1988
Anal. Chem. 1983,55, 1986-1988
Registry No. Nitrogen dioxide, 10102-44-0; nitric acid, 7697-37-2; n-propyl nitrate, 627-13-4; gold, 7440-57-5; carbon monoxide, 630-08-0; nitric oxide, 10102-43-9.
LITERATURE CITED (1) Logan, J. A.; Prather, M. J.; Wofsy, S.C.; McElroy, M. B. J . Geophys . Res. 1981, 86, 7210-7254. (2) Crutzen, P. J. Ann. Rev. Earth Planet. Scl. 1979, 7,443-472. (3) Cowling, E. B. Environ. Sci. Techno/. 1982, 76,IlOA-123A. (4) Liu, S.C., NOAA Aeronomy Laboratory, 1982,private communicatlon. (5) Hodgeson, J. A.; Bell, J. P.; Rehme, K. A.; Krost, K. J.; Stevens, R. K. "Jolnt Conference on Sensing of Environmental Pollutants"; Amerlcan Institute for Aeronautics and Astronautics, 1971;Palo Alto, CA; Paper NO. 71-1067. (6) Chisaka, F.; Yanaglhara, S.;Shlmada, I.Kikai Gijutsu Kenkyosho Shoho 1975,2 9 , 4153. (7) Futsuhara, N.; Masuda, K.; Tsuchimoto, K.; Yamaki, N. Chem. Lett.
1977,507-510. (8) Joseph, D. W.; Spicer, C. W. Anal. Chem. 1978, 5 0 , 1400-1403. (9) Kelly, T. J.; Stedman, D. H. Geophys. Res. Lett. 1979,6 , 375-378. (IO) Helas, G.; Warneck, P. J . Geophys. Res. 1981, 86, 7283-7290.
(11) Braman, R. S.; Shelley, T. J.; McClenny, W. A. Anal. Chem. 1982, 5 4 , 358-364. (12) McClenny, W. A.; Galley, P. C.; Braman, R. S.;Shelley, T. J. Anal. Chem. 1982, 5 4 , 365-369. (13) Phillips, M. P.; Sievers, R. E., Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C. Anal. Chem. 1979, 57, 1819-1825. (14) Hopper, R. T. Ceram. Ind. (Chicago) 1963 (June). (15) Bollinger, M. J. Doctoral Thesis, University of Colorado, Boulder, CO,
1982. (16) McFarland, M.; Kley, D.; Drummond, J. W.; Schemeltekopf, A. L.; Winkler, R. H. Geophys. Res. Lett. 1979, 6 , 605-608. (17) Halas, G.; Flanz, M.; Warneck, P. Int. J . Environ. Anal. Chem. 1981, 70, 155-166. (18) Schiff, H. I.; Pepper, D.; Ridley, B. A. J . Geophys. Res. 197% 8 4 , 7895-7697. (19) Kley, D.; Drummond, J. W.; McFarland, M.; Liu, S. C. J . Geophys. Res.1981, 86, 3153-3161.
(20) Huebert, B. J.; Lazrus, A. L. J . Geophys. Res. 1980, 85, 7322-7328. (21) Goldan, P. D.; Kuster, W. C.; Albrltton, D. L.; Fehsenfeld, F. C.; Connell, P.
s.; Norton, R.
B.; Huebert, B. J. Atrnos. Environ., in press.
(22) Gormley, P. G.; Kennedy, M. Proc. R . I r . Acad., Sect. A 1948, 5 2 A , 163-169. (23) Ferm, M. Atmos. Environ. 1979, 73, 1385-1393. (24) Davies, C. N. "Aerosol Science"; Academic Press: New York, 1966; pp 393-446. (25) Fuchs, N. A. "The Mechanics of Aerosols"; Pergamon Press: New York, 1964;pp 204-212. (26) Suemin, P. E.; Ivakin, 8. A. Sov. Phys.-Tech. Phys. (Engl. Trans/.) 1961, 6,359-361. (27) Wilke, C. R.;Lee, C. Y. Ind. Eng. Chem. 1955, 47, 1253-1257. (28) Lugg, G. A. Anal. Chem. 1968,4 0 , 1072-1077. (29) Stull, D. R.; Prophet, H. "JANAF Thermochemical Tables", 2nd ed.; National Bureau of Standards Reference Data Service, National Bureau of Standards: Washington, DC, 1971. (30) NASA Panel for Data Evaluation "Chemlcal Klnetic and Photochemical Data for Use in Stratospheric Modelling, #5"; JPL Publication 82-57; 1982,Jet Prooulsion Laboratorv, Pasadena, CA. (31)Johnston, H. S.;Foering, L.; Thompson, R. J. J . Am. Chem. Soc.
1953, 57, 390-395. (32) Daglish, A. G.; Eley, D. D. Roc. Int. Congr. Catal., 2nd. 1961, 2 , 16 15-1 626. (33) Cant, N.-W.; Frederickson, P. W. J . Catal. 1975, 37,531-539. (34) Stephan, J. J.; Ponec, V. J . Catal. 1978,4 2 , 1-9. (35) Eiey, D. D.; Moore, P. B. Surf. Sci. 1981, 1 I f , 325-343. (36) MacDonald, W. R.; Hayes, K. E. J . Catal. 1970, 78, 115-132. (37) Chaston, J. C. Platinum Met. Rev. 1984, 8 , 50. (38) Ong, B. G.; Mason, D. M. Ind. Eng. Chem. Fundam. 1972, 1 7 , 169-1 74.
RECEIVED for review April 18, 1983. Accepted July 11, 1983. This research in the NOAA Aeronomy Laboratory is supported in part by the Defense Nuclear Agency. R.E.S. was partially supported by the National Science Foundation Wider Grant. No. ATM81-08675.
CORRESPONDENCE Flow Injection Analysis and Cyclic Voltammetry Sir: Recently developed flow injection gradient techniques with controlled dispersion ( I , 2) offer a unique possibility to prepare solutions under controlled conditions in flowing systems. It has been pointed out in our preliminary study ( 3 ) that the combination of this feature with scanning and transient electrochemical techniques represents a potentially new way to perform electrochemical experiments. There are many designs of flow-through detectors described in the literature, mainly for continuous, on-line monitoring and for chromatographic applications. A complete bibliography of electrochemical flow through detectors can be found in the new journal dedicated to this subject (4). Although the cell described in this paper can be used for analytical purposes, it is also designed to provide information about the electrochemical properties of a segment of fluid created under the rigorous conditions of an FIA experiment. In this sense the physical volume of the detector chamber is not a critical parameter because the effectiue detector volume is given by the diffusion depletion layer created in the course of the electrochemical experiment a t the surface of the electrode. The flow rate and the scan rate are the two dominating parameters. Their influence on the appearance of the voltammogram is examined in detail. The electrochemical characteristics of this detector are described first by using the flowing stream of solution containing the constant concentration of the depolarizer, and then in the next series of experiments the depolarizer is injected into the carrier stream in the form
of a well-defined zone by the FIA technique.
EXPERIMENTAL SECTION Apparatus. A single line, flow injection analysis system (FIAstar ( 5 ) )driven by a gas of very low pressure has been used
in this work. The propellant gas was 99.99% nitrogen which was further purified by passage through hot (650 OC) copper shavings. This gas was also used to deaerate the carrier solution. The diagram of the electrochemical cell used in this study is shown in Figure 1. Here the insert shows the details of the working electrode (1): (2) reference, Ag/AgCl (saturated KC1) electrode; (3) auxiliary electrode and outlet; (4) sample inlet; ( 5 ) mercury reservoir and drop-size adjustment. As can be seen from the insert, the microelectrode is placed just below the orifice of the inlet tube. This arrangement ensures that any part of the sample plug comes into contact with the electrode surface only once. The replacement of the mercury drop is done by simply gently knocking off the old drop and then forming a new drop. The used up mercury is allowed to accumulate at the bottom of the cell compartment from where it is removed periodically. An IBM EC225 voltammetric analyzer with internal triangular voltage wave form generator WEB used in these studies. The small amount of resistance between the reference electrode and the working electrode.was compensated by the built-in IR compensator using an applied square wave as the measure of the exact compensation. Cyclic voltammograms faster than 100 mV s-l were recorded on a storage oscilloscope Tektronix, 7000 series, and photographed. This form of readout limited the precision of reported experiments to &5%, Experiments with scan rates below 100 mV s-l were recorded on Houston Model 2000 x, y recorder.
0003-2700/83/0355-1986$01.50/0 0 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12. OCTOBER 1983
1987
2
P
2
4 6 8 SWEEP RATE [volt /SeCl
mum 3. F?ot of parameter r = (Imcvs. sweep rate Y. The parameter is flow rate 0 (0.0 mLlmln; A, 0.46 mLlmin: 0.1.52 mllmin). Composition of the solution is the same as that given in Figure 2.
F w r o 1. Diagram of the electrochemical cell. The Insert shows the details of the wmking electrode (I): (2)reference, AglAgCl (saturated KCI) electrode; (3)auxiliary electrode and outlet: (4) sample inlet; (5)
mercury resewoir and drop-size adjustment.
a 4
0
-4 0
-500 mV
Flgure 2. Two types of voltammograms obtained in llowing solution: (a) hydrodynamic voltammogram, (b) cyclic voltammogram; 1 mM Fe"'(oxalate), in 0.25 M Na oxalata. pH 4.5.
Chemicals. All reagents used in this study were analytical grade. Distilled deionized water was used for preparation of all solutions. The solution of the model compound was prepared as follows: 1mequiv of Fe(NO& was dissolved in 0.25 M sodium oxalate and the pH of the solute was adjusted to 4.5 with nitric acid.
RESULTS AND DISCUSSION If the flow rate and the scan rate are chosen judiciously, i t is pwsihle to obtain either a hydrodynamic voltammogram (HV) or a cyclic voltammogam (CV). This point is illustrated hy recordings shown in Figure 2 where 1 mM Fe'"(oxalate)8 in 0.25 M N a oxalate flowing a t a rate of 0.98 mL min-' was scanned as HV by a scan rate of 50 mV 8. Then CV was obtained a t the scan rate of 1 V s-l. At this flow rate the 1 s interval required t o record a cyclic voltammogram a t 1 V s-l represents 16 pL of sample passing by the working electrode, while as much as 160 pL would be needed to record a hydrodynamic voltammogram lasting 10 s under the same flow
conditions. This simple comparison delineates the applicability of the two modes. It is possible to set the potential in the region of the mass transport limiting current and to use this device as a concentration detector for analytical purposes. Conversely, if the electrochemical or a chemical kinetic information is sought by means of cyclic voltammetry, the scan rate should be in excess of 1 V sd. The parameter which characterizes the transition from HV to CV was chosen aa r = (icn- icd/iGp where icnis the current a t the cathodic peak and io,* is the current a t the cathodic switching potential. The plot of this parameter against the sweep rate Y for various flow rates is shown in Figure 3. In stationarysolution (curve 0)there is alwap a discernible peak, in other words a cyclic voltammogram is alwap obtained. The decrease r for sweep rates >2 V s-' coincides with the increased peak separation and is indicative of the onset of the charge transfer kinetics (6, 7) of the reduction of the ferric oxalate complex. The maximum r value gives optimum signal to noise ratio. In flowing solution8 (Figure 6 (A) 0.46 mL/min: (0) 1.52 mL/min) r decreases to zero for slow sweep rates; in other words, the peak diminished and CV is transformed into HV. Thus, for this electrode configuration the usable range for CV and flow rates ahove 0.5 mL m i d requires scan rates greater than 1V s-'. It is interesting to note that even in the transient region (0.1-1.0 V s-') the ratio of corrected (6)cathodic and anodic peak currents is close to unity and the peak separation is close to 60 mV as predicted for one-electron reversible charge transfer. The reversible character of the HV is also evident from the E3/, - El/4 = 56 mV. This result can be rationalized by the fact that the disturbance of the concentration profiles by the superimposed flow affects both the oxidized and the reduced form to the same extent. The experimental results described up to this point were obtained with solutions containing the constant concentration of the depolarizer serving itself as the carrier stream. As mentioned before, the most attractive feature of the FIA/ voltammetry combination is the possibility of studying the electrochemical properties of solutions of predictably varying concentration of the depolarizer. This type of experiment using Fe(II1) oxalate as a model compound is shown in Figure 4. The potential of the working electrode was first set a t a potential (-300 mV vs. Ag/AgCl) that is situated on the diffusion limiting plateau of a hydrodynamic voltammogram. After the injection (Figure 4, S) of a 70-pL sample of 1 mM Fem(oxalat& into a carrier stream of 0.25 M sodium oxalate, pH 4.5, flowing a t the rate of 0.98 mL/min, the concentration profile was recorded. At each point of this curve the current corresponds to a defined concentration reaching the value characterized hy dispersion coefficient D which a t the peak where C? is the initial concentration of the is DpaL= C?/F
Anal. Chem. 1903. 55. 1988-1990
1988
Flgure 4. (A) Concematbn profile of sample of 1 mM Fe"'(oralate), in 0.25 M sodium oxalate. pH 4.5. Flow rate was 0.98 mL min-'. The posilwn of the vertical mrs represents time (10 s. 13.5 s. and 17.0 s. respeclhrely) and the width of lhe bars represents the duration of tb cyclic voltammograms which are shown in Figure 4 (insert). Cordbbns of the cyclic voltammograms (insert)are 3 V s-', 100 mV div-'. and p A div-'. E , = zkO.0 V.
undiluted sample. For ow experiment Dd = i*/ip = k where i* is the steady-state current obtained when the constant concentration of the depolarizer is supplied to the electrode. T h e simplest way to obtain this value is to inject a large (5Ml pL) sample of the iron(II1) oxalate to the background electrolyte so that a steady-state current value is reached. By this procedure the Y axis of Figure 4 has been calibrated in the units of concentration. In the next experiment a 70-pL sample of 1 mM Fe111(oxalate)3was again injected and single-cyclic voltammograms at 3 V s-l were recorded at times indicated by the vertical bars in Figure 4. These voltammograms are shown in Figure 4 (insert, 2 pA/div and 100 mV/div). The width of the bars in Figure 4 represents the duration of the one cvcle in the correswndine cvclic voltammomam - in F i-m e 4b (insert). T h e combination of FIA and cyclic voltammetry is attractive because of the flexibility of the former and the diagnostic power of the latter. In planning these experiments the following strategy is recommended. The choice of the FIA manifold must be made to suit the chemistry to be performed. Ample examples of various manifold configurations can be found in the basic FIA literature (8). Although the flow rate and the size of the working electrode can now be well reproduced, the geometry of the detector is less well defined and, therefore, the prediction of the absolute values of the currents is not possible at present. Fortunately, this can be circumvented, by using a well-behaved redox couple to calibrate the system. In the first step, we need to determine the maximum sweep rate a t which the couple behaves reversibly. This will establish the minimum limit of the time window of the experiment. Next, we inject a large (500 pL) sample of this I
_
standard redox couple solution, in order to establish the magnitude of current when sample dispersion equals one (cf. Figure 2), and to calibrate the Y axis in concentration units (cf. Figure 4). It follows from FIA theory and experimental evidence that it is reasonable to expect that a diluted solution of the compound of interest will always have the same dispersion in the same flow geometry. Next, by varying the sample volume, flow rate, and scan rate, we can find the CV, HV, and transition region for given experimental conditions. Finally, the injection of the sample under thus found conditions will allow a direct comparison with the electrochemical behavior of the chosen standard redox couple or to perform assays of an unknown species. T o summarize, the above observations and preliminary results demonstrate the compatibility and ease with which FIA and voltammetric techniques can be combined. It is important to realize that the experiment summarized in Figure 4 represents yet another variation on a theme of gradient FIA techniques ( I ) . The flow-through detector described here has a very small effective volume and, therefore, allows the experimental exploration of a large variety of new approaches based on combination of FIA gradient techniques and voltammetric techniques for both research and technical applications. LITERATURE CITED (1) Leach. R. A,: R i l i a s . J.: Hank. J. M. Anal.
1889-1673.
aWrm. 1983. 55.
(2) Ririi5ka. J: Hansen. E. H. Anal. CMm. Acta 1983. 145. 1-15. (3) Janata. J.: RZIEkka. J. Anal. chim. Acta 1982. 139. 105-115. (4) Current Separalbns. R. E. Shoup. Ed.. Bioanapical systems Inc.. Purdue lndu~lrlalResearch Park. 1205 Kent A".. West Lafayelle. IN 47906. (5) ROiEka. J.: Pansen. E.H.: Ramsing. A. U. Anal. l%h. Acta 1981. 134. 55-71. (8) Bard, A. J.: Faukner. L. "EIBCh~~hemlcaI Mamods": Wiley: New York. 1980. (7) Bard. A. J. "Encyclopedia a1 Ebcbochemishy of me Elements": Marcel Dekksr: New Yon. 1982 Volume IX. Pan A. ( 8 ) RuiiE*a. J.: Hansen. E. H. "Flow i n b e b n Analysis": Wiley: New YWk. 1981.
'Permanent address: Chemirhy DepRmenl A. The Technical Universw 01 Denmark. 2800 Lyngby. Denmark.
Niels Thogersen' J j i i Janata* Jaromir Riii2ka' Department of Bioengineering University of Utah Salt Lake City, Utah 84112
RECEIVED for review April 25, 1983. Accepted July 5, 1983. T h e support from the Danish Council for Science and Industrial Research to N.T. is greatly appreciated. Part of this work was supported by the NIGMS, Grant Number 22952.
Precision of the Double Known Addition Method in Ion-Selective Electrode Potentiometry Sir: One of the most important problems in analytical potentiometry is the precision in the determination of sample concentration. It is well-known that 1 mV error in the emf reading causes 4% error in the calculated sample concentration if a perfect, i.e., error free calibration line is used and the slope of the calibration line is about 59 mV/decade. Comparable alternative analytical methods, e.g., UV-visible spectrophotometry, typically allow precision and accuracy better than about 1%. There have been therefore many efforts to keep the standard deviation of the emf readings below ca.
0.1 mV and to eliminate possible sources of systematic error. One approach to achieve these goals is to use the calibration method with careful thermostatization, electrical shielding, matching of standards to samples, etc. Another approach is to use addition or subtraction methods or potentiometric titrations. In the latter cases the analyst has to balance the usually contradicting goals of high precision, high accuracy, and low cost of labor. In an earlier study (1-4) we investigated the effect of the precision of individual emf readings on the precision of the
0003-2700183/0355-1988$01.50/0 0 1983 A d c a n Chemical Sociely
