Anal. Chem. 1994,66, 3124-3130
This Research Contribution is in Commemoration of the Life and Science of I. M. Kolthoff (1894- 1993).
Unusually Strong Adsorption of Highly Charged Heteropolytungstate Anions on Mercury Electrode Surfaced Chaoylng Rong and Fred C. Anson' Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1 125
The adsorption of multiply charged, beteropolytungstateanions on mercury electrodes is extraordinarily strong. For the ChrOdum(III)-SUbStit&d anion, cu2-[P2Wi.P61Cr(OH2)]7the -, mercury surface is half-saturated at concentration of the M. In the presence of the heteropolyanion of only 5 X adsorbed anions, the rate of reduction of the unadsorbed heteropolytungstates in solution is greatly diminished. The decreased reduction rates appear to result from increases in the negative diffuse layer potentials produced by the anion adsorption. The spontaneous adsorption of heteropolytungstates on mercury may prove useful in their electroanalysis at very low levels and in electrocatalytic applications.
electron transfer. The results obtained demonstrate that, despite their high negative charges and the possible electrostatic repulsion among the adsorbed ions, a number of simple transition metal-substituted heteropolytungstates adsorb extraordinarily strongly on the surfaces of mercury electrodes to produce coatings that can interfere with the electrochemical reduction or oxidation of the complexes dissolved in solution. Diminished and distorted electrochemical responses can result from this self-inhibition process. The adsorptive behavior of one particularly strongly adsorbing complex is examined in detail in this paper, and the adsorption of several additional heteropolytungstates is also described.
Electrochemical investigations of aqueous solutions of heteropolymetalates have been carried out by many previous authors using a variety of electrode surfaces.I4 The deposition of certain of the heteropolymetalates at glassy carbon or graphite electrodes has been extensively exploited by Nadjo, Keita, and co-workers to obtain electrode surfaces with considerable catalytic activity toward the reduction of protons and some other substrates.5 In recent electrochemical studies from these laboratories in which transition metal-substituted heteropolytungstates were examined,6 measurements were restricted to glassy carbon or pyrolytic graphite electrodes because those obtained at mercury or gold electrodes were frequently less intense and more distorted than those obtained at graphite or glassy carbon electrodes. The present study was undertaken to try to identify the reasons for the poorer performance of mercury electrodes whose readily renewable, defect-free surfaces typically produce nearly ideal responses from simple electrode processes such as the reduction or oxidation of reactants like the heteropolytungstates in which bond-making or bond-breaking does not accompany the
EXPER I MENTAL SECTION Materials. The following salts of heteropolytungstatesand their iron- and chromium-substituted derivativeswere prepared according to procedures in the references cited: a-Na3[PW1 2 0 4 1 -1OHZO,~"*~' a-&[ P2w18062]-14H20,7a*7cCY-&[PWl~03gFe(OH2)].14H20,7b a-K3[SiW11039Fe(OH2)]. 14H20,7ba-&[ PW I 10&r(OH2)].11 H20,6da& [PzW 17061Fe(OH2)].15H20,7band (Y~-K~[P~WI~~~ICT(OH~)]'~ 5H20.6d The identity and purity of each salt were confirmed by IR spectroscopy and cyclic voltammetry. Triply distilled mercury (Bethlehem Apparatus Co.) was used to prepare mercury electrodes. Other chemicals were reagent grade and were used as received. Solutions were prepared from laboratory distilled water that was further treated by passage through a purification train (Millipore). Buffer solutions were prepared from 0.5 M NaHS04 (pH 1-3) or 0.5 M CH3COONa (pH 4-6). Apparatus and Procedures. Conventional electrochemical cells and instrumentation were employed. The hanging mercury drop electrode (Brinkmann Model 410) had an area of 0.014 cm2. The dropping mercury electrode had a flow rate of 0.35 mg s-l. A 0.07 cm2glassy carbon electrode (BAS, Inc.) was employed. Cyclic voltammetry was conducted with a Pine Instrument Co. potentiostat (Model RDE-3), chronocoulometry was conducted with a BAS- lOOB electrochemical analyzer and polarography was done with a PAR Model 174 potentiostat. Potentials were measured and are reported with respect to an Ag/AgCl, 3 M NaCl reference electrode with a potential of 0.21 V vs SHE. Experiments were
Contribution No. 8938 from the Division of Chemistry and Chemical Engineering. ( I ) Souchay, P. Ions MinCraux CondensCs; Masson: Paris, 1969. (2) Tourn6, C. Bull. Soc. Chim. Fr. 1967, 3196, 3199, and 3214. (3) (a) Pope, M. T.; Varga, G. M., Jr., Inorg. Chem. 1966,5, 1249. (b) Pope, M. T.; Papaconstantinou, E. Inorg. Chem. 1967, 6, 1147. (c) Varga, G. M., Jr.; Papaconstantinou, E.; Pope, M. T. Inorg. Chem. 1970, 9,662. (d) Pope, M. T. HeteropolyandIsopoly Oxometalates; Springer Verlag: New York, 1983. (4) McEvoy, A. J.; GrBtzel, J. J. Electroanal. Chem. 1986, 209, 391. (5) (a) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1985,191,441.(b) J. Electroanal. Chem. 1988, 243, 87. (c) Keita, B.; Belhonari, A,; Nadjo, L. J. Electroanal. Chem. 1993,355, 235. (d) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1993, 354, 285 and references cited therein. (6) (a) Toth, J. E.;Anson, F. C. J. Electroanal. Chem. 1988,256,361. (b) Toth, J. E.; Anson, F. C. J.Am. Chem. Soc. 1989.11I , 2444. (c) Toth, J. E.;Melton, J. D.; Cabelli, D.;Bielski, F. H. J.; Anson, F. C. Inorg. Chem. 1990,29, 1952. (d) Rong, C.; Anson, F. C. Inorg. Chem. 1994, 33, 1064.
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(7) (a) Wu, H. J. Biol. Chem. 1920.43, 189. (b) Zonnevijlle, F.; Tourn6, C. M.; Tourn6, F. T.Inorg. Chem. 1982, 21, 2751. (c) Mansuy, D.; Bartoli, J.-F.; Battioni, P.; Lyon, D. K.; Finke, R. G. J. Am. Chem. SOC.1991, 113, 7222.
0003-2700/94/0366-3 124$04.50/0 0 1994 American Chemical Society
'\ I
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0
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I
-0.2 - 0 . 4
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1. Cyckvdtammogamsofcrr[P2WIP~,CT1II(OH2)l7-recOrded In 0.5 M NaHS0,adjusted to pH 1. (A) 0.07 cm2glassy carbon electrode, (B-E) 0.014 cm2 hanging mercury drop electrode. Reactant concentration: (A, B) 0 5 ; (C) 0.01; (D) 0.05; (E) 0.10 mM. Scan rate = 50 mV s-l. The potentialaxis shown appllesto all of the vdtat?mOgra" for which the plus sign (+) marksthe point of zero current and potentiil.
conducted at the ambient laboratory temperature of 22 f 2 OC.
RESULTS AND DISCUSSION The unusual adsorptive behavior at mercury of the chromium-substitutedanion (YZ-[ P ~ W I & C ~ ( O H ~ ) (here]~after abbreviated as P2W17Cr7-) is representative of the behavior of theother heteropolytungstate complexesthat were examined in this study. The striking contrast between the cyclic voltammograms obtained for this anion a t a glassy carbon electrode and those obtained at a mercury electrode is demonstrated in Figure 1. At the glassy carbon electrode, where adsorptionof the complex is not extensive, two reversible, diffusion-controlled, two-electron waves of essentially equal magnitude are obtained (Figure 1A). Both waves arise from delocalized reduction and reoxidation of the tungsten-oxocage of the heteropolytungstate.8.9 By contrast, when a mercury electrode is employed, the first voltammetric wave is much smaller than the second (Figure lB), and the symmetrical shapeof the first waveindicatesthat it arises fromthereduction and reoxidation of a reactant confined to the electrode surface.10 The effect of changing the concentration of the heteropolyanion is shown in voltammograms C-E in Figure 1. The magnitude of the first symmetrical wave is essentially (8) (a) Pop, M. T. in Mfxed Valence Compounds; Brown, D. B., Ed., Reidel Publishing: Dordrccht, 1980. (b) Piepgraas, K.; Barrows, J. L.; Pop, M.T. J . Chem. Soc. Chem. Commun. 1989, 10. (9) (a) Kozic, M.; Hammer, C. F.; Baker, L. C. W. J. Am. Chem. Soc. 1986,108, 2748. (b) Kozic, M.; Hammer, C. F.; Baker, L. C. W. J. Am. C k m . Soc. 1986, 108,7627. (c) Kozic, M.; Casan-Pastor, N.; Hammer, C. F.; Baker, L. C. W. J. Am. Chem. Soc. 1988, 110,7697. (IO) Brown, A. P.;Anson, F.C . Anal. Chem. 1977,49, 1589.
independent of the concentration of the reactant while the peak current of the second wave responds to changes in the reactant concentration. At concentrations so small that the calculated peak currents for the reduction of the dissolved reactant are negligible, the two waves exhibit comparable magnitudes, and both have symmetrical shapes (Figure 1C). This behavioral pattern indicates that the P2W&r7- anion is strongly adsorbed on mercury and that the adsorbed layer inhibits the reduction of the dissolved anion at potentials corresponding to the first wave. However, reduction of the dissolved reactant apparently proceeds at potentials where the adsorbed anion undergoes its second two-electronreduction so the peak current for the second reduction wave remains sensitive to the reactant concentration. To test this interpretation, single potential step chronocoulometry1*was carried out with a series of increasingly concentrated solutions of the heteropolytungstates. When the potential of the mercury electrode was stepped between 0 and -0.27 V, only the first voltammetric wave shown in Figure 1B was encompassed. The resultingchronocoulometric plots of charge vs (time)*I2 had intercepts which demonstrated that the electrode surface became saturated with the adsorbed reactant at bulk concentrationsbelow 1W M (Figure 2A). The quantity of the anion adsorbed was calculated from the intercepts after subtraction of the step change in charge that resulted when the potential step experiment was repeated in the absence of the anion. The slopes of the chronocoulometric plots were insensitive to the concentration of the P2W17Cr7- anion in solution (Figure 2B, solid points), demonstrating that the adsorbed layer prevented the reduction of the unadsorbed anions in solution. However, when the potential step was extended to -0.5 V so that the second reduction wave of the adsorbed anion was encompassed, the chronocoulometric slopes reflected the bulk concentration of the reactant (Figure 2B, open points), and the values of the slopes corresponded to the reduction of the dissolved anion by four electrons as expectedon the basis of the previously reported behavior at graphite electrodes.6d The intercepts of the plots increased as expected if each adsorbed anion was reduced by four electrons. The slopes and intercepts of the chronocoulometric plots obtained from the reverse potential step (-0.5 to 0 V)" showed that the reduced heteropolyanion remained adsorbed on the electrode. It therefore seems likely that the four-electron reduction of the dissolved reactant at -0.5 V occurs by means of rapid electron transfer between the reduced adsorbed layer and the anions in solution. The driving force for such a mediated reduction of the first two-electron step is greater with the more extensively reduced anion produced at - 0 . 5 V, which might account for the elimination of the blocking character of the adsorbed layer at the more negative potential. However, the driving force for the second twoelectron step is essentially zero (the average of cathodic and anodic peak potentials for the adsorbed and dissolved reactants showed that their formal potentials were almost the same), so the intrinsic electron-transfer reactivity of the adsorbed four-electron reduced anion is apparently greater than that of its half-reduced predecessor. A similar example of redox mediation of the electrode reaction of an electrostatically (11) Anson, F. C.; Osteryoung, R. A. J . Chem. Educ. 1983,60,293.
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Vd. 66, No. 19, October 1, 1994
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0.4
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Figure 2. (A) Chronocoulometric estimates of the adsorption of aT [P2W17081Cr111(OH2)] 7- on mercury vs the concentrationof the anion. The potential was stepped from 0 to -0.27 (0)or from 0.0 to -0.5 Supporting electrolyte as in Figure 1. (B) Slopes of the V (0). chronocouiometricplots of charge vs (time)1/2vs the concentration of ~ Y [ P ~ W I ~ O ~ I C ~ ~ I ~other ( O H ~conditions ) ] ~ - : as in A.
blocked anion was reported recently.12 Adsorption Isotherm for PzW&r7-. The adsorption of the P2W17Cr7-anion on mercury is so strong that the electrode surface becomes saturated with the complex when exposed to quite small concentrations of the anion. As a result, very dilute solutions of the anion had to be used to measure its adsorption isotherm. With such low concentrations, it was necessary to expose fresh hanging mercury drop electrodes to solutions of the adsorbate for extended periods to assure that adsorption equilibrium had been attained. The procedure employed was to scan the potential of the electrode continuously between 0 and-0.3 V in a stirred solution of the adsorbate until the cathodic peak current of the developing wave stopped increasing. The electrode potential was then held at 0 V for 5 min before a single sweep voltammogram was recorded. The area under the resulting cathodic peak (using the background just ahead of the peak as a baseline) was measured to evaluate the quantity of adsorbed P2W17Cr7- present on the mercury surface at each bulk concentration of the reactant. No corrections for contributions from the dissolved reactant were necessary because such low concentrationswere involved, and, in any case, the reduction of the dissolved reactant was prevented by the adsorbed layer. (12) Lee, C.;Anson,
3126
Figure 3. Adsorption isotherm for ay[P2W170e1Cr111(OH2)]7from 0.5 M NaHS0, adjustedto pH 1. The quantities of adsorbed reactant were evaluated from the area of the first cathodic vottammetric wave with a peak at -0.15 V as recorded at 50 mV s-I.
F. C.J . Elecrround. Chem. 1992, 323, 381.
Ana&ticalChemistty, Vol. 66, No. 19, October 1, 1994
0 I
0.2
C 0
-0.2 -0.4 -0.6 -0.8 EIV
Figure 4. Quantity of ( ~ ~ [ P ~ W ~ ~ O ~ ~ C r ~ ~ ~ ( O Hvs ~ )the ] ~ initial -adsorbed electrode potential. The quantitiesof adsorbedreactant were evaluated from the area of the anodic wave at -0.15 V except for the point at E = 0 V where the cathodlc wave was used. The concentration of the polyanion was loa M. Other conditions were as in Figure 1.
The results obtained are shown in Figure 3. Thevery small adsorbate concentration that corresponds to half-saturation M, demonstrates the high affinity of of the surface, 5 X the P2W17Cr7- anion for the mercury surface. Qualitatively similar behavior was observed for most of the other heteropolytungstate anions which were studied (vide infra). The particular concentrations corresponding to half-saturation are affected by the ionicstrength andcomposition ofthe supporting electrolyte, the electrode potential, and the identity of the heteropolytungstate, but the same general pattern was exhibited by all of the anions investigated. The effect of changes in the potential of the mercury electrode on the extent of adsorption of the P2W17Cr7- anion (1 X 106 M) and its reduction products was investigated by maintaining the electrode at various initial potentials for 5 min before recording the voltammetric responses. A plot of the areas under the first voltammetric waves (cathodic for initial and final potentials between 0 and -0.3 V and anodic for initial potentials between -0.25 and -0.80 V) vs the initial electrode potential is shown in Figure 4. A sharp decrease in the adsorption occurs near the potential where the electronic
113-3 M NaHS04 pH = 3
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Figure 6. Co-adsorption of R U ( N H ~ )at ~ ~mercury + electrodes on which ( Y ~ [ P ~ W ~ ~ O ~ ~ Cis ~adsorbed. ~ ~ ~ ( HVoltammetry ~ O ) ] ~ - of (A) 0.01 mM EIV EIV and (C) 0.01 mM [ P ~ W I ~ O ~ , C ~ ~ ~ ~(B) ( H0.01 ~ O )mM ] ~ - RU(NH&~+, , Supporting electro[ P ~ W , ~ O ~ I C ~ ~ ~ ~ ( 0.01 H ~ OmM ) ] ~Ru(NH&~+. Figure 5. Effect of changes in the ionic strength of the supporting acetate buffer at pH 4.6. The voltammograms in electrolyte on the cyclic voRammogramsof ( Y T [ P ~ W ~ ~ O ~ ~ C ~ ~ ~ ~lyte: ( O 0.5 H ~M ) ]sodlum ~A and C were recorded after the electrode potential had been cycled at concentrations of (A, B) 0.01; (C, D) 0.05, and (E, F) 0.5 mM. The until a steady response was obtained (5 min). Scan rate = 50 mV s-l. current scale is 0.1 pA except for F where S = 1 pA. Scan rate =
+
50 mV
s-l.
charge on the mercury changes from positive to negative13so that simple electrostatic repulsion is the probable reason for the decrease. Effects of Ionic Strength on Adsorption and Wave Shapes of the P2W&r7- Complex. The extent of adsorption on mercury of the highly charged P2W17Cr7-anion increases as the ionic strength of the supporting electrolyte is increased, presumably because of more effective screening of the electrostatic repulsionsamong the adsorbing anions. In Figure 5 , curves A and B, voltammograms are shown for the adsorbed complex obtained from a M solution of the anion in supporting electrolytes composed of M NaHS04 or 0.5 M Na2S04 adjusted to pH 3. Note that the second reduction wave becomes somewhat narrower as well as larger at the higher concentration of supporting electrolyte. This peak sharpening is probably the result of a decrease in the electrostatic repulsion among the adsorbed reactants at the higher ionic strength. The large influence that the ionic strength of the supporting electrolyte can make on the cyclic voltammetry obtained with more concentrated solutions of the P2W17Cr7- anion is made evident in Figure SE,F. The response obtained at low ionic strength is not much different from that obtained with or 5 X M solutionsof the heteropolyanion (Figure 5A,C). (13) Delahay, P. Double Luyer ond Electrode Kinerics; Interscience New York, 1965; Chapter 4.
The symmetrical wave shapes obtained with the 5 X 10-4 M solution (Figure 5E) demonstrate that the response is dominated by the adsorbed complex despite the relatively high concentration of the reactant in the solution. Only when the supporting electrolyte concentration is increased to 0.5 M does the dissolved reactant begin to contribute to the voltammetric current at the second wave in Figure 5F. It seems likely that the reduction of the highly negatively charged anion in solution is strongly repressed by the large, repulsive diffuse layer potential that is created at the electrode surface by the adsorption of the P2W17Cr7-anions.13In other words, the heteropolyanion interferes with its own reduction at mercury electrodes at low ionic strength by adsorbing on the electrode surface and creating a negative diffuse layer potential, which decreases the concentration of dissolved P2W17Cr7-anions at the electrode surface to the point that the rate of their reduction becomes negligibly small. Similar alterations in the wave shapes of adsorbed heteropolyanionsare produced by the addition of multiply charged cations to the supporting electrolyte. For example, the effect of Ru(NH3)a3+on the surface voltammogram obtained from a l e 5 M solution of P2W17Cr7- is shown in Figure 6. The addition of only M Ru(NH3)a3+ to the supporting electrolyte results in an increase in the magnitude of the peak current and a sharpening of the second reduction wave of the adsorbed P2W17Cr7- couple (Figure 6C). In addition, a large response from the R u ( N H ~ ) ~ ~couple + / ~ +appears near -0.2 V (Figure 6C). This responseclearly originates in R u ( N H ~ ) ~ ~ + Ana&ticaiCh8mistry, Vol. 66, No. 19, October 1, 1994
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cations that are co-adsorbed on the electrode surface because the much weaker response from the M Ru(NH~)~~+/~+ couple in the absence of P2W17Cr7- is shown in Figure 6B. Measurement of the areas under the peaks for the reduction of Ru(NH&s3+ and for the first, two-electron reduction of PzW17Cr7-in Figure 6C showed that 1.6 X 10-lo mol cm-2 of the former and 6.7 X lo-" mol cm-2 of the latter complex were adsorbed on the mercury electrode. These values correspond to 46 pC cm-2 of cationic charge for R u ( N H ~ ) ~ ~ + compared with 45 pC cm-* of anioniccharge from P2W17Cr7-. B I Thus, essentially all of the anionic change of the adsorbed PzW17Cr7- anions is compensated by the co-adsorbed Ru(NH3)63+cations despite the presence of 5 X lo4 times more Na+ than Ru(NH&~+cations in the supporting electrolyte. This behavior suggests that specific chemical interactions between the P2W17Cr7- and R u ( N H ~ ) ~ions ~ +are involved. One possibility is that an insoluble salt is formed on the electrode surface. An observation supporting this interpretation was the formation of a precipitate in M solutions of P2W17Cr7- when the concentration of R u ( N H ~ ) ~was ~increased above 5 X lo4 M in 0.01 M supporting electrolyte. Diffuse Layer Effects. The reduction of 0 2 at mercury electrodes occurs at potentials where there is extensive adsorption of P2W17Cr7-. The half-wave potential for the irreversible reduction of the uncharged 0 2 molecule would be expected to shift by an amount equal to the change in the diffuse layer potential that results from the adsorption of charged ions on the electrode surface.14 We therefore examined the reduction of 0 2 at mercury electrodes to test the suggestion that the adsorption of P2W17Cr7- anions on 1 I I I I I I mercury produces a negative shift in the diffuse layer potential 0.2 0 -0.2 0 . 4 -0.6 -0.8 1 (which we have argued is responsible for the blocking of the reduction of the unadsorbed anion). In Figure 7A, a dc EfV polarogram for the reduction of 0 2 at a dropping mercury Flgurr 7. dc polarographyfor the reductionof 0.28 mM 02.Supporting electrode is shown. At supporting electrolyte concentrations electrolytes: (A) 0.01 M HCQ; (B) as In A plus 0.01 mM ( Y T [ P ~ W I ~ O ~ ~ Cr1I1(OH2)I7-;(C) 0.t MHCIO,; (0)As In C plus 0.01 mM C Y ~ - [ P ~ W I ~ O ~ ~ below ca. 0.1 M, the polarograms are distorted by large Cr111(OH2)]7-. Scan rate = 2 mV s-l;drop tlme = 5 s. polarographic maxima which prevent reliable estimates of the half-wave p0tentia1s.l~ However, the addition of 5 X 10" M P2W17Cr7-to the solution eliminates the maxima (Figure net anionic charge introduced on the electrode surface by the 7B), attesting to the strong adsorption of the anion on the adsorption of the anion. The electronicchargeon the mercury mercury surface. The shift in the potential where the cathodic surface is apparently not altered substantially by the adsorption current increases without limit to less negative potentials in of P2W17Cr7-because the background charging currents are the presence of the P2W17Cr7- anion reflects the ability of not significantly different in the presence and absence of the adsorbed heteropolytungstates to act as catalysts for the adsorbate. Therefore, the total charge on the electrode surface reduction of protons at mercury and other electrode^.^ was assumed to be the difference between the (positive) In order to obtain reliable values of Ell2 for the reduction electronic charge as measured in 0.1 M HC10416 and the of 0 2 in the absence of P2W 17Cr7-,a polarogram was recorded (negative) anionic charge of the adsorbed P2W17Cr7-anions. in 0.1 M HC104 as supporting electrolyte where no maxima The value of 4 2 = -8 mV at 0.1 V vs SCE in 0.1 M HC104 was evident (Figure 7C). A second polarogram, recorded was obtained from the data of Parsons and Payne.16 In the after mol 1-l of P2W17Cr7- was added to the solution, presence of the P2W17Cr7- anion, 42 was first calculated by exhibited a half-wave potential which was 30 mV more negative assuming that the adsorption of 0.7 x 10-lo mol cm-2 of than that obtained in the absence of the P2W17Cr7- anion P2W17Cr7-increased the net negative charge on the electrode (Figure 7D). This difference in Elp values provides an surface by 45 /IC cm-2 to obtain a value of 42 = -70 mV.17 estimate of A42, the change in the diffuse layer potential The resulting value of A42 = -62 mV is larger than the observed produced by the adsorption of the P2W17Cr7- anion.14 This value of A E l p = -30 mV. However, the likelihood that the value may be compared with the value calculated from the adsorbing P2W17Cr7- anions induce the co-adsorption of Gouy-ChapmanStern theory to gauge the magnitude of the cations from the supporting electrolyte as demonstrated for
i
~
(14) Reference 13, Chapter
~~~~
9.
(15) Heyrovsky, J.; Kuta, J. Principles of Polarography; Academic Press: New York, 1966; Chapter XIX.
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(16) (a) Parsons, R.; Payne, R. 2.Phys. Chem. N.F.1975,98,9. (b) Parsons, R. Private communication. (17) Reference 13, Chapter 3.
Table 1. Adsorption of P2WI7Cr7- and Cd(I1) at Mercury from Perchlorate or Bromlde Supporting Electrolytes’ i o * T(mol cm-Z)b
supporting electrolyte
perchlorate
[P2W17Cr7-] (M) [Cd2+] (M) E(V)c W 10-5
0 10-5
0 10-3 10-3
-0.25 -0.75 -0.75
5.1
o
6.2
D
c.
az-[P2Wi7061Fe(0H2)l7-
bromide
Cd E ( V ) c W 0
o
-0.40 -0.75 -0.75
Cd
3.8 0 4.5 2.8 1.2
o
0 The adsorption was evaluated b single potential step chronocoulometry” in supporting electrolytes of0.l M HC104 (perchlorate) or 0.3 M NaBr-lO-’ M HBr (bromide). The initial electrode potential was -0.05 and-0.22V in theperchlorateand bromideelectrol es,respectively. b Quantities of P2Wl,Cr7-(W) or Cd(II)(Cd) adsorbed(tc The potential to which the electrode was stepped.
Ru(NH3)b3+ would decrease the net negative charge per adsorbed anion and diminish A42 accordingly. In addition, the value of -8 mV for 42 in 0.1 M HC104 at 0.1 V vs SCE is the result of extensive specific adsorption of perchlorate anions.16 It is quite likely that the more strongly adsorbing P2W17Cr7- anions displace C104- anions from the electrode surface, which would also result in a smaller value of A42 than that calculated above. Overall, the experimental results seem adequate to support the conclusion that the adsorption of the P2W17Cr7- anion in 0.1 M HC104 causes 42 to become significantly more negative. At lower ionic strengths, even larger changes in 42 would result so that the observed decrease in the rate of reduction of the P2W17Cr7-anions in solution caused by their adsorption on mercury electrodes is not surprising. For example, the rate of reduction of a 7- anion is calculated to diminish by a factor of ca. lo7 if 42 is changed by -60 mV.14 Interaction of Adsorbed a~-[P2WllO6lCr(OHz)]’- with Adsorbed Cd(I1). The anion-induced adsorption of cations such as Pb2+,Cd2+, and Zn2+at mercury electrodes has been studied extensively.18 The adsorption is quite strong in bromide electrolytes, and one might expect it to be able to compete with the adsorption of heteropolyanions. With this possibility in mind, the adsorption of P2W17Cr7- was examined in the presence of Cd2+in both a perchlorate-supporting electrolyte, where there is no adsorption of Cd(II), and in 0.3 M NaBr0.001 M HBr as supporting electrolyte, where Cd(1I) is extensively adsorbed. The results are summarized in Table 1. The adsorption of the P2W 17Cr7-anion from 0.1 M HC104 is increased slightly by the addition of Cd2+ to the solution, probably because the multiply charged cation behaves similarly to R u ( N H ~ ) ~in~ diminishing + the electrostatic repulsion among the adsorbed anions. In the bromide-supporting electrolyte,the adsorption of the heteropolyanionin the absence of Cd2+ was somewhat smaller than in the C104- (or HSOd-) solutions because of the stronger adsorption of bromide anions. When Cd2+was added to the solution, the adsorption of the P2W17Cr7-complex was further depressed but not nearly as much as the adsorption of Cd(I1) wasdepressed by the presence of the heteropolyanion. The affinity of the P2W17Cr7-anion for the mercury surface is evidently great enough to result in (18) (a) Murray, R. W.; Gross, D. J. Anal. Chem. 1966, 38, 392. (b) Anson, F. C.;Christie, J. H.; Osteryoung, R. A. J . Electroanal. Chem. 1967, 13, 343. (c) O’Dom, G.W.; Murray, R. W. Anal. Chem. 1967,39, 51. (d) Anson, F. C.; Barclay, D.J. Anal. Chem. 1968,40, 1791. ( e ) Barclay, D.J.; Anson, F. C. J . Electroanal. Chem. 1970, 28, 71.
I
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-0.2
-0.4
-0.6
EIV
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0
-0.2
-0.4
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EIV
Flgure 8. Cyclic voltammograms for dilute solutions of various heteropolyanionsrecordedat Hg electrodes. The concentration of the anions was M, except for PW12OM3- which was 10“ M. Supporting electrolyte: (A) 1 M NaHSO,, pH = 0.4; (6-E) 0.5 M NaHSO,, pH = 1. I n A, the voltammogram Isthe steady-stateresponse obtalned aiter multiple scans: In D, only the FelI1/Fdl wave Is shown. Scan rate = 50 mV s-l. Current scale = 0.1 /.LA except for A where S = 0.05 PA.
the partial displacement of both adsorbed Br- anions and the CdBr2 complex which is strongly bound to them. l8 Adsorption of Other Heteropolystates on Mercury. Although the adsorbed P2W17Cr7- anion exhibits some of the clearest cyclic voltammetric evidence of the strength of its adsorption on mercury, similar behavior was exhibited by most of the heteropolytungstate anions that were examined briefly in this study. Both the unsubstituted parent anions, P2W18062~ and PW12O4o3-, as well as their transition metal-substituted derivatives are extensively adsorbed on mercury from quite dilute solutions. A representative set of voltammograms recorded with solutions so dilute that only the adsorbed complexes contribute to the responses is collected in Figure 8. It is clear that all of the complexes examined in Figure 8 are adsorbed on mercury. The effect of the adsorption on the first reduction wave of the unadsorbed complexes in solution is qualitatively similar for [PW12040]~-,[P2WI8062]”, [hWi706iFe(OHz)l7-,[P2Wi706iCr(OH2)l7-, [PW11039Fe(OHz)]&, and [SiW11039Fe(OH2)1~-. In each case, the adsorbed complex prevents or interferes with the reduction of the unadsorbed complex. The single exception to this pattern is the [PW11039Cr(OH2)]” complex, which is adsorbed but produces little or no interference with the reduction of the unadsorbed complex in solution. Since the interference in the electroreductions is believed to result from the repulsion of the anionic reactants from the electrode surface by the negative diffuse layer potential generated at the electrode surface by the adsorption of the anions, the more negative potential where the [PW11039Cr(OH2)]”- anion is reduced may weaken its adsorption and account for its lack of interference in the reduction of anions in solution. The same factor may account for the lack of interference of the other adsorbed heteropolyAnalytical Chemism, Vol. 66,No. 19, October 1, 1994
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tungstates at the second reduction waves of the dissolved anions which typically occur at potentials near or more negative than those where the strength of the adsorption diminishes (Figure 4) *
Origins of Strong Adsorption. The origin of the unusually strong interaction between heteropolytungstate anions and mercury surfaces but not glassy carbon or graphite electrodes is difficult to specify with certainty. The possibility that what is ascribed to anion adsorption on mercury might actually correspond to the formation of an insoluble mercury(I1) heteropolytungstate salt on the electrode surface was considered. However, dc polarograms recorded with dropping mercury electrodes, which were scanned toward positive potentials, showed no evidence of anodic waves attributable to the oxidation of Hg in the presence of the P2W17Cr7-anion. Even with solutions of the heteropolyanion too dilute for the electrode surface to become saturated with the adsorbed anion during the lifetime of the growing drop (e.g., 10" M), no anodic response was detected. Similarly, when hanging mercury drop electrodes were maintained at potentials as positive as 0.2 V for extended periods in solutions of P2W17Cr7and subsequently scanned to more negative potentials, the areas defined by the cathodic voltammetric peaks were no greater than were obtained after a few seconds of exposure of a freshly extruded mercury electrode to the solution of P2W 17Cr7-. The quantity of P2W17Cr7-adsorbed on mercury electrodes at saturation as determined by chronocoulometry assuming a two-electron reduction of each anion was about 0.7 X 1O-Io mol cm-2. The molecular area of the adsorbed anion estimated from the known dimension of the P2W180& anionZocorresponds to ca. 1 X 10-lomol cm-2in a close-packed monolayer. The reasonable agreement between the measured and calculated quantities of P2W17Cr7- adsorbed at saturation provides added evidence against the formation of a Hg(I1) salt of the anion on the electrode surface. Such a salt would consume far more than two electrons per anion during its (19) Rong, C.;Pope, M. T. J. Am. Chem. SOC.1992, 114, 2932. (20) Dawson, B. Acta Crystallogr. 1953, 6, 113. (21) Baker, L. C . W. In Advances in the Chemistry of Coordination Compounds; Kirschner, S., Ed.; Macmillan: New York, 1961, p 604.
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reduction to Hg(0) and the reduced anion, which would lead to a calculated saturation coverage much greater than the observed 0.7 X mol cm-2. Thus, the available evidence favors spontaneous adsorption rather than surface precipitation or Hg(I1) salt formation as the process that is responsible for the accumulation of heteropolytungstate anions on the surfaces of mercury electrodes. The driving force for the adsorption may include the water structure-breaking character of the large anions that favors their exclusion by hydrogen-bonded solvents like H2O. Electrostatic attraction of the anions to positively charged mercury surfaces is not likely to be a major factor driving the adsorption because the large diameters of the heteropolytungstatesZ0 correspond to rather small average charge densities on the anions. Indeed, their large size diminishes the extent of their solvation,21which could be an important factor contributing to their unusually strong adsorption.
CONCLUSIONS The general tendency of a variety of heteropolytungstate anions to adsorb extraordinarily strongly on mercury electrodes as demonstrated in this study may prove useful in several contexts. For analytical purposes, the pre-concentration of the anions by adsorption on mercury electrodes may allow electroanalytical determination of the anions in the range of 10-'o-10-8 M. The already documented electrocatalytic activity of many heteropolytungstate a n i ~ n s ~makes * ~ J ~the application of strongly adsorbed coatings as electrocatalysts an attractive possibility. In preliminary experiments, we have observed significant electrocatalytic activity toward the reduction of multiple electron oxidants at electrodes on which heteropolytungstates were adsorbed. Experiments to explore the scope of such electrocatalyticapplications are proceeding. ACKNOWLEDGMENT This work was supported by the National Science Foundation. Scientific Parentage of the Author. Fred C. Anson, Ph.D. under J. J. Lingane, Ph.D. under I. M. Kolthoff. Received for review April 8, 1994. Accepted June 6, 1994." ~~
Abstract published in Advance ACS Abstracrs, August 1,
1994.
