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Anal. Chem. 1001, 63,1278-1281 Irwln, J. 0.;et al. J . R . stet. Soc. 6 1954, 76, 204-222. Wallace, Davld L. I n R . A . FMwr: An m d b n ; Fkmbwg, Stephen E., Hlnkby, Davld V., Ed.; Lecture Notes in Statktlce 1; Spring er-Verlag: New York, 1980 pp 119-147. Jandera, Pavel: Kdde, Stanlslav; KoW. Stanklav. Talente 1970, 77, 443-454. Llteanu, Candln; Rk& Ion; Llteanu, Victor. Talent8 1978, 25, 593-596. Walpde, Ronald E.; Myers, Raymond H. PrObebUtty and StetkHCs for Eng/ne8fs end sc&nt/sts, 2nd ed.; Macmillan: New York, 1978. Mlller, Rupert G., Jr. Shrnnltenews Statktkil Inference, 2nd ed.; Sprlnger-Verlag: New York, 1981. Schwartz. Lowell M. Anal. Chem. 1977, 49, 2082-2068. Carter, Kenneth N., Jr. Unpublished work, Northeast Missouri State University, 1990. Brown&, K. A. steLMcel Thecxy and Melhodhkgy h S c h c e and Englneerlng, 1st ed.; Wlby: New York, 1960. Kendall, Mawice G.; Stuart, Alan. The Advenced of StetlsMw, 2nd ed.; Hafner Publishing Co.: New York. 1963: Volume 1. Sectbns 1.35, 11.3, 11.9, and 11.11.
(21) Abramowlb, Milton, Stegun, Irene A., Eds. Hencbdr of M ” h l Fmcthms; Dover: New York, 1984. (22) Schwartz. Lowell. M. Anal. Chem. 1979, 57, 723-727. (23) Franke, J. P.; de Zeeuw, R. A.; Hakkert. R. Anal. Chem. 1978, 50, 1374- 1380. (24) Seber, G. A. F. Linear Regression Analysis; Wlley: New York, 1977; pp 205-209. (25) McCullough, John 0.; Meites. Louis. Anal. Chem. 1975, 47, io8 I-1084. (26) Vbth, Ell~&6th. J . A@. physkl. 1989, 87. 390-396. (27) Wllkinson, G. N. J . R . Stet. Soc. E 1977. 39, 119-171. (28) Koschat. Martln A. Ann. Stet. 1987, 75, 482-488.
RECEIVED for review July 31,1990. Accepted March 14,1991. This work was supported in part by research startup funds and internal academic year research grants from Northeast Missouri State University.
Nitrogen Oxide Gas Sensor Based on a Nitrite-Selective Electrode Stacy A. O’Reilly, Sylvia Daunert, and Leonidas G. Bachas* Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055
Incorporatlon of dicyanocobait( 111) a,b,c,d,o,f,g-heptaProWlcobyrlnate In -4 pdy(vkryl-) “khas resulted in the development of electrodes that are sebdlvefor M e . An N0,gasbenror wasprepared by placlng this electrode behind a microporous gabpermeable membrane. NO, Is generated In the sample at pH 1.7 and, after crosdng the gacpermeabk membrane, Is trapped as nitrite by an Internal solutlon buffered at pH 2 5.5. Thls sensor Is different from the conventlonai Severlnghaus-type sensor, whlch employs a flat-bottom pH electrode. The latter senses changes in the pH of an internal unbuffered solutlon as NO, diffuses across the gas-permeabie membrane. The Severinghaus-type sensor exhlMls severe Interforewes from weak WpophHlc aclL that can cross the gas-penneabk membrane and affect the pH ol the Internal solution. The descrbd NO, sensor does not suffer from such interferences and exhibits btter detection IimHs.
INTRODUCTION In the commercially available Severinghaus-type gas sensors, a pH electrode is placed behind a gas-permeable membrane (GPM) (1-4).Gases in the sample diffuse through the GPM and change the pH of a thin film of an internal solution that is ”sandwiched”between the GPM and the pH electrode. In the case of the nitrogen oxide gas sensor, the internal solution is unbuffered and contains a relatively high concentration of sodium nitrite (5).This arrangement facilitates a direct relation between the partial pressure of NO2 (or NO) in the sample and the [H+] of the thin film (6). An inherent limitation of the commercial nitrogen oxide sensor is that other species (e.g., salicylic, acetic, and benzoic acids) can cross the membrane and alter the pH of the internal solution (6). Such an interference effect is common to all Severinghaus-typegas sensors (4). T o reduce such interferences, Meyerhoff and co-workers replaced the pH electrode behind the GPM with a polymer0003-2700/9110363-1276$02.50/0
membrane-based ion-selective electrode (ISE). Specifically, they developed ammonia, sulfur dioxide, and carbon dioxide sensors by using internal electrodes that were selective for ammonium (3,sulfite (81,and carbonate (9),respectively. In addition, they reported an air-segmented continuous-gassensing arrangement to measure NO, (IO). In the last system, NO, was generated in a flowing stream and was trapped behind a GPM by a flowing solution, which was pH 2.8 and contained hydrogen peroxide. The generated nitrate ions were detected by a flow-through nitrate-selective electrode. Finally, Coetzee and Gunaratna reported a chlorine gas sensor based on a solid-state chloride-selective electrode (11). This paper describes an NO, gas sensor that employs a nitrite-selective electrode, which is located behind the GPM and functions as the sensing element. This arrangement,along with the use of a buffered internal solution at a pH much higher than that of the sample solution, results in an improved detection limit (equal to 4 X lo-’ M) and selectivity when compared to the commercially available sensors, as well as the previously reported gas sensors.
EXPERIMENTAL SECTION Reagents and Apparatus. Vitamin BI2,2-(N-morpholino)ethanesulfonic acid (MES), tridodecylamine, dibutyl sebacate, sodium salicylate, sodium benzoate, and all the inorganic salts were obtained from Sigma (St. Louis, MO). Sodium acetate was procured from J. T. Baker (Phillipsburg,NJ). Chromatographic grade poly(viny1chloride) (PVC) was purchased from Polyscience (Warrington,PA). Bis(2-ethylhexyl)sebacate (DOS, purum) was purchased from Fluka (Ronkonkoma, NY). Tetrahydrofuran (THF), hydrochloric acid, and sulfuric acid were obtained from Fisher Scientific (Fair Lawn,NJ). 1-Propanolwas p u r c W from Aldrich (Milwaukee, WI). Sodium tetraphenylborate was obtained from Kodak (Rochester, NY). All standard solutions and the buffers were prepared with deionized (Milli-Q, Millipore, Bedford, MA) distilled water. Dicyanocobalt(II1) a,b,c,d,e,f,g-heptapropylcobyrinate, the ionophore used to prepare the nitrite-selective electrode, was synthesized by following the procedure of Murakami et al. (12). A Fisher Accumet 810 digital pH/mV meter was used to monitor the voltages. The potential was recorded on a Linear 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. Ag-AgCI
Parafilm
Syringe Body
I-mL Plastic Pipette Tip Internal Solution B Tygon Tubing 0-Ring
+I I .
Internal Solulion A .Nitrate Selective Membrane Gas-Permable Membrane
Fbwo 1. Schematic dlagram of the gas sensor based on a nitrlta
selectlve electrode. (Model 1200; Reno, NV) strip-chart recorder. Nitriteselective Electrode. Membranes were prepared by dissolving 1mg of dicyanocobalt(II1) a,b,c,d,e,f,g-heptapropylcobyrinate, 72 pL of DOS, and 33 mg of PVC in 1 mL of THF. This solution was cast in a 16-mm (i.d.) glass disk, and the solvent was evaporated at mom temperature overnight (13). The electrode was constructed by fitting the end of a l-cm3Tuberculin syringe (Becton Dickinson, Rutherford, NJ) with a 1.4-cm-long piece of Tygon tubing (R-3603). Appropriate diameter disks of the nitriteselective membrane were cut and glued to the tubing by using THF. The electrode body was filled with an internal filling solution, which was composed of 0.0100 M NaCl and 0.0100 M NaN02. A Ag-AgC1 reference electrode was placed inside the syringe body. A Fisher saturated calomel electrode (Model 13639-52)was used as the external reference electrode. Calibration of the electrodes was carried out by adding, while stirring, aliquots of known concentrations of the different anion standard solutions to a beaker containing 20.0 mL of buffer. The various buffers studied were 0.100 M MES-NaOH, pH 5.5; 0.100 M MES-NaOH, pH 6.0; 0.100 M MES-NaOH, pH 6.6; 0.100 M MES-NaOH, pH 7.0; and 0.100 M Tris-HC1, pH 7.5. The responae of the electrodes was measured by the pH/mV meter, and it was registered by the strip-chart recorder. In all the studies, the response time was defined as the time elapsed to attain 95% of the steady-state signal after each addition. NO, Gas Sensor. The configuration and construction of the sensor are depicted in Figure 1. The nitrite-selective electrode was positioned behind a gas-permeable membrane (Gore-Tex expanded PTFE ammonia diffusion membrane, pore size 0.2 pm). A buffered internal solution, 0.100 M MES-NaOH, pH 5.5, containing 0.0100 M NaCl (solution B, Figure l),was placed between the nitrite-selective electrode and the gas-permeable membrane. A Ag-AgC1 wire was used as the reference electrode and was dipped in solution B. Solution A was composed of 0.0100 M NaCl and 0.0100 M NaN02. Calibration of the sensor was performed by adding different volumes of standard solutions to a covered beaker containing 5.00 mL of a H 8 0 4 solution, pH 1.7, that was 0.04 M in Na2SOI. To avoid possible problems due to difference in the osmotic pressure between the sample solution and solution B, the osmolarity was matched at pH 5.5. For all measurements, the beaker was thermostated at 25 OC by using a Fisher Scientific Isotemp refrigerated circulator bath (Model 9500). To obtain the calibration curves of the sensor, the data were plotted as the decrease in the potential, AE (with respect to the base line), w the logarithm of the concentration of the anion present in the sample solution. Severinghaus-Type Gas Sensor. An Orion nitrogen oxide sensor (Model 95-46; Boston, MA), filled with Orion (95-46-02) internal solution, was used to compare its response characteristics with those of the proposed sensor. The response of the electrode was measured after adding different volumes of the standards to a covered beaker containing 20 mL of a H2S04,pH 1.7, that was 0.04 M in Na2S0,. For a comparison of the response times of the proposed sensor with those of a Severinghaus-type gas sensor, a sensor was prepared by using the same design as the one depicted in Figure 1. For this sensor, the internal solution A was composed of a 1.00 M citrate buffer, pH 5.5, containing 0.100 M NaCl. Solution B was composed of 0.100 M NaCl, 0.100 M NaN02, and 0.100 M NaN03. A pH-selective electrode was used instead of the nitrite-selective electrode. The pH-selective membrane was prepared by dissolving 21 mg of tridodecylamine, 122 mg of dibutyl eebacate, 1.2 mg of sodium tetraphenylborate, and 44 mg of PVC
63,NO. 13, JULY 1, 1991 1270
Table I. Selectivity Propertiee of the Nitrite-Selective Electrodea anion
W&trralOo
salicylate 3.8 thiocyanate 2.7 nitrite 1.0 5.9 x 10-9 iodide 5.9 x 10-3 perchlorate 1.9 x 10-3 bromide 1.0x 10-3 benzoate 5.9 x 104 bicarbonate 3.2 X lo-' hydrogen phosphate a Selectivity Coefficients were calculated by using the matched potential method at pH 6.6 (0.100 M MES-NaOH). in 1mL of THF. This solution was cast in a 22-mm (i.d.) glass disk. pH Stability of the Thin Film in the NO, Gas Sensor. In order to select the appropriate buffer for solution B (Figure l), the Orion NO, sensor was used. Instead of the Orion (95-46-02) filling solution, the following buffers were employed: 0.100 M MES-NaOH/0.0100 M NaC1, pH 6.0, or 0.100 M MESNaOH/0.0100 M NaC1, pH 5.5. The flat-bottom pH electrode was calibrated by using the above-mentioned buffers with the pH/mV meter set in the pH mode. Then, the pH 6.0 buffer was used as solution B, and the gas sensor was assembled. Nitrite was added to a 20-mL solution composed of 0.010 M H 8 0 4 and 0.04 M Na2S04,as discussed above, and the changes in the pH of the thin film were monitored. The same procedure was repeated for the 0.100 M MES-NaOH/0.0100 M NaC1, pH 5.5 buffer solution.
RESULTS AND DISCUSSION Hydrophobic derivatives of cobyric acid have been used previously as ionophores in the development of nitrite-selective electrodes (14-18). One such derivative, dicyanocobalt(II1) a,b,c,d,e,fg-heptapropylcobyrinate, was reported by our group to yield electrodes that have near-Nernstian responses toward nitrite (14). Table I summarizes the selectivity of the electrodes prepared by using this ionophore and DOS as the plasticizer. The selectivity coefficients were calculated by the matched-potential method (19). Specifically, the concentration of an interfering anion that produces a signal equal to the one generated by a 1X lW M nitrite solution was obtained from the respective calibration curve. The selectivity coefficients were calculated as follows:
IQ&te,anion = [nitrite]/[anion] The data in Table I demonstrate that the electrode has an excellent selectivity for nitrite over anions like bicarbonate, benzoate, etc. Some of these anions are potential interferents of the Severinghaus-design NO, gas sensor because their conjugated acids can cross the GPM and change the pH of the internal filling solution. Therefore, it appears that a nitrite electrode should be ideal for the development of a highly selective NO, gas sensor. Consequently, the gas sensor described in Figure l was constructed. Since the concentration of nitrite in the film of the proposed sensor is the measured parameter, it is important to be able to monitor the nitrite ion concentration effectively. For this purpose, the effect of the pH of the trapping solution (solution B in Figure 1)on the properties of the nitrite electrode was studied. Calibration curves of the nitrite-selective electrode (not in a gas-sensing mode) were obtained by using different buffers in the pH range of 5.5-7.5, as described in the Experimental Section. As the pH increases, the starting potentials of the electrode are shifted toward more negative values (Figure 2). This behavior can be explained by an increased level of interference from the OH- ions. For the
ANALYTICAL CHEMISTRY, VOL. 63,NO. 13, JULY 1, 1991
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>
Table 11. Response Time of the Nitrite-Based NO, Gas Sensor
-
E
2.0
-- 2 2 5 5
X
7.0 X 1.7 X 3.6 X 8.0 X 1.6 X
0 .c
c
200175150-
104-7.0 10”-1.7 10-’-3.6
X X X 10-‘-8.0 X 10-‘-1.6 X 10-’-2.8 X
10”
16 15 12 10 7 5
lo-’ 10” 10”
lo-‘ lo-’
However, the following equation can be written, which indicates that NO, and NO2 may be generated simultaneously:
-
I25 -6.0
-3,O
-4,O
-5,O
log [nitrite] 2. Callbration curves of the n i t i t w e electrode in dlfferent buffers: (0)0.100 M MES-NaOH, pH 5.5; W) 0.100 M MES-NaOH, pH 6.0; (0)0.100 M MES-NaOH, pH 6.6; 0.100 M MES-NaOH, pH 7.0; (A)0.100 M Trls-HCI, pH 7.5. The insert shows the effect of pH on the potential of the sensor at 7.0 X lod M nitrite.
(b)
+
,
-5.0
‘
,
-4,O
.
I
-3.0
-2.0
log [nitrite]
Figure 3. Stablllty of the pH of the thin fllm of buffer on the sensor sMe of the gas-permeable membrane as a function of the concen0.100 M MES-NaOH, tration of nitrlte. The buffers tested were (0) 0,0100 M NaCI, pH 5.5; )(. 0.100 M MES-NaOH, 0.0100 M NaCI, pH 6.0.
same reason, the electrodes presented better response and extended linearity at lower pH values. Calibrations were not attempted at pH lower than 5.5 because of the possible ester hydrolysis of dicyanocobalt(II1) a,b,c,d,e,f,g-heptapropylcobyrinate. In addition, as indicated by the insert in Figure 2, the change in potential per pH unit is reduced at lower pH valuea. Therefore, no significant improvement in the response of the nitrite-selective electrode is anticipated by buffering the sample solution below pH 5.5. An additional study was undertaken to determine the stability of the pH of the thin film located between the GPM and the polymeric membrane as a function of the concentration of NO, (generated by adding nitrite into a pH 1.7 solution). For this evaluation, different compositions of the internal solution B were used as described in the Experimental Section. The results obtained demonstrate that when the pH 5.5 buffer was used, the change in the pH of the thin film was somewhat less pronounced than when the pH 6.0 buffer was employed, as indicated by the difference in the slope of the two calibration curves between lob and lo4 M nitrite (Figure 3). Based on the above pH studies and the studies carried out with the nitrite-selective electrode, pH 5.5 was chosen as the internal pH for the remainder of the experiments. Calibration curves for the gas sensor were obtained by adding known amounts of nitrite to an acidic solution (pH 1.7). Because nitrogen has several oxidation states, a large number of equations are required to describe accurately the chemical equilibria involving the gas generation process (20).
+
2H+ + 2N02- NO NO2 H 2 0 (1) Other gases that may be formed include HN02, N203,N20,, and N205(20). All of these gases cross the GPM and become trapped as nitrite and nitrate ions by solution B. From the set of equations that interrelate the nitrogen-containinggases with nitrite and nitrate, it is customary to use eq 2 as an 2 N 0 2 + H20
3,O-6,O
response time, min
nitrite concn step, M
250-
NO3- + NO2- + 2H+
(2)
approximate description of the chemical reaction state behind the gas-permeable membrane (5,6). From the above equations, it is evident that, the higher the nitrite concentration in the sample, the more NO, will be generated and, therefore, the higher the concentration of nitrite behind the GPM. In addition, if solution B is buffered at a pH significantly greater than 1.7, equilibrium 2 should be shifted toward the formation of more nitrite. This buffer-trap effect should result in sensors with better detection limits. It should be noted that such an effect has been used to improve the detection limits of the NH3 and SO2 gas sensors reported by Meyerhoff (7,8). A study was undertaken to compare the proposed and the previously reported NO, gas sensors in terms of their response toward nitrite. Specifically, the sensor that was based on the nitrite-selective electrode manifested a detection limit (determined according to IUPAC recommendations (21))of 4 X lo-’ M in terms of nitrite, whereas the commercial sensor was only able to detect as low as 2 X 10” M. The data obtained by using the commercial NO, sensor agree with the detection limit claimed by the manufacturer ( 5 ) and are better than those reported by Hassan and Tadros (1 X M) (6). Meyerhoff and co-workers reported a gas-sensing system in conjunction with an air-segmented analyzer unit for the determination of NO, that employed a nitrate-selective electrode as the sensing element (IO). In order to generate nitrate ions behind the GPM, a buffer containing hydrogen peroxide was used. The equation that governs the equilibrium of this reaction is NO2- + H 2 0 2 NO3- H20 (3)
-
+
and after taking into consideration eq 2, 2N02
+ H202
2N03-
+ 2H+
(4) If it is assumed that the oxidation of nitrite to nitrate is quantitative, it should be expected that, by using the nitrate electrode, a more sensitive gas sensor should result. This is anticipated because, according to eq 4, one NO2 molecule will lead to one molecule of NO3-. However, the detection limits achieved with this gas-sensing arrangement (estimated as 5 X 10” M from Figure 5 of ref 10) were worse by an order of magnitude compared to the ones obtained with our sensor. This result was obtained because the nitrate-selective electrode does not have as good a detection limit as the nitrite electrode employed in these studies. The response times of the nitrite-based gas sensor ranged typically from 5 min at high nitrite concentration to as long as 16 min at low concentrations (Table 11). These response
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1991
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Table 111. Selectivity Properties of the Commercial and the Nitrite-Based Gar Seneoi G t 4 4 o n
anion nitrite thiocyanate
commercial sensor 1.0 0.21 9.1 x 8.3 X 3.9 x
