Bacsik, Zoltán

Balogh, Nóra

Mink, János

English

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Indirect Determination of Molecular Chlorine by FourierTransform Infrared SpectrometryZOLTA´N BACSIK,* NO´RA BALOGH, and JA´NOS MINKInstitute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u´t 59–67, H-1025 Budapest, Hungary(Z.B., J.M.); and Research Institute of Chemical and Process Engineering, University of Pannonia,Egyetem u. 10, H-8200 Veszpre´m, Hungary(N.B., J.M.)Index Headings: Fourier transform infrared spectroscopy; FT-IR spec-troscopy; Gas analysis; Chlorine gas.INTRODUCTIONNowadays, Fourier transform infrared (FT-IR) spectrometry(both extractive and open-path techniques) is widely used forthe detection of air pollutants1–6 because the method is fast,reliable, and nondestructive. It is capable of detectingsimultaneously all compounds except non-infrared activehomonuclear diatomic molecules.Molecular chlorine gas has many anthropogenic sources,including water treatment plants, chemical production facili-ties, ceramic brick industries, and paper production process-es,7–9 and it can also be formed in marine environments byreactions with sea salt particles.10–13 Various sensors have beenapplied to detect chlorine, including chemical sensors based onsemiconductor metal oxides.14 Mass spectrometry also can beused for the determination of halogen molecules at atmosphericpressure.10,15 A mist chamber and ion chromatographicdetection has been employed to detect molecular chlorineindirectly at the parts per trillion (ppt) level.16 Spectroscopictechniques (e.g., ultraviolet (UV), Raman) are also capable ofmeasuring molecular chlorine directly. An artificial chlorinecloud was investigated by Edner et al.17 by the differentialabsorption LIDAR (light detection and ranging) method.Fourier transform infrared spectrometry has already beenused to detect a homonuclear diatomic molecule. Larsen andco-workers18 measured the concentration of oxygen in a gascell by determining the concentration of ozone produced uponUV irradiation of the sample. The authors established a linearcorrelation between the concentration of oxygen in the originalsample and the concentration of ozone produced by photolysisof the sample. The aim of the present work was to investigatewhether it is possible to detect chlorine molecules by means ofFT-IR spectrometry as predicted by Hanst.19 If successful, anew analytical method can perhaps be developed based on thisinvestigation.EXPERIMENTALA Bio-Rad FTS-185 series spectrometer and a multipass gascell (Infrared Analysis Inc., Anaheim, CA) with a 25.7-meffective path length were used for the experiments. Thespecial cell was equipped with a mercury UV-radiation source,whereby ozone and reactive radicals can be produced. Thesource of the radiation was a medium-pressure quartz mercurylamp. This type of lamp emits one or two percent of itsradiation in the 185 nm mercury line and about 90% in the 254nm line, with the remainder in lines that fall in the near-UV andvisible ranges. The 185 nm photons dissociate oxygenmolecules and the resultant oxygen atoms react mainly withmolecular oxygen to form ozone. The ozone can react withother molecules, or it can absorb the 254 nm photons and bephoto-dissociated, giving oxygen atoms in the energeticsinglet-D state. These energetic atoms react with water toproduce hydroxyl radicals. The ultraviolet photons can alsodirectly photo-dissociate pollutant molecules.19The concentration of molecular chlorine gas was measuredwith a wide-range vacuum gauge (Kurt Lesker Co,KJL902074) connected to a vacuum system. Followingintroduction of a known quantity of chlorine gas and normalair (according to the analysis of the infrared spectra, this aircontained 6650 ppm water vapor, 333 ppm carbon dioxide,1.77 ppm methane, 0.32 ppm nitrous oxide, and 0.30 ppmcarbon monoxide) at atmospheric pressure into the evacuatedgas cell, the UV lamp was switched on after recording the firstsingle beam spectrum. Next, a kinetic study was carried outduring the photolysis: a spectrum was recorded every 15 s.RESULTS AND DISCUSSIONSince there are many possible reactions (e.g., Eqs. 1–5)beyond ozone and reactive radical formation under theinfluence of the UV lamp, it is not practical to try to explaineverything that could occur in the gas cell. In the course of thereactions, chlorine will be produced in atomic form and serveas a catalyst for ozone depletion. Some possible reactions aregiven here:20,21CLþ O3 ! ClOþ O2 ð1ÞOHþ O3 ! HO2 þ O2 ð2ÞClOþ HO2 ! HOClþ O2 ð3ÞHOClþ hv ! OHþ Cl ð4ÞNet O3 þ O3 ! O2 þ O2 þ O2 ð5ÞThe important point is that a band had to be identified thatbelongs to a molecule produced from the reaction of ozone orreactive radicals with molecular chlorine present in the samplegas. It was expected that the analytical band would bedeveloped in a short time with an appropriate intensity andwould not be overlapped with other bands in the spectra. Froma preliminary investigation and bearing in mind the possibleReceived 19 June 2007; accepted 7 January 2008.* Author to whom correspondence should be sent. E-mail: bacsik@chemres.hu.Volume 62, Number 3, 2008 APPLIED SPECTROSCOPY0003-7028/08/6203-000000$2.00/0 2008 Society for Applied Spectroscopyreactions, the deformation mode of hypochlorous acid (HOCl)at 1239 cm1 should be suitable for this purpose (Fig. 1).A reference spectrum of HOCl was constructed from anabsorbance spectrum of the sample, cutting out the spectralrange of 1080–1320 cm1. The shape of the HOCl band in thisreference spectrum was checked by an artificial referencespectrum of HOCl made with the aid of software22 using datafrom the HITRAN23 database. The HOCl reference spectrumwas subtracted from all the spectra measured during thephotolysis and subtraction factors were plotted as a function oftime. Linear trends were fitted to the first five points (Fig. 2).Sample spectra were recorded three times (three independentmeasurements of the chlorine concentrations with the vacuumgauge) for each point in the calibration. The measurementswere fairly reproducible (see error bars in Fig. 3).Plotting the calibration chlorine concentrations as a functionof the gradients of the lines of HOCl concentration changes(the factual concentrations of HOCl were not determined, butthe subtraction factors are directly proportional to theconcentrations according to the Lambert–Beer law in thisabsorbance range), a linear relationship (Fig. 3) was establishedfor the concentration range investigated (Eq. 6).cchlorine ¼ 1190:7b 3:248; ðR2 ¼ 0:989Þ ð6Þwhere cchlorine is concentration of chlorine (ppmv) and b is thegradient of the HOCl concentration change.CONCLUSIONBy monitoring the concentration of HOCl, clear evidencewas obtained for a correspondence with the original chlorineconcentration in the gas cell. According to our investigation,unknown chlorine concentrations can be determined from thegradient of concentration change of the HOCl produced duringthe course of the photolysis reaction in the range of about 3–18ppm, with an acceptable level of relative error (Table I). It hasbeen shown that analysis of molecular chlorine by means ofFT-IR spectrometry is therefore theoretically possible.Our future work will be focused on an investigation of theapplicability of the method over a wider concentration rangeand we will work out the exact chemometrics for measurementof molecular chlorine with analytical accuracy. It will also beimportant to investigate the influence of water (as a possiblesource of hydrogen for HOCl) and other compounds present inthe sample gas and produced during the photolysis on theoriginal chlorine concentration.24–27ACKNOWLEDGMENTSThe authors thank Ian S. Butler (McGill University, Montreal, Canada) forhis assistance in the preparation of this paper.1. G. M. Russwurm and J. W. Childers, ‘‘Open-path Fourier TransformInfrared Spectroscopy’’, in Handbook of Vibrational Spectroscopy, J. M.Chalmers and P. R. Griffiths, Eds. (John Wiley and Sons, New York,2002), vol. 2, pp. 1750–1773.2. R. L. Spellicy and J. D. Webb, ‘‘Atmospheric Monitoring Using ExtractiveFIG. 1. Quantitative determination of hypochlorous acid. (a) Samplespectrum, (b) HOCl reference, and (c) sample spectrum after subtraction ofHOCl reference.FIG. 2. Changes in HOCl subtraction factors (concentrations) during the first75 s of the photolysis at different chlorine calibration concentrations.FIG. 3. Calibration chlorine concentrations as a function of the gradients of thelines of HOCl concentration changes. The error bars represent the minimal andmaximal gradients determined from HOCl concentrations measured during thethree independent experiments.TABLE I. Calculated and measured (with the vacuum gauge) chlorineconcentrations and the error of the developed method.Gradient ofthe HOClconcentrationchangeChlorineconcentrationdetermined fromEq. 6 (ppmv)Measuredchlorineconcentration(ppmv) DifferenceRelativeerror (%)0.0051 2.83 2.99 0.16 5.350.0079 6.16 5.98 0.18 3.010.0100 8.66 8.97 0.31 3.450.0137 13.06 11.96 1.10 9.190.0148 14.38 14.95 0.57 3.810.0176 17.71 17.94 0.23 1.28Volume 62, Number 3, 2008Techniques’’, in Handbook of Vibrational Spectroscopy, J. M. Chalmersand P. R. Griffiths, Eds. (John Wiley and Sons, New York, 2002), vol. 2,pp. 1721–1749.3. D. W. T. Griffith and I. M. Jamie, ‘‘Fourier Transform InfraredSpectrometry in Atmospheric and Trace Gas Analysis’’, in Encyclopediaof Analytical Chemistry, R. A. Meyers, Ed. (John Wiley and Sons,Chichester, 2000), pp. 1979–2007.4. Z. Bacsik, J. Mink, and G. Keresztury, Appl. Spectrosc. Rev. 39, 295(2004).5. Z. Bacsik, J. Mink, and G. Keresztury, Appl. Spectrosc. Rev. 40, 327(2005).6. Z. 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Indirect Determination of Molecular Chlorine by Fourier Transform Infrared Spectrometry

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