Anal. Chem. 1985, 57, 158-162
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initial assumption, that the rate of chemical stripping by dissolved oxygen is insignificant compared to the rate of electrolyte stripping in the time scale of the experiment, is valid. We conclude from this work that the removal of oxygen is not a prerequisite to the successful application of square-wave anodic stripping voltammetry on a mercury drop electrode in analytical determinations of trace metals. Deaerating the sample substantially increases the analysis time for no improvement in sensitivity. We expect that routine determinations using this technique at far lesser concentrations and longer deposition times are possible. Extending the above conclusions to thin mercury film electrodes requires further investigations. Registry No. Cd, 7440-43-9; Pb, 7439-92-1; 02, 7782-44-7.
LITERATURE CITED (1) Brainina, Kh. 2.; Vydrevich, M. B. J . Nectroanal. Chem. 1981, 727, 1-28. (2) Vydra, F.; Stulik, K.; Julakova, E. "Electrochemical Stripping Analysis"; Wiley: Chichester, Sussex, 1976.
(3) Brainina, Kh. 2 . "Stripping Voltammetry in Chemical Analysis"; Wiley: Chichester, Sussex, 1974. (4) Yarnitsky, C.; Ouziel, E. Anal. Chem. 1976, 48, 2024-2025. (5) Clem, R. G.; Litton, G.; Ornelas, L. D. Anal. Chem. 1973, 45, 1306- 13 16. (6) Bond, A. M. Talanta 1973, 20, 1139-1152. (7) Rubel, S.; Wojciechowski, M. Anal. Chlm. Acta 1979, 709, 67-72. (8) Wojciechowski, M.; Rubel, S.; Falkowska, W. Anal. Chim. Acta 1982, 74 7 , 387-392. (9) Batley, G. E. Anal. Chim. Acta 1981, 124, 121-129. (10) Wang, J.; Dewald, H. D. Anal. Chem. 1984, 5 6 , 156-159. (11) Sinko, I.; Doiezal, J. J . Electroanal. Chem. 1970, 25, 53-76. (12) Christie, J. H.;Turner, J. A,; Osteryoung, R . A. Anal. Chem. 1977, 49, 1899-1903. (13) Turner, J. A.; Christie, J. H.; Vukovic, M.; Osteryoung, R. A. Anal. Chem. 1977, 4 9 , 1904-1908. (14) Brumleve, T. R.; O'Dea, J. J.; Osteryoung, R. A,; Osteryoung, J. Anal. Chem. 1981, 5 3 , 702-706. (15) Hoare, J. P. I n "Encyclopedia of Electrochemistry of the Elements"; A. J., Bard, Ed.; Marcel Dekker: New York, 1974; Vol. 11, Chapter 5.
RECEIVED for review August 9, 1984. Accepted October 9, 1984. This work was supported by loan of equipment from EG & G PARC and by financial support by the National Science Foundation under Grant CHE8305748.
Adsorptive Stripping Voltammetry of Riboflavin and Other Flavin Analogues at the Static Mercury Drop Electrode Joseph Wang,* Den-Bai Luo,~Percio A. M. Farias? and Jawad S. Mahmoud
Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
Controlled adsorptive accumulation of rlboflavln, flavln mononucleotlde (FMN), and alloxazine on the static mercury drop electrode provldes the basis for the dlrect stripping measurements of these compounds In the nanomolar concentratlon level. Dlfferentlal pulse voltammetry, followlng 30-min preconcentratlon, ylelds a detection llmit of 2.5 X M riboflavln. The adsorptive strlpplng response is evaluated wlth respect to experimental parameters such as preconcentration tlme and potentlal, bulk concentratlon, stirrlng rate, presence of surfactants, and others. The relatlve extent of adsorption and the resulting response are rather sensltlve to the composition of the slde chain of the flavln compound. Wlth 2-mln preconcentratlon, the procedure provides 34, 15, and 7 signal enhancement factors for the analysis of rlboflavln, FMN, and alloxazlne, respectlvely. Best results are obtained uslng a 0.001 M NaOH electrolyte. The reproducibillty of the determlnatlon (at the 4 X lo-' M level) expressed In terms of the relatlve standard devlatlon ranges from 2 % to 8 % .
Riboflavin and flavin coenzymes are compounds of great biological importance. Their redox behavior is important from both an analytical and mechanistic view. Their electrochemical behavior was reviewed by Dryhurst (1) and recently by Ksenzhek and Petrova (2). Flavin compounds exhibit a twoLelectron transfer upon reduction. The reduction process UNESCO Scholar. Present address: Department of Chemistry, South-Central Institute for National Minorities, Wuhan, People's Re ublic of China. rDepartment of Chemistry, Pontificia Universidade Catolica do Rio de Janeiro, Rio de Janeiro, Brazil.
proceeds with the formation of large amounts of semiquinone. Adsorption of flavin compounds was reported at mercury (3, 4) and solid (5) electrodes. Preferential accumulation of the reactant or product, or adsorption of both, is indicatedaccording to Brdicka theory-from the postwaves and prewaves present. Hartley and Wilson (3) and Gorton and Johansson (5) showed that well-defined cyclic voltammograms can be obtained after transferring the electrode with the accumulated flavin compound to a blank solution. Traditionally the adsorption phenomenon has led to anomalous results and has been regarded as undesirable in polarographic measurements. A sensitive method is required for the determination of riboflavin and other flavins in biological, pharmaceutical, and food matrices. In this study we will demonstrate that significantly improved quantitation of flavin compounds can be achieved by controlling their adsorptive accumulation onto the static mercury drop electrode, followed by voltammetric measurement of the surface species. This extends the detection limit for these compounds to 2.5 X M (using 30-min preconcentration). Florence (6) described the use of cathodic stripping voltammetry for the measurement of riboflavin at the mercury pool electrode, reporting a detection limit of 1 X lov8M. The interest in adsorptive stripping voltammetry which can provide very sensitive determinations of compounds with surface-active properties has been growing recently (7). It is evident that this method can be used in trace analysis of organic compounds in a broad range of applications. For compounds that cannot be accumulated by electrolytic deposition, the adsorption approach serves as an effective alternative for the preconcentration step. The analytical utility of the method is based on rapid and reproducible accumulation of the analyte a t the surface. Various important reducible and oxidizable compounds have been measured fol-
0003-2700/85/0357-0158$01.50/00 1984 American Chemical Society
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Repetitive cyclic voltammograms for 1 X lo-' M riboflavin in an unstirred 0.001 M NaOH solution: scan rate, 100 mV/s.
Figure 1.
lowing their adsorptive accumulation onto mercury (8-11) or solid (12-1 7)electrodes, respectively, yielding detection limits over the 10-%5 X M range. Sensitive voltammetric procedures, e.g., differential pulse or square wave, are usually employed to detect the surface bound species. Patterns of molecular adsorption at electrode surfaces were reviewed (18). The present study tests the adsorptive stripping behavior of flavin compounds at the static mercury electrode. The relative extent of adsorption depends on the structure of the flavin molecule. The detectability obtained for these compounds is the lowest reported yet for adsorptive striping measurements; it compares favorably with that of the well-established anodic stripping voltammetry of trace metals. Coupled with the reproducibility of the results, this provides a useful tool for measuring riboflavin and flavin analogues at the nanomolar and subnanomolar concentration levels,.
EXPERIMENTAL SECTION Solutions and Instrumentation. The EG & G Princeton Applied Research (PAR) Model 303A static mercury drop electrode was employed with an Ag/AgCl (saturated KC1) reference electrode and a platinum wire auxiliary electrode and was interfaced to the PAR 264 stripping analyzer, as recommended by the manufacturer. Instrument settings were as follows: medium drop size (surface area 0.016 cm2);equilibrium time, 15 s; potential scan rate, 5 mV/s; pulse amplitude, 50 mV; pulse repetition, 0.5 s. Cyclic voltammograms were obtained at 100 mV/s. A PAR X-Y recorder (Model 0073) was used for the collection of experimental data. The sample cell was PAR Model 0057; the cell was covered with aluminum foil. A magnetic stirrer (Troemner Model 500) and a stirring bar (1cm long, 2 mm thick) provided the convective transport during the preconcentration. Stock solutions (1 X M) of the flavin compounds (Sigma) were prepared daily. Riboflavin and FMN were dissolved in deionized water. The alloxazine solution was prepared by dissolving the compound in 0.5 mL of 0.1 M sodium hydroxide solution and making up to 100-mL volume with deionized water. The solutions were stored in the dark at 4 "C. All solutions were prepared from deionized water and analytical grade reagents. Procedure. Ten milliliters of the supporting electrolyte solution (usually 0.001 M sodium hydroxide) was added to the cell and degassed with nitrogen for 4 min (and for 30 s before each adsorptive stripping cycle). The preconcentration potential (usually -0.2 V) was then applied to the electrode for a selected time, while the solution was stirred at' 400 rpm. The stirring was then stopped, and after 15 s the voltammogram was recorded by applying a negative-going differential pulse scan. The scan was terminated at -0.8 V, and the adsorptive stripping cycle was repeated using a new mercury drop. The entire procedure was automated, as controlled by the PAR 264 stripping analyzer. All data were obtained at ambient temperature. RESULTS AND DISCUSSION Parameters Affecting the Adsorptive Stripping Response. Figure 1 shows cyclic voltammograms for 1 x lo4
Differential pulse voltammograms for 1 X lo-' M alloxazine (A), FMN (E), and riboflavin (C) solutions. Preconcentration for 2 min at -0.2 V with 400-rpm stirring; 0.001 M NaOH solution. The dotted lines represent the direct (0 min) response. Flgure 2.
M riboflavin in an unstirred 0.001 M NaOH solution. The cathodic and anodic currents gradually increase upon repetitive scans. The peak current with the adsorbed compound at saturation is several times greater than that of the solution species alone (estimated from the first scan, designated as 1). A 10-mV separation of peak potentials is observed; for an ideal Nernstian reaction under Langmuir isotherm conditions, EPa = EPc.A postpeak is observed in the reverse or anodic scan. A plot of log (peak current) vs. log scan rate for the surface adsorbed riboflavin, over the 10-200 mV/s range, was linear; this plot has a slope of 0.48. This may be according to the model of Laviron (19) that predicts a slope of 1a t high and low scan rates and a smaller slope a t moderate scan rates. Progressive increases of the peak currents were observed in similar experiments using alloxazine and FMN (not shown). In the case of alloxazine only single cathodic and anodic peaks, with 8-mV separation, were observed. The FMN cyclic voltammograms were characterized by a small postpeak (at -0.7 V) in the forward, cathodic, scan and small prepeak (at -0.7 V) and large postpeak (at -0.45 V) in the anodic branch. The spontaneous adsorption process can be utilized as an effective preconcentration step prior to the pulse voltammetric measurement. In this way highly sensitive measurements of flavin compounds can be achieved by using the method of adsorptive stripping voltammetry. Figure 2 shows differential pulse voltammograms obtained for 1 X M alloxazine (A), FMN (B), and riboflavin (C) following 2-min adsorptive accumulation. Also shown as dotted lines is the corresponding direct (solution-phase) pulse voltammetric response. The aasorptive preconcentration results in peak current enhancements of 7.1, 15.5, and 33.8 for alloxazine, FMN, and riboflavin, respectively. Similarly, 30-9 preconcentrations are sufficient to yield peak current enhancements of 3.8, 7.7, and 6.5 for alloxazine, FMN, and riboflavin, respectively (not shown). The stripping peak potentials are similar to those of the solution-phase species: -0.61 (alloxazine),-0.54 (FMN), and -0.56 V (riboflavin). The peak half-widths are 62 (alloxazine and FMN) and 70 mV (riboflavin). Among the various electrolytes (potassium nitrate, sodium hydroxide, hydrochloric acid, and borate, phosphate, and acetate buffers) examined for the adsorptive stripping study, best results were obtained in sodium hydroxide media. The adsorptive stripping reponse strongly depends on the supporting electrolyte concentration. For example, peak current enhancements of 2.0,4.7, 12.2, and 4.4 were obtained using O.l,O.Ol, 0.001, and 0.0001 M NaOH, respectively (1 x M riboflavin, 1-min preconcentration a t -0.2 V, not shown). As the 0.001 M sodium hydroxide solution yielded also the most defined peak
160
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 I
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1 5"*
1
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0
60
120'
90
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Figure 3.
Effect of preconcentration time on the differential pulse
stripping response for 1 X lo-' M alloxazine (a),FMN (b), and riboflavin
(c). Other conditions as in Figure 2.
I
1
l
a
-02
-04
A
-06
-08
EY
Linear scan (A) and differential pulse (6)stripping voltammograms for 5 X lo-' M rlboflavin. Preconcentration for 90 s: scan rate (A), 50 mV/s. Other conditions as in Figure 2. The dotted lines represent the direct (0 s) response. Figure 4.
shape, it was used throughout this study. The effect of the electrolyte concentration on the adsorptive stripping response of other analytes is currently under evaluation. The extent of preconcentration depends on the length of time over which the adsorption is allowed to proceed (Figure 3). While for riboflavin the peak current increases linearly with the preconcentration time, the currents for FMN and alloxazine level off at periods longer than 60 s. Obviously, for optimizing the preconcentration time there would be a trade-off between sensitivity and speed. These profiles have the nature of the corresponding adsorption isotherms (as the peak current is a measure of the amount adsorbed) and would change with the b$k concentration of the analyte. The reason for the observed behavior (different rates of adsorption) is not completely understood. Based on size and solubility considerations, the largest extent of adsorption is expected for the alloxazine molecule. However, best results were obtained throughout this study for riboflavin, which is a derivative of 7,8-dimethylisoalloxazine possessing a pentitol side chain at position 10. FMN is riboflavin 5'-phosphate. The additional hydrophilic group in the FMN molecule may account for its lower extent of adsorption compared to riboflavin. The adsorption behavior of flavin compounds at mercury electrodes is complex. Most information existing in the literature describes this behavior in acidic media and not alkaline (as used in the present work). It is believed ( 4 ) that a planar orientation of the electrode and the isoalloxazine ring system is formed during the adsorption of FMN. Figure 4 compares stripping voltammograms for 5 X M riboflavin obtained in the linear scan (A) and differential pulse (B) modes. The latter offers improved sensitivity as it descriminates against the charging background current. Both stripping modes offer significantly improved sensitivity compared to conventional pulse voltammetry (dotted lines). The
0,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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Gelatin, ppm r
r
?
T
?
T
,
B
100,
E
Figure 6. Stripping voltammograms obtained after increasing the riboflavin concentration in 5 X lo-' M steps (a-e). Preconcentration for 60 s at -0.2 V with stirring of 400 rpm.
producible. Figure 6 shows differential pulse voltammograms obtained after successive standard additions of riboflavin, each addition effecting a 5 X lo4 M increase in concentration; 60-s preconcentration periods were employed. These five measurements are part of ten concentration increments from 5 x to 5 x M. Linearity between the stripping peak current and the concentration is obtained. A least-squares analysis of the standard addition data yields a slope of 7.41 f 0.23 nA/10-8 M (correlation coefficient, 0.997). Similar standard addition experiments for FMN and alloxazine over the 1 X M concentration range yielded slopes to 8 X of 3.84 f 0.60 nA/10-8 M (correlation coefficient, 0.999) and 1.91 f 0.15 nA/10-8 M (correlation coefficient, 0.9821, respectively. Obviously, the linear isotherm conditions exist for low surface coverage. Deviations from linearity are expected at higher concentrations and/or using longer preconcentration times. Accordingly, the linear range is unique to the operational conditions used in each experiment. At the micromolar and submicromolar concentration levels conventional pulse voltammetry can be utilized. However, for measurements at the nanomolar level, only adsorptive stripping voltammetry offers the desired sensitivity (Figures 5 and 6). For convenient quantitation of riboflavin concentration ranging from 1 X to 5 X M, a 0.5-8-min preconcentration is sufficient. The high sensitivity of adsorptive stripping voltammetry is accompanied by good reproducibility of the results. The precision was estimated by eight successive measurements of 4X M riboflavin (60-s preconcentration a t -0.2 V). The mean peak current found was 26 nA, with a range of 24-29 nA and a relative standard deviation of 6%. Similar studies for FMN and alloxazine (eight successive measurements of 4X M) yielded relative standard deviations of 2% and 8%,respectively. These values compare favorably with those reported for adsorptive stripping measurements at solid electrodes (12,15). This can be attributed to the reproducible area and self-cleaning properties of the static mercury drop electrode and to the automatic control provided by the PAR 264 stripping analyzer. (Besides the concomitant improvement in the precision, such automation frees the operator from the tedious manual control of the stripping procedure.) The use of magnetic stirring is sufficient for maintaining good reproducibility. These data indicate that the mercury present at the bottom of the cell (from dislodged electrodes) does not affect the precision of the results. Poor sensitivity and reproducibility were obtained in analogous experiments using a stationary glassy carbon disk electrode in a stirred solution. This is due to the higher background current associated with carbon electrodes (surface transients that are not corrected by the differential pulse mode) and memory effects (as the same surface is used in successive measurements). As expected, the presence of other surface-active compounds in the sample can influence the preconcentration process via competition for the adsorption sites. Knowledge of these changes is required for understanding and minimizing their effects. Figure 7 shows the effects of camphor (A) and gelatin (B)on the adsorptive stripping response for riboflavin and
OL
L
-L--L---L:
2 4 Camphor, ppm
Figure 7. Effect of camphor (A) and gelatin (B) on the riboflavin and FMN (7 X M) stripping peaks, respectively: preconcentrationtime, (a) 60 and (b) 30 s; other conditions as in Figure 2.
-
FMN, respectively. Significant peak current depressions (45% of the riboflavin peak and 95 % of the FMN peak) are observed on adding 5 ppm surfactant. This depression effect depends on the preconcentration period (compare (a) 60 and (b) 30 s). The peak shape and position are not affected by the presence of the surfactant. Similar depression effects were observed in adsorptive stripping measurements of other organic compunds (10, 11). They are common also in anodic stripping measurements of trace metals, where the metal deposition is inhibited (20). Prior separation of interfering surface-active compounds using gel permeation can eliminate their effect on the stripping response (21). When the fraction of the surface covered by the interfering surfactant is constant, these interferences can be corrected by the standard addition procedure. For mixttres of flavin compounds, prior separation is required due to the similarity in their peak potential. The presence of metal ions with similar peak potentials (e.g., Pb2+) does not affect the adsorptive stripping response due to their masking by the supporting electrolyte hydroxide ions. In conclusion, adsorptive stripping voltammetry is a valuable technique for ultratrace measurements of riboflavin and other flavin analogues. The method provides extremely low detection limits, similar to those obtained in trace metal measurements by anodic stripping voltammetry. Thus, it should have great potential for clinical, pharmaceutical, and food analyses. Such real analyses would probably require removal of interfering surface-active components. The inherent sensitivity of the method changes with the structure of the flavin compound according to riboflavin > FMN > alloxazine. The spontaneous adsorptive preconcentration can result also in improved selectivity, as interferences from solution-phase species, with similar redox properties, can be eliminated by using the medium-exchange approach (14,22). For example, such exchange to a blank solution (following preconcentration) can be easily accomplished using the manifold of flow injection systems (14). Such systems offer the additional advantages of speed, simplicity, small sample volumes, and automation. The use of a faster stripping mode, e.g., square-wave voltammetry, would shorten the entire manifold procedure (Le., higher sampling rate). R e g i s t r y No. FMN, 146-17-8; riboflavin, 83-88-5;alloxazine, 490-59-5.
LITERATURE CITED (1) Dryhurst, G. "Electrochemistry of Biological Compounds"; Academic
Press: New York, 1977;pp 385-391. (2) Ksenzhek, 0.S.; Petrova, S. A. Bioelectrochem. Bioenerg. 1983, 1 1 , 155.
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(3) Breyer, B.; Biegler, T. Collect. Czech. Chem. Commun. 1980, 2 5 , 3348. (4) Hartby, A. M.; Wilson, G. Anal. Chem. 1988, 38, 681. (5) Gorton, L.; Johansson, G. J. Electroanal. Chem. 1980, 113, 151. (6) Florence, T. M. J. Electroanal. Chem. 1979, 97, 219. (7) Wang, J. “Stripping Analysis: Principles, Instrumentation, and Applications”; Verlag Chemie: Deerfield BeachlWeinheirn, in press. (8) Kolpin, C. F.; Swofford, H. S., Jr. Anal. Chem. 1978, 50,916. (9) Kalvoda, R. Anal. Chim. Acta 1982, 138, 11. (10) Webber, A.; Shah, M.; Osteryoung, J. Anal. Chim. Acta 1983, 154, 105. (11) Wang, J.; Luo, D. 8.; Farias, P. A. M. J. Electroanal. Chem., in press. (12) Cheng, H. Y.; Falat, L.; Li, R. L. Anal. Chem. 1982, 5 4 , 1384. (13) Jarbawi, T. B.; Helneman, W. R. Anal. Chlm. Acta 1982, 135, 359. (14) Wang, J.; Freiha, B. A. Anal. Chem. 1983, 55, 1285. (15) Chaney, E. N.; Baldwin, R. P., Anal. Chem. 1982, 54, 2556. (16) Sirla, J. W.; Baldwln, R. P. Anal. Lett. 1980, 13, 577. (17) Wang, J.; Frelha, B. A. Anal. Chim. Acta 1983, 154, 87.
(18) (19) (20) (21) (22)
Anson, F. Acc. Chem. Res. 1975, 8 , 400. Laviron, L. J. Electroanal. Chern. 1980, 112, 1. Brezonik, P. L.; Brauner, P. A.; Sturnm, T. WaterRes. 1978, IO,605. Kalvoda, R., unpublished results. Wang, J.; Frelha, B. A. Anal. Chlm. Acta 1983, 148, 79.
RECEIVED for review July 16, 1984. Accepted September 17, 1984. P. A. M. Farias acknowledges the financial support of the National Council for Scientific Development (CNPq) of the Brazilian government. D. B. Luo acknowledges the financial support of the United Nations Educational, Scientific, and Cultural Organization (UNESCO). This work was supported by the National Institutes of Health, Grant GM30913-01A1.
Performance Characteristics and Some Applications of the Nitrogen Oxide Gas Sensor Saad S. M. Hassan* and F. S. Tadros
Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt
The performance characteristics of nitrogen oxide gas sensor with regard to the origin of response, limit of detection, response time, influence of the membrane type, and selectivity have been systematically examined. Optimum conditions for direct potentiometric determination of as low as 0.1 pmoi mL-’ nitrite are descrlbed and results wlth an average recovery of 98.2% (standard deviation f0.3 %) are obtained. The sensor is also used to monitor direct titration of micromole per liter quantities of primary aryl amines, hydrazines, and azide with sodium nitrite or sodium cobaitinitrite. Resuits with an average recovery of 98.3% (standard deviation f1.3 % ) are obtainable without any significant interference from 1000-fold molar excess of amides, imides, anilides, tertiary amlnes, and quaternary ammonlum salts.
Ion selective electrodes and gas sensors have recently been used for determination of nitrogen oxide gases and nitrites in aqueous solutions ( I ) . These monitoring systems have the advantages of simple design, rapid response, possible interfacing with automatic and computerized systems, and applicability to turbid and colored solutions with minimum interference from diverse ions. Liquid membrane ( 2 , 3 )and coated wire ( 4 ) nitrate electrodes have been used for the determination of nitrite after a prior oxidation step into nitrate. Similar procedures have also been suggested for the determination of nitrogen oxide gases in cigarette smoke (2), combustion effluents ( 5 ) ,and flowing gas mixtures (6). Reduction of nitrite with either mercury metal (7) or nitrite reductase enzyme (8,9) followed by monitoring the mercury(1) or ammonia gas released, using an iodide ion selective electrode or ammonia gas sensor, respectively, has been described. All these methods, however, involve a prior redox reaction which makes the reproducibility and accuracy of the results highly influenced by the reaction conditions. On the other hand, the development of the nitrogen oxide gas sensor has greatly simplified and enables direct measurement of nitrite in aqueous media. Most of the commercially available nitrogen oxide gas sensors consist of a flat
bottom glass electrode located immediately behind a hydrophobic gas permeable membrane, whereby a very thin film of nitrite-nitrate internal electrolyte solution is sandwiched between the electrode and the membrane (IC-12). It has been suggested that this sensor probably senses an equimolar mixture of nitric oxide and nitrogen dioxide gases (11)released from acidified nitrite solution. When the sensor is immersed in the analyte solution, the gas mixture diffuses through the membrane until the partial pressure of the gas is equal in the thin film and the sample. The equilibrium concentration of the gas in the thin film affects the level of nitrite, nitrous acid, and hydrogen ion, and the change in the hydrogen ion concentration, measured by the internal glass electrode, can be monitored as a function of nitrite concentration in the analyte solution. Although the nitrogen oxide gas sensor is commercially available since 1974, its use has, so far, been limited to the determination of nitrite in smoked fish (13), unused cutting fluids and cutting oils (I4),and water, plant, and soil extracts (15,26). The response characteristics of the sensor and other possible applications have not yet thoroughly been investigated as in the case of carbon dioxide (17) and ammonia gas (18) sensors. It is also not known exactly what gaseous species diffuse from the sample solution through the membrane and is sensed. The imprecise term nitrogen oxide (NO,) or nitrite (NOT) in the name of the sensor reflects this uncertainty. The present work was thus undertaken to examine the performance characteristics of the nitrogen oxide gas sensor, including origin of sensor response, limit of detection, response time, influence of the membrane type, and selectivity, and to explore some other possible applications to widen the scope of its uses. It can be seen that this sensor can satisfactorily be used for microdetermination of nitrite, azide, amines, and hydrazines. These compounds are of significant importance as industrial products and intermediates, propellents, and environmental pollutants.
EXPERIMENTAL SECTION Apparatus. All potentiometric measurements were carried out at 25 2 “C with an Orion microprocessor ionalyzer (Model 901) with Orion nitrogen oxide gas sensor (Model 95-46). The
0003-2700/85/0357-0162$01.50/00 Y964 American Chemlcal Society
