Ion Chromatographic Determination of Sulfide, and Thiosulfate in Mixtures by Means of Their Postcolumn Reactions with Iodine.

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1 ANALYTICAL SCIENCES AUGUST 1994, VOL Ion Chromatographic Determination of Sulfide, and Thiosulfate in Mixtures by Means of Their Postcolumn Reactions with Iodine Sulfite Yasuyuki MIURA, Mieko TSUBAMOTO and Tomozo KOH Department of Chemistry, Faculty of Science, Tokai University, Hiratsuka, Kanagawa , Japan The proposed method is based on the separation of sulfide, sulfite and thiosulfate in mixtures by using a resin-based anionexchange column and a 5X10-3 M (M=mot dm-3) sodium carbonate eluent, followed by a photometric measurement of any excess iodine (as triiodide) for its postcolumn reactions with the separated sulfur species. A postcolumn solution comprising 2.5X105 M iodine, 0.01 M iodide and 0.8 M acetic acid was continuously mixed with an effluent from the separating columns and then the absorbance for the remaining iodine (as triiodide) in the stream was monitored at 330 nm. The chromatograms obtained for sulfide, sulfite and thiosulfate ions showed negative peaks, respectively, with detection limits (as S/ N=2) of 2.09X 10-6 M ( ppm) for sulfide, 1.80X 10-6 M (0.144 ppm) for sulfite and 3.52X 10-6 M (0.394 ppm) for thiosulfate. The method was successfully applied to the determination of sulfide, sulfite and thiosulfate in hot-spring water samples. Keywords Ion chromatography, sulfide-sulfite-thiosulfate mixture separation, postcolumn reaction, iodine, water analysis hot-spring The determination of sulfide, sulfite and thiosulfate in mixtures has long been a tedious and complicated task because these species react with one another and undergo air-oxidation. In a previous studys, we investigated the stabilization conditions for sulfide and sulfite in a mixture, and developed a spectrophotometric method for the determination of sulfide, sulfite and thiosulfate in their mixtures, in which sulfide and sulfite were indirectly determined by solving simultaneous equations obtained using three different procedures for the sulfide-sulfitethiosulfate mixture. In addition, we determined spectrophotometrically thiosulfate, sulfite and sulfide completely separated from their mixtures by proper chemical treatments 2,3 In these spectrophotometric methodsl-3, sulfide was fixed as precipitates of zinc sulfides and mercury(ii) sulfide.2,3 Sample solutions containing these precipitates cannot be injected into a separating column in ion chromatography because the precipitates cause the back pressure to become considerably high. Several ion chromatographic studies4_g have appeared to separate sulfur anions using various detectors of conductivity4-', UV absorbance6'' and coulometry.8 However, the sensitivities for the determinations of sulfite and thiosulfate are poor6'8 and the elution time for thiosulfate ion is considerably long.4-6,8 Further, the chromatographic peak for sulfite could not be separated from that for sulfate.5'7 Recently, Togawa et a1.9 have used a high-performance liquid chromatography with a fluorescence detector for the determination of sulfide, sulfite and thiosulfate, in which the sulfur anions were converted into fluorescent derivatives with monobromobimane; however, this reagent is not available commercially. In this work, the conditions under which the determination of the separated sulfide, sulfite and thiosulfate is feasible have been established by measuring any excess iodine for its postcolumn reaction with the sulfur species. Compared with the spectrophotometric methodsl-3, the proposed ion chromatographic method gives a more rapid and simpler procedure for the determination of sulfide, sulfite and thiosulfate in mixtures. This method has been successfully applied to the determination of the three sulfur species in hot-spring water samples. Experimental Apparatus The Model 4000i ion chromatography (Dionex) used in this study comprised a pump with dual pistons, resinbased separating columns of Dionex HPIC-AG4A (4 mm i.d.x5 cm) and HPIC-AS4A (4 mm i.d.x25 cm) in series and a ultraviolet-visible (UV-VIS) detector. A Model DQP pump with a single piston (Dionex 35250) was also used to flow the postcolumn reaction solution. A liquid mixing tee (Dionex 24313) was used to mix the postcolumn reaction reagent with an effluent from the separating columns. The mixed solution was flowed

2 596 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 into a packed reaction coil (Dionex 36036), and then passed through a detector cell. Shimadzu recorders (Model R-111 and Model C-R3A) were used for recording the chromatograms and peak areas, respectively. The water used was distilled twice and then deionized with a Model Milli-QII instrument (Nippon Millipore Ltd.). The absorption spectrum of the postcolumn reaction solution was measured using a Shimadzu Model UV-240 recording spectrophotometer with l0-mm quartz cells. Chemicals All of the chemicals used were of analytical reagentgrade and were used without further purification. The eluents were filtered through a membrane filter (pore size, 0.2 µm). An eluent of 5X 10-3 M (M=mol dm 3) sodium carbonate used in Procedure was prepared by diluting a 0.1 M carbonate solution. A postcolumn reaction solution (2.5X105 M I M KI-0.8 M acetic acid) was prepared by adding 50 cm3 of 8 M acetic acid and 2.5 cm3 of 5X10-3 M iodine in methanol to a 250 cm3 solution of 0.02 M potassium iodide, and diluting the mixture to 500 cm3 with water. A sulfide solution was prepared from large crystals of sodium sulfide nonahydrate using oxygen-free water. To remove any trace amounts of impurities from the surface of the crystals, they were rapidly washed with water and then dried by absorption of the water with filter paper. An about 0.05 M sulfide solution was obtained by dissolving 2.40 g of the crystals in 200 cm3' of water. This solution was continuously deaerated with nitrogen gas and standardized by iodometry.10 Working standard sulfide solutions were prepared by appropriate dilution with oxygenfree water containing a small amount of sodium carbonate as a stabilizer. A 1X104 -M standard sulfide in 3X10-3 M carbonate solution could be used for 33 min after preparation. An about 0.05 M sulfite solution was prepared by dissolving 0.52 g of sodium hydrogensulfite in 100 cm3 of oxygen-free water, and was standardized by iodometry.11 Working standard sulfite solutions were obtained by suitable dilution with an acetate buffer solution containing a small amount of formaldehyde (as a stabilizer). A 5X105 M standard sulfite in 1X104 -M formaldehyde-2x10-4 M acetate buffer solution (ph 5.7) was stable for at least 90 min. An about 0.1 M thiosulfate solution was prepared by dissolving sodium thiosulfate pentahydrate in water containing a small amount of sodium carbonate (0.01%) as a stabilizer, and was standardized by iodometry a week after preparation.' 1 Working solutions of standard thiosulfate were obtained by appropriate dilution with oxygen-free water. Procedure An eluent of 5X10-3 M sodium carbonate was flowed at a rate of 0.8 cm3 min 1, and then 0.05 cm3 of a sample solution containing sulfide, sulfite and thiosulfate was injected to the separating columns. Next, a postcolumn reaction solution (2.5X105 M I M KI-0.8 M acetic acid) was flowed at a rate of 0.8 cm3 min' to the mixing tee, at which the iodine reacted with the separated sulfide, sulfite and thiosulfate anions in the effluent from the separating columns in a packed reaction coil. The absorbance of the excess of iodine (as triiodide) over the sulfur species in the stream was continuously measured at 330 nm and chromatograms and peak areas were recorded. Results and Discussion Calibration plots A series of standard solutions of sulfide, sulfite and thiosulfate were treated according to Procedure. Each calibration graph for these three sulfur species, plotted as peak-areas vs. concentrations, was linear up to 1.75X 10-4 M sulfide, 1.50X104 M sulfite and 5.00X104 M thiosulfate, respectively. Their correlation coefficients were more than The detection limits, defined as S/N=2, were 2.09X10-6 M ( ppm) for sulfide, 1.80X 10-6 M (0.144 ppm) for sulfite and 3.52X106 M (0.394 ppm) for thiosulfate. The precision was determined from five results using a 0.05 cm3 aliquot of a mixture of I.00X 10-4 M sulfide, l.00x 10-4 M sulfite and 2.00X10-4 M thiosulfate. The proposed method gave a mean value of 1.00X10-4 M with a standard deviation (SD) of 3.5X10-6 M and a relative standard deviation (RSD) of 3.5% for sulfide, 1.01X10-4 M (SD=3.0X10-6 M, RSD=3.0%) for sulfite and 2.01X10-4 M (SD= 4.1X10-6 M, RSD=2.0%) for thiosulfate, respectively. Separations of sulfide, sulfite and thiosulfate ions in mixtures When a resin-based anion exchanger was used in the separation columns, sulfite and thiosulfate were slowly eluted because they were retained strongly onto the anion-exchange resin, but sulfide was eluted fast. In order to facilitate the separation of the three sulfur anions in mixtures, an attempt was made to use carbonate solutions (1X103 -to 6X103 M) as an eluent. The obtained results are shown in Fig. 1. The elution rates greatly increased for thiosulfate and sulfite with an increase in the concentration of the carbonate solution. When a 5X10-3 M sodium carbonate solution (ph 10.9) was used, sulfide, sulfite and thiosulfate ions were eluted at 2.8, 4.7 and 9.2 min, respectively, and their chromatograms were well separated from one another and gave sharp peaks, as can be seen in Fig. 2. Stabilization of sulfide and sulfite ions Sulfide and sulfite ions are susceptible to air-oxidation in an aqueous solution. In a previous study', we established the optimum conditions for stabilization of sulfide and sulfite in mixtures, under which the sulfide was fixed as a precipitate of zinc sulfide. A suspension of zinc sulfide can not be used in ion chromatography using a carbonate eluent, because the precipitate is accumulated in the separating column. In this work, a small amount of sodium carbonate was added to a sulfide

3 ANALYTICAL SCIENCES AUGUST 1994, VOL Fig. 1 Effect of the concentration of sodium carbonate eluent on the separations of sulfide, sulfite and thiosulfate in their mixture:, 52; ;, 5032-; 0, Fig. 3 Effect of sodium carbonate on the stabilization of sulfide. A 0.05 cm3 solution of sulfide was used: 0, lx 10-4 M 52_ in water;, 1 X in a 3X104 M sodium carbonate medium. Fig. 4 Effect of the amount of formaldehyde on the stabilization of sulfite. A 0.05 cm3 solution of 5X10-5 M sulfite in varying concentrations of formaldehyde solution was used: a 5X10-5 M formaldehyde; 0, 8X10-5, 1X10-4 and 3X104 M formaldehyde;, 1X10-3 M formaldehyde. Fig. 2 Chromatograms of sulfide, sulfite and thiosulfate in a mixture: 1, 7.5X10-5 M 52-; 2, 7.5X10-5 M 5032-; 3,1.5X10-4 M solution for its stabilization. The obtained results are shown in Fig. 3. In the absence of carbonate, the sulfide gave a lower peak area, owing to its evolution as hydrogen sulfide, even 2 min after preparation and further it decreased gradually in peak area with the standing time. However, the peak area obtained for sulfide in a 3X10-3 M carbonate medium remained constant for up to 33 min. Consequently, standard

4 598 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 sulfide-3x103 M sodium carbonate solutions were employed for the calibration graph of sulfide. On the other hand, formaldehyde was used as a stabilizing agent for the sulfite solution. In establishing the optimum amounts of formaldehyde, the sulfite solution was adjusted to ph 5.7 by adding an acetate buffer solution (ph 6.2).1 The obtained results are shown in Fig. 4. The sulfite gave lower peak areas owing to its airoxidation in a 5X10-5 M formaldehyde solution, but gave higher and constant peak areas over a concentration range of 8X105 to 3X104 M formaldehyde up to 90 min after preparation. On the contrary, the sulfite in a IX10~3 M HCHO gave a much lower peak area, because the hydrolysis12 of the formaldehyde-sulfite formed was highly depressed. Hence, a 1X10-4 M formaldehyde solution was used as a medium for standard sulfite solutions. A 5X10-5 M sulfite in a 1X10-4 M formaldehyde solution proved to be stable over the ph range when allowed to stand for 7 min at various ph values. their sulfur species.1 In this study, iodine was used as a postcolumn reaction reagent in order to measure the sulfide, sulfite and thiosulfate in the effluent from the separating columns. The absorption spectrum of the postcolumn reaction solution (2.5X105 M iodine-0.01 M iodide-0.8 M acetic acid) is shown in Fig. 5; the absorbance for the iodine (as triiodide) consumed by the separated sulfide, sulfite and thiosulfate was measured Postcolumn reaction solution for sulfide, sulfite and thiosulfate Under the separation conditions of sulfide, sulfite and thiosulfate from their mixtures using AG4A and AS4A columns and a 5X10-3 M carbonate eluent, the chromatographic peak obtained for sulfide could not be separated from the pesudo-peak for water, and the peak for sulfite could also not be departed from that for sulfate. On the other hand, we have reported a method for the determination of sulfide, sulfite and thiosulafte in their mixtures, based on a spectrophotometric measurement of the excess amount of iodine for its reaction with Fig. 5 Absorption spectrum for a 2.5X10-5 M iodineiodide-0.8 M acetic acid mixture solution N Fig. 6 Effect of the concentrations of iodine, iodide and acetic acid in a postcolumn reaction solution; effect of concentration of iodine in a 0.01 M iodide-0.8 M acetic acid mixture solution (A); effect of concentration of iodide in a 2.5X105 M iodine-0.8 M acetic acid mixture solution (B); effect of concentration of acetic acid in a 2.5X10-5 M iodine M iodide mixture solution (C);, 2.5X104 M ; 0, 5X104 M

5 ANALYTICAL SCIENCES AUGUST 1994, VOL with a UV-VIS detector with a monochromatic filter of 330 nm. The effects of the concentrations of iodine, iodide and acetic acid in the postcolumn reaction solution were investigated so as to obtain a higher peak area for each anion of the sulfur species. The obtained results are shown in Fig. 6. In this experiment, each 0.05 cm3 aliquot of 2.5X104 and 5X104 M standard thiosulfate solutions was used. The peak area for thiosulfate increased with an increase in each concentration of iodine, iodide and acetic acid. The maximal area was obtained in concentration ranges of 2.5X105 to 5.OX 10-5 M iodine (Fig. 6A), 1.0X102 to 1.5X102 M iodide (Fig. 6B) and 0.50 to 1.00 M acetic acid (Fig. 6C), respectively. Therefore, a mixture solution of 2.5X 10-5 M iodine, 0.01 M iodide and 0.8 M acetic acid was used as a postcolumn reaction solution in Procedure. The peak area obtained using the postcolumn reaction solution at a flow rate of 0.4 or 0.8 cm3 min 1 was higher, but that at a flow rate of 1.6 cm3 min 1 was considerably low. Hence, the postcolumn reaction solution was flowed at a rate of 0.8 cm3 min 1 in Procedure. Determination of sulfide, sulfite and thiosulfate in hot-spring samples The effect of foreign anions was investigated on the determination of sulfide, sulfite and thiosulfate. Anions such as F-, Cl-, N03-, Br, I-, SO42-, HP042-, E2P04, C104, SCN-, H000-, CH3C00-, C2042- and tartrate did not give any chromatographic peaks at concentrations as high as 0.01 M. Nitrite ion gave a positive peak at an elution time close to that of sulfide due to the formation of iodine as a result of the oxidation of the iodide in the postcolumn reaction solution with the nitrite. However, the peak caused by nitrite could be eliminated by adding sulfamic acid (2 cm3 of 0.05 M sulfamic acid to 25 cm3 of a 1X10-3 M nitrite solution) and allowing the mixture to stand for 5 min. The chromatograms for sulfide-sulfite-thiosulfate mixtures adjusted to ph 3.4, 7.4 and 10.2 are shown in Fig. 7. These mixture samples were prepared as rapidly as possible and injected into the separating column within 30 s after preparation. The difference in the chro- Fig.7 Chromatograms for sulfide-sulfite-thiosulfate mixtures at ph 3.4, 7.4 and 10.2: 1, 5X10-5 M S2-; 2, 5X 10-5 M 5032-; 3, 5X10-5 M S Table 1 Determination of sulfide, sulfite and thiosulfate in hot-spring water samples a. Obtained using a c. Obtained using a detectable. 10-fold dilution of the original volume. b. Obtained using a 2-fold dilution of the original volume fold dilution of the original volume. d. Obtained by a spectrophotometric method.' ND=not

6 600 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 O.Olppm, RSD=1.8%). The contents of the sulfur anions in hot-spring water (sample E) obtained by the proposed method were compared with those obtained by a spectrophotometric method.1 Figure 8 shows the chromatograms obtained by adding known amounts of sulfide, sulfite and thiosulfate to sample E. The recoveries for the sulfur species added to the samples ranged from 97.8 to 100.8% for sulfide, 98.2 to 101.8% for sulfite and 97.4 to 99.7% for thiosulfate, respectively. References Fig. 8 Chromatograms of hot-spring water samples. A, obtained using the original volume of sample E; B, obtained using a 2-fold dilution of the original volume of sample E, to which sulfide, sulfite and thiosulfate were added to be 0.38, 1.12 and 5.60 ppm, respectively: 1, S2-; 2, SO32-; 3, S matograms obtained for the mixtures at the three ph values can be regarded as being negligible. The proposed method was applied to the determination of sulfide, sulfite and thiosulfate in hot-spring samples. The hot-spring waters were filtered through Toyo filter paper (No. 6), and a 0.05 cm3 aliquot of the hot-spring waters was injected into the separating columns. The results obtained are shown in Table 1. The precision was ascertained from five results obtained for a 0.05 cm3 aliquot of a hot-spring water (sample A) diluted to 10-times the original volume for the determination of sulfide, to 2 times for sulfite and not diluted for thiosulfate. The proposed method afforded a mean value of 3.64 ppm with a standard deviation (SD) of 0.06 ppm and a relative standard deviation (RSD) of 1.6% for sulfide, 1.91 ppm for sulfite (SD=0.09 ppm, RSD=4.7%) and 0.57 ppm for thiosulfate (SD= 1. T. Koh and Y. Miura, Anal. Sci., 3, 543 (1987). 2. T. Koh and K. Okabe, Anal. Sc., 8, 285 (1992). 3. T. Koh, K. Okabe and Y. Miura, Analyst [London], 118, 669 (1993). 4. J. Weiss and M. Gob!, Fresenius'Z. Anal. Chem., 320, 439 (1985). 5. M. Weidenauer, P. Hoffmann and K. H. Lieser, Fresenius' Z. Anal. Chem., 331, 372 (1988). 6. S. Ikeda, H. Satake and H. Segawa, Nippon Kagaku Kaishi, 1985, P. Kokkonen and H. Hyvarinen, Anal Chim. Acta, 207, 301 (1988). 8. A. Ono, Bunseki Kagaku, 35, 476 (1986). 9. T. Togawa, M. Ogawa, M. Nawata, Y. Ogasawara, K. Kawanabe and S. Tanabe, Chem. Pharm. Bull., 40, 3000 (1992). 10. B. J. Heinrich, M. D. Grimes and J. E. Puckett, in "Treatise on Analytical Chemistry"; ed. I. M. Kolthoff and P. J. Elving, Part II, Vol. 7, p. 75, Interscience, New York, L. V. Haff, in "The Analytical Chemistry of Sulfur and Its Compounds'; ed. J. H. Karchmer, Part I, pp , Wiley-Interscience, New York, M. Lindgren and A. Cedergren, Anal. Chim. Acta, 141, 279 (1982). (Received February 8, 1994) (Accepted May 11, 1994)

Shigeya SnTO and SUIIllO UCHIKAWA. Faculty of Education, Kumamoto University, Kurokami, Kumamoto 860

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