J. Phys. Chem. 1996, 100, 3241-3247
3241
Henry’s Law Constant Determinations for Hydrogen Peroxide, Methyl Hydroperoxide, Hydroxymethyl Hydroperoxide, Ethyl Hydroperoxide, and Peroxyacetic Acid Daniel W. O’Sullivan* Rosenstiel School of Marine and Atmospheric Sciences, Marine and Atmospheric Chemistry DiVision, UniVersity of Miami, Miami, Florida 33149
Meehye Lee, Birgitta C. Noone, and Brian G. Heikes Graduate School of Oceanography, Center for Atmospheric Chemistry, UniVersity of Rhode Island, Narragansett, Rhode Island 02882 ReceiVed: April 20, 1995; In Final Form: NoVember 13, 1995X
The temperature-dependent solubilities of hydrogen peroxide (H2O2), hydroxymethyl hydroperoxide (HOCH2O2H), methyl hydroperoxide (CH3O2H), peroxyacetic acid (CH3C(O)O2H), and ethyl hydroperoxide (C2H5O2H) were determined under conditions used in the aqueous collection and analysis of atmospheric hydroperoxides. Henry’s law was obeyed over the source concentration range employed, nominally 10-610-2 M. Measurements were made using either pH ) 3 or pH ) 6 source and collection solutions. The temperatures investigated ranged from 4 to 28 °C. A solution pH 3 was used for experiments with H2O2, HOCH2O2H, and CH3C(O)O2H since these compounds decompose in less acidic solution. The solubility of HOC2H4O2H could not be accurately determined because of its rapid decomposition in solutions with pH > 3. The Henry’s law solubility of H2O2, HOCH2O2H, CH3O2H, and CH3C(O)O2H are in agreement with prior determinations. The solubility measurement of C2H5O2H is the first of its kind. Dimensional Henry’s law constants (M/atm) can be expressed by ln(Kh) ) A/T - B, where T is in degrees kelvin. Kh at 25 °C and the A and B coefficients are compound H2O2 CH3O2H HOCH2O2H CH3C(O)O2H C2H5O2H
A 7379 ( 156 5241 ( 133 9652 ( 53 5308 ( 672 5995 ( 200
B 13.42 ( 0.54 11.84 ( 0.46 18.04 ( 0.18 11.07 ( 2.34 14.28 ( 0.70
Kh (M atm-1) 8.33 × 104 ( 3.8 × 103 311 ( 14 1.67 × 106 ( 3.5 × 105 837 ( 175 336 ( 20
The high solubility of HOCH2O2H implies it will be efficiently removed from the atmosphere by precipitation or surface deposition. Its decomposition and that of HOC2H4O2H and CH3C(O)O2H forming H2O2 near neutral pH suggest these hydroperoxides may constitute a heterogeneous source of H2O2 in atmospheric water if they are formed in the troposphere. The solubility of the listed organic hydroperoxides and the propensity of three of them to decompose at neutral pH further underscores the potential for interference’s and artifacts in the aqueous collection and nonspecific analysis of H2O2 in the atmosphere.
Introduction Hydrogen peroxide (HP) and other organic hydroperoxides, e.g., methyl hydroperoxide (MHP), hydroxymethyl hydroperoxide (HMHP), ethyl hydroperoxide (EHP), and peroxyacetic acid (PAA), are intimately involved in the odd-hydrogen and odd-oxygen cycles of the atmosphere.1,2 HP and MHP are recognized reservoirs for these radicals in the air. The removal of HP and MHP by reaction with hydroxyl (HO•) or by deposition are the principal sinks of the above radicals.1 Hydroperoxides are involved in the conversion of sulfurous, nitrogenous, and carbonaceous compounds to their acidic forms in atmospheric water.3-6 Furthermore, hydroperoxides are proposed plant toxins,7-9 although their efficacy in this regard is under debate.10 The atmospheric chemistry of hydroperoxides, beyond HP and MHP, is somewhat speculative.11-13 The gas-phase kinetics of HP and MHP formation and destruction have been studied (e.g., Atkinson et al.14 and references therein), and their atmospheric concentration appears to be in agreement with X
Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3241$12.00/0
photochemical models when the effects of rapid transport or heterogeneous processes are negligible.15-17 The simpler alkyl hydroperoxides are believed to be produced by the reaction of hydroperoxyl with alkyl peroxyl radicals and lost by photolysis and reaction with HO.11,18,19 The formation of the hydroxyalkyl hydroperoxides is less certain but appears to involve reactions between ozone and an alkene and is facilitated by the presence of water.5,6,9 Gas-phase destruction mechanisms have not been suggested for the latter. In the lower troposphere, heterogeneous processes are comparable to gas-phase mechanisms in the loss of hydroperoxides. The aqueous destruction and deposition of the hydroperoxides is dependent upon their uptake by wet aerosol, cloud, precipitation, or particulate surfaces. The uptake of atmospheric species in general is limited by the volume of atmospheric water present, its composition, and the solubility of the atmospheric constituent in aqueous solution.20 Weslie et al.21 estimate deposition rates to terrestrial surfaces based upon solubility and the same is done in estimating surface deposition rates to sea water.22 The Henry’s law solubility of HP has been determined by several groups23-28 (Table 1), and Hoffmann et al.29 have © 1996 American Chemical Society
3242 J. Phys. Chem., Vol. 100, No. 8, 1996
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TABLE 1: Determinations of the Temperature Dependence of Kh for Various Hydroperoxides and Their Thermodynamic Parametersa HP
MHP HMHP PAA EHP a
author
A
B
∆H° (kJ/mol)
∆G° (kJ)
ln Kh (25 °C)
Schumb (1955) Hwang and Dasqupta (1985) Lind and Kok (1986, 1994) Staffelbach and Kok (1993) National Bureau of Standards this work Lind and Kok (1986, 1994) this work Staffelbach and Kok (1993) this work Lind and Kok (1986, 1994) this work this work
6990 7920 6338 7514
12.27 15.44 9.74 13.63
7379 5322 5241 10240 9652 5896 5308 5994
13.42 12.14 11.84 20.03 18.04 13.28 11.07 14.28
58.1 65.8 52.7 62.5 54.9 61.3 44.2 43.6 85.1 80.2 49.0 44.1 49.8
-27.7 -27.6 -28.6 -28.7 -28.4 -28.1 -14.2 -14.2 -35.5 -35.5 -16.1 -16.7 -14.4
11.17 11.12 11.52 11.57 11.46 11.33 5.71 5.74 14.32 14.33 6.50 6.73 5.82
The thermodynamic parameters are calculated at 298.15 K, a pressure of 1 atm, and ionic strength of 0.001 M.
recommended a preferred value. Determinations of organic hydroperoxide aqueous solubilities are fewer in number (Table 1). Lind and Kok27,30 determined the Henry’s law solubility of MHP and PAA as a function of temperature. Staffelbach and Kok28 and Zhou and Lee31 determined the Henry’s law solubility of HMHP as a function of temperature and pH. Their respective determinations vary by over an order of magnitude depending upon solution pH. Clearly, additional measurements of hydroperoxide solubilities are needed in order to improve our understanding of the behavior of these compounds in the atmosphere. In this paper, we present Henry’s law solubility determinations for HP, MHP, HMHP, and PAA as a function of temperature. Comparisons are made between our determinations and those from earlier works. We also present the first determination of the Henry’s law constant as a function of temperature for EHP, and an evaluation of the difficulties encountered in the determination of the Henry’s law constant for 1-hydroxyethyl hydroperoxide. The implications of our results to the partitioning of atmospheric hydroperoxides and atmospheric chemistry are then briefly discussed. Experimental Section A. Methods. Compound Synthesis and Standardization. HP (30%, J. T. Baker Chemical Co.) and PAA (32%, Aldrich Chemical Co.) were used as received. MHP was synthesized from HP and dimethyl sulfate (Eastman Kodak, Rochester, NY) in the presence of potassium hydroxide using similar methods as Kok et al.33 and Hitz.40 The nucleophilic addition of HO2anion at the methyl group on dimethyl sulfate in the presence of strong base was employed to synthesize MHP. Dimethyl sulfate (10 mmol) was mixed with 20 mmol of 30% HP in a flask immersed in an ice bath; while this was stirred, 20 mmol of 40% KOH were added. The reaction was initiated by slowly heating the mixture to 60 °C. The mixture was kept at 60 °C until bubble formation stopped. During the reaction N2 was passed through the solution at 5 mL/min; this flow was slowly increased to 500 mL/min near the reaction end point. The N2 gas was passed through 50 mL of 18 MΩ Milli-Q (MQ) water in a West-Gaeke bubbler immersed in an ice bath to capture MHP. This procedure separates MHP from HP as a result of the large difference in their Henry’s law constants. EHP was synthesized with the same procedure by replacing dimethyl sulfate with diethyl sulfate.32 HMHP was synthesized by the reaction of HP with paraformaldehyde (ICN Biochemicals) at elevated pH.33,40 Paraformaldehyde solution (500 mL, 9.3 × 10-3 M) was mixed with 500 mL of a 0.01 M HP solution at room temperature. The pH was adjusted to 10 by slowly adding 1 mL of a 0.1 M NaOH
solution. The reaction was allowed to proceed for 1 h, after which the pH was reduced to 3 by the addition of 0.5 mL of 1 M H2SO4. The resulting solution contains a mixture of bis(hydroxymethyl) peroxide (BMHP) and HMHP. No further separations were performed.33,34 The synthesized organic hydroperoxides are stable for nearly a year if stored in brown borosilicate glass bottles and refrigerated at 4 °C. HMHP decomposes faster than the other organic hydroperoxides. A stock solution of HMHP of 2-3 mM is stable for 3-4 months.32 Since HMHP’s half-life at pH 6 is 33 min and about 23 days at pH 3, it was synthesized before use. The hydroperoxide stock solutions were standardized by KMnO4 and Na2S2O3/KI titrations.35 Analytical Method. Gas-phase hydroperoxides were collected with a continuous-flow glass scrubbing coil and quantified using an HPLC fluorometric method.32 Hydroperoxides were separated by reverse phase HPLC on a 5 µm Inertsil ODS-2 PEEK column (4.6 mm × 250 mm, Metachem) followed by a postcolumn derivatization reaction forming a fluorescent dimmer. The peroxidase catalyzed dimerization of p-hydroxyphenyl acetic acid occurs in the presence of peroxy functional groups at elevated pH. Formation of the fluorescent dimmer is proportional to the concentration of a given hydroperoxide, and resulted in a precision less than 3% for analysis of the hydroperoxides in this study. The detection limit, defined as three times the standard deviation of the blank, for the individual hydroperoxides in aqueous concentrations are 1.2 × 10-9 M for HP, 1.5 × 10-9 M for HMHP, 2.9 × 10-9 M for MHP, 16 × 10-9 M for PAA, and 19 × 10-9 M for EHP. Corresponding gas-phase detection limits are 5, 7, 13, 72, and 84 pptv, respectively, with an air sample flow rate of 2 slpm (standard liters per minute) and a collection solution flow rate of 4 × 10-4 L/min. The gas-phase detection limits for the latter three hydroperoxides depend on the collection efficiency and will vary with temperature, pressure, air sample flow rate, and collection solution flow rate.34 B. Apparatus. The apparatus used to determine the Henry’s law constants for the hydroperoxides was composed of a hydroperoxide gas-phase generator, a collection system for the gas-phase hydroperoxides, and the HPLC technique described above for quantifying the individual hydroperoxides (Figure 1). Zero air was passed through a hopcalite trap (Mine Safety Apliance) and through a mass flow controller (MKS Instrument Corp.) to the hydroperoxide generator. A modified glass collection coil was employed as the generator.36 The air stream flowed through the coil as a solution of known hydroperoxide concentration was continuously pumped through the coil and to waste. The glass collection coil consisted of two parallel coil columns connected at the base (Figure 1). The gas stream
Henry’s Law Constant Determinations for Peroxides
Figure 1. Diagram of the apparatus used to perform the Henry’s law determinations for different hydroperoxides. The portion enclosed in the shaded rectangle was thermostated to (0.05 °C. Zero air was used as the carrier stream. 1 standard liter per minute (slpm) via a mass flow controller (MFC) was passed through the generation coil simultaneously with the concentrated peroxide solution. The peroxide laden air exiting the generation coil was combined with 4 slpm zero air and passed through tandem collection coils A and B and then to waste. Simultaneously, collection solution was passed through the collection coils with a peristaltic pump and captured in Teflon vials. Aliquots of the captured solutions were analyzed by HPLC. The mass flow controllers, pressure, and temperature were monitored continuously with a personal computer.
entered at the top of the first coil column and passed a tee where the aqueous generation (collection) solution entered the coil. The gas stream forces the solution to form a thin film on the coil surface that migrates down the first coil column and up the second and into a V-shaped, cyclone gas-liquid separation chamber. Formation of a thin film on the surface of the glass coil promotes equilibrium between the gas and aqueous phases. The gas-phase flows out the top and the aqueous phase was drawn out the bottom of the cyclone separator using a peristaltic pump (Ismatec). The flow rates of all the gas lines were controlled by mass flow controllers (MKS Instrument Corp.) and calibrated with a digital flow meter (Gilibrator, Gilson Instruments). During all of the experiments, gas-flow rate, pressure, and coil temperature were continuously monitored and recorded on a personal computer. The temperatures in the cyclone separators were monitored with thermisters (Deban Enterprises, Inc.), and the pressure was determined with an absolute pressure transducer (MKS Instruments Corp.). The generation and collection coils and solutions were immersed in a constant temperature bath thermostated to (0.05 °C. C. Experimental Conditions. Reagents and Solutions. All solutions were prepared with 18 MΩ Milli-Q (MQ) water and ACS reagent grade chemicals unless otherwise noted. Plasticware and glassware were thoroughly cleaned with laboratory detergent and rinsed with copious amounts of MQ water, since the presence of trace metals promotes the decomposition of hydroperoxides. The HPLC mobile phase was 1 × 10-3 M H2SO4 with 1 × 10-4 M EDTA (ethylenediaminetetraacetic acid) to complex trace metals. The fluorescence reagent was prepared in 0.42 M KHP (potassium hydrogen phthalate) buffer adjusted to pH 5.8 with 1 M NaOH, and was 2.2 × 10-2 M POPHA (p-hydroxyphenylacetic acid, Fairfield Chemical Co.) and 8.3 × 104 units/L horseradish peroxidase (type II, Sigma Chemical Co.). Collection solutions of pH 3.0 were prepared by incremental additions of 1 M HCl to 1 L solutions of 5 × 10-3 M KHP. Collection solutions of pH 6.0 were prepared by incremental additions of 1 M NaOH to 1 L solutions of 5 ×
J. Phys. Chem., Vol. 100, No. 8, 1996 3243 10-3 M KHP. The pH was determined on the NBS scale with an Orion pH meter and Ross combination electrode. Standard solutions of the hydroperoxides were prepared by volumetric serial dilutions of the stock solutions with MQ water. Synthesis and preparation of the stock hydroperoxide solutions are discussed above. HP, MHP, and EHP standards are stable for at least 24 h. PAA and HMHP decompose at neutral pH, and standards for these hydroperoxides were prepared in 1 × 10-3 M HCl. This procedure reduced the rate of decomposition of standards but did not eliminate decomposition. Consequently all standards were prepared immediately before use. Experimental Procedure. For each experimental run at a given temperature the coils and generation solution were allowed to come to thermal equilibrium with the water bath for several hours prior to the experiment. During experiments conducted near 24 °C, the room temperature was increased to 28 °C to prevent condensation from occurring in tubing lengths between glass coils that were not immersed in the thermal bath. Zero air was used as a dilution gas (4 slpm) and as the carrier gas in the gas-phase hydroperoxide generator (1 slpm). The flow through the collection coils was 3 slpm with the remaining gas vented to waste prior to the coils (Figure 1). Collection solution, zero air, and system blanks were determined with the addition line vented to waste. After changing the addition valve to the add position, the concentration of hydroperoxides in the collection solutions were determined at 2.5-5 min intervals. Data were collected as long as several hours after equilibrium was achieved. The equilibrium gas phase peroxide partial pressure in atmospheres was plotted versus the generation solution concentration at each temperature for the individual peroxides to determine the Henry’s law constant with eq 3. Linear relationships were observed for all the peroxides with the slope being equal to the Henry’s law constant. Within the uncertainty of our measurements, the resulting intercepts were not significantly different from zero for any of the peroxides examined in this study. Collection Efficiency. Extraction of gas-phase hydroperoxides depends on the solubility of the hydroperoxides as well as the collection conditions. During the collection process temperature, aqueous flow rate, and gas volume flow rate influence the collection efficiency. The collection efficiency (CE) of the continuous coil stripping system is described by the following equation:
CE ) VaKhRT/(Vg + VaKhRT)
(1)
where Va and Vg are the aqueous- and gas-phase volumetric flow rates in L/min, R is the ideal gas constant, and T is temperature in degrees kelvin. Since Kh for HP and HMHP is large, the collection efficiency for these hydroperoxides under the conditions of the experiment was >99%. The lower Henry’s law constants for the remaining hydroperoxides results in collection efficiencies that are less than 99% and that vary depending on temperature and volume flow rate. In addition to the above theoretical calculation of collection efficiency, CE can be determined experimentally by analyzing the hydroperoxide concentration in tandem stripping coils, where CE ) (1- [hydroperoxide ]coil2/[hydroperoxide ]coil1). Tandem collection coils were employed in this work to experimentally determine the CE for each hydroperoxide and to provide an additional method to calculate the Henry’s law constants for hydroperoxides that do not have collection efficiencies approaching 100% by solving eq 5 for Kh. Organic Peroxide Decomposition. Several of the organic hydroperoxides, HMHP and PAA, studied in this work are
3244 J. Phys. Chem., Vol. 100, No. 8, 1996
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TABLE 2: Experimental Results for the Determination of Kh (M atm-1) HP MHP
HMHP PAA
EHP
T (K)
ave
std dev
% uncertaintya
nb
278.15 291.15 297.15 277.15 283.15 291.15 297.15 278.15 291.15 297.15 278.15 283.15 291.15 297.15 278.15 283.15 291.15 297.15 278.15 283.15 291.15 297.15
4.98E+05 1.48E+05 9.20E+04 1.20E+03 7.70E+02 4.83E+02 3.31E+02 1.72E+07 3.62E+06 1.88E+06 3.32E+03 1.90E+03 1.25E+03 9.37E+02 1.45E+03 8.99E+02 5.72E+02 3.53E+02 1.53E+03 9.69E+02 5.52E+02 3.74E+02
1.48E+04 5.02E+03 1.52E+03 2.94E+01 5.70E+01 4.91E+01 1.11E+01 9.78E+05 3.99E+04 4.89E+04 1.42E+02 1.20E+02 2.45E+01 7.65E+01 4.36E+01 1.52E+01 1.42E+01 9.76E+00 1.55E+01 9.72E+00 5.95E+00 5.05E+00
0.01 0.19 0.29 1.45 3.23 2.58 0.52 2.57 0.17 0.75 1.84 2.80 1.25 5.15 0.99 0.01 1.85 1.08 0.47 0.56 0.19 0.32
35 32 22 7 15 32 20 13 10 11 12 14 6 10 12 14 6 10 15 21 10 16
a Percent uncertainty of the mean at the 95% confidence level was calculated using the standard deviation as the population σ, % uncertainty ) (tsn-1/2; the values for t were taken from statistical tables for the appropriate n value.42 b Number of determinations used to calculate the mean and standard deviation.
unstable and decompose at collection solution pH’s in excess of 5.27,35 PAA decomposes primarily by hydrolysis,27 and HMHP decomposes into HP and formaldehyde.35 The decomposition of these compounds is inhibited at low pHs and temperatures. Lind and Kok27 report that the complete hydrolysis of PAA requires 1-2 days. This is very long relative to the time required to complete an analysis in this work; consequently no corrections were applied to account for PAA hydrolysis. PAA has a pKa ) 8.2,41 deviations from Henry’s law as a result of dissociation at the collection solution pH of 6 would approach 0.63% which is well within our experimental uncertainty. The deviation from Henry’s law is insignificant with pH 3 collection solutions. The half-life for the decomposition of HMHP is 2.3 × 107[H+] (min). The Henry’s law determinations for HMHP were performed with pH 3 collection and generation solutions ensuring no loss of HMHP as a result of decomposition during the experiment.
Figure 2. Temperature dependence of the Henry’s law constant for H2O2. The solid line is the regression line and the dashed lines are the 95% confidence limits for our data. The symbols represent data from this study (b), Lind and Kok (1),27,30 Hwang and Dasqupta (2),26 Staffelback and Kok ([),28 and Schumb (9).24
Results All of the hydroperoxides examined in this work obeyed Henry’s law over the concentration range examined, nominally 1 µM to 5 mM. The Henry’s law constant for HP and HMHP were determined at 5, 18, and 24 °C and over the aqueous concentration ranges of 0.5-5 and 11-13 mM, respectively. This resulted in 89 separate determinations of Kh for HP and 34 determinations of Kh for HMHP. The results are presented in Table 2 and Figures 2 and 3. The Kh for MHP was determined at 4, 10, 18, and 24 °C resulting in 74 determinations of Kh over the concentration range 1.0-50 µM (Table 2, Figure 4). The Henry’s law constant for PAA and EHP were determined at 5, 10, 18, and 24 °C over the concentration ranges 7.8-348 and 7.8-87 µM, respectively. This resulted in 42 determinations of Kh for PAA and 62 determinations of Kh for EHP (Table 2 and Figures 5 and 6). The mean values of Kh for the individual hydroperoxides at each temperatures have been
Figure 3. Temperature dependence of the Henry’s law constant for MHP. The solid line is the regression line and the dashed lines are the 95% confidence limits for our data. The symbols represent data from this study (b) and Lind and Kok (9).27
fit to a Van’t Hoff equation form:
Kh ) e(A/TK-B)
(2)
where TK is the temperature in degrees kelvin, giving equal weight to data at each temperature. All of the fits had an r2 > 0.99 with the exception of PAA, r2 ) 0.96. The fitting coefficients for each peroxide are given in Table 3 with confidence limits.
Henry’s Law Constant Determinations for Peroxides
J. Phys. Chem., Vol. 100, No. 8, 1996 3245
Figure 4. Temperature dependence of the Henry’s law constant for PAA. The solid line is the regression line and the dashed lines are the 95% confidence limits for our data. The symbols represent the mean (b) and errors bars are the standard deviation from this study. The dash and dotted line are data from Lind and Kok (9).27
Figure 6. Temperature dependence of the Henry’s law constant for EHP. The solid line is the regression line and the dashed lines are the 95% confidence limits. The symbols represent the mean of replicate determinations and the errors bars are the standard deviation from this study using two HPLC systems equipped with C18 columns form different manufacturers; Column 1 (Metachem) (+), and Column 2 (Phenomenex) (0).
TABLE 3: Results for the Linear Fits of the Temperature and Kh Data compound HP MHP HMHP PAA EHP
Figure 5. Temperature dependence of the Henry’s law constant for HMHP. The solid line is the regression line, and the dashed lines are the 95% confidence limits for our data. The symbols represent the mean (b) and errors bars are the standard deviation from this study. The symbols represent data from Staffelback and Kok (3)28 and Zhou and Lee (9).31
Discussion The partitioning of a compound between a gas and liquid phase at equilibrium is described by Henry’s law, which relates the gas-phase partial pressure of a compound to its mole fraction in solution in the limit of zero concentration. Henry’s law is approximately valid for any solute in a dilute aqueous solution and is particularly applicable to the solubility of gases in liquids. Henry’s law can be applied to the solubility of hydroperoxides expressed as
[RO2H](aq) ) KhP(RO2H)
(3)
where [RO2H](aq) and P(RO2H) represent the aqueous concentration and partial pressure of a particular hydroperoxide, and Kh is the thermodynamic Henry’s law proportionality constant. Application of this equation with concentration and partial
B A B A B A B A B A
coefficients
std error
lower 95%
upper 95%
13.42 7379 11.84 5241 18.04 9652 11.07 5308 14.28 5994
0.54 156 0.46 133 0.18 53 2.34 672 0.70 200
20.30 5394 13.84 4667 20.38 8979 21.14 2418 15.99 5504
6.54 9364 9.84 5815 15.71 10326 1.01 8199 12.57 6485
pressure rather than with activity and fugacity assumes ideal behavior in both phases. An additional condition implicit in applying this equation is that the chemical form of the compound is identical in both phases. All of the hydroperoxides examined in this work obeyed Henry’s law over the concentration range examined, nominally 10-6-5 × 10-3 M. Accurate determinations of the Henry’s law constants for atmospheric hydroperoxides are necessary to fully evaluate the partitioning of hydroperoxides between gas and aqueous phases within the atmosphere. Since HP and organic hydroperoxides have been shown to be potentially important oxidants for S(IV) in atmospheric aqueous phases, accurately determining the Kh for these species will impact our ability to model sulfur oxidation in the atmosphere. Several determinations of the Kh of HP have been made prior to this study. The earliest was by Scatchard et al.,23 where the Henry’s law constant was determined from vapor pressure measurements over solution compositions from 5 to 1400 M HP and from 318 to 378 K. Martin and Damschen37 extrapolated these measurements to the temperature range 273-298 K, they indicate that the accuracy of the Henry’s law constant determined from this extrapolation is limited to about 40% as a result of the uncertainty in the vapor pressure of HP. Hwang and Dasgupta26 performed the first studies of the Henry’s law behavior of HP in dilute aqueous solutions as a function of temperature. These workers employed a novel technique for generating gas-phase HP as well as several different analytical techniques to determine the HP concentration. Lind and Kok27 and recently, Staffelbach and Kok28 determined the temperature dependence of Kh for HP. Their
3246 J. Phys. Chem., Vol. 100, No. 8, 1996 results are higher than the results of Schumb24 and Hwang and Dasgupta.26 Although the agreement between the Kh determined at 298 K by Lind and Kok27 and Staffelbach and Kok28 is very good, at lower temperatures there is a large difference between the Henry’s law constants, 43% at 277 K, as a result of the difference in the temperature dependence observed in the two studies. The uncertainty in the Kh for HP of nearly 40% determined by various investigaters prompted us to redetermine the temperature dependence of the Kh for HP. We determined the Henry’s law constant for HP as a function of generator concentration from 5 × 10-6 to 5 × 10-2 M HP at pH 3 and 6, and from 277 to 297 K. Our result for Kh at 298 K of (8.33 ( 0.38) × 105 M atm-1 is lower than Lind and Kok,27,30 (1.00 ( 0.09) × 105 M atm -1, and Staffelback and Kok,28 (1.06 ( 0.16) × 105 M atm -1 (Figure 2). All the determinations of the Kh of HP converge at lower temperatures to within their respective uncertainties with the exception of Staffelback and Kok.28 Staffelback and Kok28 reported a pH dependence in the Henry’s law constant for HP. We performed determinations with pH 3 and 6 generation and collection solutions and observed no statistically significant pH dependence. Further examination of their data with ours does not indicate a pH dependence between pH 2 and 7. A pH dependence in the Henry’s law constant would not be expected until the solution pH approached the pKa of HP of 11.7.38 Given the experimental uncertainty in Kh, the pH dependence would not be discernible until the solution pH’s were in excess of pH 10.7. Staffelback and Kok28 also determined the dependence of the Henry’s law constant for HP on sulfuric acid and ammonium sulfate concentration. The Henry’s law constant decreased with increasing sulfuric acid concentration which follows qualitatively the change in the activity of water. In ammonium sulfate solutions, the Henry’s law constant increased with increasing ammonium sulfate concentrations which the authors attribute to an association between HP and ammonia or ammonium in solution. This behavior needs further characterization if the role of HP in aerosol particles and its impact on sulfur oxidation in the marine boundary layer are to be fully understood. Lind and Kok27,30 also determined the Henry’s law constant for MHP and PAA resulting in a Kh at 298 K of 302 ( 36 and 662 ( 116 M atm-1, respectively. Their data for MHP and PAA are lower than our values of 311 ( 14 and 837 ( 175 M atm-1, respectively, but with in the uncertainty of the measurements. During the experiments with PAA the recovery of PAA varied by as much as 20% after sufficient time had passed for equilibrium to be established. This type of signal instability had been encounter before associated with degradation of the C18 column and is the major contributor to the uncertainty in the Kh for PAA determined in this work. The coefficients for the temperature dependence of the Henry’s law constants for MHP and PAA are presented in Table 2 and Figures 3 and 4, respectively. Prior to this work there have been two determinations of the Henry’s law constant for HMHP by Staffelback and Kok28 and Zhou and Lee.31 Zhou and Lee31 determined Henry’s law constants of 6.2 × 105 and 5.0 × 105 M atm-1 at 283 and 295 K, respectively. These values are lower than those determined by Staffelback and Kok28 of (10.4 ( 2.1) × 106 and (2.38 ( 0.48) × 106 M atm-1 at 283 and 295 K, respectively. Our values determined with a similar analytical method as that used by Staffelback and Kok are in very good agreement with their results, (9.49 ( 1.99) × 106 and (2.37 ( 0.50) × 106 M atm-1 at 283 and 295 K, Figure 5. The lower values determined by Zhou and Lee31 are likely a result of the incomplete separation
O’Sullivan et al. of HMHP from other hydroperoxides during the hydrolysis separation technique they employed to quantify the individual peroxide concentrations. The Henry’s law constant for EHP was determined using two different HPLC systems which were equipped with C18 columns from different manufactures. There was no difference in the observed concentrations of EHP between the two systems (Table 2 and Figure 6). The Kh at 298 K for EHP is 336 ( 20 M atm-1, which is slightly larger than that of MHP. MHP has been observed to range from 15 to 2000 pptv in the atmosphere. The similar Henry’s law constants between MHP and EHP indicate that EHP will be partitioned in the aqueous phase similar to MHP. Consequently, EHP can be readily determined with the collection strategies presently employed in atmospheric chemistry. Determining the Henry’s law constant for 1-HEHP is complicated by its decomposition. At pH 3 its concentration will decrease by 10% over 2.5 h, which is the time needed to perform an experiment.32 The change in soure concentration over the experiment increases the difficulty in determining an accurate Kh for this species. Thermodynamic Parameters. Since eq 2 has the form of the Van’t Hoff equation, the slope from the linear fit of ln Kh as a function of temperature is equal to ∆H/R where ∆H is the standard heat of solution and R is the gas constant. In addition the free energy of solution can be calculated from the equilibrium constant since -∆G ) RT ln Kh. The enthalpies and free energies of solution for the various hydroperoxides are presented in Table 1 at 298.15 K. The enthalpy of solution for HP, ∆H ) 61.3 kJ/mol, is in good agreement with the value of 62.5 kJ/mol determined by Staffelback and Kok.28 However our value is considerably higher than the NBS value of 54.9 kJ/ mol.39 The National Bureau of Standards (NBS) value was determined from the work of Scatchard et al.,23 which was extrapolated to 298.15 K from work performed from 318 to 378 K. Excluding Lind and Kok,27,30 the recent work having measurements at or around 298 K give a mean ∆H of 63.2 ( 2.3 kJ/mol. The good agreement by various investigators employing different analytical techniques lends a great deal of confidence to the temperature dependence of Kh for HP determined by combining these data sets. For the suite of hydroperoxides there is better agreement between the individual determinations of the free energy of solution than for the enthalpy of solution. Atmospheric Implications. The additional determinations of the temperature dependence of the Henry’s law constants for the HP, MHP, PAA, and HMHP provides an improved theoretical basis for evaluating the efficiency of the atmospheric sampling strategies currently employed by atmospheric chemist sampling for these species. These data will also contribute to an enhanced understanding of the partitioning of hydroperoxides between atmospheric gas and aqueous phases, allowing a better assessment of the role organic peroxides may play in the oxidation of sulfur dioxide in atmospheric aerosols. The similarity between the Kh for EHP determined in this work and that of MHP indicate that the current atmospheric sampling methods will capture EHP if it is present in the atmosphere in concentrations in excess of 20-30 pptv. Further work is need to examine the ionic strength and solution composition dependence of the Henry’s law constants for the hydroperoxides to be able to fully access the importance of hydroperoxides in the oxidation of S(IV) in aerosols. Conclusions The temperature-dependent solubilities of HP, HMHP, MHP, PAA, and EHP were determined under conditions used in the
Henry’s Law Constant Determinations for Peroxides aqueous collection and analysis of atmospheric hydroperoxides. Henry’s law was obeyed over the source concentration range employed, and the solubilities of HP, HMHP, MHP, and PAA are in agreement with prior determinations. The solubility measurement of EHP is the first of its kind and indicate that EHP will behave similarly to MHP with respect to EHP’s solubility in aqueous systems. The solubility of HOC2H4O2H could not be accurately determined because of its rapid decomposition in solutions with pH > 3. The temperature dependence of the Kh for the hydroperoxides was determined from 4 to 28 °C. These results were fit to an equation of form ln(Kh) ) A/TK - B, where TK is in degrees kelvin. The dimensional (M atm-1) Henry’s law equilibrium constant can be calculated for the hydroperoxides examined in this work with the coefficients given in Table 1. The high solubility of HMHP implies it will be efficiently removed from the atmosphere by precipitation or surface deposition. Its decomposition and that of HOC2H4O2H and PAA forming HP near neutral pH suggest these hydroperoxides may constitute a heterogeneous source of HP in atmospheric water if they are formed in the troposphere. The solubility of the listed organic hydroperoxides and the propensity of three of them to decompose at neutral pH further underscores the potential for interference’s and artifacts in the aqueous collection and nonspecific analysis of HP in the atmosphere. Acknowledgment. The work presented here was performed in support of field measurements at Harvard Forest and the URI on grants from the Electric Power Research Institute (RP202312) and from the National Aeronautics and Space Administration through the Global Tropospheric Experiment (NAG 11222 and NAG 11395). The authors would like to thank two anonymous reviewers, whose comments strengthened the manuscript. References and Notes (1) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210-7254. (2) Kleinman, L. I. J. Geophys. Res. 1991, 96, 20721-20733. (3) Penkett, S. A.; Jones, B. M. R.; Brice, K. A.; Eggleton, A. E. Atmos. EnViron. 1979, 13, 123-137. (4) Lind, J. A.; Lazrus, A. L.; Kok, G. L. J. Geophys. Res. 1987, 92, 4191-4207. (5) Horie, O.; Neeb, P.; Limbach, S.; Moortgat, G. K. Geophys. Res. Lett., submitted. (6) Neeb, P.; Horie, O.; Limbach, S.; Sauer, F.; Moortgat, G. K. The effect of water vapor on the ozonolysis of simple alkenes under atmospheric conditions; poster presented at the 8th CACGP symposium of the IAMAP Committee on Atmospheric Chemistry and Global Pollution; Fuji-Joshida, Japan, Sept 5. (7) Gab, S.; Hellpointer, E.; Turner, W. V.; Kofte, F. Nature 1985, 316, 535-534. (8) Ennis, C. A. Tellus 1990, 42B, 183-199. (9) Hewitt, C. N.; Kok, G. L. J. Atmos. Chem. 1991, 12, 181-194. (10) Polle, B. EnViron. Sci. Technol. 1994, 28, 812-815. (11) Calvert, J. G.; Mandronich, S. J. Geophys. Res. 1987, 92, 22112220.
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