Determination of sulfite in water samples by flow injection analysis with fluorescence detection

Available online at www.sciencedirect.comCHINESEScienceDirectC HEMICALL .ETTERSELSEVIERChinese Chemical Letters 21 (2010) 1457-1461www.elsevier.com/locate/ccletDetermination of sulfite in water samples by flow injectionanalysis with fluorescence detectionLi Qian Yin, Dong Xing Yuan , Min ZhangState Key Laboratory of Marine Environmental Science, Environmental Science Research Centre,Xiamen University, Xiamen 361005, ChinaReceived 18 March 2010AbstractA fast, sensitive, and reliable method for the determination of sulfite (SO3- ) in fresh water and seawater samples wasdeveloped. The proposed method was based on the reaction of o-phthalaldehyde (OPA)- sulfite-NH3 in alkaline solution, with flowinjection analysis and fluorescence detection. The experimental parameters were investigated in pure water and seawater matrixes.The detection limits (S/N = 3) were 0.006 μmol/L in pure water and 0.018 μmol/L in seawater for SO32- . The method wassuccessfully applied to analyze SO3- in the samples of rain water and fue gas desulfurization seawater.C 2010 Dong Xing Yuan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.Kerwords: Sufite; Fluorophotometry; FIA; Flue gas desulfurization seawater; RainwaterSulfur dioxide (SO2) is a harmful gas that can result in acid rain and other environmental impairment. SO2 inatmosphere is discharged mostly from coal burning. The methods for SO3- /SO2 determination are mostly in the fieldof food and air analysis. For example, formaldehyde absorbing-pararosaniline spectrophotometry method is widelyapplied in SO3- /SO2 determination for air, water and food analysis [1,2]. However, the detection limits of existingmethods are high and not suitable for samples of low SO3- concentration analysis. On the other hand, the existingmethods rarely consider the interference from complicated matrix samples such as seawater.The technology for seawater fue gas desulfurization is widely adopted by coal-fired power plants in coastal areas.SO2 in the flue gas is absorbed by fresh seawater and transfers into aqueous phase as SO32- . Therefore, it is necessaryto determine the concentration of SO3'- in the seawater to evaluate the process of desulfurization, the efficiency ofSO32- /SO4- conversion, and monitor the impact of waste seawater containing SO3on sea area. Thus, a methodsuitable for the determination of SO32- in complicated seawater matrix is needed.The method adopted in this research was based on the reaction of sulfite with o-phthalaldehyde (OPA) andammonium. OPA reacts with molecules containing primary amino group in the presence of mercaptoethanol/thiol andin an alkaline medium to give a highly fluorescent product called isoindole [3]. Zhang and Dasgupta [4] modified themethod by replacing mercaptoethanol with sulfite, and the OPA- sulfite -NH3 reaction has been adopted for a variety ofapplications, such as determination of ammonium in seawater [5,6] and atmospheric ammonia [7] and amino acids inCorresponding author.E-mail address: yuandx @ xmu.edu.cn (D.X. Yuan).中国煤化工1001-8417/$ - see front matter◎2010 Dong Xing Yuan. Published by EIsevier B.V. onCHCNMHGAll rights reserved.doi: 10.1016/.cclet.2010.06.0291458L.Q. Yin et al./Chinese Chemical Letters 21 (2010) 1457 -1461clinical samples [8]. The isoindole derived from OPA- -sulite- -NH3 reaction can be determined at ^ex = 362.5 nm andλem= 423.0 nm with a fuorescence detector [5]. However, studies of sulfite determination on the basis of OPA -sulfite- NH3 reaction are few. Tzanavaras et al. [9] determined total sulfite in wines based on the reaction of OPA andNH4Cl in basic medium with sequential injection technique.In this work, the reaction of OPA- -sulite- NH3 in alkaline solution was applied to determine SO32-:in watersamples. The reagents and instrumental parameters were investigated to suit the analyses of fresh water and seawater,respectively. .1. ExperimentalI.I. ReagentsOPA, NH4Cl, ethanol, Na2SO3, HCHO, Na2HPO4, borax and NaOH were purchased from Sinopharm ChemicalReagent Co., China. Pure water(18.2 MA cm) was obtained from a Milli-Q water purification system (Millipore Co.,USA). Reagent 1 (R1), containing 10 mmol/L of OPA, was made by dissolving 1.34 g OPA in 75 mL ethanol anddiluting to 1 L with pure water. Reagent 2 (R2), containing 2.5 mmol/L of NH4Cl, was made by dissolving 0.134gNH4Cl in 1 L pure water, Na2HPO4 was used as buffer at a concentration of 34.4 mmol/L, pH was adjusted withNaOH solution. R2 was used for freshwater measurement. Reagent 3 (R3), containing 2.5 mmol/L of NH4Cl, boraxwas used as buffer at a concentration of 13.5 g/L, pH was adjusted with NaOH solution. R3 was used for seawatermeasurement.1.2. StandardsA 10 mmol/L SO3- stock solution was made daily by dissolving sodium sulfite solid (its purity was determinedbefore use) in 100 mL of 10 mmol/L HCHO solution.1.3. SamplingThe rain water sample was collected in Xiamen University campus, November 12, 2009. The seawater sampleswere obtained from the aeration tank of flue gas desulfurization waste seawater inside a power plant and sea area nearto waste seawater discharging outlets. Samples were added with HCHO right after collection to reach a concentrationof 1 mmol/L HCHO in samples, and filtrated through a filter membrane of 0.45 μm pore diameter before analyzing.Samples with high SO3- concentration should be diluted before analysis.1.4. Analytical equipmentsHH-1 water bath device (Shunhua Instrument Inc., Jintan, China), a 4-channel LEAD-1 peristaltic pump (LongerPrecision Pump Inc., Baoding, China), a 6-way Vici valve (Valco Co, USA), and a flow-through RF-10A XLfluorescence detector (Shimadzu Co., Japan).The flow injection analysis (FIA) schematic diagram is shown in Fig. 1. Samples or standard solutions werepumped into sampling loop at flled position. Pure water or seawater was used as a carrier at injection position to putthe sample zone forward. R1 merged with R2 in the connecting tube at first, and then mixed with sample stream in aheated mixing coil (65 °C). The sample throughput was 20h-，with triple determination for each. The finalfluorescent product was detected with the fluorescence detector.2. Results and discussion2.1. Spectral characteristic中国煤化工The maximum wavelengths of excitation and emission wereMHCNMH Gnm, respectively,determined with a scanning Varian Cary Eclipse fuorescence spectrophotometer (Varian Co, USA).L.Q. Yin et al./Chinese Chemical Letters 21 (2010) 1457- -14611459DPCMC|FwtWMH→wRWater BathR2/R3-S: sample or standard solution; C: carrier (pure water or seawater); P: peristatic pump;V: 6-way valve; MC: mixing coil; D: flow-through fluorescence detector;W: waste; DP: data analysis system.Fig.1. Schematic diagram of FIA-fluorescence determination for SO3- detection. S: sample or standard solution; C: carrier (pure water orseawater); P: peristaltic pump; V: 6-way valve; MC: mixing coil; D: flow-through fuorescence detector; W: waste; DP: data analysis system.2.2. Reagent compositionOptimal parameters for fresh water analysis were studied by analyzing SO32- at a concentration of 1 μmol/L inpure water matrix. Carrier was pure water. The results showed that the fluorescence intensity increased with OPAconcentration in the range of 1- -20 mmol/L, the chosen concentration was 10 mmol/L for it was satisfactory enoughfor sensitivity. The optimal concentration of NH4C in R2 was 2.5 mmol/L. Solution pH had a significant effect on thesignal. The sensitivity of the detection would be substantially decreased when the pH of R2 was outside the range of10.3- 11.2. Final pH of R2 was adjusted to 10.70 with NaOH solution. Since SO32- is easy to be reduced, HCHO waschosen as a protective agent for SO32- . The fluorescence intensity decreased slowly as the concentration of HCHOincreased in the range of 1-1500 μmol/L. Considering the amount of SO32- in real samples, concentration of HCHOwas chosen at 1000 μumol/L for its acceptable influence on sensitivity and adequate amount for protection.Temperature, loop/coil length, and flow rate: Increasing the reaction temperature was effective in accelerating thedesired reaction in the range of 30-75 °C. 65。C was chosen as the water bath temperature for bubbles would forminside the system and cause erroneous signals under higher temperature. Optimal length of the sampling loop andmixing coil were found to be 73 cm and 257 cm, corresponding to 0.66 mL and 2.31 mL, respectively. The optimalflow rates were 0.45 mL/min for R1, R2, R3 and carrier, 0.47 mL/min for sample and standard, respectively.2.3. Performance in pure water analysisUnder the optimum conditions, there was a good linear relationship between SO32- concentration and signal forure water matrix. The linear range, linear equation and detection limit (S/N=3) were 0.1- 20 μmol/L,F = 22.9404C+0.5121 (n=6, R2= 0.99998), 0.006 μmol/L, respectively. Precision of this method was alsoexamined, pure water spiked at 1 μmol/L and 10 μmol/L SO32- was determined continuously for 7 times, the RSDswere 1.65% and 1.02%, respectively.2.4. Interference studyThe potential interference existing in real water samples was studied. The permission concentrations of coexistingions for the determination of 1 μumol/L SO32- with deviation less than士5% was listed in Table 1. It can be also seenfrom Table 1 that the interference of Ca'+, SO42- and CO3- was significant (deviation over士5%), therefore, it ishighly recommended to use the real water, which has similar characteristics with samples and is free of SO3- , as theTable 1Coexisting and interference ions and the interference level.Coexisting and interference ions K+Na+A13+C1NO3~中国煤化工~ so,2CO32-Added concentration (μmol/L)10,000 20,0003.7 10,000 10,000MHCNMHG10,000 1700Interference level (%)<+5<+5 <士5<士5 <土5 <士5 <士) <土) - 12.9+21.8+10.31460L.Q. Yin et al./Chinese Chemical Letters 21 (2010) 1457 -1461matrix to prepare standard solutions and as the carrier solution. The SO3- free water could be obtained by exposingthe water in the air for several days.In previous studies, mercaptoethanol was used with OPA as the reagent for the determination of ammonia andamino acid [3], so that the interference of mercaptoethanol was studied. It was found that the signal produced by10 μumol/L mercaptoethanol was equal to 0.05 μmol/L SO32- . Thus the interference of mercaptoethanol can beignored for it rarely exists in natural waters and it has lttle influence on SO32- determination.2.5. Application in rain water analysisThe method described above was applied in analyzing the rain water sample. In order to eliminate the interferencein rain water matrix, the determination was carried out with standard addition method, and the SO3- concentrationwas 1.26 μmol/L.2.6. pH and buffer for seawater analysisHigh pH in the test solution or sample, for example, seawater, would cause precipitation. Thus, R2 was substitutedby R3. The optimal pH of R3 and buffer were investigated. It was found that pH 9.52 was the best for R3. Theprecipitation of hydroxide was found, when mixing Na2HPO4 with sea water. For the analysis of seawater, the buffer inR3 was borax and its optimal concentration was 13.5 g/L. Carrier was seawater. Other experimental parameters werethe same as those in fresh water analysis.2.7. Performance in seawater analysisThe developed method was applied in analyzing the waste seawater of fue gas desulfurization. A typical calibrationseries was shown in Fig. 2. The linear range, linear equation and detection limit were 0. 1-20 μmol/L,F = 7.9031C + 0.0549 (n= 7, R2 = 99999), 0.018 μmol/L, respectively. Seawater spiked at 10 μmol/L SO3- wasdetermined continuously for 5 times, the RSD was 0.73%. Recoveries from seawater matrix were 93.5- 103.9%, whichwere acceptable.2.8. Comparison with reference methodThe two samples were collected from the aeration tank of a flue gas desulfurization waste seawater. Comparison ofthe analytical results of the proposed method and reference method (pararosaniline spectrophotometric method) [10]was shown as Table 2. There was no significant statistical difference between the two methods with the pairedStudent's t-test at 95% confidence level.y= 7.9031 x+ 0.0549合150R2 = 09999150- :100100-Concentration of sulfte (umol/L)B5CDBlankAB204060|I中国煤化工Time (min)Fig. 2. Typical signal output and calibration curve of seawater spiked at 0.1-20 umol/LMYHCNMH G,:I/L:(C) 1 umo/L;(D)2 μmol/L; (E) 5 pumol/L; (F) 10 μmol/L; (G) 20 μmol/L.L.Q. Yin et al./Chinese Chemical Leters 21 (2010) 1457 -14611461Table 2Comparison of analytical results of the proposed method and a reference method [10].SampleCp土S.D. (n =4, μmol/L)Calculated 1-valueCritical t-value (P = 0.95)Proposed methodReference method10.56士0.1610.61土2.420.02.4574515.05士0.1315.30士1.290323. ConclusionA sensitive, fast and reliable method was developed for the determination of SO3- in fresh water and seawatersamples. It showed no significant difference with the classical pararosaniline spectrophotometry method. This methodwas suitable for the analysis of SO3- in acid rain, the evaluation of fue gas desulfurization and aeration efficiencythrough SO3- determination, as well as the analysis of other water samples with complicated matrix.References[1] M. Kass, A. Ivaska, Anal. Chem. Acta 449 (2001) 189.[2] M.A. Segundo, A.O.S.S. Rangel, Anal. Chim. Acta 427 (2001) 279.[3] M. Roth, Anal. Chem.43 (1971) 880.[4] G. Zhang, PK. Dasgupta, Anal. Chem. 61 (1989) 408.[5] N. Amornthammarong, JZ. Zhang, Anal. Chem. 80 (2008) 1019.[6] R.J. Watson, E.C.V. Butler, L.A. Clementson, et al. J. Environ. Monit. 7 (2005) 37.[7] N. Amornthammarong, J. Jakmunee, J. Li, et al. Anal. Chem. 78 (2006) 1890.[8] M.C. Roach, M.D. Harmony, Anal. Chem. 59 (1987) 411.[9] PID. Tzanavaras, E. Thiakouli, D.G. Themelis, Talanta 77 (2009) 1614.I 10] Ministry of Environmental Protection, PRC, HJ 482-2009.中国煤化工MYHCNMH G .