1508
Anal. Chem. 1982, 5 4 , 1508-1515
Quantitative Aspects of Ion Exchange Partition of Redox Cations into Organosilane-Styrenesulfonate Copolymer Films on Electrodes J. R. Schneider and Royce W. Murray" Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514
Cross-llnked, stable polyanlonlc films of the copolymer (MPS-SS) of styrene sulfonate and y-methacryloxypropyltrlmethoxysllane have been prepared on PI electrode surfaces and used to electrostatlcally trap Ru(NH,):,' Cr(bpy)?', and methyl vlologen. The quantltles of trapped Ions are measured by thelr electron transfer reactlons wlth the electrode, and these results are used to determlne distribution coefficients between the MPS-SS fllm and the dllute bathlng solutlon of the redox Ion. For Ru(NH,):+ and MV2+,K, Is 2400 and 100 In 0.1 M KCI, respectively. Relative values of K, were measured by palrwlse competltlve partltlonlng. I n general, electrostatlc trapplng follows a reactlvlty pattern conslstent wlth Its essentlal Ion exchange character. The MPS-SS copolymer Is used wlth poly(v1nytferrocene) to prepare a bllayer electrode In whlch the Inner polymer fllm contalns no flxed redox sltes.
Electrochemical reactions of thin, electroactive polymer films in which a few to many hundreds (and more) monomolecular layers of redox species undergo electron t,ransfers with the electrode have been described for a large number of polymeric films (1-21) since the initial reports of such phenomena by Miller (I) and Bard (2). Many of these polymeric films contain the redox species covalently attached to, or an integral part of, the polymeric lattice. Such materials are referred to as "fixed-site redox polymers", the dynamic mobility of the redox site being limited by that of the polymer chains. An alternative approach to multilayer electroactive films was later introduced by Oyama and Anson (22-241, who (ion exchanged) partitioned highly charged redox ions into polycationic (22,231 and polyanionic (24)polymer films on electrodes. These polymeric films retain the redox ion sites primarily by electrostatic means. This "electrostatic trapping" has a number of advantages, including the versatility of incorporating a variety (25-28) of ionic redox species into a given polyionic film. We would like to point out that ion exchange based electrode coatings also have the potential technological advantage of being regenerable. That is, it is conceivable that electrostatically trapped, electrocatalytically reactive redox ions which have undergone chemical decay as a result of electrocatalytic usage over millions of redox cycles can be flushed from the polyionic film and replaced by fresh redox ion catalyst, without disassembly of the electrochemical reactor. Electrostatic trapping has a potentially serious disadvantage in that ion exchange is, intrinsically, a chemically reversible step. Thus, unless the solution in which the electrode is bathed contains a very small concentration of fresh redox ion (catalyst), leaching of the redox ions from the film may ultimately occur. The research described here had several purposes, the first of which was devising a polymeric material which was suitable
for preparing stable polyanionic films on electrodes. No polyanionic films had been reported at the inception of this work, and those recently described by Anson and co-workers (24) have limited stability. We have found that films prepared from the vinyl copolymer MPS-SS
8
S O i As P h i
I
c=o I
OCH2CH2CH2Si(OCH3)3 because they can be cross-linked via the organosilane function, serve as effective, electrostatically trapping polyanionic films and are also quite stable on the electrode surface. The second purpose of this study was to quantitatively study the extent to which various cations partition into the polyanionic polymer film. There have been as yet no quantitative partition measurements made for electrostatic trapping of either cationic or anionic ions into films on electrodes. Such data are clearly important with respect toleaching as mentioned above in connection with electrocatalytic applications. Also, we wanted to illustrate how electrostatic trapping is primarily an ion exchange reaction and follows known patterns of such reactions (29, 30). The cations chosen for study represent a range of charge and size and are observable at individually noninterfering reduction potentials: Ru(NH3)2+, C r ( b ~ y ) ~and ~ +MV2+ , (bpy is 2,2'-bipyridine; MV2+is methyl viologen). Thirdly, we were interested in the selectivity of partitioning from baths containing mixtures of redox ions, based on the polyanionic film's partition coefficient characteristics toward the ions and on control of the electrode potential and correspondingly the charge on the redox cation. Such selectivity phenomena might have analytical utility. To this end, we examined the competitive partitioning of pairs of the above cations into the polyanionic MPS-SS film. Wrighton et al. (31) have recently described competitive partioning of pairs of anionic redox species into a polycationic film, but the resulb were not analyzed on a partition coefficient basis. Fourthly, the polyanionic MPS-SS film has been applied to preparation of a new type of redox bilayer electrode (16, 17) in which the innermost film, the polyanionic polymer, contains redox sites present only by virtue of the ionic mobility and partitioning properties of redox cations.
EXPERIMENTAL SECTION Copolymer Preparation. The polyanionic copolymer, MPS-SS, is the product of the copolymerization of the tetra-
0003-2700/82/03541508801.2510 0 1082 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
1509
Table I. Elemental and XPS Analysis of the Copolymer MPS-SS of Styrenesulfonate (SS) and y -Methacryloxypropyltrimethoxysilane (MPS)
(SS:MPS) molar ratio
copolymer no. 1
monomer feed ratioa 2.1
elemental analysis b 1.7
2
1.1
1.1
3
6.7
1.8
4
1.1
S/Si by X P S c 0.92,e 1.8 0.98,e 1.7, 1.5 1.8 0.55, 0.69 1.0,0.99 0.7ije 0.77,e l . O e 0.68 0.90
pretreatment d none
H*0
heptane H*O heptane none aqueous 0.1 M KCl H20 heptane
Based on Galbraith analysis a Calculated from molar ratio of starting monomers; (SS/MPS) assuming equal reactivity. results for relative S/Si atom content; copolymer (% C = 63,1, H = 5.62, S = 4.35, Si = 2.27); copolymer 2 (% C = 60.0, H = 6.15, S = 3.80, Si = 3.00); copolymer 3 (WC = 60.4, H = 5.55, S = 4.91, Si = 2.45). Ratio based on S 2p and Si 2s (or Si 2p) XPS band areas at 168 eV and 154 eV (or 102 eV), respectively. Theoretical XPS S/Si sensitivities are (1.745/0.865) x ((1254- 168)/(1254- 102))0.75 = 1.93 for S Zp/Si 2p and (1.745/0.855)((1264- 168)/(1254 - 154))0*’5 = 2.02 for S 2p/Si 2s, taking cross sections ( 3 2 ) and escape depth terms ( 3 3 , 3 4 ) into account. Poly(MPS-SS) film was treated prior to XPS by immersion in the solvent listed for 5-10 min, and dried for 30 min in a vacuum oven at 50 “C. e Si determined from 1 0 2 eV Si 2p peak area. All other determinations were based on 154 eV Si 2s peak area. phenylarsenic salt of styirenesulfonate (SS) with 7-methacryloxypropyltrimethoxysilane ( M P S ) (ca. 1M) (Petrarch) in degassed N,N-dimethylformamide (DMF) at 65-85 OC in an oil bath for 1-2.5 h with AIBN (azobis(isobutyronitrile),ca. 0.01 M) as radical indicator. The postassimn salt of styrenesulfonate (Fluka) was recrystallized from 95% ethanol and converted to the tetraphenylarsonium salt to render it soluble in DMF. The polymerization reaction is ended by dropwise addition of anhydrous diethyl ether to the solution, precipitating the copolymer, MPS-SS. The solvent wm then decanted,the polymer redissolved by dropwise addition of methanol and reprecipitated by addition of diethyl ether. This prociedure was repeated twice to purify the product, which was dried by dissolving the precipitate in a minimum volume of methanol, transferring the solution to a round-bottom flask, doubling the solution volume by adding benzene, and then freezing the solution by placing the flask in dry ice. The round-bottoim flask is connected to a trap cooled to dry ice/methanol temperature, and the frozen solution evacuated to sublime the benzene-methanol mixture away. The copolymer MPS-SS is soluble in acetonitrile, DMF, and methanol. Molecular weights were not evaluated since the material is further polymerized in the electrode preparation step. The elemental analyses (Galbraith Laboratories) for MPS-SS are given in Table I (see footnote b) and are converted in Table I into monomer ratio in ithe copolymer, based on relative S/Si atom content. The analytically observed % C are lower than expected (from % Si) and % H higher than expected for all three samples, probably due to imperfect removal of methanol. Electrode Preparation. Teflon-shrouded platinum disk electrodes (area ca. 0.11-0.15 cm’) polished with either 1-pm diamond paste or 0.55-pm alumina on a polishing wheel are cleaned and superficially oxidized by either electrochemical anodization in 1M sulfuric acid (35)or treatment with concentrated nitric acid. Films of the copolymer, MPS-SS, are prepared on the platinum surface by placing a droplet (ca. 5-20 pL) of methanol solutioncontainingMPS-SS on the surface and allowing the methanol to evaporate into a nearly methanol-vapor-saturated space over a period of ca. 3-5 h and sometimes overnight. The film is cross-linked on the electrode surface via formation of siloxane -SiOSi- linkages by either exposure to HC1 vapors (36) or “heat curing” overnight in a 50 “C vacuum oven. For the bilayer experiment, a poly(vinylferrocene) film is coated on top of the MPS-SS filrn using the same droplet evaporation method. Reagents. Hexaammineruthenium(II1) tribromide was synR~~)(~b,p y ) ~ ( C l Oand ~)~, thesized as per ref 37; C ~ ( b p y ) ~ ( C l O F e ( b p ~ ) ~ ( C lwere O ~ )courtesy ~ of D. R. Rolison, H. Abrufia, and P. Denisevich, respectively; and l,l-dimethyl-4,4’-bipyridinium chloride (MV2+,methyl viologen; Aldrich), potassium chloride (Ultrapure;Alfa Organics),and lithium chloride (GFS Chemicals) were used as received. Tat rabutylammonium bromide (Aldrich)
was recrystallized from 95% ethanol. Poly(viny1ferrocene)was M solution (based on courtesy of P. Denisevich as a ca. monomer content) in methylene chloride. Electrochemistry, A conventional two-compartment electrochemical cell with Luggin capillary used a saturated sodium chloride calomel electrode as reference (NaSCE) and a platinum wire as auxiliary electrode, respectively. Electrochemical instrumentation was conventional. Cyclic voltammetry was used as the electrode characterizing technique. Instruments. X-ray photoelectron spectra (XPS) were obtained with a DuPont 650B spectrometer. A 24B Perkin-Elmer H-NMR spectrometer was used to obtain spectra of copolymer no. 2, and the individual monomers. Vinyl proton peaks present in the 5.0-7.0 ppm chemical shift range for the two monomers are not present for the MPS-SS copolymer.
RESULTS AND DISCUSSION Preparation and Characterization of MPS-SS Copolymer and Its Films. For monomer feed ratios with proportions of styrenesulfonate of 2:l and less, the copolymer composition was the same as the monomer feed ratio as shown by the analytical results in Table I. Higher percentages of the sulfonate monomer could not be incorporated into the copolymer under our reaction conditions (see copolymer no. 3). Use of longer polymerization reaction times or higher temperatures produced insoluble copolymer products. Consequently, the available MPS-SS copolymer has a limited range of composition. Presumably the proportion of sulfonate groupings and the degree of siloxane cross-linking could be changed by blending MPS-SS with linear sulfonated polystyrene but films prepared from such blends were not investigated here. Vinyl proton peaks present in the 5.0-7.0 ppm chemical shift range for the two monomers are absent in solutions of MPS-SS copolymer. This indicates that the predominant mode of polymerization is vinyl coupling, as represented above in the MPS-SS structure, as opposed to polymerization through the -Si(OR) functions. We believe the latter groups are preserved in MPS-SS copolymer preparations, which are methanol soluble, and condense only after HC1 exposure or heat curing in the film-making process. Ionomers tend to swell and dissolve in polar solvents such as water unless their molecular weight is quite large (4). This is probably the origin of instability reported (24) for films prepared from linear sulfonated polystyrene (which we verified). The organosilane copolymer component was calculated to correct this property and was successful. Freshly evaporated MPS-SS films on Pt electrodes can be redissolved by
1510
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
solution of a redox ion such as ruthenium hexaammine, Ru(NHJ63+,which also contains 0.1 M supporting electrolyte, leads to the following ion exchange equilibrium:
S 2D 168
+
Pt-/-MPS-SS( N S O ~ - ) J K + ) ~ +z~R u ( N H ~ ) + ~~+ Pt-/-MPS-SS( "SO~-),(RU(NH~)~~+),(K+),+~-~~ 3zK+ (1) I
183OeV e
I i;'YC ;UE*CI"I
I
1
142.3eV
Figure 1. X-ray photoelectron spectroscopy of copolymer film on a platlnum dlsk electrode. The blnding energies Shown correspond to S 2p (1 68 eV) and Si 2s (154 eV).
rinsing with methanol. The films however become quite insoluble, in solvents ranging from water to methanol to benzene, following brief exposure of the film to HCl vapor or to mild heat-curing. The latter procedures are reckoned to condense the -Si(OR) functions to -SiOSi- bonds, cross-linking the vinyl polymer chains. Film stability and adhesion to the Pt surface is also presumably aided by formation of Pt/OSi- bonds to the superficially oxidized Pt surface as established in previous monolayer studies (35). Films with superior smoothness (visually glossy) are obtained by very slow evaporation of the solvent and good wetting of the electrode surface by the polymer solution. A procedure developed by Nakahama (36) in which evaporation occurs into a near-saturated bath of solvent vapor produced a satisfactorily slow solvent evaporation. Both plain and superficially oxidized Pt surfaces are well-wet by methanol solutions. Films prepared in this manner which contain 10-20 pg/cm2 of polymer (sulfonate group coverage of (1-2 x mol/cm2) displayed excellent mechanical stability as well as general insolubility. The -SiOSi- cross-linking produces a mechanically rather rigid f i i , which with thicker films caused visible cracking and peeling of a number of films upon exposure to aqueous electrolyte solution with its associated swelling stresses. This was not a difficulty for films of the 10-20 pg/cm2 coverage as long as they were not over-exposed to HCl vapor (>2 min). The mechanical cracking problem was fairly common with films which had 100 pg/cm2 or more of polymer. Partitioning results reported here are for the mechanically stable, thinner MPS-SS copolymer films. Copolymer MPS-SS films exhibit XPS spectra for sulfur and silicon like that in Figure 1. Pt 4f bands for the underlying Pt electrode were absent or very weak. The relative areas of the 168 eV S 2p and 154 eV Si 2s (or 102 eV Si 2p) bands, corrected for elemental cross section and photoelectron kinetic energy effects,are given in Table I. Relative S/Si atom populations established from the Si 2s bands agree, for films rinsed in heptane, fairly well with the bulk monomer S/Si ratio as established by elemental analysis. This is interesting since the XPS analysis samples only the outermost ca. 50 A of the copolymer film. The XPS resulta for S/Si do systematically vary according to previous exposure of the copolymer film to aqueous vs. heptane solvent (see especially copolymers no. 2 and 4). This result suggests that polymer chains at the polymer surface tend to align themselves differently upon exposure to solvents which are polar and penetrate and swell the polymer (e.g., water), vs. solvents which are nonpolar and are less imbibed by the polymer (e.g., heptane). In water, the alignment apparently favors positioning (on the average) of the siloxane and silanol centers further into the aqueous environment relative to that of the styrenesulfonate centers, producing apparent S/Si ratios lower than the bulk composition. We do not know whether this has any consequence(s) in the electrochemical or partitioning behavior of the film. Behavior of Redox Ions Partitioned into MPS-SS Films. Exposure of an MPS-SS copolymer film to an aqueous
+
where p is the amount of K+ (and C1-) present in the film in excess of that required for neutralization of the sulfonate groups. The reaction assumes that penetration of the film by Ru(NH,),S+ results in ejection of K+ on a 1:l (charge) basis. The quantity rRu(mol/cm2)of R u ( N H ~ ) redox ~ ~ + ion incorporated into the MPS-SS film should by conventional ion exchange reasoning (29,30) depend on the redox ion charge and size, on its concentration CRUBol in the bathing solution, on the thickness or coverage of the MPS-SS film which determines the quantity rsoa of sulfonate groups present, and on the concentration of competing K+ ions present via the equilibrium constant K+ of the ion exchange reaction. The concentrationCR,& (mol/L) of Ru(NH3)T in the film can be calculated from I'Ru (which we shall measure) and electrode area A by the relation
The weight of MPS-SS copolymer in the film is known by evaporating in the film-making step a known volume of standard solution. Flotation experimentson bulk, cross-linked samples of MPS-SS in contact with water indicate a density near unity. rRu is determined by cyclic voltammetric detection of the R U ( N H ~ ) ~ electrochemical ~+/~+ reaction of the partitioned redox ion, after transfer of the electrode to a 0.01 M KC1 solution containing no dissolved Ru(NH3)$+,as described below. All ion exchange equilibrations were conducted in 0.1 M KC1. In a typical experiment, the Pt/MPS-SS electrode is immersed in 5 mL of an unstirred 0.1 M KCl bathing solution containing (0.5-20) X M Ru(NHS)$+for 1min, which by our experimentation suffices for equilibration according to reaction 1. The complex ion solutions are freshly prepared daily. The electrode is removed, briefly rinsed with distilled water, shaken dry, and placed in an electrochemical cell containing aqueous 0.01 M KC1 (and no RU(NH,)~"). (The low electrolyte concentration was employed to minimize leaching of the electroactive ion after transfer by minimizing competition of K+ for electrostatically bound R u ( N H ~ ) ~ ~ + . ) After about 1 min, the electrode potential is scanned and the current-potential curve for the second cyclical scan is recorded. The original, R ~ ( N H ~ ) ~ ~ +film - f r ecan e be regenerated, for reexposure to a bathing solution with the same or different CRUSo1,by 1-5 min exposure to a stirred 1 M KCl solution, which suffices to reverse reaction 1. The cyclic voltammetry which results from partitioning Ru(NH3),3+into a Pt/MPS-SS film from a series of Ru(NH& concentrations in the 0.1 M KC1 bathing solution is shown in Figure 2. The cathodic current peak represents the oneelectron reduction of R u ( N H ~ ) and ~ ~ +obviously is systematically increased by equilibration of the film with higher CRUSo1.A t these concentrations of Ru(NH3)2+in the bathing solution, we conclude that reaction 1is not forced completely to the right; the film's sulfonate sites are not saturated with Ru(NH3),3+. At potential sweep rates in the range 20-200 mV/s, the peak currents of cyclic voltammograms like those in Figure 2 are proportional to sweep rate. The charges under the peaks (and thus the measured quantity of partitioned Ru(NH&~+)are independent of potential sweep rate. These are features of
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Is
1511
C
E vs N a ' X E Flgure 2. Cyclic voltammograms at 100 mV/s of Ru(NH,):+ partitioned into a MPS-SS film on a platinum electrode in aqueous 0.01 M KCI containing no Ru(NH3):+. The MPS-SS film had been immersed in a 0.1 M KCI bathlng solutloin containing Ru(NH,):+ at concentrations and 0.5 X 10" M for curves A-D, 4.0 X 2.0 X low5,1.0 )< respectively. Coverages (I',q , mol/cm2) are 2.9 X 1.8 X lo-', 1.35 X lo-', and 7.5 X 1WYo for curves A-D. S = 45.4 pA/cm2.
Nernstian equilibration of' the electroactive Ru(NH3)2+species trapped in the MPS-SS film with the potential applied to the Pt electrode and provide evidence that the cyclic voltammetric r R u yields a measuremeint of the total amount of Ru(NH3)2+ in the film. Translation of r R u into the concentrations of Ru(NH&~+in the MPS-SS film, C R ~ P O ' Y , using eq 2, gives values of 0.02-0.1 M, much larger than the concentration of Ru(NH3)2+in the bathing solutions. Clearly KEqof reaction 1 is >>1. The partitioned Ru(NH3)2+is in 0.01 M KC1 leached from the MPS-SS film a t an insignificant rate, as was shown by the following experiment. Following a measurement of r R u in a Pt/MPS-SS electrode, the electrode was disconnected and allowed to rest in the solution (5 mL) for 2 weeks. A remeasurement of r R u for this electrode revealed no measurable difference from the initial value. This same electrode was sequentially reexposed to five 5-mL portions of fresh 0.01 M KC1, each for 1 day, and again the remeasured r R u was equal to the initial one. The above results show firstly that the C R u P o l Y for Ru(NH3)63+from the ion exchange reaction 1 in the bathing solution is not disturbed, by leaching, upon transfer of the electrode to 0.01 M KCl for the cyclic voltammetric measurement with brief potential cycling. Secondly, CRuPolY is responsive to CR$O1. Tlhese are necessary aspects of quantitative evaluation of the equilibrium constant of reaction 1in the 0.1 M bathing solution, as expressed by the distribution coefficient
KD :=
CRUPOlY
/ CRusol
(3)
Such results will be given below. Ion exchange reactiolns of methyl viologen, MV2+,and of Cr(bpy),3+ with Pt/MPS-SS films in 0.1 M KC1 bathing solutions were detected rind quantitatively observed by cyclic voltammetry in a similar manner and with similar results. These three redox catilons were selected for dissimilar electrochemical potentials, so that ion exchange partitioning of pairwise mixtures of them could be quantitatively examined. Figure 3a shows the voltammetry of the three dissolved redox ions at naked Pt; Figure 3b shows voltammetry of the three redox ions individually partitioned into Pt/MPS-SS films; and Figure 3c shows voltammetry of the three redox ions simultaneously partitioned from an equimolar (5 X lo4 M) solution mixture in 0.1 hrl KC1 into a Pt/MPS-SS film. The current peak at the most positive potential corresponds to R u ( N H ~ ) ~ ~the + / ~middle +, current peak to the Cr(bpy),3+lz+ reaction, and the most negative current peak to the MV2+I3+ reaction. It is evident from Figure 3c that Ru(NH3)2+is more highly selected by the polyanionic film than Cr(bpy),3+,which is accordingly more highly selected than MV2+. Several other interesting observationsshould be mentioned before proceeding to the partition coefficient results. First,
Figure 3. (a) Cyclic voltammogram of a mixture of dissolved RIA(",):+ (peak A), Cr(bpy),,+ (peak B), and MV2+ (peak C) at naked platinum disk electrode in aqueous 0.1 M KCI, 100 mV/s, S = 45.4 pA/cm2. (b) Overlay of cyclic voltammograms of Ru(NH,)2+, Cr(bpy),,+, and MV2+ individually partitioned into MPS-SS film on a platinum disk electrode: Ru(NH in aqueous 0.01 M KCI, 20 mV/s, S = 9.1 pA/cm2; Cr(bpy);' in aqueous 0.01 M Bu,NBr, 200 mV/s, S = 90.0 pA/cm2, with iR compensation; MV2+ in aqueous 0.01 M KCI, 50 mV/s, S = 181.8 pA/cm2, with iR compensation. (c) Cyclic voltammogram of a mixture of Ru(NH,):+, Cr(bpy):+, and MV2+ partitioned Into MPS-SS from an equimolar concentration (5 X M) solution of the three, in aqueous 0.01 M KCI, 50 mV/s, S = 90.0 pA/cm2.
)zt
A
1
,
\ ' '0'
E vs NaSCE
Flgure 4. Cyclic voltammograms of Ru(NH&~+that had been partitioned from 2.5 X M Ru(NH3):+ into a MPS-SS film, in 0.01 M KCI, 0.1 M KCI, and 1.0 M KCI, curves A-C: 20 mV/s; S = 9.1 pA/cm2.
it is evident from Figure 3a,b that the formal potentials for the dissolved ions C ~ - ( b p y )(-0.48 ~ ~ + V vs. NaSCE) and MV2+ (-0.69 V) are nearly the same as those for electrostatically trapped Cr(bpy),3+(-0.51 V) and MV2+(-0.67 V) but that for dissolved R u ( N H ~ ) (+0.18 ~ ~ + V) is quite different from Ru(NH3)Z+(-0.35 V) in the MPS-SS film. Differences between and E0lsurf of' dissolved and electrostaticaly trapped redox ions have been previously observed by Anson (24) and Kuo (26),and are attributed to differences between the strengths of the ion association interactions of the oxidized vs. reduced forms of the ion with the polymer counterionic environment. The electrostatic stabilization of the Ru(NH3):+ ion would logically be greater than that of the Ru(NH3)?+ion, causing it to be reduced at a more negative potential in the MPS-SS film as compared to the solution form. This electrostatic shift of reduction potential for Ru(NH3)2+is greatly attenuated in more concentrated electrolyte solution, as shown
1512
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Table I1 Equilibrium Solution and Partitioned Concentrations of Ru(NH,),~+in Poly(MPS-SS) Polymer Film and Bathing 0.1 M RCI Solution. r R u Measured by Cyclic Voltammetry in 0.01 M KCl
1.25
16 1000 800 i 0.0 16 1000 1000 i 65 0.75 14.5 930 1240 * 43 0.50 770 1540 ?: 64 12 450 1800 t 64 0.25 7.0 0.10 3.7 240 2400 f 65 Equilibrium Concentrations of MVZtin Poly(MPS-SS) and Bathing 0.1 M KCl Solution. r M V Measured by Cyclic Voltammetry in 0.01 M KCl
5
, j,
1.0
E vs NaSCE
Figure 5. Cyclic vokammograms of MV2+and ferrocyanide in aqueous 0.01 M KCI at (---) clean platinum electrode, S = 45.4 pA/cm2, and (-) a MPS-SS film on platinum with 5 X lo-' mol/cm2, S = 227 pA/cm2, 100 mV/s.
-
rso3
exchange reactions of the ions in the film (22,25-28). Under the conditions of Figure 2, since peak currents are proportional to potential scan rate, these processes must occur rapidly compared to the experimental time scale. Electrochemical charge transport rate effects in these films can be observed with thicker films rSOa > lo-' mol/cm2) but were not investigated as a part of this study. 4.0 4.5 220 55 i 4 2.0 3.1 153 76i 4 Fourthly, it is to be expected that the polyanionic MPS-SS 1.0 1.8 87 87 ?: 1 films should electrostatically exclude electroactive anions in 0.80 1.5 65 81 f 3 preference to partitioning of electroactive cations. Figure 5 0.40 0.83 40 100 * 3 illustrates this distinction for a mixture of ferrocyanide and a Calculated from eq 2. Calculated from eq 3. MV in contact with a bare (---) and Pt/MPS-SS film (-) -~ electrodes. The preference of the film to allow electrochemical reaction of MV2+vs. ferrocyanide exceeds a factor of 10. A in Figure 4,where voltammetry of the trapped ion in contact with 0.01, 0.1, and 1.0 M KCl electrolyte concentrations is small electrochemical wave for ferrocyanide oxidation is observable, which indicates that the anionic exclusion process compared. The respective potentials of the cathodic peaks is imperfect and/or that the film contains a few pinholes and in the initial voltammetric scan are -0.34, -0.28, and -0.20 V, cracks through which the electroactive anion can diffuse to the latter approaching that observed for the dissolved ion. Figure 4 also shows that the less tightly bound RU(NH~)~'+ the Pt surface, Finally, our calculations of KD from eq 2 and 3 assume that is rapidly leached from the film at the higher electrolyte the film density is uniform and not seriously affected by eq concentrations, since the current peaks for the oxidation scan 1,that there are not regions of the films which ion exchange are smaller than the initial reduction scan. but are totally electroinactive, and that KD does not vary Secondly, while Ru(NHJ:+, MV2+,and Cr(bpy)QB+remain greatly with the (probably variable) amount of siloxane Sistably partitioned into MPS-SS films in contact with 0.01 M 0-Si cross-linking in the film. These assumptions probably KC1 (with little or no potential cycling), continuous long term account for the observed experimental variability in KD potential cycling causes a slow leaching of Ru(NH&~+and MV2+from the film. For instance, ca. 50% loss of R u ( N H ~ ) ~ ~ + (Tables 11-IV). KDcan differ by approximately a factor of 2 from batch to batch of prepared electrodes, but within a occurred into a 0.01 M KCl electrolyte after 1h of continuous batch of electrodes prepared a t the same time is much more potential scanning, a cumulative time of ca. 30 min with the reproducible. Also, there was no obvious correlation of K D electrode in the Ru(NH3):+ reduction state. This loss ocwith polymer composition over the narrow range of monomer curred into an electrolyte medium initially free of the redox ratio available (Table I). ion. Much more rapid loss occurs for Cr(NH3):+; about half Distribution Coefficients for Ru(NH&~+and MV2+ is lost following the first scan. inta MPS-SS Films. The distribution coefficients (eq 3) Thirdly, the quantity of R u ( N & ) ~ ~partitioned + into the for these ions into Pt/MPS-SS fiis from 0.1 M KC1 solutions MPS-SS film in Figure 2A, is equivalent to, approximately, were evaluated from voltammograms like Figure 2 as a 15 monomolecular layers of this redox ion. Presumably the function of CR?' and Cwso1. The results for KD and CR~P''Y electrochemical reactions of these multilayers of redox ions and Chlpo1Yare given in Table I1 and are plotted as isotherms occur by some combination of diffusion and electron self_ I _
Table 111. Ratio of Distribution Coefficients of Redox Ionsa Partitioned into MPS-SS Films from Pair Mixtures, under Potential Control, in 0.1 M KC1; rsMeasured by Cyclic Voltammetry in 0.01 M KC1
p' (x104
(X10'0 ri mol/ cmZ) 12.4 7.3 5.0 2.2 8.8 6.1
(X10'0 rj mol/
ciPolY
cjPolY
( x ~ Y (x103 M)b
M) 3.7 6.1 1.8 1.0 2.8 5.7 3.6 1.0 2.4 1.6 3.3 1.1 Cr3+ 2.5 1.1 5.1 1.1 era+ 4.3 1.4 Ru3+ Cr3t 2.9 2.3 1.7 3.6 2.9 Ruz+ cr3+ 2.3 Calculated from eq 2. a Ru = Ru(NH,), ; Cr = Cr(bpy), ; MV = methyl viologen. from eq 4. i
j
Ru3 RuZ
+
MVZt MV" MVZ+ MVZt
M)
cjsol
(x104 M) 4.5 4.5 5.7 5.7 2.2 2.2
cma)
I .
KD~'
KD,jb
607 360 221 98 187 127
40 62 29 43 64 79
aji d
15.0 ?: 5.8 7.6 f 2.3 f 2.9 f 1.6 i
Calculated from eq 3.
2 0.7 0.4
0.3 0.3 0.1
Calculated
ANALYTICAL CHEMISTRY, VOL. 54,
NO. 9, AUGUST 1982
1513
I
Table IV. Ratio of Dist,ribution Coefficients of Redox Ionsa Partitioned into MPS-SS Films from Pair Mixtures under Potential Control, in 0.1 M KC1; rsMeasured by Cyclic Voltammetry in 0.01 M KC1 r ( x 1O1O mol/cm2) cso1(X104M) iscan aji d 1 j i j no. i j KD,iC KD3j i j 45 t 10 3187 75 80 15 16 3.1 Ru3+ MVat 1 0.25 2.0 40 k 4 70 2787 70 14 2 14 2.9 4.9 ? 0.2 840 172 21 34 7.0 4.3 Ruz" MVZ+ 1 Cr3+
MVZ+
Cr2+
MVZ+
Ru3*
Cr3+
RuZ+
Q3+
2 1 2 1 2 1 2 1 2
0.25
2.0
0.25
2.0
0.25
2.0
0.25
0.25
0.25
0.25
4.8 6.9 2.5 0.96 0.56 9.8 9.4 3.0 2.5
6.5 1.8 2.0 5.7 4.9 5.6 3.3 6.2 3.8
23 34 12 4.7 2.7 47 46 14 12
31 8.8 9.8 28 24 27 16 30 18
920 1360
f
188
f 1900 1820 580 486
5.8 t 0.4 31 e
157 44 49 140 120 1080
1.3e 1.8 f 0.3
f 0.48 i 0.05
1200
f
Calculated from eq 2. Calculated from eq 3. Calculated a Ru = Ru(NH,), ; Cr = Cr(bpy),; MV = meth 1 viologen. from eq 4. e Based on. single determination. ~ K for D Cr falls by ca. 2~ on 2nd potential scan indicating leaching loss. ____
1
T
Figure 6. Plot of CMv" VS. CMvm. Slope Is K D for MV2+ partitioning Into MPS-SS.
in Figures 6 and 7. 'The isotherm plots make clear that saturation phenomena become important a t the higher concentrations of CWso1 and CRusol, so that the most meaningful distribution coefficient data are KDvalues determined at the lower Csolvalues. From these (Table 11) we can say that the Pt/MPS-SS film show a strong affinity for both cations, i.e., KD>> 1, but that for (2400), the smaller, more highly charged ion, exceeds the affinity of the MPS-SS copolymer for MV2+ (100) by approximately a factor of 24. These distribution coefficient results are the first reported for ion exchange reactions in thin, polyionic films on electrodes. The isotherm in Fipure 7 for R U ( N I - I ~ )partitioned ~~+ into the MPS-SS film appears to achieve a plateau. The value of 3xrRu (the multiplier accounting for the ion's charge) is however, depending on the electrode, only about 20-40% of the total number of sulfonate sites present (I'R,). The available electrostatic binding sites apparently do not serve as the limiting factorn on how much Ru(NH3)83+can be incorporated into the film in reaction 1. 'Perhaps not all of the sites in the cross-linked MPS-SS lattice are sterically able to cluster, in a 3/1 charge-compensating manner around the Ru(NH3):+ cations. We do observe an increase in rRuof approximately a factor of 2 when RU("~)~'+ is partitioned from a bathing solution containing no KCl,\but even in this case complete charge equivalence was not achieved. We should note that thew experiments were conducted at low
Flgure 7. Plot of C R p 'vs. CRUPOIy.Slope is KD for Ru(NH,)?+
par-
titioning Into MPS-SS. CRUso1 in the bathing solution. Oyama and Anson (23) have described a higher level of charge saturation in an electrostatic trapping experiment but employed a much larger redox ion concentration in the bathing solution. Competitive Partitioning Experiments. In order to further explore the ion exchange selectivities of MPS-SS films among the three redox ions Ru(NH3)2+, C r ( b ~ y ) ~and ~+, MV2+,we equilibrated 0.1 M KC1 solutions of these ions with Pt/MPS-SS films and determined the CplY for each ion from its individual cyclic voltammetric wave. Results for the Ru"/MV2+, Cr3+/MV2+,and Ru3+/Cr3+competitive partition experiments are given in Tables I11 and IV. In most cases the data are averages of three experiments. The data in Table IV are more meaningful since the solution concentrations of C E O 1 employed there were lower and less influenced by the saturation effects indicated in Figures 6 and 7. Table IV also shows results for l? of the electroactive ions obtained during the first and the second cyclic potential sweeps. For Cr( b ~ y ) ~r ~on+ the , first cyclical potential scan is ca. 2 times that in the second cyclical scan, apparently due to leaching upon reduction, and KDvalues are taken from the former rcr values. Differences between rRu and rMV in the first and second cyclical scans are minor. Each of the above pairwise competitive experiments for Ru3+/MV2+,Cr3+/MV2+,and Ru3+/Cr3+was carried out with
1514
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 A
E vs. NaSCE
E vs. NaSCE
!!'+
Figure 8. Cycllc voltammograms of a mixture of Ru(NH and MV2+ partltloned from 0.1 M KCI solution containing 4.5 X 10 M MV2+and 1.0 X lo4 M Ru(NH,)?+ into a MPS-SS film on a platinum electrode in aqueous 0.01 M KCI at 100 mV/s, S = 22.7 pA/cm2. During partltloning, the electrode was potentiostated for 10 min at 0 V (curve A) and -0.51 V (curve B). For curve A, rRu = 1.2 X lo-' mol/cm2, rw = 3.7 X 1 0 - l ~mot/cm2; for curve B, rRu = 7.3 x mol/cm2, ruv= 5.7 x io-^
the electrode potentiostated at 0 V vs. NaSCE in the bathing solution. Following transfer of the equilibrated electrode to a cell containing 0.01 M KCl and cyclic voltammetric analysis of the amount of each electroactive ion partitioned into the film, the film was stripped of the electroactive ions (by immersion in 1 M KCl) and returned to the bathing solution. The Pt/MPS-SS film was then reequilibrated with the bathing solution, this time with a potential being applied between the electrochemical waves of the redox ion pair. In this case, the competitive partition reaction becomes that between the reduced form of the more easily reduced ion and the oxidized form of the other ion. The electrode is then transferred to 0.01 M KC1 for cyclic voltammetric analysis of r as before. This experiment is illustrated in Figure 8, which clearly shows by reference to Figure 8A (Ru3+/MV2+competition) that selectivity of the film for the ruthenium complex is substantially diminished by potentiostating of the electrode to reduce the film to its Ru(NH3),2+form (Figure 8B). Results for such reduced ion/oxidized ion pairwise experiments, Ru2+/MV2+,Cr2+/MV2+,and Ru2+/Cr3+are given in Tables I11 and IV. The following observations can be made from the data in Tables I11 and IV. First, both at high (Table 111) and low (Table IV) bathing concentrations Csol,the individual results for KDfor R u ( N H ~ )and ~ ~ +MV2+in their mixtures, 607 and 40 (Table 111),and 2800 and 75 (Table IV), respectively, are roughly comparable to those measured singly (Table 11) for the redox ions at the corresponding Cso',1000 for Ru(NH3)Z+ and 55 for MV2+(high CSo1)and 1800 and 76 (low CSo1),respectively. That is, the ion exchange partitioning of Ru(NH3)63+and MV2+ions occurs into the MPS-SS copolymer film without substantial interference by or interaction with the other ion. This is somewhat surprising for the MV2+ion, given that (in Table 111) it is being partitioned into a film in M R U ( N H ~ ) ~which ~ + , from Figure the presence of 1 X 7 is a sufficiently high concentration to saturate the MPS-SS film with the latter substance. Secondly, the ratio of distribution coefficients
I Figure 9. Cycllc voltammetric response of a bilayer electrode, with =1X mol/cm2, and outer layer PVFer, inner film MPS-SS, rFer = 1 X io-' mo1/cm2, in aqueous 0.1 M LiCl containing 5 x IO-^ M and Bu,NBr. Peak A corresponds to PVFer, peak B is the oxidation of Br-, peak C Is the PVFer* untrapping peak, and peak D corresponds to Ru(NH,),~+: 20 mV/s; S = 45.4 bA/cm2.
rsq
In summary, these results show that it is possible to use electrochemical reactions to examine the affinity of an ionic polymer film for various electroactive ions. Distribution and selectivity ratios for the ions can be established, and the partitioning reactions (and KD and a / ) can be altered in situ by reducing one of the two partitioning ions at the same time partitioning occurs. Description of quantitative aspects of reaction 1 in this manner is thus quite parallel to that appropriate for a conventional ion exchange process. That is, it is not clear that the chemical equilibria involved in "electrostatic trapping" of redox ions by polyanionic films on electrodes (23)are different in any essential way from those operative in partitioning of similar ions into ion exchange beads in column chromatography. It should thus be possible to deploy the substantial available body of ion exchange selectivity data to more optimally design "electrostatic trapping" reactions of redox ions at electrode surfaces coated with polyionic films, for electrocatalytic, analytical, and other purposes. It does not of course necessarily follow that ion exchange kinetics on electrodes are the same as those in beads. We have not attempted to address kinetics here, but note that the scanning of electrode potential has important kinetic effects (26) Polymer Bilayer Electrode. We have shown (17,18)how layers of two redox polymers coated on electrodes display rectifying properties based on redox conductivity characteristics of the inner redox polymer layer. The outermost layer of redox polymer is forced to undergo electron transfers (trapping and untrapping reactions) with the electrode through and at the energies of the electron levels of the inner layer of redox polymer. We have employed inner layers of redox polymer which contain fixed redox sites contributing either one or two of the redox levels needed for trapping and untrapping of redox levels of the outer layer. As further proof of our redox conductivity interpretations (18)of bilayer electrode phenomena, we have employed the MPS-SS copolymer as the inner film of a bilayer electrode. Poly(viny1ferrocene)is used as the outer layer. The redox KD,i/KD,j (4) levels in the inner film are provided by adding bromide and R u ( N H ~ ) ions ~ ~ +to the solution. The response of the bilayer expresses the selectivity between various pairs of redox ions. electrode is shown in Figure 9. Sweeping the potential beThe values of a/ in Tables I11 and IV show that the charge tween 0 and +0.7 V vs. NaSCE reveals only a very small wave on the ion more significantly affects affinity than ion size. That is, arj' for the Ru3+/MV2+and Cr3+/MV2+pairs are both for the ferrocene couple, which probably arises from a few pinhole imperfections in the MPS-SS copolymer film. (The much larger than those for the Ru2+/MV2+and Cr2+/MV2+ Eo'eurf for PVFer in the absence of the MPS-SS film is popairs. ajifor ions of like charge are in fact generally not far tential A.) If the potential is swept from 0 to -0.57 V, only from unity. The MPS-SS shows only modest chemical or steric selectivity in comparison to the effect of ion charge. the R u ( N H ~ ) ~ ~wave + / ~is+ observed (potential D). In a potential sweep into the positive potential region B, Thirdly, the order of selectivity for partitioning of the redox ions is R u ( N H ~ )> ~R ~ +u ( N H ~ )i= ~~ C+ r ( b ~ y ) , ~>+C r ( b ~ y ) , ~ + the small amount of bromide which penetrates the inner MPS-SS film becomes oxidized, transported by diffusion (or i= MV2+.
Anal. Chem. 1982,
charge transport) through the M P S S S F i n to the PVFer film, and causes the charge trapping reaction Br,
+ 2PVFer
-
2Br-
+ 21?VFert
(5)
No significant reduction peak for the PV!Fert product of this reaction is seen at potential A on the subsequent negative potential sweep, but a t potential C, where a small amount of R U ( N H ~ ) ~is' +generated, the untrapping reaction Ru(NH3):+
+ PVl?er+
-
+
R U ( N H ~ ) ~ ~PVFer + (6)
occurs to give a prominent current peak. As expected from voltammetry theory foir the bilayer (18), the untrapping current peak at potentid C shifts to a more negative Epeak when larger quantities of Br2 and thus trapped PVFer+ are generated by a more prolonged positive potential excursion ((- - -) vs. (-) where the latter corresponds to generation of a larger quantity of trapped PVFer+). Figure 9 confirms that the inner film of a bilayer electrode need not contain (18) fixed redox sites. Its redox conductivity can be provided entire1:y by mobile redox ions, which adds to the flexibility of the electrochemicaltrapping and untrapping phenomena observable (at spatially structured polymer film electrodes.
ACKNOWLEDGMENT Helpful discussions with K. W. Willman and S. Nakahama are acknowledged.
LITEXATURE CITED Merz, A.; Bard, A. J. J . Am. Chem. SOC. 1978, 100, 3222. Van De Mark, M. R.; hAlller, L. L. J. Am. Chem. SOC. 1978, 700,
3223. Peerce, P. J.; Bard, A. J. J. Elecfroanal. Chem. 1980, 174, 89 and references cited thereln. Oyama, N.; Anson, F. C. J. Electrochem. SOC. 1980, 127, 640. Oyarna, N.; Anson, F. C. J. Am. Chem. SOC. 1979, 101, 1634. Dautartas, M. F.; Evans, J. F. J. Elecfroanal. Chem. 1980, 709, 301.
54, 1515-1518
1515
(7) Kaufman, F. B.; Patel, V.; Engler, E. M. J. Elecfroanal. Chem. 1980, 773, 193. (8) Kaufman, F. 8.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q. J. Am. Chem. SOC.1980, 102, 483. (9) DeGrand, C.; Lavlron, E. J . Elecfroanal. Chem. 1981, 777, 283. (IO) Samuels, G. J.; Meyer, T. J. J. Am. Chem. SOC. 1981, 103, 307. (11) Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1981, 20, 27. (12) DeGrand, C.; Miller, L. L. J. Am. Chem. SOC. 1980, 102, 5728. (13) DeGrand, C.; Mlller, L. L. J. Elecfroanal. Chem. 1981, 117, 267. (14) Daum, P.; Lenhard, J. R.; Rolison. D. R.; Murray, R. W. J . Am. Chem. SOC. 1980, 102, 4649. (15) Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.; Schultz, F. A,; Murray, R. W. J. Am. Chem. SOC. 1980, 102, 7629. (16) Abruiia, H. D.; Denisevlch, P.; Umaiia, M.; Meyer, T. J.; Murray, R. W. J . Am. Chem. SOC. 1981, 703, 1. (17) Daum, P.; Murray, R. W. J. Phys. Chem. 1981, 85, 389. (18) Denlsevlch, P.; Wlllman, K. W.; Murray, R. W. J . Am. Chem. SOC. 1981, 703,4727. (19) Wrighton, M. S.;Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Haas, 0.; Leaa, K. D.: Nadio. L.: Palazzotto. M. C. J . Elecfroanal. Chem. 1978. 87;629. (20) Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. SOC. 1980, 102, 5123. (21) Lewls, N. S.;Bocarsly, A. E.; Wrighton, M. S. J. Phys. Chem. 1980, 84, 2033 and references cited therein. (22) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (23) Oyama, N.; Anson, F. C. J. Electrochem. SOC. 1980, 127, 247. (24) Oyama, N.: Shimomura, T.; Shlgehara, K.; Anson, F. C. J. Elecfroanal. Chem. 1980, 772, 271. (25) Rubenstein, I.; Bard, A. J. J . Am. Chem. SOC. 1980, 702, 6641. (26) Kuo, K. N.; Murray, R. W. J . Elecfroanal. Chem. 1982, 1 3 f , 37. (27) Faccl, J.; Murray, R. W. J. Elecfroanal. Chem. 1981, 724, 339. (28) Facci, J.; Murray, R. W. J. Phys. Chem. 1981, 82, 2870. (29) Helfferich, F. "Ion Exchange"; McGraw-HIII: New York, 1962. (30) Heftmann. E., Ed. "Chromatography", 2nd ed.; Reinhold: New York, 1967. (31) Bruce, J. A.; Wrlghton, M. S. J. Phys. Chem., in press. (32) Scofleld, J. H. J. Electron Spectrosc. Relat. Phenom. 1978, 8 , 129. (33) Penn, D. R. J. Elecfron Spectrosc. Relat. Phenom. 1978, 9, 29. (34) Wagner, C. D. Anal. Chem. 1977, 4 9 , 1282. (35) Lenhard, J. R.; Murray, R. W. J. Am. Chem. SOC. 1978, 100, 7870. (36) Nakahama, S.,Unlverslty of North Carolina, unpublished results, 1979. (37) Cotton, F. A., Ed. "Inorganlc Synthesis"; McGraw-Hill, New York, 1972;Vol. X I I I , p 208.
RECEIVED for review December 11,1981. Accepted April 6, 1982. This research was supported in part by a grant from the National Science Foundation.
Ultratrace Determination of Vapor-Phase Nitrogen Heterocyclic Bases in Ambient Air Jeanette Adams, Elliot L. Atlas, and Choo-Seng Glam" Department of Chemistry, Texas A&M University, College Statlon, Texas 77843
Methods have been denreloped to sample and quantltate vapor-phase nltrogen heterocyclic bases in amblent alr. A high-volume sampllng procedure which employs Florlsll as an adsorbent Is used to trap azaarenes. The analytical procedure provides simple and reproducible determinationsof trace levels of the two- and three-rlnged nitrogen-contalnlng compounds. The procedure Involves carefully controlled evaporatlve concentration oil eluates from Florlsll and a "mlnl" acid-base extraction. The data obtalned from the analysis of amblent alr Indicate {thatvapor-phase levels of azaarenes are many times higher than those previously reported based only on particulate matter. Thus, reports which did not measure vapor-phase ooncentratlons have slgnlflcantly undermeasured the total concentrations of atmospherlc nitrogen heterocycles.
Nitrogen heterocyclic bases, such as isoquinoline and acridine, were orginally isolated in the 1800s from bone oil and coal tar (1). This class of compounds has since been found 0003-2700/82/0354-1515$01.25/0
both as products of various coal utilization technologies (2-5) and as contaminants in their effluents and process waters (6-8). They have been isolated from creosote wood-preserving wastes (9) and from aerosols produced from coal incineration and automobile exhaust (10). They also have been identified in samples of ambient air particulate matter from around the world (11-16). Many nitrogen heterocyclic bases, thought to be derived from pyrolytic processes (17, 18),have been shown to be toxic, carcinogenic, and mutagenic to terrestrial and aquatic organisms (19-26). Because of the characteristically higher level of organic nitrogen in coal and its byproducts as compared to other fossil fuels (27),increasing use of coal and synthetic fuels (28) may significantly increase the levels of nitrogen heterocycles in the environment. Specifically, the higher level of benz[c]acridine in European air particulate matter as compared to that of the United States has been attributed to the combustion and processing of coal (29). Fused-ringnitrogen heterocyclic bases, azaarenes, have been typically collected by high-volume sampling with glass fiber 0 1982 American Chemlcal Society
