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Mass transport effects in electroanalysis of La3+ with alizarin derivatives at quaternized polyvinylpyridine modified electrodes
145
.I. Electroannl. Chem., 262 (1989) 145-160 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Mass transport effects in electroanalysis of La3+ with alizarin derivatives at quatemized polyvinylpyridine modif ied electrodes Kwok-Keung Shiu and D. Jed Harrison * Department (Received
of Chemistry
University of Alberta, Edmonton, Alberta T6G 2G2 (Canada)
20 June 1988; in revised form 28 November
1988)
ABSTRACT Electrochemical and spectroelectrochemical data show that at pH 9.2, La3+ forms complexes with alizarin red S and alizarin complexone in solution that do not change coordination number on reduction of the ligands to the hydroquinones. Modified electrodes sensitive to La3+ have been prepared by ion exchange of the redox active ligands alizarin red S or alizarin complexone into quatemized polyvinylpyridine films on glassy C electrodes. Distribution coefficients for extraction of various species into QPVP were found to be 1.2 x lo4 for alizarin red S, 2.4 x lo4 for alizarin complexone, 4.4~ lo4 for the alizarin complexone. La3+ complex, and 2.7 x 10’ for Fe(CN)i- , over the concentration range 0.02 to 1 pM in aqueous solution. Complexation of La 3f by polymer bound quinone occurs in the film, but the low film cation permeability results in low La3+ transport rates despite the presence of a coordinating ligand. Auger electron depth profiles show limited penetration of La3+ in the modifying film. Thin film ( - 3 nm) electrodes can be used to determine La3+ over the range 0.2 to 1 mM at pH 9.2. The analytical response arises in part from changes in transport properties in the film following complex formation.
INTRODUCTION
Chemically modified electrodes (CME) have been the subject of intense research for a number of years [l-S]. The nature of physical phenomena governing CME behavior has been examined in detail and many characteristics are now well understood. However, while several papers have appeared [6-161, exploration of the analytical utility of redox and ionic polymer modified electrodes has been relatively limited. A comprehensive understanding of factors that govern the performance of polymer modified electrodes as analytical tools, such as the properties of the polymer matrices, is still emerging [16-181. In many cases [8,10,11] the coulombic interactions within the polymer do not alone provide sufficient insight into the differing substrate transport behavior of the systems.
* To whom correspondence
0022-0728/89/$03.50
should be addressed.
8 1989 Elsevier Sequoia
S.A.
146
c$$r&~~~
*::; 0
0
ALfZARlN RED S
ALIZARIN COMPLEXONE
The large number of analytical complexation reactions developed for which one, or several, of the species involved is electroactive makes the development of ligand complexation based modified electrodes very attractive [6,8,10]. An experimental underst~ding of the factors that influence electrode performance will be important in utilizing the range of ligands available. These include the reversibility of the analytical reaction within an electrode film [6,9,10,18], and the effect of the polymer matrix on complexation [18] and mass and electron transfer processes [4,5,173 within the polymer. Equally important is an understanding of the influence of polymer charge and function~ty, in terms of coulombic and pe~sel~ti~ty effects, on the overall extraction process of an analyte into an electrode film. As part of a study of the effects of ligand properties, polymer charge, and film thickness on analytical complexation reactions at CMEs we have examined the electrochemically reversible anthraquinone derivatives alizarin red S (ARS) and alizarin complexone (AC), ion exchanged into partially quaternized polyvinylpyridine (QPVP) films on carbon electrodes, C/[QPVP],,,,. The alizarin derivatives have rich analytical metal complexation chemistry, and have been used in solution to determine electroinactive metals electrochemically [19-231, and as calorimetric reagents for a number of metal species [24-281. It is known that protonated polyvinylpyridine (PVP) is relatively permselective [3], excluding at least partially many cations from the polymer bulk, and cationic QPVP has similar properties [8,17,29]. The ability of an additions ligand incorporated in the polymer film, with a strong affinity for the metal ion, to overcome exclusion of the ion by the polymer matrix can thus be examined readily using QPVP. The influence of the matrix is a question of fundamental interest in the design of analytically useful modified electrodes, and the important role of film thickness on electrode performance in such a situation is illustrated in this paper. In addition it is found that a change in film transport properties can be used as a source of analytical ~formation.
Effect of complex formation on E,,, The formation and the electrochemistry of the La3+-quinone complexes discussed involves several equilibria that depend on pH and the stoichiometry of the oxidized and reduced forms of the Iigand and complex. The complexation reaction of metal, M, with the quinone, QHN, (N = total number of acid sites on quinone Q) can be written for various stoichiometries as M+xQH
N * M(QHN-JX
x=(0,1,
.*.,
x)
+ 4 H+
0)
147
where X is the highest order of complexation and 4 is the number of protons displaced on formation of a given complex stoichiometry. The effect of the formal concentration of M and QHN on shifts in the half .wave potential, E,,;, upon complex formation has been analyzed recently [30] using a formulation similar to that developed by Casassas and Eek [31]. When only one protonated form of the oxidized and reduced forms of the complex and the free ligand are significant, and the one-to-one complex is dominant for the oxidized ligand-metal complex, then eqn. (30) of ref. 30 can be used to describe the shift in half wave potential, AE,,,, for the ligand in the presence of the metal.
AE,,, = g
ln
1 + ,0;-;,i~$=[M] 1,x
C
(2)
PNR+*-i’,jaFIMlj
R, T and F have their usual meanings and activity coefficients are assumed to be unity. The LYE and ai”; are the fraction of the i th and i' th deprotonated forms of the quinone and_ hydroquinone derivatives, respectively. The term /3p_i,i is the overall pseudo formation constant [30] for the 1: 1 complex of the oxidized form of the ligand, as defined by Casassas and Eek [31]. The term /?t+2_ij,j is the overall pseudo formation constant for the jth complex with the i’ deprotonated form of the reduced ligand. The index j is given by j = 1, l/2, l/3.. . l/X, where X is defined in eqn. (1). The use of this formulation is described in ref. 30 and follows that introduced by Casassas and Eek [31]. Note that when formation of the oxidized form of the complex is significant, the condition PF_i,iay[M] >> 1 will hold. From a study of the effect of [M] on AE,,, at constant quinone concentration it is possible to determine whether the coordination number of the complex changes on reduction of the quinone. A similar but less general analysis has been presented by Florence and Belew for the case where only one complex stoichiometry is considered [32]. EXPERIMENTAL
Polyvinylpyridine (PVP) (molar mass 500,000 g) was a kind gift from Reilly Tar and Chemicals Corp. Quaternized PVP was obtained by reaction of benzyl chloride with PVP in refluxing methanol (22 h). Extent of quatemization, controlled by the ratio of benzyl chloride/PVP (0.66) used, was determined to be 64% by elemental analysis. (Found: Cl 12.12%, N 7.45!%, UC.: Cl 12.20%, N 7.53%). Alizarin red S and alizarin complexone were used as received (Aldrich). Water was twice distilled from alkaline permanganate. All other chemicals were reagent grade. Cyclic voltammetry and rotating disc voltammetry were performed with a Pine Instruments RDE4 potentiostat, Pine MSR rotator, and a Kipp and Zonen BD90 X-Y recorder. A PAR 174A polarographic analyzer and a Houston Instruments X-Y recorder were used for differential pulse (DP) voltammetry, with a scan rate of
148
1 mV/s, modulation amplitude of 5 mV and 0.5 s drop time. Chronoamperometry was performed with a PAR 273 with an IBM-PC-XT interfaced as controller and data acquisition system. Glassy carbon rods (Atomergic Chemetals, New York) were sealed in teflon sleeves, polished with alumina (Beuhler) to a 1 pm final polish, and rinsed in an ultrasonic bath. Pt flag electrodes for Auger electron analysis were contacted with Cu leads, and sealed in epoxy (Hysol-Clear) to leave one exposed face. A stock 2 mM QPVP (expressed in units of vinylpyridine monomer), 0.05 mM a,a’-dibromop-xylene methanol solution was prepared, and dilution of this to give 25 PM QPVP was necessary to obtain thin polymer films. In a closed container, saturated in methanol vapor, a few drops of polymer solution were slowly evaporated (- 3 h) on an electrode surface and then baked at - 60°C for 4 h in air. Evaporation in a methanol atmosphere produced more uniform films. Addition of cx,a’-dibromo-pxylene and baking at 60” C gave much more durable QPVP films, as reported previously [17]. To determine polymer coverage an electrode was soaked for 5 min in 1 mM K,Fe(CN), (0.1 it4 trifluoroacetate, pH 2), rinsed, transferred to pH 2 blank solution, and a cyclic voltammogram recorded at 10 or 20 mV/s. The total polymer coverage, expressed in pyridine units, I*,,, was calculated assuming total charge compensation by Fe(CN)i[33]. Distribution coefficients, K,, taken as the ratio of concentration of redox ion in the QPVP film to solution concentration, were determined following the method of Szentirmay and Martin [34]. At concentrations less than 1 PM, electrodes were allowed to equilibrate in stirred solution for 24 to 30 h, which was sufficient time to reach a nearly constant film concentration. At low concentrations, the QPVP film was sometimes partially lost from the electrode, and results were discarded when this occurred. At concentrations greater than 1 FM, equilibration times of 1 to 4 h proved sufficient. A thin layer spectroelectrochemical cell was prepared by sealing with epoxy (Clear-Hysol) a Hg plated Pt gauze (80 mesh, Johnson-Matthey) working electrode, Pt counter electrode, and Ag/AgCl pseudo reference electrode, between two microscope slides. Deaerated solution was introduced through a hole drilled in one of the slides at the bottom of the cell, which was sealed with electrical tape, and the top of the cell was then sealed with Parafilm. The cell was placed in a Hewlett-Packard UV-Vis photodiode array spectrometer and spectra were recorded every 2 min for 30 to 40 min. Reductions were usually complete by 15 to 20 min as evidenced by obtaining a constant spectrum. A Physical Electronics scanning Auger spectrometer, model 595 (Surface Analytical Laboratory, Dept. of Physics, Simon Fraser University) was used for Auger electron survey and depth profile analyses of surface modified Pt electrodes. A 3 keV electron beam, at a current of 50 to 100 nA, served as excitation source. Depth profiles were obtained by sputtering with a 3 keV Ar+ beam while scanning the element peaks of interest. Energy windows were C (240 to 300 eV), Pt (53 to 69 ev), La (70 to 90 eV and 600 to 647 ev). A Pt substrate was used to allow determination of the polymer/substrate interface during depth profiles.
149
RESULTS AND DISCUSSION
A variety of elcctroinactive metals [19-23,35-381 including several of the lanthanides [19-22,36-381, have been determined electrochemically using redox active ligands. Both alizarin red S (ARS) and alizarin complexone (AC) form complexes with La 3+ that show a shift in half-wave potential, AE,,,, for the quinone at the dropping Hg electrode. A knowledge of the stoichiometry of both oxidized and reduced forms of the complex is important if the shift in E1,2 is to be used to aid in identifying the metal complexed. However, no information on the reduced complexes has been reported, and there is insufficient and conflicting data on the oxidized complexes under the solution conditions employed. Further, it was necessary to determine the potential shifts of the complexes at carbon electrodes since these could differ relative to Hg due to the strong adsorption believed to occur at Hg. The cyclic voltammetric curves in Fig. 1 demonstrate that formation of the La * AC complex in solution with a 2 : 1 La3+ to AC ratio results in a new species with a peak reduction potential, Epr, of - 0.88 V vs. SCE, a shift of - 165 mV at a C electrode, similar to results at Hg. The peak heights are reduced; however, this effect was not further explored. Note the complex shows quasi-reversible electro-
I
I 0
I
I
!
I
I -0.4
,
I -0.8
I
I
,
I -1.2
,
E/Vvs.SCE
Fig. 1. Cyclic voltammetry at a naked glassy C electrode in 0.2 mM alizarin complexone (AC) (and 0.2 mM AC, 0.4 mA4 La3+ (- - -) in 0.1 M NH,, 0.1 M NH,Cl, pH 9.2 at a scan rate of 12 mV/s.
150
chemical behavior, with a peak separation, AEr, of 150 mV. With ARS as the redox active ligand a very similar electrochemical response is obtained; a shift in EPr of -135 mV is observed when 0.2 mM La3+ is added to a 0.1 mM ARS, pH 9.2 solution. A 0.1 M NH,Cl, 0.1 M NH,OH buffer solution, adjusted to pH 9.2 with HCI, was found to give optimum results for La3+ complexation by ARS and AC in solution. This ammonia buffer mixture was used in subsequent measurements using [QPVPI surfmodified electrodes unless indicated otherwise. Formation of La3+ complexes of AC and alizarin complexone 5-sulphonate with stoichiometries of 1: 1 and 2 : 2 has been reported [22,27,28], and when the metal to ligand ratio in solution is 1: 1, the 2 : 2 complex is dominant [22,27]. A detailed study of the 5-sulphonate derivative of alizarin has also been reported at pH 6.2 [27] (note that ARS is the 3-sulphonate derivative of alizarin). We undertook a spectrophotometric study of the stoichiometry of La3+ and ARS at pH 9.2. The data are in agreement with those for alizarin 5-sulphonate, in that complexes of both 1: 1 and 1: 2 metal to ligand ratio are formed. A detailed analysis of the absorbance data such as that done by Deane and Leonard [27] for alizarin 5-sulphonate and La3+ was not performed. However, it is reasonable to expect the 1: 1 ratio we observe for a La3+ and ARS complex in fact corresponds to formation of a 2 : 2 species, in the same manner as proposed for alizarin 5-sulphonate and La3+ [27]. For the latter ligand the extent of formation of the (alizarin 5-sulphonate . La), complex is very nearly constant as the ratio of La3+ to the ligand increases above 1 [27], and there is no evidence to the contrary for the ARS and La3+ complex. Formation of alizarin 5-sulphonate and alizarin complexone 5-sulphonate complexes with La3+ becomes less favourable at lower pH, and does not occur below pH 3 to 4 [27]. We find no spectroscopic or electrochemical evidence for formation of La3+ complexes of AC or ARS at pH 2 or less, consistent with the results for the 5-sulphonate derivatives. Consequently, La3+ complexation by AC or ARS in a polymer coated electrode (see below) at pH 9.2 can be completely reversed by transferring the electrode to a pH 2 solution. The extent of ionization of the quinone-La3+ complexes, and the number of protons involved in eqn. (1) has not been reported due in part to the complicated equilibria involved. We found dichloromethane, benzene, or diethylether extract less than 0.5% of any AC and ARS complexes of La3+ formed in aqueous pH 9.2 solutions with a 1: 1 metal to ligand ratio, suggesting that the complexes are charged. The fact that complexes can be extracted into the [QPVP],,,, film is consistent with anionic species formation. Both spectroelectrochemical and electrochemical measurements were undertaken to evaluate the stoichiometry of the reduced La3+ complex. Spectroscopic studies of the reduced forms of the free ligands and complexes were obtained with a thin layer spectroelectrochemical cell with an optically transparent electrode. The spectrum of both free AC at 0.2 mM in pH 9.2 buffer, and the complex formed in a 0.2 mM AC, 0.2 mM La3+ mixture at pH 9.2 is shown in Fig. 2a. The respective solutions were then exhaustively electrolyzed to generate the hydroquinone form of AC. Comparison of the two spectra in Fig. 2b, and the data in Table 1, indicates that a
151 0.20 a OXIDIZED 0.15
FORM , pH 9 2
-AC
0.2 mM
---AC
0.2mM
,
La 0.2mM
% f g
0.10
z 2 0.05
b REDUCED FORM , pH 9.2 Cl. 1s
-
AC 0.2 mM
---
AC 0.2mM
,
La 0.2mM
g g$ 0.10 0 i?ci 4: 0.05
WAVELENGTH
/ nm
Fig. 2. (a) Spectra of the quinone form of alizarin complexone (AC) and the La3+ complex in a thin layer spectroelectrochemical cell. (b) Spectra of the hydroquinone form of AC and the La3+ complex following exhaustive electrolysis at a Hg coated Pt mesh in the spectroelectrochemical cell. TABLE
1
Values of h,,,
a for quinone
and quinone.La3+
Species
&nax/~
AC AC.La(l:l)’ ARS ARS.La(l:l)’
334,520 553 334,520 536
oxidized
form
complexes
reduced 397,451 401,457 398,466 398,468
form b
(sh) d (sh) d
’ Wavelength of absorbance peak maximum. b Reduced form prepared by exhaustive electrolysis of oxidized form in thin layer spectroelectrochemical cell. ’ Complex formed in a solution 0.2 mM in metal and ligand. d This peak for the free ligand becomes a broad shoulder with lower absorptivity ( - 50%) for the complex.
152
complex with different spectral properties from the free hydroquinone persists upon reduction of the AC, La3+ mixture. The spectra for the La. ARS complex in oxidized and reduced form were obtained in similar fashion, and indicate a complex is also formed between the hydroquinone form of ARS and La3+, Table 1. While the peaks do not shift for the hydroquinone form of ARS in the presence of La3+, the absorptivity at 398 nm decreases - 50% and the shoulder at 468 nm becomes broader and about 40% less intense, extending to longer wavelengths as is the case for AC - La in the reduced form, Fig. 2b. An electrochemical study of the dependence of E,,*, determined from the peak potential of a differential pulse voltammogram, on the concentration of La3+ was also performed. With a constant concentration for either AC or ARS of 20 @I, E 1,2 was found to be constant within f 2 mV over a La3+ concentration range of 30 to 100 PM. Under these conditions, where the 1: 1 complex is dominant for the oxidized form, eqn. (2) can be applied, although since the system is quasi-reversible, eqn. (2) must be viewed as an approximate relationship for this case. Despite this consideration, lack of a change in AE,,, for the complex, when combined with the spectral data, provides evidence that the stoichiometry remains unchanged for both oxidized and reduced species in the presence of excess La3+ [32]. Polymer modified surfaces The simplest route to preparation of a modified electrode is through ion exchange of anionic ARS or AC into a quatemized polyvinylpyridine film [l-3,8,10,29] on a C electrode surface, to give a quinone modified electrode, C/[QPVP . ARS] surf or C/[QPVP . AC] surf. At pH 9.2, more than 98% of ARS is present in the 2 - form [24], while based on the pK,s for AC, approximately 80% is present as the dianion and 20% is in the 3 - form [39]. The alizarin derivatives are strongly bound in the polymer film, as determined from the partition isotherm for extraction of electroactive ions over a concentration range of 20 04 to 1 mM. Values of the apparent distribution coefficients, K,, are reported in Table 2. At concentrations of 1 @I or less the isotherm is linear and the values of K, are of the order of lo4 to 105, while above 0.1 mM the polymer nears saturation and K, decreases. For Fe(CN)i-/4equilibrium is established and a true distribution coefficient is measured. However, even after 24 h soaking in 2 M KCl, 70-80% of the quinone remains in the film, whereas all of the Fe(CN)),-14can be removed. Consequently, while the measured distribution coefficients indicate clearly the very strong affinity of [QPVP],,,, for the quinones and the 1: 1 La . AC complex they may not reflect the true value of K,. This is shown in Fig. 3 by the cyclic voltammetric response of a C/[QPVP],,,, electrode in a blank solution, pH 9.2, containing only ammonia buffer, following soaking in a 1 mM Fe(CN)i-, 1 mM ARS, pH 9.2 solution. The ratio of surface coverages, JYARs/l?FtiCNj;-, for such an experiment is typically 3.5 to 4, despite the fact that the distribution coefficients favor Fe(CN)z- over ARS by a factor of - 2. Apparently the irreversibility of the quinone extraction results in its dominance in the film, demonstrating the poor values. The hydrophobic nature of the predictive power of the apparent K,
153 TABLE 2 Distribution ratio, K,, in the QPVP matrix Ion
KD
a
0.02-l Fe(CN):Fe(CN);AR!? AC*AC.La(l:l)d a Distribution b Determined ’ Determined d Prepared as
pi%4b
2.7 x 10’ 1.2 x 104 2.4x lo4 4.4x104
lmh4’ 411 266 156 372
ratio, K, = cp/c,, at pH 9.2, 0.1 M NH,Cl, 0.1 M NH,OH supporting electrolyte. from the slope of the linear portion of the partition isotherm over the range 0.02 to 1 pM. at 1 mM. a 1: 1 mixture of AC and La3+.
quinones, their relatively large size, and the distance of separation of the multiple charge sites on the ions, may decrease mobility once bound in the film, compared to inorganic complexes and the singly charged quinones studied previously [33]. Several qualitative studies have shown that cations such as Ru(NH,)z+ and Co(bipyridyl), 3+ do not easily p enetrate [QPVP],,, or protonated [PVP],,,, films [3,8,17,29,33], as shown by a 90 to 99% reduction in cyclic voltammetric peak heights. A knowledge of the permeability or K, for La3+ in [QPVP],,,, films would aid in determining whether metal transport or extraction in the film is a limiting process in complex formation. Unfortunately, due to the difficulty in preparing pin-hole free films, attempts to determine the permeability of La3+ quantitatively by the method of Anson and co-workers [40], br of Eu3+i2+ by rotating disk voltammetry [l-3] were unsuccessful. However, cyclic voltammetry showed that the current for Eu3+12+, an ion very similar to La3+, was reduced at least fiftyfold at a [QPW surfelectrode compared to a naked surface. Large quantities of ARS or AC can be accessed electrochemically within the films, but when IpY 2 lo-* mol/cm2, only 40 to 80% of the quinone present is electroactive. This was determined by comparing the apparent I,, coverage determined by exchanging Fe(CN):and either ARS or AC sequentially into the polymer films from 5 mM solutions, and recognizing that lYFe(oNj;- is a relatively reliable indicator of the total pyridinium sites present [33]. When I’,, < lo-* mol/cm2, the charge in the films can be totally compensated by the alizarin derivatives, on the basis of the electrochemistry, which indicates that all of the quinone present is electroactive. The apparent diffusion coefficient for charge transport, Dct, by ARS or AC in the QPVP film has been determined by chronoamperometry in solutions of supporting electrolyte, pH 9.2, and by rotating disc voltammetry [2,3-5,331, in 5 mM quinone, pH 9.2. The results indicate a slow charge transport process, with Dct varying from 0.2 to 1 x lo-” cm2/s for both free ligands in relatively thick polymer = lo-* to 10e7 mol/cm2). In comparison, DC, for oxidation of Fe(CN)zfilms ( rpy at pH 9.2 in the same polymer is 1 to 3 X lo-” cm2/s.
154
C/QPVP.AC J
C/OPVP.(ARS. -4 -
Fe (CN),“)
0.1 M NH4Cl 0.1 M NH4OH
Q, L
0
40.4
I
I
I
0
I
L
-0.4
I -0.8
I -0.4
E/Vvs.SCE
Fig. 3. A C/[QPVP.ARS,
Fe(CN):-lSuti
-0.6 EIV
-0.8
-1 .o
VS SCE
electrode response in pH 9.2 solution of supporting electrolyte
alone, following equilibration in a 1 mM ARS, 1 mM K,Fe(CN),, is - 2 at pH 9.2, and it clearly competes strongly with Fe(CN)i-
pH 9.2 solution. The charge on ARS for ion exchange sites in the polymer.
Fig. 4. Differential pulse voltammogram of high coverage, ppy = 2~ lo-* mol/cm*, C/[QPVP.AC],,,, electrode in blank, pH 9.2 supporting electrolyte () and with 1 mM La’+ added (- - -). Scan rate 1 mV/s, modulation amplitude 5 mV, drop time 0.5 s.
Thick film behavior Initial experiments with relatively high coverage of polymer films, 5 x 10h9 to 5 X lo-’ mol/cm2 of pyridine sites, demonstrated that a La,. AC,, complex is formed within the [QPVP . AC],,,r film. A differential pulse (DP) voltammogram of a C/[QPVP - AC1surfelectrode in a blank, pH 9.2 solution results in a peak at -0.64 V for the surface bound quinone, Fig. 4. Transfer of the electrode to a 1 mM La3+ solution, pH 9.2, gives a differential pulse voltammogram with a peak at -0.64 V for AC, and a new peak at - -0.82 V, as shown in Fig. 4. Similarly a WQPVP . A=1 surfelectrode develops a new peak at - -0.81 V on exposure to La3+, with the same ratio of free ligand to complex peak current as observed for AC modified electrodes. These results are consistent with partial conversion of the quinone, Q, in the film to a La,. Q, complex, although the stoichiometry of the complex is undetermined. The extent of complex formation appears small, and the peak current for the complex is relatively independent of La3+ concentration, showing a high degree.of scatter in i, as La3+ concentration is varied from 0.05 to 2 mM. Given the difficulty with pinhole formation in the film, the current for the complex may arise from complex formed near the electrode surface in the vicinity of a pinhole. This would account for the variability observed.
155
In contrast, if a C/[QPVP],,,, electrode is exposed to a 0.2 mM AC, 0.2 mM La3+, pH 9.2 solution, in which a 1 : 1 complex already exists, then a DP voltammogram shows a single peak at - 0.83 V. Exposure of a C/[QPVP . AC],,,r electrode to the same solution does not result in significant formation of the complex in the film. These results indicate that the complex is durable within the film, and can diffuse into an electrode which initially contains no strongly bound species such as AC or ARS. However, the complex is apparently unable to diffuse rapidly in a film which is pre-saturated in quinone. This may reflect a change in film structure when the bulky quinone acts as counterion; a well documented occurrence [l-5,29] for more highly charged ions such as Fe(CN)i-/4in polycationic films. An Auger electron depth profile of a thick film Pt/[QPVP . ARS],,,, electrode, I,, = 6.5 X lo-’ mol/cm2, exposed to 1 mM La3+, pH 9.2, is shown in Fig. 5a. The depth profile reveals that La is present primarily at the solution/polymer interface and does not permeate the entire film. Depth profiling of Pt/[QPVP. AClsurf electrodes prepared in the same manner also show this phenomenon. Depth profiles of Pt/ [QPW surfelectrodes exposed to 1 mM La3+, pH 9.2, with no quinone
(b) Pt
2 min
C
CH:
La L.3
SPUTTER
TIME
Fig. 5. Auger electron depth profile of (a) high coverage Pt/[QPVP. ARS],,,, electrode, lYpy= 6.5 X lo-’ mol/cm*, and (b) low coverage Pt/[QPVP. ARS],,,, electrode, rpy = 1 X 10W9 mol/cm*, after exposure to 1 mM La3+, pH 9.2 solution.
156
present show that La3+ does not penetrate the [QPVP],,,, film. In some cases a small La signal is observed on an initial survey scan, but the signal is - 30 times smaller than that for Pt/[QPVP - ARS],,,, electrodes exposed to La3+ at pH 9.2, and is not present after even a brief Ar+ sputter (- 5 s). A Pt/[QPVP],,, electrode exposed to a 0.1 mM ARS, 0.1 mM La3+, pH 9.2 solution was also examined and the depth profile indicated a uniform La 3+ distribution up to the polymer/electrode interface. This confirms the conclusion, based on electrochemical measurements (see above), that the complex is readily transported in a fresh [QPVP],,,, film containing no quinone. For modified electrodes exposed sequentially to ARS or AC, then La3+ solutions, depth profiling demonstrates that La3+ transport is a serious problem within [QPVP . ARS or AC],,, films, despite the ability of the quinone containing films to extract La3+ from solution. Thin film behaviour To offset the problem of low La3+ transport rates, much thinner QPVP films have been used, with Fry G 2 X lop9 mol/cm2. Figure 5b shows a depth profile of a Pt/[QPVP . ARS],,, electrode, I,,, = lop9 mol/cm2, that was exposed to a 1 mM La3+, pH 9.2 solution. The Pt signal is observed in an initial survey scan, indicating film thickness of less than 3 nm or the presence of pinholes in the film. This is consistent with the measured I,,, value for which a thickness of - 3 nm is expected; additionally the uniformity of such a thin film is uncertain. Significantly, the La signal declines at nearly the same rate as that for C, indicating La is present throughout the film. The electrochemical response (see below) is also consistent with nearly complete permeation of La3+ within thin [QPVP . ARS],,,, films. A DP voltammogram in a blank, pH 9.2 solution of a thin fihn C/[QPVP . AC],,,f electrode with I,, = 2.0 X 10e9 mol/cm2 shows a reduction peak at -0.71 V vs. SCE, Fig. 6. This is equal to EPr for the solution species and is - 60 mV more negative than for AC in thicker films. The difference in potentials suggests the environment in the thin polymer film differs from that of the bulk polymer and is perhaps more “water-like” or open in structure. However, the thickness dependence of the potential shift was not examined further. The most striking effect upon immersion of the electrode in La3+ solution is the large increase in peak height, which is accompanied by a voltage shift of E,, to -0.80 V in 0.4 mM La3+, pH 9.2. We find that the change in i, for a C/[QPVP . AC or ARS],,,, immersed in La3+, pH 9.2 solution is proportional to La3+ concentration over a limited range. A plot of the increase in i,, determined as i, for the quinone in a La3+ containing solution minus the peak current in a blank solution, i, - i,([La3+] = 0), is shown in Fig. 7 for both quinone ligands. Between exposure to La3+ solutions, the concentration of quinone in the polymer is recovered by immersion in a 5 mM solution of ARS or AC. Repeated transfers between a quinone solution and a solution of a given La3+ concentration do not result in further change in i, in the La3+ solution. Consequently, the increase in i, with increasing [La3+] cannot result from an increase in complexing agent within the polymer film during re-exposure to quinone solutions.
157
1.6
1.2
Q,
0.8
0.4
01
I
I
-0.4
,
-0.6
1
1
-0.6 ElVvs
I
-1 .o
SCE
Fig. 6. Differential pulse voltammogram of C/IQPVP.AC],,,, electrode, PpY = 2X10W9 mol/cm, in blank, pH 9.2 supporting electrolyte (- - -), and after transfer to a 0.4 mM La3+, pH 9.2 solution See text for a discussion of electrode preparation. ( -).
I’
”
’ .
”
‘I
”
6.4
4.6:
/\\\
3.2
.
-
l-6 1.6
/:,/
0
0
,
/ 0.4
, 0.6
,
, 1.2
,
,
,
:
1.6
[La3’] /mM Fig. 7. Dependence of net peak reduction current, expressed as i, - ip([La3+ ] = 0). on La3+ concentration determined by differential pulse voltammetry at pH 9.2 for (0) C/[QPVP-ARS],,,, electrode with electrode with Pry = 2 x 10e9 mol/cm2. Solid ppY =1.2X10e9 mol/cm2 and for (A) C/[QPVP.AC],,,r lines are for clarity only, to connect data points.
158
Using chronoamperometry the diffusion coefficient for charge transfer in the [QPVP . AC, La3+],,,r film was determined as a function of [La3’] in solution. The term cDz,12, where c is the concentration of the ligand in the film, increases as the La3+ solution concentration increases up to 0.8 mM, and then decreases at higher concentrations. These data are consistent with the observed changes in differential pulse peak height [41] at the modified electrode. The cause of the effect is uncertain, but, since it does not arise from an increase in complexing agent in the film, it is most likely due to changes in film crosslinking and structure as the La,. AC,, complex is formed. Increases in Dct as the extent of ionic crosslinking decreases are a common observation for electrostatically bound redox species. Since the anionic charge of the quinone . La complex will be lower than that of the free quinone, ionic cross-linking would be expected to decrease as more La3+ is extracted into the film. The decline in signal observed at high [La3’] at the modified electrode is consistent with a decline in i, observed at naked electrodes in 0.2 mM ARS solution when [La3’] is greater than 1 mM. This may be due to the complex series of hydrolysis equilibria for La3+ at high pH that includes formation of polynuclear La species [42] as the La3+ concentration increases. Additionally, in solutions with [La3’] > 1 mM and [La3’]/[Q] > 2, we observe formation of a purple precipitate upon standing for 24 h.
CONCLUSION
The Auger depth profile results demonstrate two important features of the La. Q systems. The first is the evidence that mass transport of La3+ in the film, and not charge transport between redox sites, is the factor that dictates the requirement for thin films. The other relates to the mechanism of charge transport in the film. For electrostatically bound redox species it is difficult to determine whether site-site exchange of the electroactive species, or electron transfer through self-exchange is the dominant mechanism of charge transport. In the case of the La. Q systems, site-site exchange of the complex would lead to a uniform distribution of La3+ in the film, in contrast with the data obtained by Auger depth profiling, Fig. 5a. Consequently, at least for the complex, the so-called electron hopping mechanism between redox species arising from electron self exchange must be the dominant form of charge transfer in the film. A unique feature of the response of [QPVP. Qlsurf films to La3+ is that the analytical response arises in part from the change in film transport properties as La3+ is extracted into the polymer film. The increase in Dct could result from an increase in the self-exchange rate, k,, for the complex relative to the ligand; however, based on the quasi-reversible nature of the cyclic voltammetry for the solution complex, Fig. 1, a decrease in k, is more likely. An increase in the mobility of the complex in the film, due to the decreased charge of the complex relative to the free ligand, is a more likely cause of the increase in DC, as [La3’] increases. The increase in mobility would then lead to enhanced site-site exchange, or more
159
frequent collisions resulting in increased rates of electron self-exchange. The latter suggestion is consistent with the Auger depth profile data, as discussed above. In the design of modified electrodes for analysis of metal ions, attention should be given to controlling permselectivity of the polymer matrix to reduce problems associated with substrate transport. This may be accomplished through increasing the solvent compatibility (swelling) of the film, varying the charge of the ionic sites, or by incorporating a high density of functional groups that can form labile complexes with the analyte, in addition to the presence of a strongly complexing analytical agent. ACKNOWLEDGEMENTS
We wish to thank J. Buechler and Reilly Tar and Chemical Corporation for a generous gift of 400,000 and 500,000 g molar mass polyvinyl pyridine, and B. Heinrich of Simon Fraser University (SFU) for arranging access to the SFU Surface Analytical Laboratory. We thank the Central Research Fund (University of Alberta) and the Natural Sciences and Engineering Research Council of Canada for support of this research. REFERENCES 1 R. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1983, pp. 191-368. 2 W.J. Albery and A.R. Hillman, AMU. Rep. C, R. Sot. Chem., 78 (1981) 377. 3 N. Oyama and F.C. Anson, J. Electrochem. Sot., 127 (1980) 640. 4 C.P. Andrieux and J.-M. Saveant, J. Electroanal. Chem., 134 (1982) 163. 5 F.C. Anson, J.-M. Saveant and K. Shigehara, J. Phys. Chem., 87 (1984) 214; J. Am. Chem. Sot., 105 (1983) 1096. 6 H.C. Hurrell and H.D. Abruiia, Anal. Chem., 60 (1988) 254. 7 A.R. Guadalupe, S.S. Jhaveri, K.E. Liu and H.D. Abruiia, Anal. Chem., 59 (1987) 2436. 8 M.J. Gehron and A. Brajter-Toth, Anal. Chem., 58 (1986) 1488. 9 L.M. Wier, A.R. Guadalupe and H.D. Abruha, Anal. Chem., 57 (1985) 2011. 10 A.R. Guadalupe and H.D. Abruha, Anal. Chem., 57 (1985) 142. 11 J.A. Cox and P.J. Kulesza, J. Electroanal. Chem., 175 (1984) 105; Anal. Chim. Acta, 165 (1983) 71. 12 G. Sittampalam and G.S. Wilson, Anal. Chem., 55 (1983) 1608. 13 M. Petersson, Anal. Chim. Acta, 147 (1983) 359. 14 J.F. Price and R.P. Baldwin, Anal. Chem., 52 (1980) 1940. 15 G.T. Cheek and R.F. Nelson, Anal. Lett., 11 (1978) 393. 16 L.D. Whiteley and C.R. Martin, Anal. Chem., 59 (1987) 1746. 17 J.K. Doblhoffer and R. Lange, J. Electroanal. Chem., 229 (1987) 239. 18 C.M. Lieber and N.S. Lewis, J. Am. Chem. Sot., 107 (1985) 7190. 19 X.-X. Gao, N. Li, K. Jiao, L. Zhang and M. Zhang, Kexue Tongbao (Eng. Ed.), 29 (1984) 616. 20 J. Li and Z. Zhao, Fenxi Huaxae, 12 (1984) 669. 21 N.-Q. Li, Z. Li and X.-X. Gao, Acta Chim. Sin., 41 (1983) 351. 22 G. Ali Qureshi, Bull. Sot. Chim. Belg., 90 (1981) 9. 23 T.M. Florence, Y.J. Farrar and H.E. Zittel, Aust. J. Chem., 22 (1969) 2321. 24 M.S. Masoud, M.S. Tawfik and S.E. Zayan, Synth. React. Inorg. Met.-Org. Chem., 14 (1984) 1 and references cited therein.
160 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
L.J. Csanyi, Mikrochim. Acta, 1 (1980) 277. M.A. Leonard and T.S. West, J. Chem. Sot., (1960) 4477. SF. Deane and M.A. Leonard, Analyst, 102 (1977) 340. F.J. Langmyhr, KS. Klausen and M.H. Nouri-Nekoui, Anal. Chim. Acta, 57 (1971) 341. N. Gyama, T. Shimomura, K. Shigehara and F.C. Anson, J. Electroanal. Chem., 112 (1980) 271. K.K. Shiu and D.J. Harrison, J. Electroanal. Chem., 260 (1989) 249. E. Casassas and L. Eek, J. Chim. Phys., 64 (1967) 971. T.M. Florence and W.L. Belew, J. Electroanal. Chem., 21 (1969) 157. D.J. Harrison, K.A. Daube and M.S. Wrighton, J. Electroanal. Chem., 163 (1984) 93. M.N. Szentirmay and C.R. Martin, Anal. Chem., 56 (1984) 1898. H.H. Willard and J.A. Dean, Anal. Chem., 22 (1950) 1265. S.M. Palmer and G.F. Reynolds, Z. Anal. Chem., 216 (1966) 202. T.M. Florence and G.H. Aylward, Aust. J. Chem., 15 (1962) 64. M.L. Thakur, Talanta, 21 (1974) 771. F. Ingman, Talanta, 20 (1973) 135. J. Redepenmng and F.C. Anson, J. Phys. Chem., 91 (1987) 4549. A.P. Brown and F.C. Anson, Anal. Chem., 49 (1977) 1589. R.M. Smith and A.E. Martell (Eds.), Critical Stability Constants, Vol. 4, Plenum Press, New York, 1976, p. 2.
.I. Electroannl. Chem., 262 (1989) 145-160 Elsevier Sequoia S.A., Lausanne - Printed
in The Netherlands
Mass transport effects in electroanalysis of La3+ with alizarin derivatives at quatemized polyvinylpyridine modif ied electrodes Kwok-Keung Shiu and D. Jed Harrison * Department (Received
of Chemistry
University of Alberta, Edmonton, Alberta T6G 2G2 (Canada)
20 June 1988; in revised form 28 November
1988)
ABSTRACT Electrochemical and spectroelectrochemical data show that at pH 9.2, La3+ forms complexes with alizarin red S and alizarin complexone in solution that do not change coordination number on reduction of the ligands to the hydroquinones. Modified electrodes sensitive to La3+ have been prepared by ion exchange of the redox active ligands alizarin red S or alizarin complexone into quatemized polyvinylpyridine films on glassy C electrodes. Distribution coefficients for extraction of various species into QPVP were found to be 1.2 x lo4 for alizarin red S, 2.4 x lo4 for alizarin complexone, 4.4~ lo4 for the alizarin complexone. La3+ complex, and 2.7 x 10’ for Fe(CN)i- , over the concentration range 0.02 to 1 pM in aqueous solution. Complexation of La 3f by polymer bound quinone occurs in the film, but the low film cation permeability results in low La3+ transport rates despite the presence of a coordinating ligand. Auger electron depth profiles show limited penetration of La3+ in the modifying film. Thin film ( - 3 nm) electrodes can be used to determine La3+ over the range 0.2 to 1 mM at pH 9.2. The analytical response arises in part from changes in transport properties in the film following complex formation.
INTRODUCTION
Chemically modified electrodes (CME) have been the subject of intense research for a number of years [l-S]. The nature of physical phenomena governing CME behavior has been examined in detail and many characteristics are now well understood. However, while several papers have appeared [6-161, exploration of the analytical utility of redox and ionic polymer modified electrodes has been relatively limited. A comprehensive understanding of factors that govern the performance of polymer modified electrodes as analytical tools, such as the properties of the polymer matrices, is still emerging [16-181. In many cases [8,10,11] the coulombic interactions within the polymer do not alone provide sufficient insight into the differing substrate transport behavior of the systems.
* To whom correspondence
0022-0728/89/$03.50
should be addressed.
8 1989 Elsevier Sequoia
S.A.
146
c$$r&~~~
*::; 0
0
ALfZARlN RED S
ALIZARIN COMPLEXONE
The large number of analytical complexation reactions developed for which one, or several, of the species involved is electroactive makes the development of ligand complexation based modified electrodes very attractive [6,8,10]. An experimental underst~ding of the factors that influence electrode performance will be important in utilizing the range of ligands available. These include the reversibility of the analytical reaction within an electrode film [6,9,10,18], and the effect of the polymer matrix on complexation [18] and mass and electron transfer processes [4,5,173 within the polymer. Equally important is an understanding of the influence of polymer charge and function~ty, in terms of coulombic and pe~sel~ti~ty effects, on the overall extraction process of an analyte into an electrode film. As part of a study of the effects of ligand properties, polymer charge, and film thickness on analytical complexation reactions at CMEs we have examined the electrochemically reversible anthraquinone derivatives alizarin red S (ARS) and alizarin complexone (AC), ion exchanged into partially quaternized polyvinylpyridine (QPVP) films on carbon electrodes, C/[QPVP],,,,. The alizarin derivatives have rich analytical metal complexation chemistry, and have been used in solution to determine electroinactive metals electrochemically [19-231, and as calorimetric reagents for a number of metal species [24-281. It is known that protonated polyvinylpyridine (PVP) is relatively permselective [3], excluding at least partially many cations from the polymer bulk, and cationic QPVP has similar properties [8,17,29]. The ability of an additions ligand incorporated in the polymer film, with a strong affinity for the metal ion, to overcome exclusion of the ion by the polymer matrix can thus be examined readily using QPVP. The influence of the matrix is a question of fundamental interest in the design of analytically useful modified electrodes, and the important role of film thickness on electrode performance in such a situation is illustrated in this paper. In addition it is found that a change in film transport properties can be used as a source of analytical ~formation.
Effect of complex formation on E,,, The formation and the electrochemistry of the La3+-quinone complexes discussed involves several equilibria that depend on pH and the stoichiometry of the oxidized and reduced forms of the Iigand and complex. The complexation reaction of metal, M, with the quinone, QHN, (N = total number of acid sites on quinone Q) can be written for various stoichiometries as M+xQH
N * M(QHN-JX
x=(0,1,
.*.,
x)
+ 4 H+
0)
147
where X is the highest order of complexation and 4 is the number of protons displaced on formation of a given complex stoichiometry. The effect of the formal concentration of M and QHN on shifts in the half .wave potential, E,,;, upon complex formation has been analyzed recently [30] using a formulation similar to that developed by Casassas and Eek [31]. When only one protonated form of the oxidized and reduced forms of the complex and the free ligand are significant, and the one-to-one complex is dominant for the oxidized ligand-metal complex, then eqn. (30) of ref. 30 can be used to describe the shift in half wave potential, AE,,,, for the ligand in the presence of the metal.
AE,,, = g
ln
1 + ,0;-;,i~$=[M] 1,x
C
(2)
PNR+*-i’,jaFIMlj
R, T and F have their usual meanings and activity coefficients are assumed to be unity. The LYE and ai”; are the fraction of the i th and i' th deprotonated forms of the quinone and_ hydroquinone derivatives, respectively. The term /3p_i,i is the overall pseudo formation constant [30] for the 1: 1 complex of the oxidized form of the ligand, as defined by Casassas and Eek [31]. The term /?t+2_ij,j is the overall pseudo formation constant for the jth complex with the i’ deprotonated form of the reduced ligand. The index j is given by j = 1, l/2, l/3.. . l/X, where X is defined in eqn. (1). The use of this formulation is described in ref. 30 and follows that introduced by Casassas and Eek [31]. Note that when formation of the oxidized form of the complex is significant, the condition PF_i,iay[M] >> 1 will hold. From a study of the effect of [M] on AE,,, at constant quinone concentration it is possible to determine whether the coordination number of the complex changes on reduction of the quinone. A similar but less general analysis has been presented by Florence and Belew for the case where only one complex stoichiometry is considered [32]. EXPERIMENTAL
Polyvinylpyridine (PVP) (molar mass 500,000 g) was a kind gift from Reilly Tar and Chemicals Corp. Quaternized PVP was obtained by reaction of benzyl chloride with PVP in refluxing methanol (22 h). Extent of quatemization, controlled by the ratio of benzyl chloride/PVP (0.66) used, was determined to be 64% by elemental analysis. (Found: Cl 12.12%, N 7.45!%, UC.: Cl 12.20%, N 7.53%). Alizarin red S and alizarin complexone were used as received (Aldrich). Water was twice distilled from alkaline permanganate. All other chemicals were reagent grade. Cyclic voltammetry and rotating disc voltammetry were performed with a Pine Instruments RDE4 potentiostat, Pine MSR rotator, and a Kipp and Zonen BD90 X-Y recorder. A PAR 174A polarographic analyzer and a Houston Instruments X-Y recorder were used for differential pulse (DP) voltammetry, with a scan rate of
148
1 mV/s, modulation amplitude of 5 mV and 0.5 s drop time. Chronoamperometry was performed with a PAR 273 with an IBM-PC-XT interfaced as controller and data acquisition system. Glassy carbon rods (Atomergic Chemetals, New York) were sealed in teflon sleeves, polished with alumina (Beuhler) to a 1 pm final polish, and rinsed in an ultrasonic bath. Pt flag electrodes for Auger electron analysis were contacted with Cu leads, and sealed in epoxy (Hysol-Clear) to leave one exposed face. A stock 2 mM QPVP (expressed in units of vinylpyridine monomer), 0.05 mM a,a’-dibromop-xylene methanol solution was prepared, and dilution of this to give 25 PM QPVP was necessary to obtain thin polymer films. In a closed container, saturated in methanol vapor, a few drops of polymer solution were slowly evaporated (- 3 h) on an electrode surface and then baked at - 60°C for 4 h in air. Evaporation in a methanol atmosphere produced more uniform films. Addition of cx,a’-dibromo-pxylene and baking at 60” C gave much more durable QPVP films, as reported previously [17]. To determine polymer coverage an electrode was soaked for 5 min in 1 mM K,Fe(CN), (0.1 it4 trifluoroacetate, pH 2), rinsed, transferred to pH 2 blank solution, and a cyclic voltammogram recorded at 10 or 20 mV/s. The total polymer coverage, expressed in pyridine units, I*,,, was calculated assuming total charge compensation by Fe(CN)i[33]. Distribution coefficients, K,, taken as the ratio of concentration of redox ion in the QPVP film to solution concentration, were determined following the method of Szentirmay and Martin [34]. At concentrations less than 1 PM, electrodes were allowed to equilibrate in stirred solution for 24 to 30 h, which was sufficient time to reach a nearly constant film concentration. At low concentrations, the QPVP film was sometimes partially lost from the electrode, and results were discarded when this occurred. At concentrations greater than 1 FM, equilibration times of 1 to 4 h proved sufficient. A thin layer spectroelectrochemical cell was prepared by sealing with epoxy (Clear-Hysol) a Hg plated Pt gauze (80 mesh, Johnson-Matthey) working electrode, Pt counter electrode, and Ag/AgCl pseudo reference electrode, between two microscope slides. Deaerated solution was introduced through a hole drilled in one of the slides at the bottom of the cell, which was sealed with electrical tape, and the top of the cell was then sealed with Parafilm. The cell was placed in a Hewlett-Packard UV-Vis photodiode array spectrometer and spectra were recorded every 2 min for 30 to 40 min. Reductions were usually complete by 15 to 20 min as evidenced by obtaining a constant spectrum. A Physical Electronics scanning Auger spectrometer, model 595 (Surface Analytical Laboratory, Dept. of Physics, Simon Fraser University) was used for Auger electron survey and depth profile analyses of surface modified Pt electrodes. A 3 keV electron beam, at a current of 50 to 100 nA, served as excitation source. Depth profiles were obtained by sputtering with a 3 keV Ar+ beam while scanning the element peaks of interest. Energy windows were C (240 to 300 eV), Pt (53 to 69 ev), La (70 to 90 eV and 600 to 647 ev). A Pt substrate was used to allow determination of the polymer/substrate interface during depth profiles.
149
RESULTS AND DISCUSSION
A variety of elcctroinactive metals [19-23,35-381 including several of the lanthanides [19-22,36-381, have been determined electrochemically using redox active ligands. Both alizarin red S (ARS) and alizarin complexone (AC) form complexes with La 3+ that show a shift in half-wave potential, AE,,,, for the quinone at the dropping Hg electrode. A knowledge of the stoichiometry of both oxidized and reduced forms of the complex is important if the shift in E1,2 is to be used to aid in identifying the metal complexed. However, no information on the reduced complexes has been reported, and there is insufficient and conflicting data on the oxidized complexes under the solution conditions employed. Further, it was necessary to determine the potential shifts of the complexes at carbon electrodes since these could differ relative to Hg due to the strong adsorption believed to occur at Hg. The cyclic voltammetric curves in Fig. 1 demonstrate that formation of the La * AC complex in solution with a 2 : 1 La3+ to AC ratio results in a new species with a peak reduction potential, Epr, of - 0.88 V vs. SCE, a shift of - 165 mV at a C electrode, similar to results at Hg. The peak heights are reduced; however, this effect was not further explored. Note the complex shows quasi-reversible electro-
I
I 0
I
I
!
I
I -0.4
,
I -0.8
I
I
,
I -1.2
,
E/Vvs.SCE
Fig. 1. Cyclic voltammetry at a naked glassy C electrode in 0.2 mM alizarin complexone (AC) (and 0.2 mM AC, 0.4 mA4 La3+ (- - -) in 0.1 M NH,, 0.1 M NH,Cl, pH 9.2 at a scan rate of 12 mV/s.
150
chemical behavior, with a peak separation, AEr, of 150 mV. With ARS as the redox active ligand a very similar electrochemical response is obtained; a shift in EPr of -135 mV is observed when 0.2 mM La3+ is added to a 0.1 mM ARS, pH 9.2 solution. A 0.1 M NH,Cl, 0.1 M NH,OH buffer solution, adjusted to pH 9.2 with HCI, was found to give optimum results for La3+ complexation by ARS and AC in solution. This ammonia buffer mixture was used in subsequent measurements using [QPVPI surfmodified electrodes unless indicated otherwise. Formation of La3+ complexes of AC and alizarin complexone 5-sulphonate with stoichiometries of 1: 1 and 2 : 2 has been reported [22,27,28], and when the metal to ligand ratio in solution is 1: 1, the 2 : 2 complex is dominant [22,27]. A detailed study of the 5-sulphonate derivative of alizarin has also been reported at pH 6.2 [27] (note that ARS is the 3-sulphonate derivative of alizarin). We undertook a spectrophotometric study of the stoichiometry of La3+ and ARS at pH 9.2. The data are in agreement with those for alizarin 5-sulphonate, in that complexes of both 1: 1 and 1: 2 metal to ligand ratio are formed. A detailed analysis of the absorbance data such as that done by Deane and Leonard [27] for alizarin 5-sulphonate and La3+ was not performed. However, it is reasonable to expect the 1: 1 ratio we observe for a La3+ and ARS complex in fact corresponds to formation of a 2 : 2 species, in the same manner as proposed for alizarin 5-sulphonate and La3+ [27]. For the latter ligand the extent of formation of the (alizarin 5-sulphonate . La), complex is very nearly constant as the ratio of La3+ to the ligand increases above 1 [27], and there is no evidence to the contrary for the ARS and La3+ complex. Formation of alizarin 5-sulphonate and alizarin complexone 5-sulphonate complexes with La3+ becomes less favourable at lower pH, and does not occur below pH 3 to 4 [27]. We find no spectroscopic or electrochemical evidence for formation of La3+ complexes of AC or ARS at pH 2 or less, consistent with the results for the 5-sulphonate derivatives. Consequently, La3+ complexation by AC or ARS in a polymer coated electrode (see below) at pH 9.2 can be completely reversed by transferring the electrode to a pH 2 solution. The extent of ionization of the quinone-La3+ complexes, and the number of protons involved in eqn. (1) has not been reported due in part to the complicated equilibria involved. We found dichloromethane, benzene, or diethylether extract less than 0.5% of any AC and ARS complexes of La3+ formed in aqueous pH 9.2 solutions with a 1: 1 metal to ligand ratio, suggesting that the complexes are charged. The fact that complexes can be extracted into the [QPVP],,,, film is consistent with anionic species formation. Both spectroelectrochemical and electrochemical measurements were undertaken to evaluate the stoichiometry of the reduced La3+ complex. Spectroscopic studies of the reduced forms of the free ligands and complexes were obtained with a thin layer spectroelectrochemical cell with an optically transparent electrode. The spectrum of both free AC at 0.2 mM in pH 9.2 buffer, and the complex formed in a 0.2 mM AC, 0.2 mM La3+ mixture at pH 9.2 is shown in Fig. 2a. The respective solutions were then exhaustively electrolyzed to generate the hydroquinone form of AC. Comparison of the two spectra in Fig. 2b, and the data in Table 1, indicates that a
151 0.20 a OXIDIZED 0.15
FORM , pH 9 2
-AC
0.2 mM
---AC
0.2mM
,
La 0.2mM
% f g
0.10
z 2 0.05
b REDUCED FORM , pH 9.2 Cl. 1s
-
AC 0.2 mM
---
AC 0.2mM
,
La 0.2mM
g g$ 0.10 0 i?ci 4: 0.05
WAVELENGTH
/ nm
Fig. 2. (a) Spectra of the quinone form of alizarin complexone (AC) and the La3+ complex in a thin layer spectroelectrochemical cell. (b) Spectra of the hydroquinone form of AC and the La3+ complex following exhaustive electrolysis at a Hg coated Pt mesh in the spectroelectrochemical cell. TABLE
1
Values of h,,,
a for quinone
and quinone.La3+
Species
&nax/~
AC AC.La(l:l)’ ARS ARS.La(l:l)’
334,520 553 334,520 536
oxidized
form
complexes
reduced 397,451 401,457 398,466 398,468
form b
(sh) d (sh) d
’ Wavelength of absorbance peak maximum. b Reduced form prepared by exhaustive electrolysis of oxidized form in thin layer spectroelectrochemical cell. ’ Complex formed in a solution 0.2 mM in metal and ligand. d This peak for the free ligand becomes a broad shoulder with lower absorptivity ( - 50%) for the complex.
152
complex with different spectral properties from the free hydroquinone persists upon reduction of the AC, La3+ mixture. The spectra for the La. ARS complex in oxidized and reduced form were obtained in similar fashion, and indicate a complex is also formed between the hydroquinone form of ARS and La3+, Table 1. While the peaks do not shift for the hydroquinone form of ARS in the presence of La3+, the absorptivity at 398 nm decreases - 50% and the shoulder at 468 nm becomes broader and about 40% less intense, extending to longer wavelengths as is the case for AC - La in the reduced form, Fig. 2b. An electrochemical study of the dependence of E,,*, determined from the peak potential of a differential pulse voltammogram, on the concentration of La3+ was also performed. With a constant concentration for either AC or ARS of 20 @I, E 1,2 was found to be constant within f 2 mV over a La3+ concentration range of 30 to 100 PM. Under these conditions, where the 1: 1 complex is dominant for the oxidized form, eqn. (2) can be applied, although since the system is quasi-reversible, eqn. (2) must be viewed as an approximate relationship for this case. Despite this consideration, lack of a change in AE,,, for the complex, when combined with the spectral data, provides evidence that the stoichiometry remains unchanged for both oxidized and reduced species in the presence of excess La3+ [32]. Polymer modified surfaces The simplest route to preparation of a modified electrode is through ion exchange of anionic ARS or AC into a quatemized polyvinylpyridine film [l-3,8,10,29] on a C electrode surface, to give a quinone modified electrode, C/[QPVP . ARS] surf or C/[QPVP . AC] surf. At pH 9.2, more than 98% of ARS is present in the 2 - form [24], while based on the pK,s for AC, approximately 80% is present as the dianion and 20% is in the 3 - form [39]. The alizarin derivatives are strongly bound in the polymer film, as determined from the partition isotherm for extraction of electroactive ions over a concentration range of 20 04 to 1 mM. Values of the apparent distribution coefficients, K,, are reported in Table 2. At concentrations of 1 @I or less the isotherm is linear and the values of K, are of the order of lo4 to 105, while above 0.1 mM the polymer nears saturation and K, decreases. For Fe(CN)i-/4equilibrium is established and a true distribution coefficient is measured. However, even after 24 h soaking in 2 M KCl, 70-80% of the quinone remains in the film, whereas all of the Fe(CN)),-14can be removed. Consequently, while the measured distribution coefficients indicate clearly the very strong affinity of [QPVP],,,, for the quinones and the 1: 1 La . AC complex they may not reflect the true value of K,. This is shown in Fig. 3 by the cyclic voltammetric response of a C/[QPVP],,,, electrode in a blank solution, pH 9.2, containing only ammonia buffer, following soaking in a 1 mM Fe(CN)i-, 1 mM ARS, pH 9.2 solution. The ratio of surface coverages, JYARs/l?FtiCNj;-, for such an experiment is typically 3.5 to 4, despite the fact that the distribution coefficients favor Fe(CN)z- over ARS by a factor of - 2. Apparently the irreversibility of the quinone extraction results in its dominance in the film, demonstrating the poor values. The hydrophobic nature of the predictive power of the apparent K,
153 TABLE 2 Distribution ratio, K,, in the QPVP matrix Ion
KD
a
0.02-l Fe(CN):Fe(CN);AR!? AC*AC.La(l:l)d a Distribution b Determined ’ Determined d Prepared as
pi%4b
2.7 x 10’ 1.2 x 104 2.4x lo4 4.4x104
lmh4’ 411 266 156 372
ratio, K, = cp/c,, at pH 9.2, 0.1 M NH,Cl, 0.1 M NH,OH supporting electrolyte. from the slope of the linear portion of the partition isotherm over the range 0.02 to 1 pM. at 1 mM. a 1: 1 mixture of AC and La3+.
quinones, their relatively large size, and the distance of separation of the multiple charge sites on the ions, may decrease mobility once bound in the film, compared to inorganic complexes and the singly charged quinones studied previously [33]. Several qualitative studies have shown that cations such as Ru(NH,)z+ and Co(bipyridyl), 3+ do not easily p enetrate [QPVP],,, or protonated [PVP],,,, films [3,8,17,29,33], as shown by a 90 to 99% reduction in cyclic voltammetric peak heights. A knowledge of the permeability or K, for La3+ in [QPVP],,,, films would aid in determining whether metal transport or extraction in the film is a limiting process in complex formation. Unfortunately, due to the difficulty in preparing pin-hole free films, attempts to determine the permeability of La3+ quantitatively by the method of Anson and co-workers [40], br of Eu3+i2+ by rotating disk voltammetry [l-3] were unsuccessful. However, cyclic voltammetry showed that the current for Eu3+12+, an ion very similar to La3+, was reduced at least fiftyfold at a [QPW surfelectrode compared to a naked surface. Large quantities of ARS or AC can be accessed electrochemically within the films, but when IpY 2 lo-* mol/cm2, only 40 to 80% of the quinone present is electroactive. This was determined by comparing the apparent I,, coverage determined by exchanging Fe(CN):and either ARS or AC sequentially into the polymer films from 5 mM solutions, and recognizing that lYFe(oNj;- is a relatively reliable indicator of the total pyridinium sites present [33]. When I’,, < lo-* mol/cm2, the charge in the films can be totally compensated by the alizarin derivatives, on the basis of the electrochemistry, which indicates that all of the quinone present is electroactive. The apparent diffusion coefficient for charge transport, Dct, by ARS or AC in the QPVP film has been determined by chronoamperometry in solutions of supporting electrolyte, pH 9.2, and by rotating disc voltammetry [2,3-5,331, in 5 mM quinone, pH 9.2. The results indicate a slow charge transport process, with Dct varying from 0.2 to 1 x lo-” cm2/s for both free ligands in relatively thick polymer = lo-* to 10e7 mol/cm2). In comparison, DC, for oxidation of Fe(CN)zfilms ( rpy at pH 9.2 in the same polymer is 1 to 3 X lo-” cm2/s.
154
C/QPVP.AC J
C/OPVP.(ARS. -4 -
Fe (CN),“)
0.1 M NH4Cl 0.1 M NH4OH
Q, L
0
40.4
I
I
I
0
I
L
-0.4
I -0.8
I -0.4
E/Vvs.SCE
Fig. 3. A C/[QPVP.ARS,
Fe(CN):-lSuti
-0.6 EIV
-0.8
-1 .o
VS SCE
electrode response in pH 9.2 solution of supporting electrolyte
alone, following equilibration in a 1 mM ARS, 1 mM K,Fe(CN),, is - 2 at pH 9.2, and it clearly competes strongly with Fe(CN)i-
pH 9.2 solution. The charge on ARS for ion exchange sites in the polymer.
Fig. 4. Differential pulse voltammogram of high coverage, ppy = 2~ lo-* mol/cm*, C/[QPVP.AC],,,, electrode in blank, pH 9.2 supporting electrolyte () and with 1 mM La’+ added (- - -). Scan rate 1 mV/s, modulation amplitude 5 mV, drop time 0.5 s.
Thick film behavior Initial experiments with relatively high coverage of polymer films, 5 x 10h9 to 5 X lo-’ mol/cm2 of pyridine sites, demonstrated that a La,. AC,, complex is formed within the [QPVP . AC],,,r film. A differential pulse (DP) voltammogram of a C/[QPVP - AC1surfelectrode in a blank, pH 9.2 solution results in a peak at -0.64 V for the surface bound quinone, Fig. 4. Transfer of the electrode to a 1 mM La3+ solution, pH 9.2, gives a differential pulse voltammogram with a peak at -0.64 V for AC, and a new peak at - -0.82 V, as shown in Fig. 4. Similarly a WQPVP . A=1 surfelectrode develops a new peak at - -0.81 V on exposure to La3+, with the same ratio of free ligand to complex peak current as observed for AC modified electrodes. These results are consistent with partial conversion of the quinone, Q, in the film to a La,. Q, complex, although the stoichiometry of the complex is undetermined. The extent of complex formation appears small, and the peak current for the complex is relatively independent of La3+ concentration, showing a high degree.of scatter in i, as La3+ concentration is varied from 0.05 to 2 mM. Given the difficulty with pinhole formation in the film, the current for the complex may arise from complex formed near the electrode surface in the vicinity of a pinhole. This would account for the variability observed.
155
In contrast, if a C/[QPVP],,,, electrode is exposed to a 0.2 mM AC, 0.2 mM La3+, pH 9.2 solution, in which a 1 : 1 complex already exists, then a DP voltammogram shows a single peak at - 0.83 V. Exposure of a C/[QPVP . AC],,,r electrode to the same solution does not result in significant formation of the complex in the film. These results indicate that the complex is durable within the film, and can diffuse into an electrode which initially contains no strongly bound species such as AC or ARS. However, the complex is apparently unable to diffuse rapidly in a film which is pre-saturated in quinone. This may reflect a change in film structure when the bulky quinone acts as counterion; a well documented occurrence [l-5,29] for more highly charged ions such as Fe(CN)i-/4in polycationic films. An Auger electron depth profile of a thick film Pt/[QPVP . ARS],,,, electrode, I,, = 6.5 X lo-’ mol/cm2, exposed to 1 mM La3+, pH 9.2, is shown in Fig. 5a. The depth profile reveals that La is present primarily at the solution/polymer interface and does not permeate the entire film. Depth profiling of Pt/[QPVP. AClsurf electrodes prepared in the same manner also show this phenomenon. Depth profiles of Pt/ [QPW surfelectrodes exposed to 1 mM La3+, pH 9.2, with no quinone
(b) Pt
2 min
C
CH:
La L.3
SPUTTER
TIME
Fig. 5. Auger electron depth profile of (a) high coverage Pt/[QPVP. ARS],,,, electrode, lYpy= 6.5 X lo-’ mol/cm*, and (b) low coverage Pt/[QPVP. ARS],,,, electrode, rpy = 1 X 10W9 mol/cm*, after exposure to 1 mM La3+, pH 9.2 solution.
156
present show that La3+ does not penetrate the [QPVP],,,, film. In some cases a small La signal is observed on an initial survey scan, but the signal is - 30 times smaller than that for Pt/[QPVP - ARS],,,, electrodes exposed to La3+ at pH 9.2, and is not present after even a brief Ar+ sputter (- 5 s). A Pt/[QPVP],,, electrode exposed to a 0.1 mM ARS, 0.1 mM La3+, pH 9.2 solution was also examined and the depth profile indicated a uniform La 3+ distribution up to the polymer/electrode interface. This confirms the conclusion, based on electrochemical measurements (see above), that the complex is readily transported in a fresh [QPVP],,,, film containing no quinone. For modified electrodes exposed sequentially to ARS or AC, then La3+ solutions, depth profiling demonstrates that La3+ transport is a serious problem within [QPVP . ARS or AC],,, films, despite the ability of the quinone containing films to extract La3+ from solution. Thin film behaviour To offset the problem of low La3+ transport rates, much thinner QPVP films have been used, with Fry G 2 X lop9 mol/cm2. Figure 5b shows a depth profile of a Pt/[QPVP . ARS],,, electrode, I,,, = lop9 mol/cm2, that was exposed to a 1 mM La3+, pH 9.2 solution. The Pt signal is observed in an initial survey scan, indicating film thickness of less than 3 nm or the presence of pinholes in the film. This is consistent with the measured I,,, value for which a thickness of - 3 nm is expected; additionally the uniformity of such a thin film is uncertain. Significantly, the La signal declines at nearly the same rate as that for C, indicating La is present throughout the film. The electrochemical response (see below) is also consistent with nearly complete permeation of La3+ within thin [QPVP . ARS],,,, films. A DP voltammogram in a blank, pH 9.2 solution of a thin fihn C/[QPVP . AC],,,f electrode with I,, = 2.0 X 10e9 mol/cm2 shows a reduction peak at -0.71 V vs. SCE, Fig. 6. This is equal to EPr for the solution species and is - 60 mV more negative than for AC in thicker films. The difference in potentials suggests the environment in the thin polymer film differs from that of the bulk polymer and is perhaps more “water-like” or open in structure. However, the thickness dependence of the potential shift was not examined further. The most striking effect upon immersion of the electrode in La3+ solution is the large increase in peak height, which is accompanied by a voltage shift of E,, to -0.80 V in 0.4 mM La3+, pH 9.2. We find that the change in i, for a C/[QPVP . AC or ARS],,,, immersed in La3+, pH 9.2 solution is proportional to La3+ concentration over a limited range. A plot of the increase in i,, determined as i, for the quinone in a La3+ containing solution minus the peak current in a blank solution, i, - i,([La3+] = 0), is shown in Fig. 7 for both quinone ligands. Between exposure to La3+ solutions, the concentration of quinone in the polymer is recovered by immersion in a 5 mM solution of ARS or AC. Repeated transfers between a quinone solution and a solution of a given La3+ concentration do not result in further change in i, in the La3+ solution. Consequently, the increase in i, with increasing [La3+] cannot result from an increase in complexing agent within the polymer film during re-exposure to quinone solutions.
157
1.6
1.2
Q,
0.8
0.4
01
I
I
-0.4
,
-0.6
1
1
-0.6 ElVvs
I
-1 .o
SCE
Fig. 6. Differential pulse voltammogram of C/IQPVP.AC],,,, electrode, PpY = 2X10W9 mol/cm, in blank, pH 9.2 supporting electrolyte (- - -), and after transfer to a 0.4 mM La3+, pH 9.2 solution See text for a discussion of electrode preparation. ( -).
I’
”
’ .
”
‘I
”
6.4
4.6:
/\\\
3.2
.
-
l-6 1.6
/:,/
0
0
,
/ 0.4
, 0.6
,
, 1.2
,
,
,
:
1.6
[La3’] /mM Fig. 7. Dependence of net peak reduction current, expressed as i, - ip([La3+ ] = 0). on La3+ concentration determined by differential pulse voltammetry at pH 9.2 for (0) C/[QPVP-ARS],,,, electrode with electrode with Pry = 2 x 10e9 mol/cm2. Solid ppY =1.2X10e9 mol/cm2 and for (A) C/[QPVP.AC],,,r lines are for clarity only, to connect data points.
158
Using chronoamperometry the diffusion coefficient for charge transfer in the [QPVP . AC, La3+],,,r film was determined as a function of [La3’] in solution. The term cDz,12, where c is the concentration of the ligand in the film, increases as the La3+ solution concentration increases up to 0.8 mM, and then decreases at higher concentrations. These data are consistent with the observed changes in differential pulse peak height [41] at the modified electrode. The cause of the effect is uncertain, but, since it does not arise from an increase in complexing agent in the film, it is most likely due to changes in film crosslinking and structure as the La,. AC,, complex is formed. Increases in Dct as the extent of ionic crosslinking decreases are a common observation for electrostatically bound redox species. Since the anionic charge of the quinone . La complex will be lower than that of the free quinone, ionic cross-linking would be expected to decrease as more La3+ is extracted into the film. The decline in signal observed at high [La3’] at the modified electrode is consistent with a decline in i, observed at naked electrodes in 0.2 mM ARS solution when [La3’] is greater than 1 mM. This may be due to the complex series of hydrolysis equilibria for La3+ at high pH that includes formation of polynuclear La species [42] as the La3+ concentration increases. Additionally, in solutions with [La3’] > 1 mM and [La3’]/[Q] > 2, we observe formation of a purple precipitate upon standing for 24 h.
CONCLUSION
The Auger depth profile results demonstrate two important features of the La. Q systems. The first is the evidence that mass transport of La3+ in the film, and not charge transport between redox sites, is the factor that dictates the requirement for thin films. The other relates to the mechanism of charge transport in the film. For electrostatically bound redox species it is difficult to determine whether site-site exchange of the electroactive species, or electron transfer through self-exchange is the dominant mechanism of charge transport. In the case of the La. Q systems, site-site exchange of the complex would lead to a uniform distribution of La3+ in the film, in contrast with the data obtained by Auger depth profiling, Fig. 5a. Consequently, at least for the complex, the so-called electron hopping mechanism between redox species arising from electron self exchange must be the dominant form of charge transfer in the film. A unique feature of the response of [QPVP. Qlsurf films to La3+ is that the analytical response arises in part from the change in film transport properties as La3+ is extracted into the polymer film. The increase in Dct could result from an increase in the self-exchange rate, k,, for the complex relative to the ligand; however, based on the quasi-reversible nature of the cyclic voltammetry for the solution complex, Fig. 1, a decrease in k, is more likely. An increase in the mobility of the complex in the film, due to the decreased charge of the complex relative to the free ligand, is a more likely cause of the increase in DC, as [La3’] increases. The increase in mobility would then lead to enhanced site-site exchange, or more
159
frequent collisions resulting in increased rates of electron self-exchange. The latter suggestion is consistent with the Auger depth profile data, as discussed above. In the design of modified electrodes for analysis of metal ions, attention should be given to controlling permselectivity of the polymer matrix to reduce problems associated with substrate transport. This may be accomplished through increasing the solvent compatibility (swelling) of the film, varying the charge of the ionic sites, or by incorporating a high density of functional groups that can form labile complexes with the analyte, in addition to the presence of a strongly complexing analytical agent. ACKNOWLEDGEMENTS
We wish to thank J. Buechler and Reilly Tar and Chemical Corporation for a generous gift of 400,000 and 500,000 g molar mass polyvinyl pyridine, and B. Heinrich of Simon Fraser University (SFU) for arranging access to the SFU Surface Analytical Laboratory. We thank the Central Research Fund (University of Alberta) and the Natural Sciences and Engineering Research Council of Canada for support of this research. REFERENCES 1 R. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1983, pp. 191-368. 2 W.J. Albery and A.R. Hillman, AMU. Rep. C, R. Sot. Chem., 78 (1981) 377. 3 N. Oyama and F.C. Anson, J. Electrochem. Sot., 127 (1980) 640. 4 C.P. Andrieux and J.-M. Saveant, J. Electroanal. Chem., 134 (1982) 163. 5 F.C. Anson, J.-M. Saveant and K. Shigehara, J. Phys. Chem., 87 (1984) 214; J. Am. Chem. Sot., 105 (1983) 1096. 6 H.C. Hurrell and H.D. Abruiia, Anal. Chem., 60 (1988) 254. 7 A.R. Guadalupe, S.S. Jhaveri, K.E. Liu and H.D. Abruiia, Anal. Chem., 59 (1987) 2436. 8 M.J. Gehron and A. Brajter-Toth, Anal. Chem., 58 (1986) 1488. 9 L.M. Wier, A.R. Guadalupe and H.D. Abruha, Anal. Chem., 57 (1985) 2011. 10 A.R. Guadalupe and H.D. Abruha, Anal. Chem., 57 (1985) 142. 11 J.A. Cox and P.J. Kulesza, J. Electroanal. Chem., 175 (1984) 105; Anal. Chim. Acta, 165 (1983) 71. 12 G. Sittampalam and G.S. Wilson, Anal. Chem., 55 (1983) 1608. 13 M. Petersson, Anal. Chim. Acta, 147 (1983) 359. 14 J.F. Price and R.P. Baldwin, Anal. Chem., 52 (1980) 1940. 15 G.T. Cheek and R.F. Nelson, Anal. Lett., 11 (1978) 393. 16 L.D. Whiteley and C.R. Martin, Anal. Chem., 59 (1987) 1746. 17 J.K. Doblhoffer and R. Lange, J. Electroanal. Chem., 229 (1987) 239. 18 C.M. Lieber and N.S. Lewis, J. Am. Chem. Sot., 107 (1985) 7190. 19 X.-X. Gao, N. Li, K. Jiao, L. Zhang and M. Zhang, Kexue Tongbao (Eng. Ed.), 29 (1984) 616. 20 J. Li and Z. Zhao, Fenxi Huaxae, 12 (1984) 669. 21 N.-Q. Li, Z. Li and X.-X. Gao, Acta Chim. Sin., 41 (1983) 351. 22 G. Ali Qureshi, Bull. Sot. Chim. Belg., 90 (1981) 9. 23 T.M. Florence, Y.J. Farrar and H.E. Zittel, Aust. J. Chem., 22 (1969) 2321. 24 M.S. Masoud, M.S. Tawfik and S.E. Zayan, Synth. React. Inorg. Met.-Org. Chem., 14 (1984) 1 and references cited therein.
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