Langmuir 1991, 7, 3197-3204
3197
Electrochemical, Thermodynamic, and Mechanistic Data Derived from Voltammetric Studies on Insoluble Metallocenes, Mercury Halide and Sulfide Compounds, Mixed Silver Halide Crystals, and Other Metal Complexes following Their Mechanical Transfer to a Graphite Electrode+*$ Alan M. Bond*,'and Fritz Scholz*32 Department of Chemical and Analytical Sciences, Deakin University, Geelong, Victoria 321 7, Australia, and Department of Chemistry, Humboldt University, Hessische Strasse 1-2, Berlin 1040, Germany Received August 7, 1990. In Final Form: March 21, 1991 The water-insoluble,or sparingly soluble compounds,ferrocene, cobaltocene,mercury dithiophosphate[Hg(Rzdtp)z],mercury and lead dithiocarbamate complexes [Hg(Rzdtc)z,Pb(Rzdtc)~], and mercury and silver halides and chalcogenates can all be mechanicallytransferred onto a paraffin-impregnatedgraphite electrode. In this way, the redox chemistry can be studied directly, commencing with the solid-state form of the compound. Additionally, normally inaccessible thermodynamic data may be obtained from voltammetric measurements. In each case, the mechanism for reduction or oxidation involvesa restructuring of the solid-electrode interface by electron transfer, diffusion, chemical, and adsorption reactions. In the case of ferrocene and cobaltocene, but only after multiple scans, data are directly comparable to that obtained in solution on the water-soluble form of the oxidized compounds. For Hg(Et2dtp)z and M(Et2dtc)z (M = Pb, Hg) formal potentials and stability constants can be calculated, the latter being in full agreement with literature values. The first scan of square wave or differential pulse voltammograms of CY- and p-Hg(Etzdtc)Zand red and black HgS illustrate the importance of solid-state effects. Studies on mixed crystals of AgC1-AgBr are consistent with potentiometric data and allow a distinction to be made between mixed crystals and mechanical mixtures and provide a method to determine the bromide to chloride ratio in the mixed crystals.
Introduction Voltammetric studies are commonly undertaken on solution-soluble species. However, many compounds are insoluble in important solvents such as water, and electrochemical studies on such compounds have therefore been restricted (e.g. refs 3-7) in such media. Recently, it has been shown that solids may be mechanically transferred to carbon electrodes and voltammograms can be obtained.&12 I t is apparent from these studies, which have been directed toward analytical applications using differential pulse voltammetry, that a wide range of solid compounds are amenable to direct voltammetric investigation in an extremely simple manner. + Presented in part at the Symposium on Organometallic Electrochemistry at the American Chemical Society Meeting held in Washington DC, August 1990. 1 Submitted for inclusion in the Arthur Adamson Festschrift Issue of Langmuir. (1)Deakin University. Present address: Department of Chemistry, La Trobe University, Bundoora, Victoria 3083 Australia. (2) Humboldt University. (3) Kulesza, P. J.; Galus, Z. J. Electroanal. Chem. Interfacial Electrochem. 1989,269, 455. (4) Brainina, Kh. Z.; Neiman, E. Ya.; Slepushkin, V. V. Inuersionnye Elektroonoliticheskie Metody, Khimia: Moscow, 1988. (5) Xiang, M. J.Electroanal. Chem. Interfacial Electrochem. 1988, 242, 63. (6) Gruner, W. Mikrochim. Acta 1986, I, 301. (7) Zakhrachuk, N. F.; Illarionova, I. S.; Lel'kin, K. P. Zh. Anal. Khim. 1988, 43, 1224. (8)Scholz, F.; Nitschke, L.; Henrion, G. Naturwissemchaften 1989, 76,71. (9) Scholz, F.; Nitschke, L.; Henrion, G. Damaschun, F. Naturwissenschaften 1989, 76, 167. (10) Scholz, F.;Nitschke, L.; Henrion, G. Fresenius' 2. Anal. Chem. 1989,334, 56. (11) Scholz, F.;Nitschke, L.; Henrion, G. Damaschun, F. Fresenius' 2. Anal. Chem. 1989,335, 189. (12) Scholz, F.; Nitschke, L.; Henrion, G. Electroanalysis 1990,2,85.
0743-7463/91/2407-3197$02.50/0
In this work, detailed studies on the mechanism of the redox processes of mechanically transferred solids are reported. The compounds investigated were carefully chosen to vary in solubility, structure, and redox chemistry. Initially, the oxidation process of sparingly soluble ferrocene and cobaltocene was examined and compared to the solution (reduction) electrochemistry of their oxidation products. These two examples represent cases where the oxidized form of the compound is water soluble and the reduced form nonionic and relatively water insoluble. Subsequently, a report is presented on the behavior of extremely insoluble compounds, which exist in different modifications (a-and p-Hg(Etedtc)z, red and black HgS). Studies on AgC1-AgBr mixed crystals further emphasized the importance of the structure of the solids. Finally, studies on Hg(Etzdtp)z, Hg(Etzdtc)z, Pb(EtZdtc)z, and related complexes (dtp- = dithiophosphate, dtc- = dithiocarbamate) provide evidence for the involvement of coupled chemical reactions in voltammetric studies of these insoluble compounds. All obtained results can be related to conventional electrochemistry in the solution phase. The systematic account provided by a study of a range of solid compounds demonstrates the widespread applicability of the proposed method to the electrochemical study of water-insoluble compounds.
Experimental Section All reagentsused were of analytical reagent grade purity. Millipore quality water was used for the preparationof the electrolyte solutions. The synthesis of the dithiocarbamate complexes is described elsewhere.'3-'5 The reference electrodewas Ag/AgCl (13) Bond, A. M.; Colton, R.; Dillon, M. L.; Moir, J. E.; Page, D. R. I n o g . Chem. 1984,23, 2883.
0 1991 American Chemical Society
Bond and Scholz
3198 Langmuir, Vol. 7, N o . 12, 1991 (saturated NaCl) or saturated calomel (SCE) and the auxiliary electrode platinum wire. AgC1-AgBr mixed crytals were prepared by melting appropriate amounts of bom compounds together. The resulting ingots had a polycrystalline structure. Other compounds were used as supplied by the manufacturer. Voltammetric experiments were performed at 25 1 OC with either a BAS 100, a BAS l00A (Bioanalytical Systems, West Lafayette, USA), a PAR Model 174A (Princeton Applied Research Corp., Princeton, NJ) or an ECM 700 (Academyof Science,Berlin, GDR) instrument. Experiments with the analog PAR instrument use a linear ramp, whereas the other instruments employ a staircase waveform. However, for calculations of potentials presented in this paper, agreement between the linear and staircase methods is better than A 2 mV. The preparation of the paraffin-impregnated graphite electrodes (PIGE) employed in this work is described in detail elsewhere.lWl2Briefly, spectrographic grade graphite rods (0.5 cm diameter, 3 cm length) were soaked in liquid paraffin (mp 70 "C) under vacuum until gas bubbles ceased to be evolved from the graphite rod. In the present study, 1-2 mg of the sample was placed on a glass plate and the lower flat surface of the PIGE was pressed and rubbed on the area containing the sample. The actual form of the sample (powder or crystals) was not found to be important in the case of the solids studied in this work. Superfluous solid substance was gently wiped off the electrode with tissue paper. Finally, the electrode was carefully immersed into the electrolyte solution so that only the prepared electrode surface made contact with the solution. A number of analogous experiments to those described in detail with a PIGE were performed with normal glassy carbon and pyrolytic graphite electrodes to ensure that the presence of paraffin is not essential to observe the voltammetric response described in this paper. At the PIGE, the components adhere to the surface for longer periods, but apart from this feature, no major differences were observed.
1100 p A
(a)
*
Results and Discussion Oxidation of Solid Ferrocene. Ferrocene has a low solubility of approximately 3 X 10" M in aqueous solution'6 and is therefore sparingly soluble, rather than being complete insoluble in water. However, the oxidized ferrocenium cation is very soluble. The studies on this welldefined Fc/Fc+ (Fc = ferrocene) redox couple provide considerable insight into the mechanism of the electrochemistry of solids attached to the graphite electrode. Figure l a shows the first to fourth scans (scan rate = 50 mV s-l) of a cyclic voltammogram of solid ferrocene attached to the graphite electrode and using aqueous NaF as the electrolyte. On the first scan, the oxidation process is extremely drawn out and no well-defined oxidation peak is observed. For all cycles after the first, both the oxidation (Epox) and reduction (&Ied) peaks are well-defined and both shift continuously toward more negative potentials, but approach a constant value after approximately 20 scans. The asymmetric peak shaped oxidation and reduction responses observed after the initial scans are consistent with the presence of linear diffusion of solutionsoluble species generated by the voltammetric experiment. After reaching the "steady state" with respect to the positions of the peak potential, the current gradually decreases in magnitude with increasing numbers of scans until eventually the response vanishes completely. For these "steady state" conditions, and with a scan rate of 50 mV s-', the peak separation is 82 mV (Epox= +274 mV, Epred = 192, ipox/ipred = 3.6). Similarly, the changes are not accelerated noticeably with respect to the number of scans by using a higher scan rate (e.g. 500 mV s-'). These ~~~
~
(14) Bond, A. M.; Colton,R.; Hollenkamp, A. F.; Hoskins, B. F.; McGregor, K. J . Am. Chem. Soc. 1987,109, 1969. (15) Bond, A. M.; Colton, R.; Hollenkamp, A. F. Znorg. Chem. 1990, 29,1991. (16) Bond,A. M.; McLennan, E. A.; Stojanovic, R. S.; Thomas, F.G. Anal. Chem. 1987,59, 2853.
0
POTENTIAL vs.Ag/AgCI \ V I
3.
1.
T
20pA
-1
(b)
0.5
P O T E N T I A L vs.Ag/AgCI [ V I
Figure 1. Voltammograms for oxidation of solid ferrocene mechanically transferred to a paraffin-impregnated graphite electrode. Electrolyte is aqueous 0.1 M NaF. (a) Cyclic voltammogram, first to fourth cycles, scan rate = 200 mV s-l. (b) Square wave voltammogram, amplitude = 25 mV, frequency = 14 Hz (tenth scan).
results imply that an electrochemically induced process is coupled with the oxidation-reduction cycle. Figure l b shows a square wave voltammogram (tenth scan) for oxidation of ferrocene. With square wave voltammetry, a progressive shift of peak potential toward more negative potentials is observed with each scan until eventually a constant value also is observed after approximately 20 scans. The peak potentials for the oxidation and reduction direction scans of cyclic square wave voltammograms for ferrocene attached to the electrode are given in Table I, along with the corresponding values obtained for oxidation of ferrocene dissolved in the solution at the saturated solution concentration level at graphite and glassy carbon electrodes. The latter potentials have the same values as obtained from cyclic voltammograms for reduction of a solution of the highly soluble ferrocenium cation. Important points to note are (i) there is a very close agreement between the data for the voltammetry of solid and dissolved ferrocene under "steady state" (20th scan) conditions and (ii) all data are consistent with presence of weak ferrocene adsorption,'G which is stronger for the paraffin-impregnated graphite electrode than for a glassy carbon electrode (peak potentials shifted toward more positive values). In the solution phase'6 the mechanism is well established to be Fcadss Fcbulk+ Fc'
+ e- (Fc = ferrocene)
(1)
When an experiment is begun with solid ferrocene, the initial scans are different from those observed in a solution phase study. However, under 20th cycle conditions, the two experiments are almost indistinguishable with respect to the peak potentials and correspond to the reaction given in eq 1. Additionally, it should be noted that in the case of the solid, after many cycles, the response vanishes
Voltammetric Studies of Insoluble Compounds
Langmuir, Vol. 7, No. 12, 1991 3199
Table I. Peak Potentials and Half Width, W1/2, of Cyclic Square Wave Voltammograms for Ferrocene at 20 OC dissolved ferroceneb solid ferrocenea PIGE Enox,V vs AgIAgC1 WUZ, mV
PIGE EDox, V vs AgIAgC1
glassy carbon Enox,V vs AdAgC1 Wl12. mV oxidation scan 0.252 i 0.002 140 i 5 0.260 i 0.002 160 i 5 0.204 i 0.002 95 5 reduction scan 0.244i 0.002 130 i 5 0.248i 0.002 150f 5 0.206 i 0.002 105 i 5 Measured from the 20th scan after transfer of solid ferrocene onto a PIGE with 0.1 M NaF as the electrolyte. * Measured in the solution phase with a PIGE and a glassy carbon electrode using a saturated solution of ferrocene dissolved in 0.1 M NaF.
W m mV
~~
(I
Scheme I diffusion into bulk 3all3b
I dissolved
species
I -0
/SOLID\
500
/v
ADSORBATE
VS.
Ag/AgCI ( V )
ELECTRODE
completely, whereas this does not occur in the solution study. Controlled potential oxidative electrolysis a t +0.5 V vs Ag/AgCl of solid ferrocene leads to a fast removal of all ferrocene from the electrode surface and the resulting bulk concentration of the solution-soluble ferrocenium cation is too low to be detected by cyclic voltammetry or spectrophotometry. This confirms that the amount of solid material attached to the surface is exceedingly small. A model to explain the electrochemistry of solid ferrocene is shown in Scheme I. Initially, the compound is present as a solid which is confined to the surface. However, since ferrocene has a finite solubility, reaction l a will occur, and it is this process that can initiate other pathways in the presence of an applied potential. For example, application of a positive potential will drive reactions l a and 3a in the forward direction and lead to a removal of the solid from the electrode surface. Subsequent reduction will drive reactions 3b and 2b and lead to a restructuring of the ferrocene attached to the surface. However, in any given oxidation-reduction cycle,an overall loss of material occurs from the electrode surface via net diffusion (reaction 3a). The paraffin must play the key role in initially holding the solid to the surface, whereas the adsorbed species formed during the course of the redox chemistry is bound to the surface by chemical forces in the usual way so that ultimately the process conforms to eq 1. In principle, the presence of paraffin could lead to partitioning of the compound across the water-paraffin interface. Under these circumstances, the experiments would represent voltammetry of an electroactive compound a t the immiscible phases of water-paraffin and paraffin-graphite. However, analogous experiments with ferrocene and other compounds at glassy carbon and pyrolytic graphite electrodes and indeed platinum elect r o d e ~ ~provide ’ equivalent data to that obtained when paraffin is present. In the absence of paraffin, the main difference is that the compounds do not adhere to the surface as strongly and prolonged experiments are not possible as is the case with PIGE. Additionally, it is unlikely that KN03 could enter the paraffin film to provide sufficient conductivity for voltammetry between the two (17)Bond, A. M.; Bobrowski, A.; Scholz, F. J . Chem. Soc., Dalton Trans. 1991,411.
T
T -
(b)
POTENTIAL0
.
V vs.Ag/AgCI -10 500
Figure 2. Cyclic voltammetry (scan rate = 200 mV s-l) for oxidation of solid cobaltocene mechanically transferred to a paraffin-impregnated graphite electrode. Electrolyte is aqueous 0.1 M NaF’. (a) First to fifteenth cycles. (b) After prolonged cycling and with background current subtracted from the response. immiscible phases to occur. Thus, if such a mechanism does occur a t all, then it is likely to be a minor pathway in the overall reaction scheme. In the present paper, this possible mechanism has therefore been neglected. Oxidationof Solid Cobaltocene. It is well-known that cobaltoceneis easily oxidized to the cobaltoceniumcation in organic solvents according to eq 2,18 although only very limited data are available in aqueous media.lg The Cc s Cc’
+ e- (Cc = cobaltocene)
(2)
electrochemical behavior of solid cobaltocene mechanically attached to the graphite electrode using NaF as the electrolyte resembles that of ferrocene but has some important differences which enable further aspects of the mechanism of the electrochemistry of solids attached to graphite electrodes to be understood. For cobaltocene, prolonged cycling (Figure 2a) is accompanied by a pronounced decrease in the peak separation, with oxidation and reduction peak ratios remaining close to unity and with the value of (Epox + Epred)/2being constant a t (-1.08 f 0.01) V vs Ag/AgCl. This average value of oxidation
(la) Geiger, W. E., Jr.; Smith, D. E. J.Electroanal. Chem. Interfacial Electrochem. 1974,50, 31. (19) Ovsyannikova, E. V.; Krishtalik, L. I.; Alpatova, N. M.; Shirokii, V. L.; Leonova, E. V. Elektrokhimiya 1989,25, 1348.
Bond and Scholz
3200 Langmuir, Vol. 7, No. 12, 1991
'
and reduction potentials is the formal potential, EfO,for the Cc+/Cc redox couple which suggests that all factors influencing the oxidation and reduction components of the experiment are symmetrical. In contrast, in the case of ferrocene, the ratio of peak currents is well-removed from unity and potentials for oxidation are more positive than E f O . The smaller the amount of solid cobaltocene attached to the electrode, the less is the peak separation for the first cycle. However, after about 15 cycles, the peak separation attains a value of (60 f 5) mV, which is the value expected for a reversible diffusion-controlled one-electron oxidation process. After continuing the cycling for lengthy periods, the peak currents for the diffusion-controlled process decrease to negligible values. However, in the absence of the diffusion-controlled pathway and after background subtraction, a process with a peak separation of close to 0 V becomes observable (Figure 2b), with the peak potential for both the oxidation and reduction processes being equal to -1.08 V vs Ag/ AgCl and equal to Ef". As in the case of the earlier calculation of Efofrom data involving a diffusion component, this result again implies complete symmetry of the oxidation and reduction component of the experiment when both oxidized and reduced forms are adsorbed. The zero peak separation indicates that very small amounts of cobaltocene are immobilized on the electrode surface. This condition leads to complete restriction of diffusion, and as theoretically expected for a reversible process, equal peak potentials for the oxidation and reduction scan directions are therefore observed. When cobaltocene was oxidized to the cobaltocenium cation with oxygen in 0.1 M NaF solution, reductive cyclic voltammograms of soluble M solutions of the cobaltocenium cation gave an Efo value of 1.09 f 0.01 V vs Ag/AgCl. When cobaltocene is transferred to the surface of a glassy carbon electrode, the voltammograms are very similar to those observed with a paraffin-impregnated graphite electrode. The calculated Ef"value a t the glassy carbon electrode is -1.07 f 0.01 V vs Ag/AgCl. The major differences between oxidation of solid cobaltocene and ferrocene and with reference to Scheme I are as follows: (i) during the first approximately 20 cycles, the voltammetry of cobaltocene does not contain evidence for a reversible adsorption process coupled to the charge transfer process (i.e. reactions 2a and 2b are absent); (ii) (Epox+ EPred)/2 = Efoand is virtually constant for all experiments; (iii) i,ox/ipr~ is unity; (iv)very small amounts of cobaltocene are surface confined and eventually voltammograms with zero peak separation are observed corresponding to a reversible surface reaction that is not coupled to diffusion. In contrast, the adsorption and diffusion pathways are coupled in the case of ferrocene. Reduction of Solid Mercurous Chloride. When mercurous chloride (calomel), which is much less soluble in water than ferrocene or cobaltocene, is mechanically transferred to the surface of the graphite electrode, phenomena additional to those already described for the sparingly soluble ferrocene and cobaltocene are observed. However, the low solubility does not preclude a voltammetric response from being observed. On the first cycle, starting at a potential of +0.4 V vs Ag/AgCl and scanning in the negative potential direction, no significant faradaic reduction current is observed until a fairly negative potential of -0.5 to -1.0 V vs Ag/AgCl is reached. Provided that the switching potential is very negative (-0.5 to -1.0 V vs Ag/AgCl) then during the second and subsequent cycles, a new and well-defined process is observed at potentials less negative than the switching potential. In
3.
3.
POTENTIAL Vs.Ag/AgCI(V
1
3.
Figure 3. Cyclic voltammograms (scan rate = 100 mV s-l) for reduction of HgZC12 mechanically transferred to a paraffinimpregnatedgraphite electrodefollowing two initial cycles (not shown) with a switching potential of -1.0 V vs Ag/AgCl. The third cycle is labeled. Electrolyte for (a) is saturated aqueous KN03and for (b) 0.1 M aqueous KN03.
0.1 M KN03, a positive shift in peak potentials combined with a decreasing peak separation is observed with each consecutive scan (Figure 3). The decreasing peak separation effect is believed to be mainly due to a decreasing iR drop accompanying the decrease in peak currents with each scan because in the more conducting saturated KNO3 solution (Figure 31, less change in peak separation is observed during prolonged cycling. If the first cycle is switched at 0 V vs Ag/AgCl, no response is observed in the initial or subsequent cycles,which impliesthat a plating of mercury onto the electrode is necessary during the first scan to observe the response shown in Figure 3 and that this can only be achieved at very negative potentials. Figure 4 shows a cyclic voltammogram obtained at a scan rate 2 mV after enough cycles have been achieved to reach a "steady state response". Figure 4b gives the square root of the wan rate dependence of the "steady state response" which demonstrates the steady state response is almost completely diffusion controlled at scan rates above 100 mV s-l. Evidence for chloride diffusion away from the electrode is obtained by noting the presence of a mercury stripping peak a t +0.8 V vs Ag/AgCl on the reverse of oxidation scan direction when this potential region is included in the cyclic voltammetry. Elementary mercury can be regarded to be a chloride-deficient mercury species. The mechanism for reduction of solid HgZC12 is therefore concluded to be as follows: During the first cycle calomel is reduced irreversibly at the graphite electrode a t very negative potentials (eq 3) to form mercury droplets
-
graphite
Hg,Cl, +2eThe reverse reaction
mercury
H g + 2e-
(3)
Hg + 2C1Hg,Cl, + 2e(4) occurs reversibly on mercury (eq 4) forming solid calomel attached to the mercury droplets and not to the graphite electrode. Loss of chloride, via diffusion, leaves a surplus
Langmuir, Vol. 7, No. 12,1991 3201
Voltammetric Studies of Insoluble Compounds
T
//
25OpA
-0.4J POTENTIAL V S . Ag/AgCI ( V I
0.:
< E
.-&
VS.
AglAgCl ( V
I
Figure 5. Cyclic voltammetry (scan rate = 200 mV s-l first, second, and third cycles) for reduction of solid Hg(Etzdtp)z mechanically transferred to a paraffin-impregnated graphite electrode. Electrolyte is aqueous 0.1 M KN03.
case with calomel may be regarded as being very insoluble in water. All four compounds exhibit parallel electrochemical behavior after the solids are transferred to a graphite electrode from which thermodynamically significant data can be obtained. Figure 5 shows the first three cycles for reduction of Hg(Et2dtp)z with 0.1 M KN03 aqueous solution as the electrolyte. On the first scan, a reduction response (eq 6) is observed at very negative potentials
0.1
-
graphite
ML,+2e-
M + 2L-
(6)
(L = Etzdtp, M = Hg) during which elemental mercury is deposited onto the graphite surface. The subsequent cycles include a chemically reversible couple mercury
ML2+ 2eFigure 4. (a) 'Steady state" cyclic voltammogram (scan rate = 2 mV 8-l) for reduction of HgZC12 mechanically transferred to a paraffin-impregnated graphite electrode. (b) Dependence of .,i red and ipoxfor the 'steady state" voltammogram on the square root of scan rate. Electrolyte is saturated aqueous KN03.
of liquid mercury attached to graphite which is oxidized irreversibly (eq 5) at much more positive potentials
Confirmation of the identity of this process was made by conducting experiments with mercurous and mercuric nitrate and noting that the peak potential for the mercury stripping process is in complete agreement with the literature.20 In contrast to oxidation of ferrocene or cobaltocene, where the electroactive product of electrolysis provides a diffusion and/or adsorption component, in the case of reduction of calomel it must be the nonelectroactive chloride ion which diffuses away from the electrode because both the electroactive oxidation product (HgpC12) and the electroactive reduction product (Hg) are almost insoluble in aqueous media and are immobilized at the electrode surface. Reduction of Solid Mercury Diethydithiocarbamate, [Hg(Etzdtcz)z], Lead Diethyldithiocarbamate, [Pb(Etzdtc)z], Lead Pyrrolidinecarbodithioate, [Pb(pydtc)2],and Mercury Diethyldithiophosphate, [Hg(Etzdtc)~],Complexes. The compounds H g ( E t p d t ~ ) ~ , Pb(Etzdtc)z, Pb(pydtc)2, Hg(Etsdtp12, and (HgL2) have very small solubility products (e.g. pK,(Hg(Etzdtc)z) = 43.5, pKS(Pb(Et2dtc)2)= 21.712' and therefore as is the (20) Brainina, Kh. Z. Zh. Anal. Khim. 1964,19, 810. (21) Kotrlg, S.;Sucha, L. Handbook of Chemical Equilibria in Analytical Chemistry; Ellis Horwood: Chichester, 1985.
e M + 2L-
(7)
at around 0 V vs Ag/AgCl and an oxidation peak at +0.8 V vs Ag/AgCl due to stripping of surplus mercury (some L- has escaped by diffusion). From the average value of the oxidation and reduction peaks of the chemically reversible couple (eq 7), it is possible to derive E f O , assuming that factors affecting the potential of both halves of the redox couple are symmetrical. This value can be determined relative to the Eo valuez2for the reaction for the noncomplexed metal (eq 8) M2++ 2e- s M (8) and therefore it is possible23 to calculate the overall conditional stability constant, 02, for the reaction
+
@a
M2+ 2L- s ML,
(9)
with relatively weak hydroxy and nitrate complexation being neglected. Pb(Etzdtc12, Pb(pydtc)z, and Hg(Et2dt& give analogous voltammetry to that described for Hg(Et2dtc)z so that the stability constants of these complexes also can be determined. Values of redox potentials used in the calculations under conditions given in Table I1 were as follows: Eopb2+p,= -0.125 V vs NHE (normal hydrogen electrode);22E O H ~ ~ = + /0.853 H ~ V vs NHE;22 Efo[Hg(Etzdtp)z] = -0.187 V vs Ag/AgCl; Ef0[Hg(Et2dtC)2]= -0.480vvs Ag/AgC1;Efo[Pb(Etzdtc)z] = -0.852 V vs Ag/AgCl; Efo[Pb(pyrrdtc)2]= -0.837 V vs AgCI; with (Ag/AgCl, satd. NaCl) being +0.203 V vs NHE. Table I1 summarizes the data obtained for the stability constants. The extremely good agreement with literature (22) Standard Potentials in Aqueous Solution; Bard, A. J., Parsons,
R., Jordon, J., Eds.; Marcel Dekker: New York, 1985.
(23) Heyrovskp, J.; Kuta, J. Principles of Polarography; Academic Press: New York, 1966.
3202 Langmuir, Vol. 7, No. 12, 1991
Bond and Scholz
-0.7 D
9 1-1
-1.500
10.
0W-
-0.7
-0.5
E:(H20) VS.
Ag/AgCI ( V I
Figure 7. Plot of formal potentials of mercury dithiocarbamate complexes in aqueous 0.1 M KN03 solution calculated from cyclic voltammograms of solids mechanically transferred to a graphite electrode versus formal potentials calculated from cyclic voltammograms at platinum electrodes for reduction of compounds dissolved in dichloromethane (0.1 M Bu4NC104). Data for water are taken from ref 27 and data for dichloromethane are taken from ref 11.
n - 0:s
A,
-116
P O T E N T l AL
vs. A g / A g C I ( V )
Figure 6. Cyclic voltammetry (scan rate = 200 mV 5-l) for reduction of Pb(Etzdtc)z (a) as solid Pb(Et~dtc)2mechanically transferred to a paraffin-impregnated graphite electrode (electrolyte is aqueous 0.1 M KN03) and (b) as a 5 x M solution of Pb(Et,dt& in dichloromethane (0.1 M BQNClO,) at a platinum electrode. See ref 15 for further details. Table 11. Comparison of Stability Constants Determined by Voltammetric Measurements on Solids Attached to a Graphite Electrode with Literature Values complex Hg(Et2dtp)Z in 0.3 M KNOs, 40% EtOH Hg(Et2dtc)Z in aqueous 0.1 M KN03 Pb(Et2dtc)z in aqueous 0.1 M KN03 Pb(pydtc)Z in aqueous 0.1 M KN03
this work 28.3 0.4
27.7
ref 24
38.2 f 0.2
38.07 f 0.3
25
17.7 f 0.2
17.7 (polarography) 18.3 extraction 17.1 (polarography) 16.8 (extraction)
26
*
17.2 f 0.7
lit. value
-1.0
-0.5
-1.5
P O T E N T I A L vs. Ag/AgCl IV \
Figure 8. Square wave voltammogram (first scan, amplitude = 25 mV, frequency = 14 Hz) for reduction of solid (A) a-Hg(Etndtc)z and (B) &Hg(Etzdtc)z mechanically transferred to a paraffinimpregnated graphite electrode. Electrolyte is aqueous 0.1 M KN03. Vvs.SCE
-E
26
shows confirms the interpretation of the voltammetry. Data obtained on a much wider series of complexes will be reported elsewhere.z7 Figure 6 a comparison of cyclic voltammograms of solid Pb(Et2dtc)z at a graphite electrode with an aqueous electrolyte and as c dissolved solution in dichloromethane at a platinum electrode. The remarkable similarity implies that parallel voltammetric behavior occurs under both sets of conditions even though one set of experiments commences with a solid and the other with a solution-soluble species. Figure 7 is a plot of the formal potentials calculated from voltammograms of the solid for a wide range of mercury dithiocarbamate complexes in waterz7 (24) Toropova, V. F.; Cherkasov, R. A.; Savel’eva, N. I.; Pudovik, A. N. Zh. Obshch. Khim. 1970,40, 1043. (25) Kemula, W.; Hulanicki, A.; Nawrot, W. Rocz. Chem. 1964, 38, 1065. (26) Scharfe,R. R.; Sastri, V. S.; Chakrabarti, C. L. Anal. Chem. 1973, 45, 413. (27) Bond, A. M.; Scholz, F. J. Phys. Chem., in press.
-7r
V
\i
v
Figure 9. Differential pulse voltammograms (first scan) for reduction of solid mercury compounds mechanically transferred to a paraffin-impregnated graphite electrode. Electrolyte is aqueous 0.1 M oxalic acid. For HgS the pH was adjusted to 5.0.
versus the formal potentials obtained when the complexes are dissolved in dichloromethane.’l The linear correlation further confirms that the voltammetry after the first scan of the solid mechanically transferred to the graphite electrode becomes dominated by solution reactions. Reduction of Compounds Existing in Structurally Different Forms. It is well-knownthat Hg(Ehdtc)z exists
Voltammetric Studies of Insoluble Compounds
Langmuir, Vol. 7, No. 12, 1991 3203
v
- E 0
I [
-E
-
vs.SCE 0.5
V v s SCE
v
-E 0
vs.SCE 0.5
t ! : : ! ! : : : : I
Figure 10. Differential pulse "stripping" voltammogramsof solid mercury compounds mechanically transferred to a paraffinimpregnated graphite electrode and reduced at -2.0 V vs SCE for 20 s. Electrolyte is 0.1 M oxalic acid. Above right: Voltammogram of HgO in a solution to which M NazS has been added.
Reduction of Compounds Existing in Structurally Different Forms. It is well-knownthat Hg(Etzdtc)zexists in several different crystallographic forms.28 Figure 8 contains the square wave voltammograms (first scan) of trace amounts of the a and /3 modifications mechanically transferred to a graphite electrode. The a form gives Epred = -1.074 f 0.003 V vs Ag/AgCl (average of seven measurements) while the (3 form gives Epred = -1.025 f 0.005 V vs Ag/AgCl (average of seven measurements) and has a small shoulder at a less negative potential. Both the peak potential and the peak shape are obviously influenced by the structure of the solid compound. Figure 9 includes the reductive differential pulse voltammetric scans of very insoluble black and red modifications of mercury sulfide. The differences in the voltammetry of the mercury sulfide compounds are pronounced and the order of the potentials is in agreement with the well-known higher chemical reactivity of black HgS compared to red HgS. Clearly, the initially observed voltammetry of the solid form includes information on the solid component. Reduction of Solid Mercury and Silver Halides and Chalcogenates. Figure 9 showsthe reduction (differential pulse voltammograms) of several mercury compounds (for example eq 10) to elementary mercury and anions. The anions
-
HgX + 2e- Hg + X2(10) are water soluble and can either diffuse away from the electrode surface or be adsorbed onto the electrode surface. The reduction potentials for these compounds are a (28) Iwasaki, H.Acta Crystallogr. 1973, B29, 2115.
Ag Br
7 AgC'
Y
f
Figure 11. Differential pulse voltammograms of solid silver compounds mechanically transferred to a paraffin-impregnated graphiteelectrode: (a)reduction mode; (b)strippingmode after a preliminary reduction at -2.0 V vs SCE for 20 8. Electrolyte is 0.1 M oxalic acid. function of electrochemical and chemical kinetics, thermodynamics, and surface activities on graphite. Figure 10 gives differential pulse stripping or inverse voltammograms obtained after a preliminary reduction of the mercury compounds to form elemental mercury on the graphite electrode. Since the electrolyte contains 0.1 M oxalic acid, the formation of mercury oxalate can be observed in all cases, except for reduction of HgS. Apparently, the sulfide ions are so strongly adsorbed on the deposited mercury that they do not escape from the surface via diffusion. Figure 10 also includes the stripping voltammogram obtained from solid HgO in a solution containing deliberately added sulfide ions. In this case,
3204 Langmuir, Vol. 7, No. 12, 1991
Bond and Scholz
potential depending on the molar ratio of chloride to bromide (Figure 12). The data are in full accordance with potentiometric studies29 and show essentially the same potential shift with change in molar ratio. The agreement with potentiometric and voltammetric data confirms the reversibility. Thus, it is obviously possible to distinguish in a simple and rapid manner mixed crystals of AgClAgBr from physical mixtures of the two compounds. In the latter case, separate voltammetric responses are found for the constituents.
’ O.: \
ln
LI 2 W
\\
I-
::
Conclusion 0
0.5 m o l a r ratio m
1
Br’ Figure 12. Peak potential of differential pulse voltammograms obtained for reduction of solid AgC1-AgBr united crystals mechanically transferred to a paraffin-impregnated graphite electrodeas a function of the molar ratio for bromide. Electrolyte is 0.1 M oxalic acid.
the voltammogram obtained from HgO is identical with that observed with HgS, confirming the importance of the solid-adsorption chemical interactions. The peak potential of the labeled peaks in Figure 10 follow the order of stability constants of these compounds as is expected from the previous discussion. The silver compounds exhibit parallel behavior to that described for the mercury compounds (Figure 11). Since AgCl and AgBr have peak potentials differing by about 300 mV under conditions of differential pulse voltammetry, it is possible to study the voltammograms of mixed crystals of AgC1-AgBr. These mixed crystals give voltammograms showing only one single peak with the peak
The voltammetric data for all water-insoluble or sparingly soluble compounds studied in this work exhibit behavior that can be explained by reaction Scheme I or a related reaction scheme. In each case, the electrochemical perturbation generates at least one solution soluble species. The differences in the specific behavior of the compounds arise from different chemical properties of the species participating in reaction Scheme I. Although the interaction of the different steps is fairly complicated, in many cases it is possible to extract electrochemical, thermodynamic, and mechanistic data on insoluble compounds, which may not always be easily gained from other techniques of measurement.
Acknowledgment. The work described in this paper was financially supported by the Deakin University Research Committee. The authors gratefully acknowledge this contribution. (29) Thiel, A. 2.Anorg. Allg. Chem. 1901, 24, 1.
