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bility. Some redundancy is a good feature for field instruments to possess. Finally, the sensor array provided good separation of the 22 compounds, even with the introduction of &25% random error.
ACKNOWLEDGMENT The authors wish to thank Matt Nolan for providing the histograms and Diana Hampton for organizing the 22-compound data set. Registry No. NOz,10102-44-0;NO, 10102-43-9;HzS,7783-06-4; "3,7664-41-7; CO, 630-08-0; CH&OOH, 64-19-7;CCld, 56-23-5; HCHO, 50-00-0;CH3N02,75-52-5; CHCl,, 67-66-3;SOZ, 7446-09-5; Clz, 7782-50-5;benzene, 71-43-2; cyclohexane, 110-82-7;benzyl chloride, 100-44-7;ethyl acrylate, 140-88-5;pyridine, 110-86-1; toluene, 108-88-3;tetrahydrofuran, 109-99-9;nitrobenzene,98-95-3; acetone, 67-64-1; tetrachloroethylene, 127-18-4. LITERATURE CITED Stetter, J. R.; Zaromb, S.; Penrose, W. R.; Findlay, M. W., Jr.; Otagawa, T. Proceedings Hazardous Material Spills Conference: Prevention, Behavior, Control, and Clean-up of Spills and Waste Sites, Nashville, TN, 1984. Stetter, J. R.; Penrose, W. R.; Zaromb, S.; Christian, D.; Hampton, D. M.; Nolan, M.; Billings, M. W.; Steinke, C.; Otagawa, T. Proceedlngs Second Annual Technical Seminar on Chemical Spills, Toronto, Ontario, 1985. Stetter, J. R.; Penrose, W. R.; Zaromb, S.;Nolan, M.; Christian, D.; Hampton, D.; Billings, M.; Steinke, C.; Stull, J. 0. Proceedings 1985
Digitech Conference Instrument Society of America, Boston, MA, May 14-16, 1985. Stetter, J. R.; Zaromb, S.; Findiay, M. W., Jr. Sens. Actuators 1985, 6 (4), 269-287. Stetter, J. R.; Zaromb, S.; Penrose, W. R.; Otagawa. T. Proceedings 1984 JANNAF Safety and Environmental Protection Subcommittee Meeting Proceedings, CPIA Publication 408, 1964; pp 189-194. Stetter, J. R.; Zaromb, S.;Penrose, W. R.; Otagawa, T.; Sinciair, J.; Stull, J. 4th Natlonal Symposium on Recent Advances in Pollutant Monitoring of Ambient Air and Stationary Sources, US. Environment Protection Agency, Raleigh, NC, 1984. Stuper, A. J.; Brugger, W. E.; Jurs, P. C. "Computer Assisted Studies of Chemical Structures and Biological Function"; Wiley-Interscience: New York, 1979. Tou, J. T.; Gonzalez, R . C. "Pattern Recognition Principles"; AddisonWesley: Reading, MA, 1974. Beech, G. "Fortran I V in Chemlstry"; Wiley: New York, 1975. Massart, D. L.; Kaufman, L. "The Interpretation of Analytical Chemical Data by the Use of Cluster Analysis"; Wiley: New York, 1983. Rose, S. L.; Holtzclaw, J. R. Naval Research Laboratory Report 8848, 1985.
RECEIVED for review May 29, 1985. Resubmitted November 4,1985. Accepted November 4,1985. The authors gratefully acknowledge the support of the pattern recognition work by William R. Helms, Instrumentation Section, Chief, Electronic Engineering Directorate at Kennedy Space Center (Contract No. CC-82306A). The electrochemical sensor array work was supported by J. 0. Stull, U.S. Coast Guard Office of Research and Development.
Novel Flow-Through Pneumatoamperometric Detector for Determination of Nanogram and Subnanogram Amounts of Nitrite by Flow- Injection Analysis Antonin Trojanek' and Stanley Bruckenstein*
Chemistry Department, University at Buffalo, State University of New York, Buffalo, New York 14214
A gas porous electrode structure that detects volatile electroactive specles In a flowing liquid stream Is described and evaluated for Its utlllty in flow Injection analysis. The electrode Is fabricated by deposlting a porous gold layer on one side of a porous Teflon membrane. The gold serves as the amperometrlc electrode which consumes dissolved, volatile specles that Is transported from the flowing solution through the membrane to the metallzed face where It is electrolyzed. Nltrlte ion Is determined by reactlon In the carrier stream to produce nltrlc oxlde and lodlne, and both are electroxldlred at the gold electrode. The detection llmlt is 30 pg of nltrlte Ion. Dlssolved, nonvolatlle electroactlve specles do not Interfere.
Various flow-injection analysis procedures have been developed for nitrite determination because it is an important pollutant. Most methods employ spectrophotometric detection of diazonium dyes developed by coupling nitrite with a suitable reagent (1-3). The most sophisticated methods permit simultaneous determination of both nitrite and nitrate (after reduction to nitrite) at a rate of 90 samples per hour. The lowest detection limit attained by using spectrophotoPermanent address: Jaroslav Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Acadamy of Sciences, Jilska 16, 11000 Prague 1, Czechoslovakia.
metric detection was 0.4 ppb (4),but generally detection limits 2 or 3 orders of magnitude higher are typical. Flow-through systems employing amperometric detection of nitrite reaction products were recently reported. Detection of nitrosyl bromide (formed from nitrite in concentrated acidic bromide solutions) at a glassy carbon electrode ( 5 ) gave a nitrite detection limit of 0.1 ng (4.6 ppb). Polarographic detection of Cr(III), produced by nitrite oxidation of Cr(II), had a detection limit of 15 ng (740 ppb) (6). Nygaard (7) reported the pneumatoamperometric (8,9) detection limit of nitric oxide formed during reduction of nitrite by hydroquinone was 18 nmol (corresponding to 830 ng of NOz-). The sample analysis rate was was about 20 samples per hour. The pneumatoamperometric detection of iodine produced during the reaction of nitrite with iodide (10) gave a detection limit of 7.2 ng of NO2- and a sample analysis rate of about 1 2 samples per hour. Conventional pneumatoamperometry was used in the latter two studies. The component that was to be determined, X, was reacted with a reagent, R, in a small volume of solution contained in a reaction chamber to produce a volatile, electroactive product, Y. Y was then purged from the reaction mixture by a stream of an electroinactive gas and the gas impinged on the sensing gas porous electrode. The electrode's potential was chosen to electrolyze Y. The resultant current is proportional to the amount of the species X. Pneumatoamperometry is sensitive and selective but is not ideally suited to process a large number of samples rapidly. The slow
0 1986 American Chemical Society 0003-2700/86/0358-0866$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
11 II
Electrolytic Vessel
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ana ==3
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867
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Metalized Membrane ==3
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Flgure 1. Schematic representation of the pneumatoamperometry process. The volatile species Y in the carrier stream diffuses through the pores In the membrane to the porous electrode surface in the electectrochemical cell and is oxidized or reduced.
step in the case of rapid reaction between X and R is the time required to purge Y from the liquid into gas phase. This slow purging rate produces a low sampling frequency. In this work we describe a modified pneumatoamperometric technique that is not subject to limitation imposed by the purging rate. The scheme is shown in Figure 1. X is reacted with R to form Y and the solution containing X, R, and Y flows over a gas porous electrode structure. The detecting electrode consists of a porous Teflon membrane, one side of which is covered with a porous metal. The unmetalized Teflon face contacts the aqueous liquid phase containing R, any unreacted X, and Y. Y, a volatile eledroactive species species, partitions into and through the gas pores in the membrane and reaches the metalized face that is the indicator electrode of an electrochemical cell used to detect Y. The porous metal electrode's potential is selected to be on the limiting current region for the electrolysis of Y. Hence, this electrolysis current is proportional to the amount of X from which Y was produced. It is noteworthy that even if R is electroactive, it does not yield a current provided it is nonvolatile. This feature makes possible the use of solution chemistry to prepare Y from X that would not be possible if the amperometric detector electrode contacted the flowing stream. The particular detector described in this report consists of a gas-permeable Gore-Tex membrane, one face of which is covered with a porous gold electrode. The practical application of the technique is demonstrated in a simple flow-injection system by the detection of trace amounts of nitrite. No attempts were made to optimize the flow system, as the object of this work was to demonstrate the applicability of this novel detector to flow-injection analysis.
EXPERIMENTAL TECHNIQUES Reagents. Solutions were prepared with reagent grade wter obtained from a Milli-Q reagent grade water system (Millipore Corp., Bedford, MA). The liquid carrier stream used for nitrite determination, 0.01 M KI, was prepared by dissolving an appropriate amount of potassium iodide in 2 L of oxygen-free water. The water was continuously purged by a stream of nitrogen in order to prevent iodine formation by air oxidation. Nitrite samples in 1.5 M HZSO4 were prepared from fresh 0.1 M NaN02 and 3 M HzS04stock solutions. An iodine stock solution 0.05 M in I2 and 0.2 M in KI was prepared by dissolution of iodine in iodide solution and was standardized. Instrumental Section. An operational amplifier-based three-electrode potentiostat constructed in our laboratory was used. Currents were recorded with a Heath Model SR-204 strip-chart recorder (Heath Co., Benton Harbor, MI) on the 1 V full scale range. The detector's response time was determined with a Tektronix Model 564 storage oscilloscope (Tektronix, Portland, OR). An Altex Model llOA piston pump (Altex Scientific, Inc., Berkeley, CA) was used to deliver the carrier stream. Samples were injected using a Rheodyne Model 7120 injection valve (Rheodyne, Berkeley, California) equipped with 20-pL loop and connected to the detector by a 9 in. long, 0.01 in. i.d. stainless
a
'E@Ab:;;[
D
; ; -; ;
'
H G F
Flgure 2. Pneumatoamperometricflow-through cell: (A) upper Plexiglas part; (6)metalized Gor-Tex membrane: (C) auxiliary Gore-Tex membrane (D)polyethylene spacer; (E)bottom Plexiglas part; (F) carrier stream inlet; (G) carrler stream outlet.
steel capillary. The carrier stream reservoir was placed in a thermostated bath kept at 20 "C. A flow rate of 0.5 mL/min was used, if not otherwise stated. Flow-Through Cell. The flow-through cell consists of two Plexiglas parts held together by three screws (Figure 2). The upper part serves as the electrochemical cell and contains 0.1 M sulfuric acid, a gold wire auxiliary electrode,and a Luggin capillary leading to a saturated calomel reference electrode. Electrolyte contact to the porous gold working electrode is made through a 0.1 in. diameter hole drilled in the bottom of the upper part. The metalized gold membrane is sealed to the cell bottom around the hole using two layers of Densil pressure sensitive silicone adhesive (Dennison Manufacturing Co., Framinghem, MA) through which a 0.1 in. hole is punched. This construction produced a welldefined working electrode area in contact with the sulfuric acid electrolyte without any leakage of electrolyte from the cell. The bottom part of the detector was screwed tightly to the upper half of the detector, clamping together the metalized membrane, the auxiliary membrane, and the 0.015 in. thick polyethylene spacer cut out to connect the liquid inlet and outlet. The liquid inlet (stainless steel 0.01 in. i.d.) and outlet (Teflon 0.01 in. id.) tubes also were attached to the bottom half of the cell. The inlet hole drilled in the bottom half was centered about the 0.1 in. hole in the upper half of the cell. The gold-coated membrane and auxillary membrane are pieces of porous, 0.003 in. thick Gore-Tex poly(tetrafluoroethy1ene) sheeting (W. L. Gore and Associates, Inc., Elkton, MD). Liquid enters the bottom half of the detector through the stainless steel tubing, passes through a channel to the polyethylene spacer, and exits the detector through the Teflon tubing. The main experimental problem we experienced was wetting of the membrane pores. This wetting causes a gradual decrease of the sensor's response rate and sensitivity. The wetting and the accompanying flooding of the membrane's pores appear to be facilitated by the presence of gold metal particles in the pores. Hence, we modified the original method for metalized membrane preparation (8) in the following manner. A sheet of the porous membrane (Goretex) was draped over the top and sides of a glass tube. Excess membrane was held against the tube's side by wrapping tightly with a number of turns of Teflon tape. Nitrogen was blown through the membrane with a velocity corresponding approximately to 0.3 L m i d cm-2 of membrane surface. Undiluted gold resinate (Engelhard Industries, East Newark, NJ) was brushed and heat cured in several layers until good conductivity over the metalic surface was obtained. Flowing nitrogen through the membrane during electrode preparation prevented resinate from penetrating into the pores. Membranes metalized this way retained their original nonwetting
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character far longer than when the original procedure was used. Obviously, any other method ensuring absence of metal in the pores (e.g., vacuum evaporation of a metal onto the membrane) could be used for electrode preparation. The best long-term response stability was obtained by interposing a second Gore-Tex membrane between the one with the porous metal electrode and the solution. The presence of the second membrane had a negligible effect on sensitivity and response rate. Electrical contact to the deposited porous gold was made by pressing a gold wire against a gold strip that was formed simultaneously with the circular gold electrode.
RESULTS AND DISCUSSION Electrode Pretreatment. After the electrochemical cell was filled with 0.1 M H2S04,the porous indicator electrode potential was cycled a t a rate 100 mV s-l from 0.2 V to +1.35 V for about 5 min. The same procedure was performed every day before starting measurements in order to avoid the cumulative effects of a decrease of detector response of about 8% per 24 h. The electrode potential then was set to +1.35 V and after about 40 min the detector was ready to use when the background current reached a practically constant value below 0.2 PA. Response Rate. The time of response of the detector was examined by following its response to a step change in the concentration of electroactive species in the flowing solution. A 175-pL portion of 5 X M iodine solution was injected into a flowing stream of pure water. Since the connection between injection valve and the detector was made by a 3 in. long stainless steel capillary of 0.01 in. i.d., plug flow of electroactive species toward the sensing membrane could be assumed. The time constant for response was 1.05 s at a 0.5 mL/min flow rate, which corresponds to a response volume of 8.75 pL. Detector cells were constructed with different metalized membranes and similar results were obtained. Also a cell constructed with an interposed unmetalized Gore-Tex membrane between the solution and the metalized membrane also gave similar results. Electrolysis Efficiency. The efficiency of electrolysis of the working electrode was determined by injecting 20 pL of water saturated with iodine into a flowing stream of pure water and measuring the charge for iodine oxidation by integrating the current-time response curve. Taking the concentration of iodine to be 1.14 X low3M (11) and assuming that the electrode process was the electrooxidation of iodine to iodate, an electrode efficiency of 1.8% was obtained at a flow rate of 0.5 mL m i d . Decreasing the flow rate to 0.1 mL min-l increased the efficiency to 7.3%. These efficiencies are comparable to those found in the previous pneumatoamperometric technique in which gas phase mass transport prevailed on the bare membrane side (8). Flow Rate Dependence of Response. Twenty microliters of water-saturated iodine solution was introduced into a pure stream of flowing water. The current response varied modestly with the flow rate. In the flow rate range 0.2-0.5 mL-l, the peak current response changed approximately with the 0.2 power of the flow rate and tended toward a zero power as the flow rate decreased. Effect of Iodide on the Response to Iodine. The nitrite determination described below requires detection of volatile iodine produced as a reaction product in the presence of excess iodide. Thus the effect of iodide concentration on the detector response to iodine was studied. The reaction between iodine and iodide to form triodide ion is known to proceed very rapidly to equilibrium with K = 714 L mor1 (12). In the earlier pneumatoamperometric technique involving gas purging of iodine from a solution, the presence of iodide caused the theoretically predicted decrease of response expected to accompany nonvolatile triodide formation. The reciprocals of
peak current heights were plotted vs. iodide concentration. This plot yielded a value of the equilibrium constant value, which was in good agreement with the literature (13). The flow-through pneumatoamperometric detector also exhibited a decreased sensitivity to iodine detection when iodide was present. Experiments analogous to those performed earlier (13) were repeated in the flow injection configuration. Large volume plugs (175 pL) of samples containing 2.5 X M I2 and iodide in concentration range from M to 5 X M were injected into a stream of flowing water. The peak current data were analyzed as described in ref 13 and yielded a value of 210 L molV for the equilibrium constant. We obtained this same result in several independent series of experiments and cannot account for the erroneous value we obtained. Mixing. The flow injection system we used for nitrite determination did not include a mixing coil or any other device to homogenize the reaction mixture created by injecting a sample into the carrier stream. Hence a model fast reaction, that of iodate with iodide in acid medium, was used to study mixing. Iodate reacts rapidly and quantitatively with iodide in acid media to yield 3 mol of iodine/mol of iodate (14). Thus, if acidified iodate solution is injected into a flowing KI solution, the current peak due to iodine production as determined at +1.35 V will provide information about mixing between the injected sample and the carrier stream. The current peak height for iodine detection resulting from the injection of 20 pL of acidified 5 X M KIOBinto a carrier stream containing 0.01 M KI was determined, as was the current peak height for the injection of the same volume of 5 X 10-5 M I2 into the same of carrier stream. The ratio of current peaks was 2.88, nearly the expected value of 3. Perfect mixing of the central part of the injected plug with the carrier stream does not seem to have occurred. However, the mixing that takes place in the simple flow-injection system we used is suitable for detecting reaction products formed by fast chemical reactions. Determination of Nitrite. Reaction with Iodide To Make NO and I,. Nitrite can be reduced by various reagents to nitric oxide (7), which, because of its volatility and electrochemical activity, meets the requirements for pneumatoamperometric detection. Nitric oxide is formed by reaction of nitrite with iodide in acid media 2N02- 21- 4H' = 2NO I2 + 2H20 (1)
+
+
+
In addition, since iodine is also a volatile electroactive species, this reaction was chosen for nitrite determination. Nitric oxide is electrooxidized in acidic media to form nitrate a t potentials >+1.0 V vs. SCE (7, 15). Also, iodine is oxidized to iodate at gold at +1.35 V SCE (8). Thus, the +1.35 V was chosen to detect both of these volatile products. Twenty microliters of freshly prepared nitrite solutions M to 1.5 x M NaN02 in 1.5 M H2S04, containing 0.09 to 138 ng of NO;) was injected into the carrier stream stream containing 0.01 M KI. As can be seen from Figure 3, welldeveloped current peaks were obtained even for the lowest amounts that were injected. Since the current signal decreased to 1% of its peak value about 30 s after its initial increase, more than 100 samples per hour could be analyzed under the conditions employed. The calibration curve was a straight line with a slope of 130.7 (standard deviation 1.9) nA/ng of NO2-and an intercept of 7.3 (standard deviation 1.9) nA. The reproducibility was measured by making 15 successive injections of 69 ng of NO; using equal volumes of the same solution. This experiment yielded a relative standard derivation of 0.5%. The background current noise amplitude was typically 2 nA peak to peak, and the detection limit calculated as the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
1
100nA
1 min
-1
TIME
Figure 3. Response curves for nitrite determination by reaction with iodide. Twenty microliters of sample containing (1) 4.3, (2) 1.84, (3) 0.92, and (4) 0.184 ng of NO,- injected into the carrier stream stream containing 0.01 M K I .
amount of nitrite giving a response twice this value is 30 pg of NOz- or 1.5 ppb NOz-. Provided the reaction between nitrite and iodine proceeds quantitatively, an estimate of nitric oxide detection efficiency can be made. Twenty microliters of 5 X 10" M acidic nitrite solution was injected into flowing carrier stream and the oxidation charge at the porous electrode was measured. The charge required to oxidize an equivalent amount of iodine was also determined. With these data, the electrode efficiency for nitric oxide oxidation is 12.3%. This relatively high value compared to iodine is consistent with the high partial pressure of nitric oxide dissolved in water. By use of solubility and vapor pressure data (16),partition coefficients for the relevant gas-liquid equilibria were calculated. At 20 "C the partition coefficient of iodine is 2.64 x 10-1atm L mol-', whereas that of nitric oxide is more than a thousand times higher (4 X lo2 atm L mol-'). Acidification To Make NO. A simple procedure to convert nitrite into a volatile electroactive species is acidification. Nitrite is converted into nitrous acid, which decomposes to produce nitric oxide (17).
Flow injection analysis of aqueous nitrite samples was performed by injecting them into a stream of 0.1 M sulfuric acid. Even though a higher stream acidity would probably enhance the analytical sensitivity, the possibility of corrosion of the pump by acid made this inadvisable.
869
Over the concentration range studied, 1-4000 ng of nitrite, the current response was linear with a slope of 22.84 (standard deviation 0.03) nA/ng and there was an intercept of 2.4 (standard deviation 0.3) nA. This sensitivity is about one-fifth that obtained in the iodide procedure and demonstrates that the production of nitric acid according to eq 2 is not a major source of the nitric oxide detected in the iodide procedure. The peak-to-peaknoise in these experiments was 3 nA, leading to a detection limit of 250 pg of NOz-.
CONCLUSION The flow-through gas porous electrode technique, which has been described, produces a practical and highly sensitive amperometric detector for volatile electroactive species dissolved in a flowing stream. The electrode structure allows the determination of volatile electroactive species in the presence of of nonvolatile electroactive or electrode-poisoningspecies. The technique permits using reaction chemistry in the carrier stream that is foreclosed when conventional flow-through electrode cells are used. For example, dissolved electroactive iodide ion, present in the carrier stream used in our iodate and nitrite studies, produces no electrode response since it is nonvolatile. The response time is sufficently fast to permit flow injection analysis at a rate as high 100 samples h-l. Registry No. Au, 7440-57-5; NOT, 14797-65-0. LITERATURE CITED (1) Glne', M. F.; Bergamln, H.; Zagatto, E. A,; Reis, B. F. Anal. Chim. Acta 1980, 114, 191. (2) Zagatto, E. A.; Jacintho, A. 0.; Mortattl, J. H.; Beramin, F. Anal. Chim. Acta 1980, 120, 399. (3) Van Staden, J. F. Anal. Chim. Acta 1982, 138, 403. (4) Nakashima, S.; Yagi, M.; Zenki, M.; Takaheshi, A,; Toei, K. Benseki Kagaku 1082, 31 (12) 732; Chem. Abstr. 1983, 98,673. (5) Fogg, A. G.; Bsebsu, N. K.; Abdalla, M. A. Analyst (London) 1982, 107, 1040. (6) Schothorst, R. C.; ReiJn, J. M.; Poppe, H.; den Boef, G. Anal. Chim. Acta 1083, 745, 197. (7) Nygaard, D. D. Anal. Chim. Acta 1981, 130, 391. (8) Gifford, P. R.; Bruckenstein, S. Anal. Chem. 1080, 52, 1024. (9) Gifford, P. R.; Bruckenstein, S. Anal. Chem. 1080, 52, 1028. (10) Beran, P.; Opekar, F.; Bruckenstein, S.Anal. Chim. Acta 1982, 736, 389. (11) "Handbook of Chemistry and Physics", 51st ed.; The Chemical Rubber Co.: Cleveland, OH, 1970; p 8-124. (12) "Handbook of Chemistry and Physics", 51st ed.; The Chemical Rubber Go.: Cleveland, OH, 1970; p B-96. (13) Beran, P.; Bruckenstein, S. Anal. Chem. 1980, 52, 2207. (14) Kolthoff, I . M.; Sandell, E. B.; Meehan, E. J.; Bruckentein, S. "Quantitative Chemical Analysis", 4th ed.; The Macmillan Co.; New York, 1969; p 849. (15) Kosek, J. A. Thesis, part I, State University of New York at Buffalo, 1979. (18) "Handbook of Chemistry and Physics", 61st ed.; CRC Press, Inc.: Boca Raton, FL, 1980, pp B-106, 125 and D-200. (17) Bailar, J. C.; Emeleus, H. J.; Nyholm, R.; Trotman-Dickenson, A. F. "Comprehensive Inorganic chemistry"; Pergamon Press: New York, 1973; Vol. 2, p 372.
RECEIVED for review August 21,1985. Accepted November 8,1985. The work was supported by the Air Force Office of Scientific Research under AFOSR Grant No. 83-0004.
