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(13) G. G. Rohrback and W. E. Reed, "Evaluation of Extraction Techniques for Hydrocarbons in Marine Sediments", Publication No. 1537, Institute of Gsophysics and phnetary Physics, University of California at Los Angeles, Los Angeles, Calif., 1976. (14) J. W. Farrington and B. W. Tripp. "A Comparison of Analysis Methods for Hydrocarbons in Surface Sediments", in "Marine Chemistry in the Coastal Environment", T. M. Church, Ed., ACS Syrnp. Ser., No. 18, 1975, Chanter 15.
(15) D. Van de Meent, W. L. Maters, J. W. De Leeuw, and P. A. Schenck, Ora. Geochern.. 1. 7 119771. (16) H. k h u m a n n , "Reactions o f Elemental Sulfur with Inorganic, Organic, and Metal-Organic Compounds" in "Sulfur in Organic and Inorganic Chemistry", A. Senning, Ed., Marcel Dekker, New York, 1972, Chapter 21. (17) S.Jensen, L. Renberg, and L. Reutergardh, Anal. Chem., 49, 316 (1977). (18) J. K. Bartlett and D. A. Skoog, Anal. Chem., 26, 1008 (1954).
(19) F. K. Kawahara. Environ. Sci. Techno/.,5 , 235 (1971). (20) L. Gasco and R. Barrera, Anal. Chim. Acta., 61, 253 (1972). (21) M. T. J. Murphy, B. Nag, G. Rouser. and G. Kritchevsky, J. Am. Oil Chern. Soc., 42, 475 (1965). (22) W. E. Haines, G. L. Cook, and J. S. Ball, J. Am. Chem. Soc., 78, 5213 (1956). (23) R. Burwood and G. C. Speers, Estuarine CaastaiMar. Sci., 2, 117 (1974).
RECEIVED for review October 10, 1978. Accepted December 26, 1978. This study was supported by U.S. Department of Energy Contract AT(45-1)-2225-T40. This is Contribution No. 1059 from the Department of Oceanography, University of Washington, Seattle, Wash.
Voltammetric Ion Selective Electrode for the Determination of Nitrate James A. Cox" and George R. Litwinski Department of Chemistry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 7
A sensor which uses an anionexchange membrane to enclose a small volume electrolysis cell has been demonstrated to be suitable for nitrate determinations. The three-electrode cell includes a constrained mercury column indicator electrode; filter paper, which Is impregnated with a 0.1 M KCI-0.01 M ZrOCI, electrolyte, serves as the constraining barrier and as the spacer for the thin-layer electrolysis chamber. The anion-exchange membrane sheath permits transfer of nitrate from the sample into the electrolysis chamber by Donnan dialysis and excludes several species which would otherwise interfere. Controlled potential electrolysis at -1.25 V vs. Ag/AgCI provides the sensing current, the value of which is proportional to the sample concentration of nitrate. As a steady-state current is not developed with the present design, a defined current sampling time is used. Linear response over 3 orders-of-magnitude nitrate concentration is obtained. The detection limit using the current at 8 mln is 6.7 X 10" M NO,-.
Membrane-clad voltammetric sensors have been designed for the determination of certain molecular species, especially dissolved gases. T h e Clark oxygen electrode is probably the best known ( 1 , 2 ) . This system functions by using a neutral membrane t o separate the sample from a small volume electrochemical cell. Oxygen diffuses into the cell where it is continuously reduced by controlled potential electrolysis a t a platinum electrode; a steady-state current is developed in proportion t o the oxygen concentration in the sample. Comparable sensors for ionic species have not been previously reported. T h e present design is based upon our earlier work in which nitrate was determined by linear potential scan voltammetry a t a hanging mercury drop electrode in the presence of a La(II1) catalyst ( 3 ) . Donnan dialysis was used t o transfer a controlled fraction of the nitrate from the sample into the 0.01 M LaC134.1 M KC1 electrolyte prior to the voltammetric scan in order t o increase the overall sensitivity (through preconcentration) and eliminate interference by cations and sur0003-2700/79/0351-0554$01 .OO/O
factants. In the present work ZrOClz has been used as the catalyst since, unlike in the La(II1) system, the mercury surface does not become passivated and dissolved oxygen does not interfere with the development of the nitrate reduction current (4). EXPERIMENTAL The instrumentation consisted of a three-electrode polarograph which was constructed with Teledyne Philbrick Model 1027 operational amplifiers and a Hewlett-Packard Model 15101B strip chart recorder. The chemicals were ACS Reagent Grade and were used without further purification. Because of the known effect of aging ( 5 - 3 , the electrolytes were prepared from a 0.1 M ZrOCll stock solution which had been stored for a t least 3 weeks. The ion-exchange membranes and the poly(viny1 acetate) and poly(viny1 alcohol) neutral membranes were obtained from RAI Research Corporation, Hauppauge, Long Island, N.Y. The ion-exchange membranes were pretreated by the general procedure of Blaedel and Kissel (8) except that they were stored in 0.1 M KC1-0.01 M ZrOClz prior to use. The membrane-clad voltammetric sensors (Figure 1) utilized Ag/AgCl reference, platinum counter, and constrained mercury column indicator electrodes. The system was constructed by cementing a 5-mm 0.d. (2-mm i.d.1 glass tube into a 12-mm o.d. tube with Torr Seal low vapor pressure resin, Varian Associates, Vacuum Division, Palo Alto, Calif. The tbbes were placed concentrically. The reference and counter electrode wires were also cemented into the assembly. The resin face and outer tube of the sensor were machined at a 45' angle; the mercury tube was left planar. A pair of disks of Whatman No. 40 (medium) filter paper were placed over the mercury tube and resin face. Prior to assembly they were impregnated with the KC1-ZrOCl2 electrolyte. The paper served as the constraint for the mercury and as a spacer. An ion-exchange membrane was firmly drawn over the entire end of the assembly. The membrane was fixed in place by Teflon tape and a small plastic hose clamp. Mercury was poured into the inner tube and electrical contact was made by inserting a platinum wire. The length of the complete assembly was about 12 cm. The assembly was stored in supporting electrolyte between experiments. As co-ion penetration occurs extensively with a high ionic strength solution on each side of the membrane, this step regenerates the inner solution and removes the electrolysis products. At least 5 min is allowed for this step. 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
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Flgure 1. Design of the voltammetric sensor. (a) Pi counter electrode; (b) constrained Eg indicator; (c) Ag/AgCI reference; (d) epoxy; (e) retainer ring; (f) paper spacer and supporting electrolyte reservoir; (9) anion-exchange membrane
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Figure 2. Linear scan voltammetric reduction of nitrate. (1) Hanging mercury drop; (2) paper-constrained mercury column indicator electrode. Solution: M NO,-, 0.1 M KCI, 0.01 M ZrOCI,; scan rate, 50 mV S-'
RESULTS A N D D I S C U S S I O N Preliminary experiments were performed to determine whether the described sensor would respond to nitrate. In these experiments, the electrode was dipped into a lo-' M N a N 0 3 solution for 15 min, and a linear potential scan voltammogram was subsequently obtained at a 10 mV s-' scan rate. A nitrate reduction current plateau was observed in the range -1.0 to -1.3 V; a t more negative potentials discharge of the supporting electrolyte occurs (Figure 2). Comparable experiments performed with a typical polarographic cell and a hanging mercury drop electrode produce a peak a t -0.9 V with the Zr(IV) catalyst in agreement with the results of our recent mechanistic study (9). T h e high resistance of the constrained Hg electrode and immobilized electrolyte are responsible for the potential shift and lack of a distinct peak. As the intended application uses controlled potential electrolysis, the absence of a peak current is not important, but since the paper-constrained Hg indicator electrode does not permit complete resolution of the nitrate reduction current from the supporting electrolyte discharge, other constraining materials were tested. A mercury column with a terminal coarse glass frit ( I O ) gave similar linear scan voltammetry results; however, it was difficult to obtain good solution contact when the ion-exchange membrane was attached. Neutral membranes (poly(viny1 alcohol), poly(viny1 acetate), and cellophane) were also employed to enclose the Hg column (11); because of slow diffusion of nitrate through these materials, a nitrate reduction current was not observed. Subsequently only filter paper was used as the Hg constraint and spacer.
:/mlr,
6
Figure 3. Current-time behavior of the nitrate voltammetric ion selective electrode. (1) NaNO,, l o 4 M; (2) blank (distilled H,O). Sample volumes, 100 mL. Supporting electrolyte, 0.1 M KCI-0.01 M ZrOCI,, Electrolysis potential, -1.25 V vs. Ag/AgCI. Samples were deaerated for 5 min prior to measurement and were stirred
Based upon the data in Figure 2, experiments were performed in which the nitrate sensor was dipped into deaerated samples of various concentrations; the controlled potential electrolysis current a t -1.25V was monitored. Typical results are shown in Figure 3. T h e above electrolysis potential was selected because it yielded the greatest signal-to-background ratio. Unlike the case of polarographic oxygen electrodes, steady-state currents were not obtained with the present sensor design. Decreasing the ratio of electrode-to-membrane areas, which decreases the time to reach steady state for the oxygen electrode (2), was not effective. Hence, working curves were prepared using the electrolysis current measured a t a prescribed time after contact to the stirred sample. Under such conditions, linear working curves were obtained. For example, with an 8-min current sampling time linear least-squares analysis of a 7-point curve over the range 1.0 x to 1.2 X M NO3- yielded the following: slope, 1.91 f 0.05 X lo6 nA/M; intercept, 0.:34 nA; and correlation coefficient, 0.998. The slope was unchanged up to 5 X M NO3-. Five replicate blank trials (distilled water instead of a nitrate sample) yielded a current of 0.42 i 0.01 pA with the 8-min current sampling time. The detection limit (using the criterion of the concentration which yields a current of twice the blank uncertainty) was 6.7 X M NO3-. With a 2-min sampling time, the working to 1 x lo-*M NO3-. A comparable least range was 8 X squares study a t the lo-* M level gave a slope of 1.37 f 0.04 X lo3 pA/M, correlation coefficient of 0.9985, and intercept of 8 X kA. The major limitations of the present electrode design are the lack of a steady-state current and the physical instability of the constrained mercury indicator electrode. Both problems are associated with the Zr(1V) catalyst. The constant decrease in the blank current, which is shown in Figure 3, is probably the result of some hydrogen discharge which occurs a t -1.25 V. Evidence for this discharge is the increasing current beyond -1.1 V in Figure 2 with the paper-constrained mercury indicator. The consumption of proton during the subsequent controlled potential electrolysis a t -1.25 V would cause the current contribution from proton reduction to decrease with time. If the electrolysis cell could be constructed with a low internal resistance, a nitrate reduction potential could be defined a t which the background electrolysis is negligible (see Figure 2). Such an electrolysis cell would presumably yield a linear scan voltammetric response identical to that of the hanging mercury drop electrode i-E curve in Figure 2. In this case an electrolysis potential of -1.0 V would result in nitrate reduction without concurrent proton reduction. Steady state would then likely be achieved. Such a cell could be made with a solid indicator electrode. Several solid electrode types were
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investigated by linear scan voltammetry with the Zr(1V) catalyst, including wax-impregnated graphite, mercury-coated graphite and platinum, solid silver amalgam (12),and glassy carbon. I n each case, the electrolyte discharge obscured the nitrate reduction current. An improved catalyst would permit resolving the nitrate current from the background a t a solid electrode. The commonly used La(II1) and UOzClzcatalysts (13, 14) failed because of passivation of the electrode and a low signal-to-background ratio, respectively. Presently catalysis by a mixed Cd, Cu deposit on graphite, which was reported by Bodini and Sawyer (15), is being tested in the membrane-clad sensor. Species which can interfere with the measurement, and the extent to which they will interfere, can be predicted from previous work. T h e anion-exchange membrane will exclude neutral species, except for those of low molecular weight, and cations ( 3 ) . Anions which are electroactive a t -1.25 V will interfere. Fortunately these are not abundant in most samples, b u t nitrite, which often is present with nitrate, is in this category. Measurements in nitrite-containing samples must be performed after a separation or by an alternative approach such as with an enzymatic method (16). Anions which can affect the Zr(1V)-catalysis of the nitrate reduction will interfere. For example, sulfate in high concentrations (ca. M) will shift the nitrate reduction beyond the discharge of
the supporting electrolyte (9). If the sample is not deaerated, the reduction of dissolved O2 will add to the background current.
LITERATURE CITED L. C. Clark, Jr., Trans. Am. Soc. Arfif. Intern. Organs, 2 , 41 (1956). I. Fatt. "The Pohrographic Oxygen Sensor", CRC Press, Cleveknd. Ohio, 1976. G. L. Lundquist, G. Washinger, and J. A. Cox, Anal. Chem., 47, 319 (1975). J. A. Cox and F. Brumgard, Southern Illinois University-Carbondale, Carbondale, IIi., unpublished results, 1976. H. W. Wharton, J . Electroanal. Chem.. 9, 134 (1965). V. M. Klyuchnikov, L. M. Zaitsev, S. S. Korovin, and I. A. Apraksin, Zh. Neorg. Khim. (Engl. Trans/.), 17, 1593 (1975). R. L. Young, J. E. Spell, H. M. Siu, and R. H.Phillip, Environ. Scl. Techno/., 9 , 1075 (1975). W. J. Blaedel and T. R. Kissel, Anal. Chem., 44, 2109 (1972). J. A. Cox and A. Brajter, Electrochim. Acta, in press. J. H. Clausen, G. B. Moss, and J. Jordan, Anal. Chem.. 38, 1398 (1966). E. Pungor, G. Nag, and 2s. Feher, J. flectroanal. Cbem.,75, 241 (1977). 2. Stojek and 2. Kublik, J. flectroanal. Chem., 6 0 , 349 (1975). J. W. Collat and J. J. Lingane, J. Am. Chem. Soc., 76, 4214 (1954). I.M. Kolthoff, W. E. Harris, and G. Matsuyama. J. Am. Chem. SOC., 66, 1782 (1944). M. E. Bodini and D.T. Sawyer, Anal. Chem., 49, 485 (1977). C. H. Kiang, S. S. Kuan, and G. G. Guilbauit, Anal. Chem., 50, 1319 (1978).
RECEIVED for review November 13,1978. Accepted January 15, 1979. This work was supported by the Water Resources Center, University of Illinois, Project A-087-ILL, in the OWRT Allotment Program.
Spectroelectrochemical Determination of Heterogeneous Electron Transfer Rate Constants Douglas E. Albertson and Henry N. Blount" Brown Chemical Laboratory, The University of Delaware, Newark, Delaware
1971 1
Fred M. Hawkridge" Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284
The theory underlying the single potential step spectroelectrochemical determination of heterogeneous electron transfer rate constants is presented. The resulting expressions are experimentally verified using the quasi-reversible oxidation of ferrocyanide at tin oxide optically transparent electrodes at pH 7.00. For this model system, good agreement is obtained between values of kl,hand cy determined spectroelectro= 4.6 (f0.2) X cm/s, cy = 0.328 chemically (h0.006)) and those determined by the previously reported technique of chronocoulometry ( kI,,,= 4.0 (f0.2) X lo4 cm/s, a = 0.323 (f0.008)). The present methodology was devised to quantitatively characterize and thereby compare the rates of heterogeneous electron transfer at chemically modified electrodes.
T h e recent development of chemically modified electrodes (CMEs) has stimulated considerable interest, and a variety of both novel methods of preparation and applications have been reported. Electrochemical syntheses and analyses which were intractable before the advent of CMEs have been reported, and it is expected that activity in this area will increase 0003-2700/79/0351-0556$01 .OO/O
as present methodologies are improved and new approaches are developed. Methodologies which have been applied to affect CME surfaces include adsorption (1-14), amidization (15-22), silanization (22-33), formation of ether linkages (34-36), vapor deposition of phthalocyanine ( 3 7 ) ,and binding of quinones (38). Other methods for preparing modified electrode surfaces include use of plasma treatment (39) and electrochemically driven surface reactions (40-42). Species selective electrodes have also been developed for potentiometric and amperometric analyses incorporating immobilized enzymes and bacteria. Recent reports have described advances in this area of research and this family of modified electrodes will not be discussed here (43-45). Spectroscopic and electrochemical techniques have been used to characterize the morphology of CME surfaces, the electrochemical behavior of the surface, and heterogeneous electron transfer reactions occurring a t CMEs. T h e latter problem has been addressed using cyclic voltammetry, differential pulse polarography, and chronocoulometry. However, these techniques fail to provide species-selective heterogeneous electron transfer rate parameters in an extremely important application of CMEs, namely, electrocatalysis. The absence C 1979 American Chemical Society
