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The Uptake of Formic Acid on Montmorillonite Clay: Optimization of an FTIR Study Jennifer Kim, Linda Hancock, Angela Gloyna, and Courtney D. Hatch Hendrix College Department of Chemistry Atmospheric Characteristics: •Largest natural aerosol loading in the atmosphere (~45%) •Relatively long atmospheric lifetime (~2 weeks) Impacts of Mineral Dust Aerosols • Climate aerosol effects (direct and indirect) • Reactive surface • Alters atmospheric gas phase concentrations • Alters surface composition • Strong interaction between mineral aerosol and carbonyl species. Mineral Dust Aerosols Formic Acid Chemical Properties of Formic Acid (HCOOH): • Exists as dimers • Hydrogen bonds • Forms stable ion clusters • Acts as an electrophile or nucleophile • Vapor pressure = 44.8 mmHg • Abundant organic acids in the atmosphere • Go through atmospheric transformations Reduction of Soluble Ion Interferences • • The montmorillonite clay was washed three times with 18 MΩ water or methanol in a sonic bath. • Soluble peaks can be removed from clay prior to HCOOH exposure to reduce peak overlap with formate peak? Results for Optimization of quantitative IC method: • Flow rate=0.5 mL/min • Column Temperature=30°C • Neutral pH • Injection volume=100 μL Results of pre-rinsing clay prior to HCOOH exposure: • Significant reduction of adsorbed Cl - • No significant change in adsorbed NO 3 - • Irreversible adsorption • Improved IC resolution • Used washed clay for future adsorption studies to reduce ion interferences. Conclusions References (1) Hatch, C. D.; Gough, R. V.; Tolbert, M. A. Atmospheric Chemistry and Physics 2007, 7, 4445-4458. (2) Wu, L.; Tong, S.; Hou, S.; Ge, M. J. Phys. Chem A 2012, 116, 10390-10396. (3) Frinak, E. K.; Mashburn, C. D.; Tolbert, M. A. Journal of Geophysical Research 2005, 110, D09308. (4) Rubasinghege, G.; Ogden, S.; Baltrusaitis, J.; Grassian, J. Phys. Chem A 2013, 117, 11316-11327. Acknowledgements Support: This work was supported by the Morris and Ann Henry Odyssey Professorship and the Hendrix College Odyssey Program. We would also like to thank Linda Hancock and Angela Gloyna for their contributions to this work. Chloride % by mass Unwashed 6.29% Washed 3.29% Methanol Washed 3.32% Grinded 3.07% Nitrate % by mass Unwashed 2.03% Washed 1.93% Ion Chromatography Analysis Optimization Measurements Peak Area # of Theoretical Plates Resolution Width at Half Peak Height (w 1/2 ) Chemical Properties of Montmorillonite: • Silicate layer clay • Large reactive surface area for adsorption: 83.79±0.22 ↑ /g • Cation Exchange Capacity: 84.4 meq/100g, major exchange cation Ca Montmorillonite Experimental Design Offline Ion Chromatography Analysis Soluble Condensed Phase Species Extraction: • Clay sample was collected and extracted immediately upon completion of each experiment. • Extraction of soluble species occurred through sonication in ultrapure water followed by centrifugation. • Extract diluted to 5 mL. Procedure Challenges: Low concentration Poor IC peak resolution Need for optimization of offline IC analysis Result: Washing the clay sample with 18 MΩ water and methanol decreased the amount of chloride by 50% (by mass) but no significant decrease in percent by mass of the nitrate. 40 30 20 10 0 Average Peak Area 40 30 20 10 0 Cl - Concentration (ppm) Cl Standards Washed Unwashed Methanol y = 1.6107x - 0.387 R 2 = 0.999 2.0 1.5 1.0 0.5 0.0 Average Peak Area 2.0 1.0 0.0 Cl - Concentration (ppm) 20 15 10 5 0 Average Peak Area 25 20 15 10 5 0 NO 3 - Concentration (ppm) y = 0.76108x - 0.15559 R 2 = 0.999 NO 3 - Standards Dilution Chambers • Dual transmission FTIR reaction cell • Measure condensed and gas- phase simultaneously • FA gas-phase concentration range: 12-75 ppmv Condensed Phase Gas Phase 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Absorbance 1750 1700 1650 1600 1550 Wavenumber / cm -1 Time 0.12 0.11 0.10 0.09 0.08 Absorbance 3000 2800 2600 2400 2200 2000 1800 1600 1400 Wavenumber (cm -1 ) FT-IR Spectrum of Gas-Phase Formic Acid Gas-phase formic acid FT-IR Spectrum of Condensed Phase HCOOH Peak Height of C=O at ~1700 cm -1 Peak Area of HCOOH (g) Peak area of gas phase formic acid stays fairly constant as time increases. HCOOH+H2O ⇌ HCOO-+H3O+ pK a (HCOOH)=3.75; K a =1.77×10-4 Below 3.75, HCOOH stays protonated. Above 3.75, HCOOH deprotonates to HCOO-. Result: Eluent flow rate of 0.5 mL/min and neutral pH [6.49] (do not change the pH of the formate). Resolution = 0.589 Δt r / (w 1/2 ) avg. Result: Eluent flow rate of 0.5 mL/ min and injection volume=100 μL Injection Volume Resolution at 0.5 mL/min Resolution at 1.0 mL/min 25 μL 1.545 1.484 100 μL 1.716 1.342 Variable Eluent Flow Rate (0.5 mL/min & 1.0 mL/min) Column Temperature (30°C – 50°C) pH by addition of H 2 SO 4 (pH 2-6) Injection Volume (25 μL & 100 μL) pH Study Result: Eluent flow rate of 0.5 mL/min and column temperature=30°C Column Temperature Study http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392002000400013 http://www.restauro-online.com/Formic-acid- min-85-technical-CH2O2 Concentration of HCOOH: 12 ppm Formate Calibration Curve
