76
Anal. Chem. 1980, 52, 76-80
Rotated Porous Carbon Disk Electrode W. J. Blaedel" and Joseph Wang Department of Chemistry, University of Wisconsin- Madison, Madison, Wisconsin 53 706
The electrochemical characterization and the analytical use of a rotating electrode made of a thin Reticulated Vitreous Carbon (RVC) disk are presented. The electrode is characterized by the anodic oxidation of ferrocyanide ion. Well defined current-potential curves are observed. The dependences of the limiting current on the rotation speed and on the ferrocyanide concentration have been investigated. Pulsed rotation voltammetry at the rotating RVC electrode is demonstrated at submicromolar levels of ferrocyanide and micromolar levels of NADH. Differential pulse anodic stripping voltammetry at the rotating mercury-coated RVC electrode is demonstrated for quantitation of heavy metals at the 10-nM level using 4-min deposition times. The electrode is inexpensive and easy to fabricate.
The use of solid electrodes for electroanalytical purposes has gained popularity in recent years ( I ) . Solid electrodes are particularly useful for the anodic oxidation of many organic molecules (Z ), and as substrates for mercury films used in anodic stripping voltammetry (ASV) (3). Carbon as a solid electrode material is becoming popular compared to metallic materials like platinum or gold whose behavior is complicated by the formation of oxides and adsorbed hydrogen films (I), and by their finite solubility in mercury, in the case of ASV (3). Among the various forms of carbon suitable for electroanalytical purposes, vitreous carbon (glassy carbon) possesses marked advantages because of its distinctive properties ( 4 ) . Rotated electrodes are highly advantageous for analytical applications owing to their high mass transport rates which provide great sensitivity. The most important rotated solid electrode is the rotated disk electrode (RDE), whose application to anodic oxidations and stripping determinations is developing rapidly ( 5 ) . Many other shapes of rotating electrode have been described in the literature: rotated wire ( I , p 104), rotated cone (6), rotated hemisphere ( 7 ) , and the rotated ring electrode (8). As used for analytical applications, rotated electrodes have relatively small surface areas. Rotated electrodes with large surface areas that yield high analytical currents have not been employed successfully owing to the accompanying high background currents. In recent years, various sensitive electroanalytical techniques have been developed to discriminate against various components of the background current a t solid electrodes. These techniques include sinusoidal modulation of the rotation speed (9),pulsed rotation voltammetry (PRV) ( I O ) , differential pulse anodic stripping voltammetry (DPASV) (31, subtracted ASV ( II ) , staircase ASV (12),and ASV with collection (13). Since these techniques discriminate against the major components of the background current, an increased sensitivity would result by combining them with large surface area rotating electrodes. In this paper we describe the behavior and the use of a thin rotating porous carbon disk electrode, made of Reticulated Vitreous Carbon (RVC). RVC is a newly introduced electrode material, possessing the electrochemical properties of glassy carbon together with many hydrodynamic advantages (14). I t has been employed recently as an indicator electrode for use in a variety of flowing systems (15-18). The rotating RVC 0003-2700/8G/0352-0076$01 00'0
electrode provides high analytical currents owing to the combination of the high surface area of the RVC (about 66 cm2/cm3(14))with the efficient mass-transport of the rotating electrode. The high void volume of the RVC (as high as 97%) promotes efficient circulation of the sample solution inside the electrode volume. The high background currents that accompany these high analytical currents are compensated by employing the sensitive PRV or DPASV techniques. Such a combination permits the determination of very low concentrations of electroactive species.
THEORY The limiting current for any electrode can be generalized in the following equation (19):
il = nFACM
(1)
where n is the number of electrons transferred per molecule, F is the value of the Faraday, A is the electrode surface area, C is the bulk concentration of the electroactive species, and M is the mass-transfer coefficient. For rotating electrodes, M has the following general form:
M = Kw" (2) where w is the electrode rotation speed, and K and cy are constants. K depends on the diffusion coefficient, and on the kinematic viscosity and the density of the solution, while cy depends on the flow regime and on the shape and size of the electrode. By combination of Equations 1 and 2 one obtains:
i,
= nFKACw*
(3)
The PRV technique involves switching the rotational speed of the working electrode between two values, while maintaining a constant potential. The difference in limiting currents may be described by the following equation, derived from Equation 3:
Ai, = il,H- il,L= nFKAC(wH"- wLa)
(4)
The subscripts H and 1, designate the high and the low rotation speeds, respectively. The limiting current difference, Ail, is independent of the background currents arising from nonconvective sources. For DPASV a t a thin-mercury film electrode the peak current is given by ( 3 ) : 0.138 q m Lp
= ____
t
(5)
where q, is the charge passed in the deposition of the metal (given by i]tdep, where tdep is the deposition time) and t is the pulse width. Combining Equations 3 and 5 leads to the following expression for the DPASV peak current a t any rotating mercury-coated solid electrode: ZP
- 0.138 nFKACtdepwa t
(6)
The direct proportionality between the analytical current and the electrode surface area in the PRV and the DPASV techniques, together with their capability to discriminate against the major components of the area-dependent background current, is the basis of using high surface area rotated electrodes for achieving increased sensitivity. 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
& 0.5
-
INCH
77
----
PLEXIGLAS ROTATED SHAFT
STAINLES LEAD MERCURY P O O L
GLASSY CARBON ROD EPOXY CEMENT RVC DISK
Figure 1. Rotating RVC disk electrode EXPERIMENTAL Electrodes. The electrode design is given in Figure 1. It was constructed from a thin 100-ppi RVC disk (0.086-inch (2.2-mm) thick and 0.215-inch (5.4-mm) diameter) that was obtained from Chemotronic International, Ann Arbor, Mich. (The production process was recently transferred to the Fluorocarbon Co., Anaheim, Calif.) The disk was epoxied at its perimeter (see Figure 1)onto the end of a 0.5-inch 0.d. Plexiglas rotated shaft through a thin film of nonconducting fast-setting epoxy (Epotek 349, Epoxy Technology, Watertown, Mass.). Electrical contact was achieved by pressing the RVC disk against the plane end of a rough polished, glassy carbon rod (0.1-inch (2.5-mm) diameter) that was cast in the end of the Plexiglas shaft. The inside end of the glassy carbon rod was covered by a mercury pool in which a stainless steel lead was dipped. The RVC electrode had a calculated volume of 0.051 mL and surface area of 3.4 cm2. The same RVC disk was used in all experiments, over a period of more than six weeks. All potentials are referenced to a Ag/AgCl electrode in 0.010 M KCl solution, separated from the sample solution by an agar-filled glass frit (20). A two-electrode system was used during the characterization study and the PRV application. The resistance between the electrodes, in the solution containing the supporting electrolyte, was 400 Q. Current-potential curves were corrected for iR drop. The magnitude of the iR drop (as high as 30 mV at the concentration levels that were employed) did not affect the controlled potential amperometry or the PRV data, taken at potentials on the plateau, for which the limiting current is independent of applied potential. For the ASV application, a three-electrode system was used with spectroscopic graphite as the counter electrode. Apparatus. A Sargent FS polarograph was used for generation of the current-potential curves, for controlled potential amperometry and for PRV a t the micromolar level. For PRV at the submicromolar level, potentials were applied with a simple battery-powered potentiometer, and currents were measured with a picoammeter (Model 414S, Keithley Instruments, Cleveland, Ohio), the output of which was fed through a 1000-pF capacitor into a Houston Omniscribe chart-recorder. ASV and DPASV voltammograms were recorded with a PAR 174 Polarographic Analyzer with a Houston Omniscribe X-Y recorder. The rotation assembly and the rotation counting system were described previously ( I O ) , with the exception that another motor speed controller (Model GT-21, G. K. Heller Corp., Bellerose, N.Y.) was used. Reagents. Deionized water (Continental Water System (charcoal bed filter, mixed bed deionizer, 0.2-pm Gelman filter)) was used to prepare all solutions. All chemicals were analytical reagent grade. Supporting electrolytes were 0.1 M phosphate buffer (pH 7.4), and 0.04 M HAC-NH,Ac buffer (pH 4.8). Stock solutions of 5 mM K4Fe(CN)6and of 10 mM NADH (Sigma Chemical Co., St. Louis, Mo.) were made up fresh each day. The ferrocyanide solutions were stored in the dark, and the NADH solutions were stored at 4 "C. Standard solutions, 1mM in various metal nitrates, were prepared and stored in polyethylene bottles. A 0.01 M Hg(I1) solution, for plating the mercury film, was prepared by dissolving pure mercury in nitric acid and making up to volume with deionized water. All studies were made by adding aliquots of the stock solutions to the supporting electrolyte
I
-02
1
0
0.2 0.4 APPLIED POTENTIAL, VOLTS
0.6
Figure 2. Current-potential curves for 37.5 pM K,Fe(CN)6. Scan rate, 1 V/min. Supporting electrolyte,0.1 M phosphate buffer (pH 7.4). Data corrected for background current and iR drop solution to give the desired concentration. Procedure. For studies a t the bare RVC electrode, the following procedure was employed. A 100-mL aliquot of the buffer was deaerated with nitrogen for 20 min. The outlet of the deaeration tube was located far from the working electrode to reduce the effect of bubbles on electrode behavior. During deaeration, the working electrode was pretreated (without rotation) by first applying a potential of +1.25 V for 10 min, then cycling the applied potential between -1.25 V and +1.25 V for an additional 10 min allowing 2 min at each potential. This was followed by a 1-min period at zero applied potential before use. Following pretreatment, the electrode was held at the potential for the start of the scan, or at the desired working potential on the plateau for controlled potential amperometry or PRV studies. The transient currents were allowed to decay until steady state was reached (this usually took about. 20 to 30 rnin). Following this, the measurements were made on the background solution and the analyte solution. All data presented were corrected for background. The electrode was rotated only during the measurements in order to reduce bulk depletion of the analyte concentration. PRV data were obtained by switching manually (through the motor speed controller) between a low and high value. Also, during current measurements, the nitrogen delivery tube was raised above the surface of the solution. The mercury film ASV data were obtained by co-depositing the mercury film and the trace metals on the RVC electrode in the following manner. A 4-mL aliquot of 1 M NH,Ac-HAc solution was pipetted into a 100-mL volumetric flask, followed by 0.60 mL of 1 X lo-* M Hg(I1) solution. The mixture was then diluted to volume and transferred to the cell. The solution was deaerated for 10 min, while the working electrode was held at +0.2 V. The nitrogen delivery tube was then raised above the solution, and a potential of -1.0 V was applied at the electrode while it rotated at 900 rpm. After 6 min, the potential was switched to 4.1 V and held there for 1 min. Following this conditioning, the electrode was ready for use in an analytical run. Background and sample determinations were carried out successively by applying the plating potential for a selected time determined by the sought-for concentration levels. The electrode rotation was then stopped, and after a 15-s rest period the metals were stripped from the mercury film by applying an anodic potential scan, either in the differential pulse mode or in the linear mode. The scan was stopped at +0.05 V, and this potential was maintained for 30 s before the next determination was performed. The mercury film was removed at the end of a series of experiments by holding the electrode at +0.5 V for 20 min.
RESULTS A N D D I S C U S S I O N Current-Potential Curves. Figure 2 shows representative linear scan voltammograms for the oxidation of 37.5 p M K,Fe(CN)6 a t two rotation speeds. Well defined waves and plateau regions were obtained a t both speeds. A clear trend
78
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Table I. Parameters of Current-Concentration Plotsa
dayb
12 17 18 20
500 rpm 1800 rpm sensitivity,c M,(cm/s) sensitivity,c M , (cm/s) MA/pM x pA/pM x 0.67 5 0.81 i 0.86 i 0.95 5
0.02 0.03 0.01
0.02
2.04 2.47 2.62 2.89
1.22 t 1.50 t 1.66 i 1.76 t
0.05 0.09 0.10 0.06
3.72 4.57 5.06 5.36
a Conditions: seven-point standard additions from 10 to 70 M Madded ferrocyanide: supporting electrolyte, 0.1 M phosphate buffer pH 7.4); applied potential, + 0.85 V ; steady-state mode. Day from initial use of the electrode. Tabled values include 90% confidence limits.
toward greater irreversibility is observed at the higher rotation speed, as manifested by the greater slope of the wave and by the anodic shift (=40 mV) of the half-wave potential. This phenomenon was predicted for irreversible reactions a t forced-convection electrodes (I9), and was confirmed for electrochemical reactions at other convective nonporous glassy carbon electrodes (20,21). The reversible behavior and the ease of oxidation were found to be dependent on the intensity of pretreatment and on the age of the electrode, as described later in this paper. Mass Transport. The dependence of limiting current upon rotation speed was evaluated using 37.5 MMK,Fe(CN)6 over the range of 200-2000 rpm. A log-log plot of current against rotation speed (not shown) was linear with a slope of 0.508. When these data were replotted on a linear scale, the dependence of the limiting current upon the square root of the rotation speed gave a straight line t h a t extrapolated to zero current a t zero rotation speed. Similar plots with zero intercepts characterize laminar flow at other shapes of rotating electrodes: disk ( 5 ) ,cone (6),hemisphere (7), and ring (8). The value of (Y in Equation 3 may change with the size, shape, or porosity of the RVC electrode. Study of these various geometric parameters is under investigation. Plotting the reciprocal of the limiting current against the reciprocal of the square root of the rotation speed (not shown) also yields a straight line with zero intercepts. Such a relationship was suggested as a criterion of the homogeneity of the electrode surface with respect to mass transport (22). The efficiency of the mass transport will be discussed later. Dependence of Current upon Concentration, Quantitative evaluation with the rotating RVC electrode is based on the linear correlation between the limiting current and the analyte concentration, described in Equation 3. To confirm this linearity, four separate experiments were performed on different days, each a t two rotating speeds. The results are given in Table 1. All the corresponding eight calibration curves (of current vs. concentration) are highly linear, as shown by the 90% confidence limits of the data. Such linearity is in accordance with the statement of Adams ( I , p 106), that for all rotated electrode systems, limiting current is proportional to bulk concentration. From the data shown in Table I, a clear trend toward increased sensitivity is observed with the aging of the electrode. For the nine-day time period over which these experiments were performed, the sensitivities increase with time, up to about 150% of their original values. This aging effect is probably associated with the formation of oxide groups on the RVC surface. New surfaces would probably be undersaturated with respect to these oxide groups. Since these groups are formed irreversibly (23), they are probably generated a t the large surface of the RVC during the daily pretreatments and studies. Similarly enhanced anodic oxidation of ferrocyanide, due to aging or pretreatment effects, was reported for various carbon electrodes and was attributed to the presence of the
oxide groups (21, 24). While analytical applications of the RVC electrode are based on the linear correlation between limiting current and analyte concentration, its exploitation for kinetic studies will require a thorough study of the effects of history and pretreatment of the electrode. T h e sensitivity is remarkably high, corresponding to 0.674.95 pA/pM at 500 rpm, and to 1.22-1.76 pA/pM at 1800 rpm. These values are higher than the corresponding values of about 0.02 pA/pM that have been obtained, under similar conditions, with glassy carbon disks rotated at 500 rpm (10, 25). (An RDE with surface area similar to the above described RVC electrode would require a diameter of about 2.1 cm, for which an undesirable turbulent flow is encountered (26).) When the data of Table I are normalized to the electrode surface area, values of 0.2-0.28 pA/pM cm2 are obtained a t 500 rpm, which are similar to values of 0.286 bA/pM cm2 calculated for the rotating glassy carbon disks ( I O , 25). This indicates t h a t the current depends on the electrode surface area and that the fluid flow occurs within the electrode. However, the advantages of these high analytical currents are offset by high background currents due to the high surface area of the RVC. Analytical advantages may be gained when sensitive techniques, designed to discriminate against the background currents, are coupled with the electrode operation. Such a combination will be described later in the paper. The efficiency of the mass-transport a t the rotating RVC electrode may be estimated by calculating the values of the mass-transfer coefficient a t its surface. M values may be calculated from the limiting currents in Table I, according to cm/s a t Equation 1. M values range from 2.04-2.89 X cm/s at 1800 rpm. Values around 500 rpm to 3.72-5.36 X cm/s were calculated from data reported for a glassy 3.1 X carbon disk rotated a t 500 rpm (10, 25). The agreement between these values indicates that the efficiency of mass transport a t the rotating RVC electrode approaches that a t the RDE, apparently owing to the combination of high rotational speeds with the high void volume of the RVC t h a t promotes circulation of the solution inside the electrode. PRV at the Rotated RVC Electrode. Steady-state measurements discussed in previous sections are characterized by high background currents that are troublesome when measuring very low concentrations of electroactive species. The detectability may be greatly improved by using the PRV technique, which discriminates against the major contributors to the steady-state background currents. Figure 3 is a reproduction of a pulsed-rotation chart record for 0.2 pM ferrocyanide in 0.1 M phosphate buffer (pH 7.4). The blank pulsed-rotation current (shown on the left) amounted to 110 nA, corresponding to a concentration around 0.18 pM. The compensated nonconvective component of the blank current (not shown) is high, about 5 pA, The noise level is very low, only around 2 nA. A detection limit of about 3 nM may be estimated, if it is based on a signal-to-noise ratio of 1. T o demonstrate the utility of PRV a t the rotating RVC electrode for anodic measurements of low concentrations of organic compounds, PRV current-time data were recorded for NADH a t micromolar concentration levels (Figure 4). NADH was chosen because its quantitation is the basis of the determination of many enzymes and substrates. Its electrochemical quantitation a t the 10-fiM concentration level was reported using PRV a t the RDE (24). The data presented in Figure 4 indicate the possibility of measuring NADH a t concentration levels around 1 pM, without the need for chemical mediators. The pulsed-rotation response time is around 5-10 s, which is similar to that observed a t the RDE ( I O ) . Rapid pulsed rotation measurement that would permit the achievement of subsecond cycling time, is a possibility that is now under
I5 S
7" Figure 4. Pulsed rotation response for 2 and 4 pM NADH. Rotation speeds and supporting electrolyte, as in Figure 3. Applied potential, +0.75 V. Pulsing period, 10 s investigation. Automated PRV measurements obtained under the control of a programmable calculator have been described (27). For practical applications, when a real sample may contain electroactive interferences, the sensitive pulsed-rotation technique may be coupled with a separation technique, like liquid chromatography, as was suggested recently by Kissinger e t al. (28). ASV and DPASV at the Rotating Mercury-Coated RVC Electrode. Anodic stripping voltammetry can be used in conjunction with the rotating RVC electrode coated with a thin mercury film, for the trace analysis of amalgam-forming heavy metals. RVC was found recently to be a very suitable substrate for the thin mercury film stationary electrode, as applied to ASV in a flow-through cell (18). The expected advantage of high peak currents, due to the high rotational speeds and the large surface area, was offset by high background currents, due mainly to charging current which is directly proportional to the electrode area. Application of the DPASV technique might discriminate effectively against the charging current, giving high sensitivity. The idea of combining sensitive stripping approaches with large surface area electrodes was suggested recently (12, 29), but was not exploited experimentally. Figure 5 illustrates typical ASV and DPASV curves for some common cations present a t the 10-nM level. The peak currents obtained with these two modes are remarkably high (corresponding to values of 1-3 pA/nM). Using the linear scan mode, the detectability is not much improved because of the accompanying high charging current. In contrast, the absence of charging current in the differential pulse mode results in improved detectability, as shown by the sharp and high peaks of cadmium and lead. Such an improvement is not noticeable for copper, whose stripping peak is partly obscured by the background current of mercury oxidation, which is not compensated by the differential pulse mode. The low noise level in the DPASV curve permits the quantitation of 1 nM of lead and cadmium using 4-min deposition periods. The analysis of subnanomolar concentration levels would require longer deposition periods. Since the deposition step in anodic stripping analysis is a coulometric process, some bulk depletion of the cations may occur. Study of this depletion effect was performed by measuring the limiting oxidation current for ferrocyanide over a 1-h time period. During the usual deposition times (3-5 min) used for the stripping measurements at the 1-10 nM levels and a t 900 rpm, a small bulk depletion occurs (3-5%). The
/
Pb(ll)
CdCll)
I
I
i
I
-0.4 0 APPLIED POTENTIAL, VOLTS Flgure 5. Characteristic stripping vottammograms for 0.65 ppb copper, 2.1 ppb lead, and 1.1 ppb cadmium. Four-minute depositions at -1.15 V; pH 4.8 acetate buffer; 900 rpm. (a) Linear scan ASV; 50 mV/s. (b) DPASV; amplitude, 25 mV; repetition time, 0.5 s; scan rate, 5 mV/s. Dotted lines represent blank solutions
-0.8
1-h experiment results in about a 40% depletion of the bulk concentration. This exploratory DPASV demonstration indicates the improved sensitivity obtained with the large surface area rotated electrode. Future work will include a more detailed characterization of stripping analysis a t the rotated mercury-coated RVC electrode.
LITERATURE CITED Adams, R. N. "Electrochemistry at Solid Electrodes": Marcel Dekker: New York, 1969; p 104, p 106. Tomilov, A. P.; Mairanovskii, S. G.; Fioshin, M. Y.; Smirnov, V. A. "Electrochemistry of Organic Compounds"; J. Wiley: New York, 1972. Copeland, T. R.; Skogerboe, R. K. Anal. Chem. 1974, 4 6 , 1257A. Zittel, H. E.; Miller, F. J. Anal. Chem. 1965, 3 7 , 200. Opekar, F.; Beran, P. J . Nectroanal. Chem. 1976, 69, 1. Kirowa-Eisner, E.; Gileadi, E. J . Nectrochem. SOC. 1976, 123, 22. Chin, D. T. J . Electrochem. SOC. 1971, 118, 1764. Kadija, I.V.; NakiC, V. M. J . Nectroanal. Chem. 1972, 3 4 , 15. Miller, 8.; Bruckenstein, S. A n d . Chem. 1974, 4 6 , 2026. Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1978. 50, 476. Ariel, M.; Wang, J. "Improved Anodic Stripping Voltammetry"; NBS Special Publication (1974 Symposium on Accuracy in Trace Analysis), Vol. 11, p 881 (issued 1976). Eisner, U.; Turner, J. A,; Osteryoung, R. A. Anal. C h m . 1976, 48, 1608. Laser. D.: Ariel. M. J . Nectroanal. Chem. 1974. 49. 123. "Reticulated Virieous Carbon (RVC)"; Chemotronics Internationai: Ann Arbor, Mich., 48104, 1976. Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 5 1 , 353. Biaedel, W. J.; Wang, J. Anal. Chem. 1979, 51, 799. Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 5 1 , 1045.
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Anal. Chem. 1980, 52, 80-83
(18) Blaedel, W J.; Wang, J. Anal. Cbem. 1979, 57,1724. (19) Jordan, J.; Javick, R. A. Electrochim. Acta 1962, 6, 23. (20) Blaedel, W. J.; Jenkins, R. A . Anal. Cbem. 1975, 4 7 , 1337. (21) Blaedel, W. J.; Schieffer, G.W. J . Electroanal. Chem. 1977, 80, 259. (22) Thirst. H. R.: Harrison. J. A. " A Guae to the Studv of Electrode Kinetics": Academic Press: New York, 1972; pp 86-87: Laser, D.; Ariel, M. J . flectroanal. Cbem. 1974, 52, 291. Blaedel, W. J.; Mabbott, G. A. Anal. Cbem. 1978, 50, 933. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974, 46, 1952. Bogotskaya, I. A . Dokl. Akad. Nauk SSSR 1952, 85, 7057. Engstrom, R. C.; Blaedel, W. J. Cbem., Biomed., Environ. Instrum. 1979, 9 ,61.
(28) Davis, G. C.; Holland, K. L.; Kissinger, P. R. J . Liquid Cbromatogr. 1979, 2, 663. (29) Kryger, L.; Jagner, D. Anal. Chim. Acta 1975, 78, 251.
RECEIVED for review August 3: 1979. Accepted October 15, 1979. This work was funded in part by the University Sea Grant Program under a grant from the Office of Sea Grant, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and by the State of Wisconsin.
Determination of Arsenic(III), Arsenic(V), Monomethylarsonate, and Dimethylarsinate by Differential Pulse Polarography after Separation by Ion Exchange Chromatography F. T. Henry"' and T. M. Thorpe' Department of Chemistry, Miami University, Oxford, Ohio 45056
Monomethylarsonate (MMA) and dimethylarsinate (DMA) were determined at trace levels by differential pulse polarography (DPP). The arsenicals were isolated through interactions with ionexchange resins and digested in perchloric acld. Reduction by SO, allowed quantitation as As( 111). Recoveries averaged 98% for MMA and 100% for DMA with relative standard deviations of less than 5 % in each case. The detection limits were 18 ppb and 8 ppb, respectively. Inorganic As(II1) was determined directly either in 1.0 M HCI or in 1.0 M HC104. Total Inorganic arsenic (As(tot)) was measured by DPP after the reduction of As(V) to As(II1) with SO,. The concentration of Inorganic As(V) was evaluated as the difference between the results for As(tot) and inorganic As(II1).
Interest in determining the levels of individual compounds of arsenic in environmental samples arises from the recognition of their toxicity and possible carcinogenicity. Measurement of total arsenic content does not establish a n effective base from which to estimate hazard because it fails to reflect variations in toxicity, extent of transport, and bioavailability which occur with respect to chemical form. It has been shown ( I ) that arsenic(V), arsenic(III), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) are present in natural water systems. (The structures of DMA and MMA are shown in Figure 1.) Moreover, a dynamic relationship exists whereby oxidation-reduction and biological methylation-demethylation reactions (2-5) provide pathways for the interconversions of the arsenicals. Analytical methods capable of distinguishing between the predominant species of arsenic are necessary if immediate and potential impacts are to be accurately assessed. A variety of techniques have been used to obtain speciation data for the aforementioned forms of arsenic. Spectrophotometry (6-9) and gas chromatography (10-13) have been Present address, Ross Laboratories, 625 Cleveland Avenue, Columbus, Ohio 43216. 'Present address, The Procter & Gamble Company, Sharon Woods Technical Center, 11530 Reed Hartman Highway, Cincinnati, Ohio 45241. 0003-2700/80/0352-0080$01 .OO/O
useful for one or more of the arsenicals, but have failed t o determine all four species. The procedure developed by Braman and co-workers ( I ) for generation and selective volatilization of arsines resolved the four forms of arsenic. However, molecular rearrangements (14) and incomplete recoveries a t low concentrations (15, 16) have been reported. Yamamoto (17) and Dietz and Perez (18) observed that DMA has a strong affinity for acid-charged cation-exchange resins. Elton and Geiger (19) used this fact to separate MMA and DMA prior t o determinations of the organoarsenicals by differential pulse polarography (DPP). The authors reported detection limits of 0.1 Fg/mL and 0.3 kg/mL, respectively. Henry, Kirch, and Thorpe (20) reported a method for the determination of As(III), As(V), and total inorganic arsenic by DPP. As(II1) was measured directly in 1 M HCIOl or 1 M HC1 (21). Total inorganic arsenic was determined in either of these supporting electrolytes after the reduction of electroinactive As(V) with aqueous sulfur dioxide. As(V) was evaluated by difference. Sulfur dioxide was selected because it reduced As(V) rapidly and quantitatively, and excess reagent was readily removed from the reaction mixture. This paper describes a procedure for the determination of As(V), As(III), DMA, and MMA. As(V) and As(1II) are determined by the method of Henry, Kirch, and Thorpe (20). DMA and MMA are isolated through interactions with ionexchange resins, digested in hot, concentrated perchloric acid, reduced by sulfur dioxide, and quantitated as As(II1) by DPP.
EXPERIMENTAL Instrumentation. A Princeton Applied Research Corporation (Princeton, N.J.) Model 174A polarographic analyzer and a Hewlett-Packard (Avondale, Pa.) Model 7040A X-Y recorder were used for all APP determinations. The flow rate of the DME was 0.845 mg/s. A 2.0-s drop time was employed for all measurements. Other instrumental parameters are as previously described (20). Reagents. High purity arsenic trioxide was obtained from ROC/RIC (Belleville, N.J.). Reference purity dimethylarsinic and monomethylarsonic acids were provided by the Ansul Company (Weslaco, Tex.). All other chemicals were reagent grade. Triply distilled, deionized water was used to prepare all solutions. Preparation of Ion-Exchange Columns. Cation, Dowex 50 W-X8, and anion, AG 1-X8, exchange resins were obtained from 0 1979 American
Chemical Society
