Environmental Laboratory Exercise Analysis of Hydrogen Peroxide by Fluorescence Spectroscopy Judith Weinstein-Lloyd Chemistry/Physics Program, SUNYICollege at Old Westbuly, Old Westbuly, NY 11568
Jai H. Lee Environmental Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973 a s 03, NOZ, HN03, and HzOz are easier to measure than their highly reactive free-radical precursors. Such measurements in conjunction with model calculations provide information about free radical concentrations. The effect of acidic precipitation on streams and lakes in the Northeastern United States and Canada was welldocumented during the 1980's (8, 9), as were the consequences to building materials and the ecosystem (9-11). It has been demonstrated that precipitation in these areas contains sulfuric acid, the oxidation product of SOz generated during the combustion of sulfur-rich coal. The principal reactions involved in acid rain formation are illustrated in Figure 1.Although gas-phase oxidation of SOz by OH. radical to acid formation, aqueous.phase oxidation pathways are believed to he more important, as illustrated by the increase sOzoxidation rates in power plant plumes that impact fog banks (12,13). , principal aqueous-phase oxidants for soz are H ~ o ~03, and, in the presence of transition metal ions, oz,with HzOz playing the most important role in the Northeast (14). Its large Henrys Law coeff~cient,7.4 x lo4 MIatm, ensures substantial partitioning of Hz02 into the aqueous phase (15). Field studies have demonstrated that cloud droplets typically contain either dissolved SOz or HzOz, but not both, suggesting t h a t t h e reaction between them goes to completion during a typical cloud lifetime (161, and laboratory studies have elucidated the mechanism of t h e H20z-S(IV) reaction, which exhibits pH-dependent kinetics (17,181. Because HzOz may be the limiting reagent in the vicinity of SOz emitters, predicting the extent of SOz oxidation requires accurate determination of Hz02 in the atmosphere (19). Recent evidence has emerged suggesting that organic peroxides are present in the atmosphere and also play a role in S(IV) oxidation (20,211. I n this e x ~ e r i m e n t .students analyze precipitation samples for trace concentrations of Hz02 using a newly developed fluorescence technique (22). / I Analysis is based on the ~roductionof OH.-radical by the reaction between HzOz and ferrous ion (Fenton's reSulfate agent) with subsequent radical scavaerosol Dewitiw enging by benzoic acid:
Gaseous hydrogen peroxide is found in parts-per-billion by volume concentrations in clean and polluted atmospheres. Studies show that HzOz and organic peroxides are deleterious to plant tissues (I, 2), and may play a part in forest decline in Europe (3).A number of analytical methhave heen developed for determining atmospheric peroxides; these methods and their applications in the field are the subject of two recent reviews (4,5). ~ ~ofhighiconcentrations ~ ~ ofozone,dhydroear~ons, ~ ~ that occur in ur. particulate matter and oxides of ban areas during the summer, sometimes referred to a s 'cLosAngeles smog", are complex chemical events initiated by sunlight and involving the free radicals OH* and HOz* a s intermediates. Figure 1illustrates the key chemical reactions involved in the production of photochemical air pollution. In spite ofthe central role played by free radicals, low ambient concentration makes their measurement in the field diff~cult.One of the ways we verify our understanding of photochemical air pollution is by comparing field measurements to the results of computer models (e.g., 6,7) that predict the time evolution of key chemical species under specified meteorological conditions. Products such
Nlrate aemwl
Deposition
',
Fe(I1)+ H,O,
+ OH. + OH- + Fe(II1) (1)
Figure 1. Principal reactions of gas-phase species implicated in the formation of acid rain and photochemical pollution. OH.
(2)
+ C,H,COOH + C,H,(OH)COOH.
Volume 72 Number 11 November 1995
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I n the p n w n c e of02 and dissolved iron, the cyclohrxadimyl radical formed in re;ictlon 2 produces fluorescent hydrox\.benzox isomers 12.71. The substantial ~~~~" ~ ~ acid - - OIIBAI ~ increase i n fluoreicence intensity of OHBA upon comnlexation with Al(II1) ion is ex~loitedto increase sensitivity (24). Unlike standard sp~ctroscopicand titrimetric techniques, Fenton-OHBAanalysis is sensitive enough for the determination of ambient peroxide i n precipitation, which typically varies from to 104M. And, in contrast to a more widely used, highly sensitive enzyme-based method (251, the reagents are inexpensive and easily available, and reagent solutions are stable, making this experiment well-suited for the undergraduate laboratory. ~
~
Experimental procedure' Ramwater samplei; are collected a s closc to the time of the srheduled (!xperiment as posiible, and pH measured. To account for matrix effects, samples are spiked with small nliquots ofa known hydrogen peroxide standard and a~ n~n l"-v ~ standard additions methods - e ~~ -ronventional -~dbv ~" ~ ~ (26). For good precision, the instructor should make a preliminary determination of the Hz02 concentration and use a peroxide standard which, when diluted, will approximate the unknown concentration. Although HzOz can be purchased in assayed 3% solution from Mallinkrodt (St. Louis. MO). the concentration eraduallv declines, so i t is advisable to titrate the 3% solition against standardized KMn04. We have found that HzOz keeps its titer for several months when refrigerated and protected from contamination. If rainwater collection is impractical, a n HzOz unknown can be prepared by the instructor. A multichannel peristaltic pump is used to deliver the product of reaction between simple, fluorescence reagent (iron(I1) sulfate and benzoic acid) and enhancing reagent (Al(II1) i n acetate buffer) to a fluorometer flow-through cell, as illustrated in Figure 2. The reaction coils i n the figure are lengths of Teflon tubing that introduce a twominute delay to allow the reactions to proceed substantially toward completion. If a pump i s unavailable, equal volumes of the sample and fluorescence reagent can be mixed, and a third equal aliquot of enhancing reagent 'Adetaiied experimental procedure is available from J. W: L. upon request. RECORDER
added. The reaction with fluorescence reagent should be timed for three minutes; the AI(II1) complexation will be complete in 1 5 min. Maximum fluorescence intensity of the hydroxybenzoic acid-aluminum complex under t h e experimental conditions described above is obtained with hex=305 and L,=410 nm (10). The low peroxide concentrations determined in this experiment call for careful analytical work and attention to detail. Afew precautions are noted below Many common impurities,including detergents, fluoresce. It is imoartant to clean and rinse thorouehlv all dassware that will came into contact with peroxide samples. Rinsing with nitric or hydrofluoric acid may be necessary to remove traces of organic impurities. Background fluorescence can be minimized by using freshly prepared reagents that have not absorbed HzOzfrom the air, and protecting solutions from light. Students should be cautioned to prepare all reagents from the same water for consistent results. Rainwater should not have im~actedtrees or other obiects are minimized. Safety Considerations Safety glasses and protective clothing should he worn for the proced;re. Concentrated sulfuric and acetic acids, required far reagent preparation, must he used with caution. .Cancentrated nitric or hvdrafluorie acid. which mav be
Results A typical graph of fluorescence intensity versus volume of added standard HzOz is shown in Figure 3. For this rain sample, the concentration of HzOz, determined from the ratio of intercept to slope, a s described in this Journal (261, is 1.2 x 10" M. On rare occasions, rain samples show no measuratde H202 because it has heen rirrim>dby :in t!~c(~.;s of dissolved S02. When this occurs, cahbrauon cJrves may exhibit nonlinearity due to reaction between SOz a n d added HzOz. I t should be noted that the Fenton-OHBA method shows limited sensitivity to some organic perox-
FLUOROMETER
ENHANCING REAGENT
-
FLUORESCENCE REAGENT PEROXIDE WPLE
PUMP
Figure 2. Experimental setup for H202 measurement. Fluorescence reagent contains 0.8 mM FeSO, and 2.0 mM benzoic acid in pH I .9 Sulfuricacid; enhancing reagent contains 6.0 mM AI(NO,), in 0.10 M acetate buffer, pH 3.8. Peroxide sample preparation is described in text.
1054
Journal of Chemical Education
mL H202 ADDED
Figure 3. Results of standard addition analysis of H 0 in recipita2 3M) added to tion. Abscissa shows milliliters standard H202 (4.5x 1025 mL samples
ides in addition to HzOz. However, its high solubility and greater stability under ambient conditions make HzOz the predominant peroxide observed in precipitation. Conclusion This experiment is appropriate for an upper-level instrumental analysis or integrated laboratory course. Although e. Drocedure usi t can be conducted a s a s i m.~ l auantitative ing commercial HzOz unknowns, the experience can be broadened bv havine students collect and handle authentic environmental samples, examine the excitation and emission s ~ e c t r u mof the fluorescent product, or analyze samples for S ( N ) and other inorganic ions. Blank solutions may exhibit substantial background t h e use of the most sensitive fluorescence, scales on some fluorescence detectors. This, coupled with low :lmlicmt peroxide conrentratitmi may challenge instrument detertion limits in this experiment Unlike mo?t ronventionnl laboratory exercises that specificall? are designed to avoid such conditions, this rxp,!rimt!nt gn.es studenti wluahls prnctlcal experience with sonic of the otherwile iibstract concepts ule teach; i.e, nose, detection limit ;ind scllsitivlty. Rerause hydrogen peroxide is ubiquitous in the atmosphere and highly soluble, the experiment also illustrates tht. critical imDonance o f hlank mrnsurernents. The Fenton method has been used recently to monitor peroxide concentrations i n air, cloudwater, precipitation and surface waters in a number of field missions (27,281. Our students have carried out this experiment for several years with good results. They are enthusiastic about collecting and analyzing authentic precipitation samples. The experiment also serves a s a point of departure for discussions on environmental chemistry and environmental regulation. The 1990 Clean Air Act mandated a 10-million ton per year reduction in SOz emissions from 1980 levels through improved scrubber technology and the use of SOz emissions trading allowances, a market-based approach toward environmental imnrovement. Each emitter is Drovided with a number of emission allowances that can be used to emit SOz or can be sold to other emitters by plants that successfully reduce their own emissions. Both HzOz and SOz are required for the production of acid rain, as illustrated i n Figure 1, but peroxide must be present i n excess if the mandated SOz reductions are to have the desired effect. Chemistry students who understand the conc e ~of t the limiting reagent know that the total amount of 4 ) be decreased by lowering re(e.g., ~ 2 ~ 6 cannot actant concentration (e.g., SOz) if HzOz is a limiting reagent. Details such as this illustrate the role of careful scientific work in formulating public policy (29).
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8. "Interim Assessment: The Causes and Effects of Aeid Deposition:' National Acid F'mipitationhsessment Prprpam. Government Printing Office: Washmgton. DC, 1987. 9. Hicks. B. B.. Ed. Drpoailion. Both We1 ond Dry: Aeid Precipitation Series Volume 4, Butteworth: MA. 198C lo. Tomanson, G. H E N a f f f A d d Depmi,ii" in I h h F " ~ s t 1 " r E " m p ~ ~ " d N ~ ~ t h A m e ~ . i m : Lerner Publications: MN. 1990. 11. Allen. R. Allen. C. R.: Sherwood. S. Today's Chem~afo i Work 1993.2.28-38. 12. Eat0ugh.D. J;Arthurn.R. L.;Etough, N. L.; Hill. M. W.;Mangelsan. N. E:Richter, B. E.: Hansen, L. DEnviron. Sei. Technol. 1984.18.855-859. 13. Bizjak,M.;Benner W.H.:Hanren.A.D.A.:Hrcek.D.:Hudnik,Y:Novakov,TAtmos. Enuimn. 1988,22.2861-2862. 14. Schwartz. S. E. In Sol, NO and NO2 Ozidorion Mpehonisms:Atmospherk Considerations; Calvert. J . G., Ed.. Butterworth: MA. 1984. 15. Lind. J. A : Kok, G. L. J. Oeoph.ys. ROS.1886.91.7889-7895. 16. Daum. P. H.: Schwartr, S. E.; Kellx T J.: Newman. L. Alrnos. Enuimn. 1984. 18.
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