ANALYST, JANUARY 1988, VOL. 113 139

Chemiluminescence Method for the Direct Determination of Sulphur Dioxide

Norimichi Takenaka, Yasuaki Maeda* and Makoto Munemori Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Prefecture, Sakai 591, Japan and Danian Zhang East China Institute of Chemical Technology, Department of Environmental Engineering, Mei Long Road, Shanghai, People's Republic of China

~

A method for the determination of SO2 based on the fact that it enhances the chemiluminescence produced by the reaction of luminol with H202 has been developed. The calibration graph for SO2 was a straight line with a correlation coefficient of 0.998 from 1 to 1000 p.p.b. (VN). The detection limit (signal to noise ratio = 3) was 0.6 p.p.b. and the relative standard deviation for ten measurements of 10 p.p.b. of SO2 was 5.3%. The method required 1 min or less for the determination of 10 p.p.b. of SO2. Ozone and nitrogen dioxide interfered but were almost completely removed by using a tube packed with glass beads coated with FeS04. Sulphur trioxide and the other gases except H2S did not interfere. Keywords: Sulphur dioxide determination; luminol chemiluminescence; sulphur trioxide

Sulphur dioxide is a major air pollutant and one of the main causes of acid rain generation. It is usually determined by the conductimetric method192 or the pararosaniline method. 1 ~ 3 Both methods can detect gaseous SO2 at p.p.b. levels but need 30-60 min for a single measurement and therefore they give only the average concentration of SO2 during the sampling time. Further, the former method is subject to interferences

. from other acidic or alkaline gases and the latter uses toxic mercury. SO2 in the gas phase can be directly determined by flame photometric495 or fluorescence methods .6*7 The flame photometric method has the disadvantages that gas chromato- graphic treatment is required because this method can detect all sulphur compounds, and also the base line of the signal is very high. The fluorescence method is influenced by aromatic hydrocarbons. Recently, chemiluminescence methods for the detection of gaseous SO2 have been developed.gl0 The method developed by Meixner and Jaeschke,g however, also uses toxic mercury and the method developed by Kato et a1.9 has a narrow dynamic range.

The reactions of luminol with oxidising agents such as H202,11J2 C10- 13 and C1214 produce chemiluminescence. This chemiluminescence is enhanced by metal cations such as C O ~ + ,I5 Fez+ 16 and Cr3+ 17 in aqueous solution and has been applied to the determination of the metal cations. In previous work, it was found that NO2 could be determined by chemiluminescence produced by the reaction of alkaline luminol solution with gaseous N0218 and also SO2 could be determined using the fact that SO2 enhances the chemi- luminescence produced by the reaction of luminol with N02.19 With the luminol - NOz - SO2 system, the detection limit is 0.3 p.p.b. for SO2, and this method needs only a few minutes for a single measurement.19 However, it is inconvenient to use a gaseous reagent for field measurements, as NO2 is a major air pollutant and 5 p.p.m. NOz is used in the measurements. Further, we found that in a luminol chemiluminescence system using H202 instead of NO2, the response time became shorter and the interference from other gases, especially from H2S, was reduced. In this paper we report the determination of SO2 based on the chemiluminescence produced by luminol with H202 as an oxidising agent.

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* To whom correspondence should be addressed.

Experimental Apparatus The experimental set-up is shown schematically in Fig. 1. This detector is essentially the same as that used with the luminol- NO2 - SO2 system19 and consists of three parts, that is, a gas supply system, a solution mixing part and a detection part. A known concentration of SO2 is prepared with a Seitetsu Kagaku SDS 201(D) standard gas dilution system. By packing with glass beads in a mixing tube (P) in which alkaline luminol solution (L) and hydrogen peroxide solution (K) were well mixed, a stable signal was obtained. The light emission was measured using a UV-31 quartz filter with a Hamamatsu Photonic R374 photomultiplier (PMT)(F), which was oper- ated at 520 V.

Reagents Luminol (5-amino-2,3-dihydrophthalazine-l,4-dione) of ana- lytical-reagent grade and other reagents (Wako Pure Chem- ical) were used without further purification. A standard

'-2 K Ail L

Fig. 1. Schematic diagram of chemiluminescence analyser. (A) Air compressor; (B) standard as cylinder of SO2; (C, , (;) silica gel and activated carbon as puri8cation tube; (D) SDS 201 standard as dilution system; reaction vessel; H F amplifier; (I) recorder; (J) peristaltic pump; (!k{ H202 solution; [I,) 1ummnol.solution; (M) drain; (N) vacuum pump; (0, Q) needle valve; (P) mlxing tube

) flow meter; (F) photomultiplier tube;

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140 ANALYST, JANUARY 1988, VOL. 113

cylinder gas (B) containing 99.5 or 95.6 p.p.m. of SO2 in N2 and obtained from Seitetsu Kagaku was used. Carbon monoxide, carbon dioxide and ethane were obtained from Gasukuro Kogyo. Gaseous ammonia and formaldehyde were prepared by evaporation in a sample bottle at reduced pressure. The concentrations of the gases were adjusted by diluting with clean air purified by passing through active carbon and silica gel.

In the experiment on interference by SO3 in the determina- tion of SO2, a mixture of SO3 and SO2 gases was used. The mixture of SO3 and SO2 was generated by decomposition of 1 g of A12(S04)3 packed in a Pyrex glass tube by heating with an electric furnace and a 1 dm3 min-1 flow of N2 through the tube. SO3 and SO2 in the effluent gas were adsorbed in distilled water using three fritted bubblers (SO3 and SO2 were completely absorbed) and the concentrations of SO3 and SO2 in the gas were determined as S042- and S032-, respectively, by ion chromatography.

A stock solution of 5 x 10-3 rnol dm-3 of luminol in 1 x 10-2 mol dm-3 sodium hydroxide was prepared and was used after keeping it for more than 2 weeks in the dark, because the signal intensity of freshly prepared luminol solution gradually increased and a stable signal intensity was obtained only after 10 days. H202 (30% in water) was diluted with re-distilled water just before use.

Procedure The chemiluminescence signal was obtained when H202 was added to alkaline luminol solution, and SO2 enhanced this chemiluminescence. For the measurement of SO;?, therefore, the background emission intensity was obtained when clean air was fed into the reaction vessel, and the total emission intensity (background intensity plus enhanced emission inten- sity by SO2) was obtained when sample air containing SO2 was fed into the vessel.

Results and Discussion Effects of Luminol, Sodium Hydroxide and Hydrogen Peroxide Concentrations on the Emission Intensity The chemiluminescence was not observed when SO2 was contacted with alkaline luminol solution or hydrogen per- oxide. However, SO2 enhanced the chemiluminescence pro- duced by the reaction of luminol solution with H202.

Both the intensities of the background emission (chemi- luminescence produced by the reaction of luminol with H202) and of the enhanced emission (chemiluminescence enhanced by SO2) varied when the concentrations of luminol, NaOH or H202 or the flow-rate of gas or solution were changed. The effect of luminol concentration on the enhanced emission intensity is shown in Fig. 2. The enhanced emission intensity increased with increase in luminol concentration, reached a maximum at 5 X 10-5-7 X 10-5 mol dm-3 and decreased at higher luminol concentrations.

The effect of NaOH concentration on the enhanced emission intensity is shown in Fig. 3. The enhanced emission and also the background emission were not observed below 1 X mol dm-3 NaOH. Above 1 X 10-4 mol dm-3 NaOH the emission intensity increased with increase in NaOH concentration, reached a maximum at 1 x 10-2 mol dm-3 and decreased at higher NaOH concentrations.

The effect of H202 concentration on the enhanced emission intensity is shown in Fig. 4. The emission intensity increased with increase in hydrogen peroxide concentration from 1 x 10-4 to 1 x mol dm-3. The effect of H202 concentration above 1 x mol dm-3 was not investigated because the enhanced emission and the background emission were too strong for the output range of the amplifier under our experimental conditions.

1 1

-6 -5 -4 -3 Log lumtnol concentrationh

Fig. 2. Effect of luminol concentration on signal intensity. SO2, 1 p.p.m.; NaOH, 1 X 10-2mol dm-3; H202, 1 X 10-3 mol dm-3. (0) Emission intensity enhanced by SO2; (A) ratio of enhanced emmion intensity to background emission intensity

10 r 120

- m .- k 5 v)

0 -4 -3 -2 -1

Log NaOH concentrationh

Fig. 3. Effect of NaOH concentration on signal intensity. SO,, 1 p.p.m.; NaOH, 1 X 10-2 mol dm-3; Hz02, 1 x 10-3 mol dm-3. (0) Emissjon intensity enhanced by SO,; (A) ratio of enhanced emission intensity to background emission intensity

.- i?

8 1.0 cn

E .-

I n A +-

I ' '0 -4 -3 -2

Log Hz02 concentrationh

Fig. 4. Effect of H202 concentration on signal intensity. SO,, 1 p m.; luminol, 5 X 10-5 mol dm-3; NaOH, 1 x 10-2 mol dm-3. (8j i mission intensity enhanced by SOz; (A) ratio of enhanced emission intensity to background emission intensity

Effect of Sodium Hydroxide Concentration in the Luminol Stock Solution The effect of NaOH concentration in the luminol stock solution was investigated as the time profile of the signal intensity of 1 p.p.m. of SO2. The NaOH concentrations in the stock solutions were 5 x 10-3 mol dm-3 (which is the same molarity as that of luminol), 1 X 10-2 mol dm-3 (which is twice the molarity and the same normality as that of luminol)

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ANALYST, JANUARY 1988, VOL. 113 141

and 4 x 10-2 mol dm-3 (which is four times the molarity and twice the normality as that of luminol). The measurement of chemiluminescence intensity was carried out at 5 x 10-5 mol dm-3 luminol, 1 x 10-2 mol dm-3 NaOH, 1 x 10-3 mol dm-3 H202 and 1 p.p.m. SO2. The results are shown in Fig. 5. For 5 x 10-3 mol dm-3 NaOH stock solution the signal intensity increased until 10 d after the preparation of the stock solution and reached a constant value, and then increased again after 30 d. For 1 x 10-2 and 4 x 10-2 mol dm-3 NaOH stock solutions the signal intensities increased until 10-20 d later and then reached constant values, and they did not increase again. The colour of the 5 x 10-3 mol dm-3 alkaline stock solution became darker 30 d after the preparation of the stock solution, but the other solutions did not become darker. Hence the molarity of NaOH in the stock solution should preferably be higher than that of luminol. The same result was obtained for KOH. The chemiluminescence emission spec- trum obtained with a stock solution that became dark 30 d after preparation was similar to that obtained with a freshly prepared stock solution. The reason why the enhancement effect increased until 10-20 d later has not yet been clarified.

.PI 15

I I I I I

Tirne/d 0 10 20 30 40

Fig. 5. Time profile of emission intensit and effect of alkaline concentration of stock solution. Sodium hyJroxide concentrations in stock solutions: (A) 5 X 10-3 mol dm-3; (B) 1 X 10-2 mol dm-3; and C 4 x 10-2 mol dm-3. Luminol concentration, 5 x 10-3 rnol dm-3; Ld 2 , 1 p.p.m., luminol, 5 x 10-5 mol dm-3; NaOH, 1 X lop2

mol dm-3; H202, 1 x 10-3 rnol dm-3

20 I i

- m g) 10 .- fa

5

1 I I I I I

0 1 2 3 4 5 Solution flow-rate/cms min-1

Fig. 6. Effect of flow-rate on signal intensity. SO,, 1 p.p.m.; luminol, 5 x 10-5 mol dm-3; NaOH, 1 x 10-2 mol dm-3; H202, 1 X 10-3 mol dm-3. Gas flow-rate: A) 0.4 dm3 min-1; (B) 0.8 dm3 min-'; (C) 1.00 dm3 min-1; (D) 1.30 6x113 min-1; and (E) 1.80 dm3 min-1

Table 1. Effect of sulphite and sulphate on the background emission intensity

Concentration/ Relative signal Compound mol dm-3 intensity

1.00 None . . , . . . . . - Na2S03 . . . . . . 4.9 x 10-5 0.65 Na2S04 . . . . . . 4.9 x 10-5 0.51

Table 2. Optimum conditions

Luminol concentration . . . . . . . . 5 x 10-5 rnol dm-3 NaOH concentration . . . . . . . . . 1 x 10-2 rnol dm-3 Hz02 concentration . . . . . . . . . . 2 x 10-3 rnol dm-3 Flow-rateof reagent solution . . . , . . 0.8 cm3min-* Flow-rate of sample gas . . . . . . . . 1 .O dm3 min-*

Effects of Flow-rate on the Emission Intensity Alkaline luminol solution and H202 solution were fed into the reaction vessel at the same flow-rate. The effect of the flow-rates of the reagent solutions at various gas flow-rates on the emission intensity is shown in Fig. 6. The signal intensity increased and then decreased with increase in the solution flow-rate at each gas flow-rate. The signal obtained at solution flow-rates between 0.6 and 1.0 cm3 min-1 was very stable, whereas at gas Bow-rates higher than 1.30 dm3 min-1 it was noisy, so a solution flow-rate of 0.8 cm3 min-1 and a gas flow-rate of 1.00 dm3 min-1 were used in subsequent studies.

Reaction Mechanism SO2 easily dissolves in alkaline H202 to produce S032- and SO$. Therefore, the effects of sulphite and sulphate anions on the chemiluminescence intensity were investigated. As shown in Table I, both ions quenched the chemiluminescence reaction of luminol with H202. Therefore, the enhancement effect of SO2 must be applied before SO2 is converted into sulphite and sulphate anions. As shown in Fig. 3, the decrease in the emission intensity at high NaOH concentrations is probably due to the increase in sulphite or sulphate concentra- tion with increase in the dissolution of SO2 at high pH.

The emission spectrum of the enhanced chemiluminescence produced by SO2 was measured with a Hitachi Model MPF-2A fluorimeter. The spectrum (360-540 nm, Lax. = 435 nm) was similar to that obtained from the reaction of luminol with H202 in the, presence of Cu2+, which was explained as being emitte'il by the aminophthdate ion.12 Stauff and Jaeschke20 reported that chemiluminescence is emitted from excited SO2* in the triplet state with an emission spectrum (450-600 nm) in the oxidation reaction of sulphite ion by perrnanganate anion. The difference in these chemi- luminescence spectra shows that SO2 is not the emitter of the chemiluminescence, but rather a sensitiser or catalyst of the production of chemiluminescence from the oxidation of luminol in the luminol - Hz02 - SO2 system. Further investiga- tions are required in order to clarify the mechanism in detail.

Calibration Graph for SOz The optimum conditions for the determination of SO2 are summarised in Table 2. Under these conditions the ratio of the enhanced emission intensity from 1 p.p.m. SO2 to the chemiluminescence intensity from the reaction of lurninol with Hz02 was about 20, and the calibration graph for SO2 was a straight line with a correlation coefficient of 0.998 from 1 to

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142 ANALYST, JANUARY 1988, VOL. 113

I 100 p.p.b. I

Fig. 7. Signal of sulphur dioxide

Table 3. Comparison of results obtained using different methods

Concentration of SO2 found, p.p.b.

Gas and concentration Conductimetric present, p.p.b. Proposed method method

soz, 10 . . . . . . . . 9.9 9.8 SOZ,20 . . . . . . . . 18.8 20.3 SOz,4O . . . . . . . 40.6 39.9 SO2,lO + NH3,lOO . . 9.9 7.1 SO2,20 + NHJ, 100 . . . . 19.5 17.0

Table 4. Interferences from other gases. SOz concentration, 1 .O p.p.m.

Concentration, Relative signal Gas p.p.m. intensity

None c o z . . . . . . . c02 . . . . . . . . NH3 . . . . . . . .

NO;! . . . . . . . Nz0 . . . . . . . . HCHO . . . . . . C3H6 . . . . . . . . O3 . . . . . . . 0 3 . . . . . . . . HC1 . . . . . . . . Cl2 . . . . . . . . HIS . . . . . . .

NO . . . . . . .

100 500

1 .o 0.5 0.5 0.3 0.1

0.12 0.28 0.2 0.08 0.9

100

1 .oo 1.02 1.27 0.96 1.05 2.43 0.96 0.89 0.97 3.8 4.64 1.01 1.13 2.45

1000 p.p.b. A typical signal recorded on the chart is shown in Fig. 7. The proposed method requires only 1 min or less to determine 10 p.p.b. of SO2 and is much faster than the method based on the luminol - NOz system and other methods. The relative standard deviations for 10 measurements of 100 and 10 p.p.b. of SO2 were 0.63 and 5.3%0, respectively. The detection limit (signal to noise ratio = 3) calculated from this value for 10 p.p.b. of SO2 was 0.6 p.p.b.

The proposed method was compared with a conductimetric method. As shown in Table 3, when the sample gas contained only SO2 the results obtained by both methods were in good agreement with the real concentration, but when the sample gas contained SO2 and NH3, the results obtained with the proposed method were in better agreement with the real concentration than were those obtained with the conducti- metric method.

Interference by Other Gases

Interference by other gases was investigated. As shown in Table 4, NO, NH3 and many other gases did not interfere, but

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Table 5. Removal of NOz and O3 interferences

Relative signal intensity

Without With FeS0, tube FeS04

Gas and concentration, p.p.b. tube 1 g FeSO, 2 gFeS04 S02,50 . . . . 1.0 0.99 0.99 SOz, 50 + N 0 2 , SO0 . . . . 2.43 1.42 1.02 SOz, 50 + 03,200 . . . . 8.53 1.21 1.03 S02,50 + NO2,500 + 03,220 8.65 1.43 1.06

Table 6. Interference from SO3

Temperature of furnace/

“C 200 300 350 420 490

Composition of sample gas mixture by ion chromatography,

p.p.m.

so2 so3 0 0

0.05 0.04 0.31 0.04 0.30 1.20 0.21 2.80

Concentration of SOz by the

proposed method, p.p.m.

0 0.07 0.28 0.29 0.17

03, NO2 and H2S interfered. The concentrations of NO2 and O3 are generally high in urban atmospheres and therefore it is necessary to remove these interferents before measuring SO2 in ambient air. The sample gas containing SO2 and NO2 or O3 was passed through a tube (5 and 10 cm long x 8 mm i.d.) packed with glass beads (1 mm 0.d.) coated with iron(I1) sulphate.21 As summarised in Table 5, SO2 was not removed by the tube, whereas 71% of NO2 and 97% of 0 3 were removed by the tube (5 cm long) with beads coated with 1 g of FeS04 and 98% of NO2 and 99% of O3 were removed by the tube (10 ern long) with beads coated with 2 g of FeS04.

The interference from SO3 was also investigated. The results for a mixture of SO3 and SO2 obtained by ion chromatography and those obtained with the luminol chemi- luminescence method are summarised in Table 6. It was found that SO3 did not interfere in the determination of SO2 by the luminol chemiluminescence method. Therefore, SO2 can be selectively determined by this method even in the presence of SO3 and may be useful for the elucidation of the SO2 oxidation process.

1.

2.

3. 4.

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6.

7.

8.

9.

10.

11.

12.

13.

References Yanagisawa, S., “Sulfur Oxides,” Japan Chemical Society, Tokyo, 1975, Chapter 11. Thomas, M. D., h i e , J. O., and Fitt, T. C . , Ind. Eng. Chem., Anal. E d . , 1946, 18,383. West, P. W., and Gaeke, G. C., Anal. Chem., 1956,28,1816. Stevens, R. K., O’Keefe, A. E., and Ortman, G. C., Environ. Sci. Technol., 1969,3, 652. Stevens, R. K., Muric, J. D., O’Keefe, A. E., and Krost, K. J., Anal. Chem., 1971,43, 827. Okabe, H., Splitstone, P. L., and Ball, J J., J. Air Pollut. Control Assoc., 1973,23, 514. Smith, W. J., and Buckman, F. D., J. Air Pollut. Control Assoc., 1981,31, 1101. Meixner, F. X., and Jaeschke, W. A., Int. J. Envrron. Anal. Chem., 1981,10,51. Kato, M., Yamada, M., and Suzuki, S., Anal. Chem., 1984,53, 2529. Meixner, F. X., and Jaeschke, W. A , Fresenius Z . Anal. Chem., 1984,317,343. Kok, G. L., Hpller, T. P., Lopez, M. B., Nachtrieb, H. A., and Yuan, M., Environ. Sci. Technol., 1978, 12, 1072. White, E. H , Zafiriou, O., Heins, H. H., and John, H. M., J. Am. Chem. SOC., 1964.86.940. Babko, A. K., Terietskaya, A., and Dubovenko, L. I., Ukr. Khim. Zh., 1966, 32, 728.

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ANALYST, JANUARY 1988, VOL. 113 143

14. Kachibaya, V. N., Siamashvili, I. L., and Mamukashvili, M. V . , Ukr. Khim. Zh., 1971,26, 1848.

15. Burdo, T. G., and Seitz, W. R., Anal. Chem., 1975,47,1639. 16. Seitz, R. W . , and Hercules, D. M., Anal. Chem., 1972, 44,

2143. 17. Seitz, R. W., Suydam, W. W., and Hercules, D. M., Anal.

Chem., 1972,44,957. 18. Maeda, Y., Aoki, K. , and Munemori, M., Anal. Chem., 1980,

52,307. 19. Zhang, D., Maeda, Y., and Munemori, M., Anal. Chem.,

1985,57,2552.

20. Stauff, L., and Jaeschke, W. A., 2. Naturforsch., Ted B , 1978, 33,293.

21. Anderson, H. H., Moyer, R. H., Sihbett, D. J., and Sutherland, D. C., US Pat., 3 659 100,1970.

Paper A71120 Received March 26th, 1987 Accepted August 6th, 1987

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