury is irreversible, and the open circuit potential is therefore ill-defined and positive to the S.S.C.E., where very little adsorption of bromocamphor occurs. An attempt was also made to study chlorocamphor in 0.1M N a F and 25% methmol by the proposed method. Chlorocamphor in this medium gives an ill-defined irreversible polarographic wave with a half-wave potential near -1.0 volt us. S.S.C.E. The potentialtime curves for this system were so illdefined that measurement of accurate transition times was impossible. Therefore, this study was not carried further. CONCLUSIONS
Although chronopotentiometry is a sensitive method for detection of surface excess of an electroactive species, its quantitrttive application is subject to several limitations. There is no a priori way of assigning the correct sequence of occurrences in the electrode reaction of adsorbed and diffusing species. Seither curvature of plots nor deviation of calculated diffusion coefficients is sensitive enough to allow experimental discrimination, especially if only small amounts of adsorption occur. Since the plot of ZT us. 71’2 yields intermediate values for surface excess, this plot would be preferred to avoid extremes of error. The plot of ZT us. 1/Z would yield a maximal value, and the plot of (Ir)”* us. Z-l/Z a minimal value of surface excess concentration. [It was pointed out by a reviemer that the more rigorous equation given by Reinmuth (21) would lead to a higher value for surface excess than the approximate Equation 6, and under some circumstances it could even exceed that obtained from the plot of Equation 3. In the recent work of Tatwanadi and Bard (26) on riboflavin, however, the rigorous equation led to results lower than given by Equation 3 or 5 but higher than given by Equation 6.1 The range of values from these plots depends on the concentration range and surface activity of the adqorbate. The effect of desorption of an organic adsorbate (Alizarin Red S) in the region of potentials involved in transition time measurement is to yield erroneously high transition times. The calculated diffusion coefficients were high and concentration-dependent.
The system should yield a well defined open circuit potential in the region of potential wftere adsorption is to be studied. The electrode process should not be subject to kinetic complications, which lead to erroneously low, or even negative, calculated values of surface concentration. LITERATURE CITED
( 1 ) h S O n , F. c . , ANA;. CHEM. 33, 1498 (1961). (2) Ibid., p. 1838. (3) Anson, F. C . , J . Am. Chent. SOC.83, 2387 (1961). (4) Bard, A.’ J., ANAL. &EM. 35, 340 (1963). (5) Bjerrum, J., “Metal Amine Forma-
tion in Aqueous Solutions,” P. Haase and Son, Copenhagen, 1941. ( 6 ) Delahay, P., “Kew Instrumental Methods in Electrochemistry,” Chap. 8, Interscience, New York, 1954.
(7) Delahay, P., Berzins, T., J . Am. Chem. SOC.75,2468 (1953). ( 8 ) Ford, J. J., M. S. thesis, Iowa State College, 1954. (91 of Chemistrv. 8th ed.. . ,N.Handbook A. Lange, ed., p. 936;‘HandbooG Publ., Sandusky, Ohio, 1952. (10) Laitinen, H. A., ANAL.CHEM.33, 1458 (1961). (11) Laitinen, H. A., Gaur, H. C., Anal. Chim. Acta 18, 1 (1958). (12) Laitinen. H. A.. Grieb. M. W..’ J. ‘ A m . Chem. $ 0 ~ .77,’5201 (1955). (13) Laitinen, H. A., Mosier, B., Ibid., 80, 2363 (1958). (14) Laitinen, H. A., Randles, J. E. B., Trans. Faraday SOC.51, 54 (1955). (15) Landsberg, R . , Kitzsche, R., Geissler, W., 2. Physik. Chem. Leipzig 222, 54 (1963). (16) Loienz, W., 2. Elektrochem. 59, 730 (19.55). \ - - - - I -
(17) Lorenz, W., Muhlberg, H., Ibid., 59, 736 (1955). (18) Mamantov, G., Delahay, P., J. Am. Chem. SOC.76, 5323 (1954). (19) Meyer, F. R., Rong, G., Angew. Chem. 52, 637 (1939).
(20) Mosier, B., Ph.D. thesis, University of Illinois, 1957. (21) Reinmuth, W., ANAL. CHEM. 33, 322 (1961). (22) Rhodes. D. R.. Ph.D. thesis. Uni. versity of Illinois, i 9 6 l . (23) Ross, J. W., DeMars, R. D., Shain, I., ANAL.CHEM.28, 1768 (1956). (24) Sand, H. J. S., Phil. Mag. 1, 45 (1901). (25) Sherman, E. O., Ph.D. thesis, University of Illinois 1963. (26) Tatwawadi, 8. V., Bard, A. J., ANAL.CHEM.36, 2 (1964). (27) Work, J. B., “Inorganic Synthesis,” Vol. 2, p. 221, PllcGraw-Hill, New York, 1946.
RECEIVEDfor review April 4, 1963. Resubmitted August 19, 1963. Accepted September 30, 1963. Abstracted from the Ph.D. thesis of L. M. Chamber8 (1963). Research supported in part by the Aeronautical Research Laboratories, Office of Aerospace Research, U. S. Air Force, under contract AF 33 (616)-5446, and in part b the National Science Foundation, un&r Grant NSF-G 21049.
Solid State Potentiostat for Controlled Pote nt ia I IEIec t roIysis FREDERICK LINDSTROM and JOE B. DAVIS Clernson College, Clemson,
S. C.
b The recent development of solid state electronic devices having electrical characteristics well suited for the job to be done in ancilytical controlled potential electrolysis has led to the design and construction of an instrument half the size of an analytical balance capable of holding the potential to within a few millivolts of any preset value up to 10 volts. The current may vary frori 0 to 5 amperes while such control i s being effected. Having no moving parts or vacuum tubes, it offers superior reliability and simplicity of operation. The instrument has been evaluated 3y using it in the analysis of several standard samples with precision and accuracy.
I
PRINCIPLE, cor trolled potential electrolysis or controlled cathode electrodeposition (5) is one of the most attractive methods For the absolute gravimetric determincttion of metals in nonferrous alloys. The problem has always been that some automatic means of controlling the electrode potential is needed to make the numerous published methods practical. Even before the electronic art had reached the stage where the design and construction of a machine for effecting control was likely t o prove fruitful, numerous more or less successful designs were advanced, as shown in recent N
reviews (2, 3). If the newer solid state devices were used, a much more satisfactory design superior to those of early workers on this problem could be developed. The current design of a simple, practical, and inexpensive potentiostat is easier, for de.cices such as transistors and diodes are natural components for such service. They are capable of handling rather large amounts of current a t low voltage. This is not so with vacuum tubes. Solid state devices are small, highly efficient and, if used within their ratings, offer unsurpassed reliability. Wadsworth (6) has dawribed an instrument employing transistors for series regulation of an 8-ampere diode power supply. It was quite stable and offered rapid response, but to control near zero current, a reverse polarity power supply had to be used to cancel the high zero current of the instrument. A potentiostat satisfactory for most published controlled cathode electrodeposition procedures must fulfill a number of requirements. It must supply about 6 volts and a current of several amperes. Accuracy of control need not be greater than about 50 millivolts, for most separations involve reduction potential differences of more than 0.1 volt. Response time must be fast enough to follow the electrode processes. A response time of several
seconds is sufficient, for changes in the electrolyte are slow. The measuring circuit must not draw an appreciable current from the reference electrode or its potential will be affected. A practical maximum current for a commercial fiber-type calomel electrode is said to be 10 microamperes (6). Regulation should be done in such a manner that minimum power lo?-5 occurs within the instrument. Shunt or series regulators, such as the Wadsworth instrument, dissipate large amounts of power relative to the output power. They are essentially variable resistors which control by wasting the excess power as heat. The Wadsworth machine mas water cooled. The motor driven variable autotransformers used in several of the early electromechanical instruments did not dissipate power as heat but did require considerable power t o drive the autotransformer. A potentiostat should offer simplicity of operation or its value as a labor saving device would be lost. As with all instruments, a potentiostat should offer reliability and loiv maintenance cost. Its first cost, maintenance cost, and operation cost must be considered in any practical design. The unit described here works on an entirely different principle. This circuit rectifies and delivers to the electrolysis cell only the amount of current needed a t a particular time in the VOL. 36, NO. 1, JANUARY 1 9 6 4
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