Anal. Chem. 1999, 71, 201-204
Guanidinium-Based Potentiometric SO2 Gas Sensor Mark D. Mowery, Richard S. Hutchins, Pedro Molina,† Mateo Alajarı´n,† Angel Vidal,† and Leonidas G. Bachas*
Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506-0055
Many gas sensors have been developed and commercialized based on the Severinghaus design.1-5 This construction uses a flat-bottom pH electrode behind a gas-permeable membrane (GPM) and has resulted in the commercialization of gas sensors for ammonia, carbon dioxide, sulfur dioxide, hydrogen sulfide, and nitrogen oxides.4,5 Additionally, a gas sensor for acetic acid based on this design has also been reported.6 Generally, these gas sensors contain an internal solution possessing a fixed concentration of the conjugate base of the weakly acidic gas of interest; in the case of the ammonia gas sensor, a fixed concentration of ammonium chloride is used. The selectivity achieved with these sensors is generated by coupling together two different selective processes. Specifically, to illicit a response from a Severinghaus-type gas sensor, the species must first be a gas or be convertible to a gas. All gaseous species can cross the GPM and enter the internal solution, where the pH electrode is contained. Inside the GPM, only those gases that can change the pH of the internal solution will be detected. Thus, any species that is not convertible to a gas or any gas that cannot be hydrolyzed inside the GPM (e.g., N2, O2, etc.) will not be detected
by the gas sensor. Although this design provides sensors with good selectivity over potentially interfering species, there are still many gaseous species that can interfere with the operation of the sensor by changing the pH of the solution behind the GPM. Almost any gas for which a Severinghaus-type gas sensor can be designed represents a potential interference for other such gas sensors. An improvement on the Severinghaus design was introduced by Meyerhoff and co-workers, who replaced the internal pH electrode of an ammonia gas sensor with an ammonium-selective electrode that incorporated nonactin as the ionophore.7,8 The selectivity of the ammonia gas sensor, thus prepared, increased by providing a means for discriminating ammonia gas from other gaseous species that can change the pH of the solution behind the GPM but do not respond at the nonactin-based ammoniumselective electrode. Several additional gas sensors have since been reported by Meyerhoff’s research group, most of which contain internal ion-selective electrodes (ISEs) for the conjugate base of the acidic gas of interest. Gas sensors for carbon dioxide,9 sulfur dioxide,10 and an air-segmented flow-through continuous NOx gas sensor that uses a nitrate-selective electrode11 have been reported. These sensors have improved selectivities over the commercially available Severinghaus gas sensors. Additional NOx gas sensors have been developed in our laboratory that are based on nitrite-12 or nitrate-selective electrodes.13 The SO2 gas sensor reported by Pranitis and Meyerhoff uses a HSO3-/SO32- -selective electrode with a mercury(II) complex of diethyldithiocarbamate as the active component.10 The HSO3-/ SO32- electrode was observed to suffer interferences from other sulfur-containing species and iodide. When this ISE was placed behind a GPM, the interference from these anions was greatly reduced. However, several ions when present at higher concentrations still interfered with the detection of SO2 gas.10 We have recently reported the development of a highly selective HSO3- electrode based on a guanidinium-containing ionophore.14 In the present article, we describe the development of a potentiometric SO2 gas sensor with the best selectivity
† Departamento de Quı´mica Orga ´ nica, Facultad de Quı´mica, Universidad de Murcia, Campus de Espinardo E-30071, Murcia, Spain. (1) Severinghaus, J. W.; Bradley, A. F. J. Appl. Physiol. 1958, 13, 515-520. (2) Ross, J. W.; Riseman, J. H.; Krueger, J. A. Pure Appl. Chem. 1973, 36, 473487. (3) Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1974, 69, 129-141. (4) Mascini, M.; Cremisini, C. Chim. Ind. (Milan) 1980, 62, 222-230. (5) Daunert, S.; Bachas, L. G.; Smith-Palmer, T. In Encyclopedia of Analytical Science; Townshend, A., Haswell, S., Lederer, M., Wilson, I., Worsfold, P., Eds.; Academic Press: New York, 1995; pp 4118-4124. (6) Hassan, S. S. M.; Ahmed, M. A.; Mageed, K. H. A. Anal. Chem. 1994, 66, 492-496.
(7) Meyerhoff, M. E. Anal. Chem. 1980, 52, 1532-1534. (8) Meyerhoff, M. E.; Fraticelli, Y. M.; Greenberg, J. A.; Rosen, J.; Parks, S. J.; Opdycke, W. N. Clin. Chem. 1982, 28, 1973-1978. (9) Greenberg, J. A.; Meyerhoff, M. E. Anal. Chim. Acta 1982, 141, 57-61. (10) Pranitis, D. M.; Meyerhoff, M. E. Anal. Chim. Acta 1989, 217, 123-133. (11) Martin, G. B.; Meyerhoff, M. E. Anal. Chim. Acta 1986, 186, 71-80. (12) O’Reilly, S. A.; Daunert, S.; Bachas, L. G. Anal. Chem. 1991, 63, 12781281. (13) Herna´ndez, E. C.; Mortensen, C.; Bachas, L. G. Electroanalysis 1997, 9, 1049-1053. (14) Hutchins, R. S.; Molina, P.; Alajarı´n, M.; Vidal, A.; Bachas L. G. Anal. Chem. 1994, 66, 3188-3192.
An SO2 gas sensor was developed by using a hydrogen sulfite-selective electrode positioned behind a gas-permeable membrane (GPM). The hydrogen sulfite-selective electrode was prepared by incorporating a multicyclic guanidinium ionophore in a plasticized poly(vinyl chloride) membrane. This gas sensor presents important advantages over the conventional Severinghaus-type SO2 gas sensor that contains a pH electrode immersed in an internal solution behind the GPM. The Severinghaus gas sensor suffers interferences from weak acids that can cross the GPM as gases and change the pH of the internal solution. In contrast, in the proposed sensor, the excellent selectivity of the HSO3- electrode and the ability of the GPM to discriminate gaseous from nongaseous species combine to generate the most selective potentiometric SO2 gas sensor reported to date.
10.1021/ac980335n CCC: $18.00 Published on Web 12/03/1998
© 1998 American Chemical Society
Analytical Chemistry, Vol. 71, No. 1, January 1, 1999 201
Figure 1. Structure of the guanidinium ionophore used in the internal hydrogen sulfite ISE.
reported to date by incorporating this HSO3- ISE behind a GPM. The remarkable selectivity of this SO2 gas sensor stems from the internal HSO3- ISE, which has been shown previously to discriminate very well against each of the anions tested as potential interferences, particularly the conjugate bases of weakly acidic gases. This paper also compares the new SO2 gas sensor with a Severinghaus sensor that has been constructed and tested in a parallel fashion. EXPERIMENTAL SECTION Reagents. The guanidinium ionophore used was prepared according to our published procedure.15 2-Nitrophenyl octyl ether (NPOE) was purchased from Fluka (Ronkonkoma, NY). Bis(2hydroxyethyl)iminotris(hydroxymethyl)methane (Bis-Tris) was acquired from Research Organics (Cleveland, OH). Poly(vinyl chloride) (PVC) was obtained from Polysciences (Warrington, PA). Sodium hydrogen sulfite was purchased from Mallinckrodt Chemical Works (St. Louis, MO). The sodium salts of hydrogen carbonate, nitrate, and thiocyanate, along with tetrahydrofuran (THF), were acquired through Aldrich (Milwaukee, WI), and the sodium acetate and sodium iodide were from J. T. Baker (Phillipsburg, NJ). Sodium nitrite, sodium chloride, sodium citrate, potassium chloride, and concentrated phosphoric acid were Fisher Scientific (Cincinnati, OH) chemicals. The sodium perchlorate, sodium salicylate, sodium fluoride, tridodecylamine, and dibutyl sebacate were obtained from Sigma (St. Louis, MO). Sodium tetraphenylborate was purchased from Eastman Kodak (Rochester, NY). All of the aqueous solutions were prepared using deionized (Milli-Q Water purification system; Millipore, Bedford, MA) distilled water. Apparatus. A Fisher Accumet 810 digital pH/mV meter was connected to a Linear strip-chart recorder (model 1200; Reno, NV) to monitor and record the changes observed in the gas sensor’s potential. The temperature was maintained at 25 °C using a Fisher Isotemp circulator bath (model 9500) and a capped jacketed beaker. Preparation of the HSO3- and pH Electrodes. The components for the HSO3- -selective membrane (1 mg of the tetrafluoroborate salt of the guanidinium ionophore shown in Figure 1, 33 mg of poly(vinyl chloride), 63 µL of the plasticizer 2-nitrophenyl octyl ether, and 0.25 mg of the lipophilic anionic salt sodium tetraphenylborate) were dissolved in 0.5 mL of THF. This membrane cocktail was cast into a glass ring with a 16-mm i.d., and the solvent was allowed to evaporate overnight at room temperature. The pH-responsive membrane used in the Severinghaus-type gas sensor was prepared in the same manner and (15) Molina, P.; Alajarı´n, M.; Vidal, A. J. Org. Chem. 1993, 58, 1687-1695.
202 Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
Figure 2. Design of the guanidinium-based SO2 gas sensor that incorporates an internal HSO3- -selective electrode. The HSO3- selective membrane is pressed firmly against the gas-permeable membrane (the gap shown between the two membranes is not drawn to scale).
contained 11.2% (w/w) of the ionophore tridodecylamine, 64.8% (w/w) of the plasticizer dibutyl sebacate, 23.4% (w/w) of poly(vinyl chloride), and 0.6% (w/w) of sodium tetraphenylborate. Both the pH and hydrogen sulfite electrodes were assembled using 1-mL Tuberculin syringes (Becton Dickinson, Rutherford, NJ) with the tip fitted with a short section of Tygon tubing (R-3603). A disk with the same outer diameter as the tubing was cut from the cast membrane and glued to the end of the tubing using THF. A Ag/AgCl wire was suspended inside the syringe barrel that contained an electrolyte solution (internal solution A) of either 0.100 M KCl (HSO3- electrode) or 0.100 M NaCl in 1.0 M citrate buffer, pH 5.5 (pH electrode). Before use, these electrodes were conditioned overnight in solution B; the composition of this solution is described below for the two types of SO2 gas sensors. SO2 Gas Sensors. Figure 2 shows the SO2 gas sensor that incorporates an internal HSO3- -selective electrode; the Severinghaus-type gas sensor was constructed in a similar fashion and used a pH electrode as the internal electrode. To assemble the SO2 gas sensors, a 5-mL plastic pipet tip, with the tip end cut off to match the outer diameter of the Tygon tubing, was fitted with a GPM (Gore-Tex expanded PTFE ammonia diffusion membrane, pore size 0.2 µm; Gore, Elkton, MD). An internal solution (internal solution B in Figure 2) consisting of either 1.00 M BisTris HCl, pH 6.0 (HSO3- electrode) or 0.010 M NaHSO3 and 0.010 M NaCl (pH electrode) was placed in the pipet tip. The selective electrodes were then inserted inside the pipet tip and pressed firmly against the GPM. A Ag/AgCl wire was immersed in this solution as the reference electrode. Procedure. Calibration experiments were performed using both the Severinghaus-type gas sensor and the gas sensor based on the HSO3- -selective electrode by placing the sensors inside a covered jacketed beaker containing 0.100 M phosphoric acid buffered at pH 2.0 using NaOH. Additions of different volumes of prepared standard solutions were made to the covered beaker through a removable porthole in the cap of the beaker. The data obtained were plotted as the change in potential, ∆E (with respect to the baseline starting potential), vs the logarithm of the concentration of the test anion. RESULTS AND DISCUSSION The guanidinium moiety has a strong affinity for oxoanions, as has been shown by their active role in many natural
Table 1. Selectivity Properties of the Hydrogen Sulfite-Selective Electrode, the Severinghaus-Type SO2 Gas Sensor, and the Hydrogen Sulfite-Based SO2 Gas Sensora pot KHSO 3 ,anion
anion
HSO3electrode
perchlorate salicylate thiocyanate iodide acetate nitrite fluoride bicarbonate
4.5 × 10-3 3.0 × 10-3 1.3 × 10-3 1.0 × 10-3 5.4 × 10-4
