Environ. Sci. Technol. 1986, 20, 524-526
Membrane-Based Flow Injection System for Determination of Sulfur( I V ) in Atmospheric Water Purnendu K. Dasgupta” and Vlnay K. Guptat Department of Chemistry, Texas Tech University, Lubbock,
Texas 79409-4260
m A single flow channel analysis system that employs the formaldehydeS(1V)-pararosaniline color reaction has been developed for the determination of aqueous S(1V). The system employs a passive cation-exchange membrane reactor for the introduction of base and a pressurized porous membrane reactor for the introduction of acidic pararosaniline. The detection limit is 1.6 X lo-’ M S(IV); the sample size is 200 pL, and the throughput rate is 15/h. Results for cloudwater samples are presented. The most widely used method for determining small quantities of aqueous S(1V) involves the Schiff reaction (1) with pararosaniline (PRA) and HCHO. The current federal reference method (2) for determining atmospheric SO2 also relies on this principle, after collecting the SO2 in a K2HgC14solution. In lieu of toxic and corrosive mercury salts, a buffered HCHO absorber has been introduced for the collection of atmospheric SO2 (3). It has been shown that the buffered HCHO absorber is much more effective than K2HgC14toward stabilizing collected S(1V) ( 4 ) . To determine S(1V) collected in HCHO absorber, direct addition of PRA is ineffective. With HCHO, S(IV) forms a stable adduct, hydroxymethanesulfonate (HMSA) (3,5),which must be decomposed to the parent constituents before the pararosaniline reaction can take place (3,6). Consequently, an automated continuous flow analysis setup generally requires a minimum of three flow channels: a blank absorber channel in which the sample containing SO2 is injected, a flow channel which adds base to decompose the HMSA, and, finally, a flow channel which adds the acidic pararosaniline. This type of arrangement was used by Kok et al. (7) for adapting the analytical process to air segmented continuous flow analysis (SCFA). In recent years, flow injection analysis (FIA) is rapidly replacing SCFA because of the simplicity of equipment and faster throughput rates (8). It is important to minimize the number of flow channels in FIA when trace determination is conducted because the primary sources of detector noise are mixing inhomogeneity and pump pulsation; overall detectability in FIA deteriorates as the number of pump channels are increased. We report herein a single flow channel FIA procedure for the above analytical system, introducing the applications of passive and active membrane reactors in FIA and demonstrate the use of the system for analysis of S(1V) in cloud water. Experimental Section The experimental arrangement is shown schematically in Figure 1. Buffered formaldehyde absorber blank is stored in collapsible plastic bag R and pumped by peristaltic pump P (Gilson Minipuls 2, Gilson Medical Electronics, Middelton, WI) at a rate of 0.25 mL/min through a six-port PTFE rotary loop type injector (Rheodyne type 50,0.5-mm connecting passages, Cotati, CA) equipped with a 200-pL loop. The pump and the injector are connected +Permanent address: Department of Chemistry, Ravishankar University, Raipur 492010, M. P., India. 524
Environ. Sci. Technol., Vol. 20, No. 5, 1986
by a length of coiled tubing D l (inside diameter of coil i= 1cm; 50-cm length). All connecting tubing are PTFE, 1.5 mm 0.d. and 0.8 mm i.d. The outlet of the valve is connected to a passive reactor system N by coiled tubing D2, 30 cm in length. D1 and D2 serve as pulse dampeners. The passive reactor N consists of a 3-cm length of a perfluorosulfonate cation exchanger membrane tube (Nafion 811X, ~ 0 . mm 7 i.d., ~ 1 . mm 0 o.d., Perma-Pure Products, Toms River, NJ) with details of connections shown in Figure 2a. The membrane tube is immersed in a 50% solution of NaOH contained in a 125 mL capacity stoppered polypropylene bottle, and the solution is stirred by means of a small stir bar and an external magnetic stirrer. The passive reactor bottle outlet is connected to the active reactor bottle T inlet via coiled delay D3,40 cm in length. The active reactor T consists of a 0.25 mm diameter quartz-fiber-filled porous polypropylene membrane tube (7 cm long, 0.4 mm id., 0.45 mm o.d., surface porosity 4076, mean pore size = 0.05 pm, Celgard X-20, Celanese Corp., Charlotte, NC) with details of connection shown in Figure 2b. The membrane tube is immersed in a solution of pararosaniline reagent containing 0.833 g/L pararosaniline hydrochloride (Fisher Scientific) in 1 M H2S04. The reagent is driven through the pores by pneumatic pressure, =6 psi. It is necessary to immerse the membrane first in a solvent that wets the membrane, e.g., methanol, and then put it in the reagent solution and pressurize the bottle to establish flow across the membrane. The active membrane reactor outlet is connected by coiled delay D4 of length 1.25 m to the photometric detector (Kratos SF 770, Ramsey, NJ) set at 580 nm. The buffered formaldehyde absorber is prepared as a 1OX stock solution by dilution of 5.3 mL of HCHO (37%), acid, 22.8 g of trans-1,2-cyclohexylenedinitrilotetraacetic and 5 g of NaOH to 1L. The pH of this reagent is ~ 4 . 8 , and it is stable for at least 3 months. This reagent formulation, due to Kok et al. (7), is superior for cloudwater S(1V) measurements compared to absorber recipes used for gas-phase SO, collection ( 3 , 6 ) ;the large concentration of chelant is necessary to prevent interference from transition metal ions, notably Mn(II), and the large concentration used permits the exploitation of the multiprotic chelant itself as the buffering agent. The reagent is diluted 10-fold with deionized water before use as carrier blank. The cloudwater sample was collected by Sonoma Technology Inc. (Santa Rosa, CA) using a cloudwater collector (9) and methodology that has been previously described ( 1 0 , l l ) . The important difference from previous experiments was that the cloud water was directly collected into a precleaned dried and individually weighed polyethylene bottle containing 1 mL of the 1OX buffered formaldehyde absorber. After collection, the bottles were individually weighed and shipped, typically under refrigeration in “Blue Ice” as a precaution, to our laboratory. Requisite amount of deionized water or lox absorber blank was added to individual bottles to make the formaldehyde concentration identical with that of the carrier blank used in the analytical setup. The dilution of the original cloudwater sample as a result of the above operations is corrected for in the results reported here. The
0013-936X/86/0920-0524$01.50/0
0 1986 American Chemical Society
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Flgure 1. Analytical system schematic. R, reservoir containing buffered formaldehyde absorber blank; P, peristaltic pump; D1, -2, -3, and -4, coiled delay tubes; V, loop-type rotary Injector; N, passive membrane reactor for NaOH introductlon: T, pressurized porous membrane reactor for acidic pararosaniline introduction;D, absorbance detector. 580 nm.
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I I
I I
I
Flgure 2. Details of membrane reactors. (a) Passive membrane reactor: A and B, PTFE inlet/outlet tubes, 1.5 mm o.d., 0.8 mm 1.d.; F, silicone rubber stopper; M, perfluorosulfonate cation exchanger membrane tube; S, stlr bar. Inset shows details of connection: J is a small length of 30-gauge PTFE microbore tube insert which allows M to be inserted inside tube A and crimped by nichrome wire crimp C from the outside. The wire crimp is covered up by PTFE tape TT. (b) Pressurized membrane reactor: D and E, PTFE inlet/outlet tubes, 1.5 mm o.d., 0.5 mm i.d.; G, pressurizing gas inlet; P, porous polypropylene membrane. The inset detail is similar to that in (a), except a 28-gauge stainless steel needle insert N is used and the membrane is filled with quartz-fiber Q.
samples were injected into the analytical system through disposable membrane filters (PTFE, 0.45 pm). Results and Discussion Because membrane reactors have never previously been described for use in FIA, a short account of their characteristics is given below. We distinguish between passive and active membrane reactors (12) in that reagent is forcibly (e.g., by pressure) introduced through the membranes in the latter type of reactors while reagent simply diffuses through the membrane in passive reactors by virtue of a concentration differential existing across the membrane. In passive reactors, a small loss of sample into the reagent also generally occurs due to the reverse outward diffusion. However, this can be prevented in special circumstances, as in this work. Excellent mixing is produced in narrow bore membrane reactors due to uniform radial entry of the reagent throughout the length of the reactor. In the present application, an NaOH flow channel and a mixing tee are replaced by the passive membrane reactor. If solutions on both side of the membrane are relatively dilute, the cation-exchange membrane prevents migration
of anions across the membrane due to the Donnan barrier presented by the fixed negatively charged sulfonate groups in the membrane matrix. Under these conditions, quantitative exchange of all cations in the carrier for sodium is only possible in a hollow tubular configuration if the carrier flow rate is very slow and a significant length of membrane is used (13), neither of which conditions are maintained here. Also merely exchanging all cations for sodium will not render the solution sufficiently alkaline (pH 111) to attain rapid decomposition of HMSA into formaldehyde and sulfite. To achieve this, free NaOH must be introduced, which can be accomplished by overcoming the Donnan barrier by employing a high concentration of NaOH (14). A very high concentration of NaOH was used in this work so that only a small length of the membrane tube was necessary, and under the experimental conditions pH of the effluent from the reactor was 512. The hydrophilic membrane, however, permits relatively facile passage of water across the membrane, and due to the existing osmotic pressure differential, a small amount of outward water transport occurs across the membrane. The consequent dilution of the NaOH reagent requires that it be replaced with fresh reagent weekly; no adverse effects have been observed for week-long operation with the same NaOH solution. However, some stirring is necessary to prevent reagent depletion in the vicinity of the membrane. Note that while the Donnan barrier is overcome with respect to the inward transport of NaOH due to the high concentration employed, the barrier remains in effect for outward diffusion of sulfite present only in low concentration, and no sample loss occurs. The alkaline carrier bearing formaldehyde and sulfite enters the porous membrane reactor where acidic pararosaniline is introduced by pneumatic pressure. We investigated porous PTFE and polypropylene membrane tubes for this application. The former contains pores that are 2-3.5 pm in mean diameter, nearly 2 orders of magnitude larger than the latter type. Because only a relatively small amount of pressure is required to achieve the necessary reagent introduction rate through the large pore membrane, some of the carrier diffuses out through the membrane. A slow polymerization reaction occurs between pararosaniline and formaldehyde and degrades membrane performance over a period of several days. These limitations were not observed with the smaller pore size polypropylene membrane; however, our experience indicates that this difference between performances of the two membrane types is somewhat unique and relevant only to the present chemistry. The polypropylene membranes are narrow bore and very thin walled and have a tendency to kink and shut off flow. This is prevented by inserting a filament inside the membrane. While we generally use nylon monofilament (13) for the construction of filament-filled tubular membrane devices, the high acidity of the reaction medium hydrolyzes nylon, and a length of quartz fiber (optical fiber) was used instead. The active reactor should be pressurized sufficiently to attain a reagent introduction rate such that the effluent pH is 1-1.05, which is optimum for this reaction (3). With no restricting tubing placed at the detector outlet, the required pneumatic pressure was found to be =6 psi. After the pararosaniline introduction, the delay to the detector represents a reaction time of approximately 2 min. Although to develop the full extent of color, greater delays, up to 10 min, are necessary ( 3 , 6 ,7), and greater dispersion is introduced by the longer delay line, increasing sample requirement and deteriorating attainable throughput rate. Also, although the absolute magnitude of the signal is Environ. Sci. Technol., Vol. 20,
No. 5, 1986 525
the delay is made much shorter. Typical calibration results at the level of 3.1 X 104-3.1 X and 3.1 X 10-7-3.1 X lo4 M S(1V) are shown in parts a and b of Figure 3, respectively. The calibration plot is nonlinear at the low end, which is a characteristic of this method (7). The LOD on the basis of S / N = 3 is approximately 1.6 X lo-’ M S(1V). The relative standard M level and above is 12.5%. deviation at the 1.6 X The results for cloudwater samples collected in the Los Angeles basin is shown in Table I.
Acknowledgments We thank L. W. Richards, Sonoma Technology Inc., for his assistance and valuable discussions. Registry No. S, 7704-34-9; H20, 7732-18-5; HCHO, 50-00-0; pararosaniline, 569-61-9.
Literature Cited
a
b
Figure 3. Typical system performance. (a) 3.1 X 10-5-3.1X lo-’ M S(IV), duplicate injection for each sample, injection frequency every 4 min, and 0.4 absorbance unit full scale (AUFS). The first peak pair is 90% of full scale. (b) 3.1 X 10-’-3.1 X lo-’ M S(IV), other parameters same as in (a) except 0.03 AUFS. Base-llne drift shown is typical under this condition: quantiiation error due to drift, however, is minimal.
Table I. S(1V) Content of Cloudwater Collected in Los Angeles, May 1985
sample ID s5 s10 s11
SU15 su19
S(IV), pM (MD)” 0.43 (f0.15) 0.39 (*0.11) 0.28 (fO.11) 0.34 0.40
Not specified for samples a Duplicate or triplicate analysis. where limited amount did not permit replicate analysis.
increased by allowing greater reaction time, signal/noise does not improve proportionally, and limit of detection (LOD) is not substantially better. We found that at least 2 orders of magnitude higher sample concentrations (up to 3.0 X M S(1V))such as that reportedly occurring in fogwater (15)can be analyzed by the present system if
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(1) Schiff, H. Justus Liebigs Ann. Chem. 1866, 140, 92-95. (2) Fed. Regist. 1971, 36 (84), 8187-8191 (April 30, 1971). (3) Dasgupta, P. K.; DeCesare, K.; Ullrey, J. C. Anal. Chem. 1980,52, 1912-1922. (4) Dasgupta, P. K.; DeCesare, K. B. Atmos. Enuiron. 1982, 16, 2927-2934. (5) Kok, G. L.; Gitlin, S. N.; Lazrus, A. L. J . Geophys. Res., in press. (6) Dasgupta, P. K. J . Air Pollut. Control Assoc. 1981, 31, 779-782. (7) Kok, G. L.; Gitlin, S. N.; Gandrud, B. W.; Lazrus, A. L. Anal. Chem. 1984,56, 1993-1994. ( 8 ) Rhiiska, J.; Hansen, E. H. “Flow Injection Analysis”;Wiley: New York, 1981. (9) Mohnen, V. A. Atmos. Technol. 1980, 12, 20-25. (10) Richards, L. W.; Anderson, J. A.; Blumenthal, D. L.; Brandt, A. A.; McDonald, J. A.; Waters, N.; Macias, E. S.; Bhardwaja, P. S. Atmos. Enuiron. 1981, 15, 2111-2134. (11) Richards, L. W.; Anderson, J. A.; Blumenthal, D. A.; McDonald, J. A,; Kok, G. L.; Lazrus, A. L. Atmos. Environ. 1983, 17, 911-914. (12) Dasgupta, P. K. In “Ion Chromatography”; Tarter, J. G., Ed.; Marcel-Dekker, New York, in press. (13) Dasgupta, P. K. Anal. Chem. 1984,56, 96-103. (14) Dasgupta, P. K.; Bligh, R. Q.; Lee, J.; D’Agostino, V. Anal. Chem. 1985,57, 253-257. (15) Munger, J. W.; Jacob, D. J.; Hoffmann, M. R. J . Atmos. Chem. 1984,1, 335-350.
Received for review August 8,1985. Accepted November 25,1985. The support of this work by the Electric Power Research Institute through RP 1630-28 is gratefully acknowledged.
