Laboratory Experiment pubs.acs.org/jchemeduc
Combining Experiment and Theory To Probe Salt Aerosol Deliquescence Joshua P. Darr* Department of Chemistry, University of Nebraska at Omaha, Omaha, Nebraska 68182, United States S Supporting Information *
ABSTRACT: Atmospheric aerosols represent an interesting and practical way for students to learn about chemical thermodynamics. This paper describes a laboratory experiment designed for physical chemistry students in which they learn about phase transitions of salt aerosols. The aerosols are generated by atomizing a salt solution, and then they are passed through a gas cell in a transmission-mode Fourier transform infrared spectrometer. Utilizing the O−H stretch of liquid water, the relative quantity of water in the aerosols can be measured. The deliquescence phase transition in which the solid aerosol particle dissolves to form a liquid droplet is evidenced by an abrupt increase in the water content of the aerosols. Multiple salts are suitable for analysis, including sodium chloride and ammonium bisulfate. Additionally, comparisons to model calculations can be made using the Extended Aerosol Inorganics Model Web site. This lab exposes students to important thermodynamic concepts such as chemical potential, activity, and phase transitions and forces them to critically interpret spectra. KEYWORDS: Upper-Division Undergraduate, Environmental Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning, Internet/Web-Based Learning, Atmospheric Chemistry, IR Spectroscopy, Phase Transitions, Thermodynamics
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practical techniques such as handling gases and using vacuum. Finally, they learn information relevant to environmental issues, such as the nature, examples, and phases of aerosols. This is all done in the context of using experiment and theory collaboratively to address scientific problems. Written reports demonstrate that students learn how thermodynamics, particularly the concepts of chemical potential and phase transitions, applies to an important scientific topic. The combination of experiment and theory used facilitates this understanding because of the diverse approaches involved and the appeal to different learning styles.
hemical thermodynamics is a major topic in both general chemistry and physical chemistry courses. Typically, chemistry laboratories that illustrate thermodynamic concepts utilize a calorimeter to monitor a temperature change induced by a chemical or physical process, though interesting exceptions do exist.1−3 Environmental chemistry represents a potentially good venue for studying thermodynamics. Environmental chemistry is also an extremely practical and relevant field where undergraduate experiments4−7 are likely to engage students. The laboratory experiment described here seeks to connect these two ideas using spectroscopy to probe an aerosol’s phase change. Aerosols are solid particles or liquid droplets suspended in a gas, and they have important implications for the environment, particularly climate.8 To pique students’ interest in thermodynamics, this experiment uses the practical context of aerosols and examines the conditions in which they undergo phase changes via a transmission-mode Fourier transform infrared (FTIR) spectrometer. Specifically, the deliquescence, or solidto-solution phase transition, of inorganic salt aerosols is measured by acquiring FTIR spectra as the relative humidity (RH) is systematically varied. The experimental results are then compared with literature values and results obtained using model calculations performed on a Web site established for the atmospheric chemistry community.9 By performing these experiments, students learn about fundamental concepts in thermodynamics such as chemical potential, activity, and phase transitions. Students also learn © 2013 American Chemical Society and Division of Chemical Education, Inc.
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BACKGROUND Many types of aerosols can be found in our environment, including smog, mineral dust, and sea salts.10 This experiment examines the deliquescence phase transition of inorganic aerosols that simulate sea salt. Deliquescence is important because it dictates the aerosol’s physical state, which in turn can affect its optical properties and chemical reactivity.11,12 The ambient conditions and relative stability of the two phases control this phase change. As the RH increases, the chemical potential, μ, or molar Gibbs energy of a solid particle remains constant, while that of the solution phase decreases. Eventually, μ for the solution will equal that of the solid particle. At this point, the particle abruptly adsorbs water and transforms into a saturated solution that is in equilibrium with the water vapor. Published: September 23, 2013 1392
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Figure 1. Schematic of the experimental apparatus (A), including an expanded version of the relative humidity system (B).
aerosols. A schematic of the flow cell apparatus used is shown in Figure 1. Similar ones are described in the literature.13,14 The apparatus was assembled by the instructor prior to the laboratory session. A parts list and assembly instructions are included in the Supporting Information. Either nitrogen, N2, or a dry air supply must be connected to the atomizer. Approximately 30 psi (210 kPa) of either gas is used for aerosol generation. The aerosols are dried as they pass through a diffusion dryer containing silica gel desiccant. Finally, the particles flow through a gas cell inside the FTIR spectrometer and then vent to a fume hood. A laser pointer can be used to confirm whether aerosols are being generated; scatter from the particles is easily visual. The quantity of water exposed to the aerosols was systematically controlled by generating a well-defined mixture of dry air/N2 and wet air/N2 with two flowmeters, Figure 1B. The wet air/N2 was generated by bubbling the gas through a vessel containing water. If more precise control over the RH is desired, two bubblers may be used in parallel. The resulting mixture was further mixed with the aerosols before they were introduced to the gas cell as shown in Figure 1A. The water content was monitored quantitatively by measuring the RH with a commercial thermohygrometer. Once the RH reached the desired value, an IR spectrum was acquired. The experiment was tested on two different commercial FTIR spectrometers with either a standard deuterated triglycine sulfate or lithium tantalate detector. Students were instructed to acquire spectra consisting of 30− 50 scans from at least 2500−4000 cm−1 at 4 cm−1 resolution. The O−H stretch of interest is a broad feature centered at approximately 3400 cm−1. Using the H−O−H bend for this experiment is also possible, though that feature lies within considerable spectral congestion, and a more sophisticated analysis would be required. Figure 2 shows representative student spectra collected in absorbance mode both before, gray trace, and after, black trace, deliquescence has occurred. The y axis is labeled as extinction because a nonzero signal can result from either absorption or scattering. Spectra in Figure 2A were acquired using NH4HSO4 aerosols. In addition to the signal ascribed to water adsorption at 3400 cm−1, two other features are present. The sharp features ranging from 3500 to 4000
The chemical potentials, μ, for each phase can therefore be set equal to one another: μH O(g) = μH O(aq) (1) 2 2 The chemical potential of the aqueous phase, μH2O(aq), can be related to the water activity, αw: μH O(aq) = μH* O + RT ln αw 2
2
(2)
With a few simple manipulations (see the Supporting Information), it can also be shown that the activity is simply the decimal form of RH,
αw =
RH 100
(3)
and the RH at which deliquescence occurs, DRH, is the water activity of the saturated solution, αws, αws =
DRH 100
(4)
Because liquid water has an easily identifiable infrared signature, it can be used to experimentally determine the salt aerosol’s DRH. This experiment’s objective, then, is for students to critically interpret infrared spectra to determine a salt aerosol’s DRH and compare their results to model calculations and scientific literature.
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EXPERIMENTAL PROCEDURE This laboratory experiment was completed in one 3.5 h laboratory session, typically with ample time remaining. A total of 28 students in four sections spanning two years worked in groups of two to three to perform the experiment. The instructor provided students with a list of possible salts to study. Included among these were sodium chloride (NaCl) and ammonium bisulfate (NH4HSO4). Another option provided was ammonium nitrate, but it was not chosen by enough groups to ensure repeatability of the results. Students then made a concentrated solution of the salt. Depending upon the type of atomizer used, the volume required may vary. Our students were asked to prepare 500 mL of a 100 g/L salt solution and then load it into the atomizer to generate the 1393
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Laboratory Experiment
HAZARDS Nitrates are oxidizers and should be collected and disposed of using the institution’s Environmental Health and Safety or equivalent office. Compressed gases should always be fastened securely to a bench or other appropriate laboratory fixture when being used. Nitrogen is an asphyxiation hazard. Most laser pointers are class 2 or 3R lasers; the beam should not be directed into one’s eye or used at eye level.
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DATA ANALYSIS It is the instructor’s discretion to decide how students should analyze the data. Depending upon his or her decision, students can plot as a function of RH either the peak height of the O−H stretching feature or the integrated area from 2800−3600 cm−1. This graph should be relatively constant at low RH and then abruptly increase near the DRH. Students should be cautioned not to confuse the water vapor or N−H stretch features for the liquid water feature at 3400 cm−1. A graph of student data obtained with NH4HSO4 aerosols is shown in Figure 3A. There is a constant signal from 28% to 41% RH and then an increase in peak height between 41% and 45% RH. This is in good accord with the DRH of 39−40% given in the literature.15 Beyond 45%, the signal continues to increase because the water partitioning onto the aerosol increases with the corresponding increase in water vapor concentration. For NaCl, Figure 3B, there is a modest increase in peak height up to 70% RH, but the peak height increases drastically at 75%, in good agreement with the literature.15 Weis and Ewing14 also observed measurable water content below the DRH, which they attributed to a porous aerosol structure. If desired, a more sophisticated analysis can also be performed by acquiring spectra of only humidified air or N2 at the same RH values and then subtracting them from the spectra containing the aerosols. This eliminates the features attributable to gaseous water vapor.
Figure 2. IR spectra obtained by students before, gray trace, and after, black trace, deliquescence has taken place for (A) NH4HSO4 and (B) NaCl aerosols. Features ascribed to IR absorption by water vapor, water adsorbed on the aerosols, and the N−H stretch in ammonium are labeled.
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cm−1 are attributed to gaseous water vapor. Additionally, the N−H stretch in NH4+ is observed near 3200 cm−1. Spectra in Figure 2B were acquired using NaCl aerosols and exhibit the same features except for the N−H stretch. Spectra should initially be acquired from approximately 20−70% RH in 10% increments giving students a rough idea where deliquescence occurs. Additional points are chosen to fine-tune the DRH value by acquiring spectra at intermediate or higher RH. Typical uncertainty in thermohygrometers is ±2−3%.
MODELING CALCULATIONS In the second year implementing this experiment, students compared their experimental results to those obtained from calculations using thermodynamic models16 from the Extended Aerosol Inorganics Model (E-AIM) Web site.9 Students used model III and performed a parametric calculation varying relative humidity. The Web site calculations assume a 1 m3 volume, and the atomizer used here gives an aerosol density of
Figure 3. Graphs obtained from student data illustrating the increase in absorbance of the O−H stretching feature as the (A) NH4HSO4 and (B) NaCl aerosols deliquesce. The experimental DRHs, open boxes, are taken as being between the first two points where a significant increase in peak height occurs. The literature values, shaded boxes, are from ref 15. 1394
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2 × 106 particles/cm3. The mean particle diameter is 0.35 μm. Using the column output format, students were able to copy the output to a spreadsheet and plot it. A number of thermodynamic parameters are given. The molalities and activity coefficients, in particular, are useful for illustrating the nonideality of the aerosols. Additionally, the molality calculation can be confirmed using the solubility of the salt. However, the main column of interest is labeled “n_H2O(aq)”, the number of moles of liquid phase water. By plotting it versus RH, students were able to compare their experimental results to those obtained using E-AIM. For NaCl, the average of student experimental DRHs was slightly lower than the literature15 and modeling values, 69.6(1.0)% versus ≈75%, where the experimental error represents the uncertainty in the RH propagated through for the replicate student data. Modeling was not available for NH4HSO4; however, student results were within error of the literature values, 39(3)% versus 39.0% or 40%.15 The source of the discrepancy for NaCl is not completely clear, but possibilities include drift or systematic error of the thermohygrometer, as well as student error in conducting the experiment. For suggestions for minimizing the error, see the Supporting Information.
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(13) Miñambres, L.; Sanchez, M. N.; Castano, F.; Basterretxea, F. J. J. Phys. Chem. A 2008, 112, 6601. (14) Weis, D. D.; Ewing, G. E. J. Geophys. Res. 1999, 104, 21275. (15) Martin, S. T. Chem. Rev. 2000, 100, 3403. (16) Clegg, S. L.; Brimblecombe, P.; Wexler, A. S. J. Phys. Chem. A 1998, 102, 2155.
ASSOCIATED CONTENT
* Supporting Information S
Student handout; instructor notes, including a parts list and assembly instructions. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author would like to thank the Physical Chemistry I Laboratory students that participated in the testing and development of this experiment, as well as Edmund Tisko, the other instructor of the course.
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REFERENCES
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