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 NO3
- • 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 (w1/2)
Chemical Properties of Montmorillonite:
• Silicate layer clay • Large reactive surface area for
adsorption: 83.79±0.22 𝐦↑𝟐 /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
Ave
rage
Pea
k A
rea
403020100Cl- Concentration (ppm)
Cl StandardsWashedUnwashedMethanol
y = 1.6107x - 0.387R2
= 0.999
2.0
1.5
1.0
0.5
0.0Ave
rage
Pea
k A
rea
2.01.00.0Cl- Concentration (ppm)
20
15
10
5
0
Ave
rage
Pea
k A
rea
2520151050 NO3
- Concentration (ppm)
y = 0.76108x - 0.15559R2
= 0.999
NO3- 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
Abso
rban
ce
1750 1700 1650 1600 1550Wavenumber / cm-1
Time
0.12
0.11
0.10
0.09
0.08
Abs
orba
nce
3000 2800 2600 2400 2200 2000 1800 1600 1400Wavenumber (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+ pKa (HCOOH)=3.75; Ka=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 Δtr / (w1/2)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 H2SO4 (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