Elusive Carbonic Acid: A Determination of its Vapor Pressures and Enthalpy of Sublimation for

Mars and BeyondAriel S. Lewis1,3, Paul D. Cooper2,3, Marla H. Moore3, Reggie. L. Hudson1,3

1 Department of Chemistry, Eckerd College, 2 Department of Chemistry and Biochemistry, George Mason University, 3 Astrochemistry Branch, NASA Goddard Space Flight Center.

Identifying frozen acids, such as formic acid (HCOOH) acetic acid (CH3COOH), and carbonic acid (H2CO3) on solar system surfaces has been more difficult than identifying solid H2O, H2O2, O2, or O3., partly due to the paucity of low-temperature data for the acids. It is known that HCOOH, CH3COOH, and H2CO3 are formed through the ion irradiation and far-UV photolysis of various ice mixtures – formic acid is made in H2O + CO ices, acetic acid is likely produced in CH4 + CO2 ices, and carbonic acid is produced from both ion irradiation and UV-photolysis of H2O + CO2 ices. The formation and thermal evolution of these acids is important in understanding the surface chemistry of icy satellites. Of particular interest is the possible formation, stability, and evolution of carbonic acid on Mars, given that planet’s known polar H2O and CO2 ice reservoirs.

0.0054 0.0056

-15.0

-14.5

-14.0

-13.5

-13.0

-12.5

-ΔHS = slope x 8.314

= -8346.2 ± 196.8 x 8.314ΔH

S = 67.9 ± 1.6 kJ /mol

ln Vapor Pressure

1/T (K-1)

ACETIC ACID

4000 3000 2000 10000.0

0.2

0.4

0.6

0.8

1.0

Absorbance

Wavenumber (cm-1)

633 cm-1

ACETIC ACID

EXPERIMENTAL

Formic and acetic acid ices were prepared by injecting pure liquid samples of each acid onto a cold KBr window (12 K) in a vacuum system. Carbonic acid ices were prepared by layering KHCO3 and HBr solutions on the cold finger, and warming to ~240 K to promote their reaction. Vapor pressures were calculated from the loss rate of each acid at a particular temperature, taken from the decrease in intensity of a chosen IR spectral band. The rate of loss, J, is the number of molecules leaving a unit area per unit time, determined from decreases in a spectral feature’s intensity. The vapor pressure p was calculated by using the equation below, where M is the molecular mass of the species, T is the temperature, and k is the Boltzmann constant.

MkT

pJ

π2=

Heats of sublimation were calculated by applying the Clausius-Clapeyron equation over the measured temperature range.

RESULTS FOR ACETIC AND FORMIC ACIDS

680 660 640 620 600 580

0.0

0.1

0.2

t = 13640

t = 10540

t = 4340

Absorbance

Wavenumber (cm-1)

t = 0 ACETIC ACID

Heats of Sublimation, ΔHs (kJ/mol)

This work Calis van Ginkel et al.

Stephenson & Malanowski

Airoldi & DeSouza

Coolidge Stull

Formic Acid 62.5 ± 5.6 62.2 ± 1 60.5 - 60.7 60.1

Acetic Acid 67.9 ± 1.6 67.6 ± 1 54.5 67 ± 1 - -

Carbonic Acid 66.3 ± 6.6 - - - - -

Carbonic Acid:Results and Conclusions

800 750 700 6500.0

0.2

0.4

0.6

0.8

t = 1260t = 840

t = 420

Absorbance

Wavenumber (cm-1)

t = 0

FORMIC ACID

4000 3000 2000 10000.0

0.5

1.0

1.5

2.0

Absorbance

Wavenumber (cm-1)

718 cm-1

FORMIC ACID

0.0056 0.0058 0.0060

-14.5

-14.0

-13.5

-13.0

-12.5

-12.0

-11.5

-ΔHS = slope x 8.314

= -7516.0 ± 679.4 x 8.314ΔH

S = 62.5 ± 5.6 kJ /mol

1/T (K-1)

ln Vapor Pressure

FORMIC ACID

0 2000 4000 6000 8000 100000.0

0.2

0.4

0.6

0.8

1.0

1.2

166 K168 K

170 K172 K

174 K

Normalized Intensity

Time (sec)

FORMIC ACID

176 K

References

Airoldi, C.; DeSouza, A.G., J. Chem. Soc., Dalton Trans., 1987, 12, 2955.; Calis-Van Ginkel, C.H.D.; Calis, G.H.M.; Timmermans, C.W.M.; DeKruif, C.G.; Oonk, H.A.J., J. Chem. Thermodyn., 1978, 10, 1083.; Coolidge, A.S., J. Amer. Chem. Soc., 1930, 52, 1874.; Stephenson, R.M.; Malanowski, S., Handbook of the Thermodynamics of Organic Compounds, Elsevier: New York, 1987.; Stull, D.R., Organic compounds, Ind. Eng. Chem., 1947, 39, 517.

PREPARING CARBONIC ACID

The table at the bottom of the center panel compares sublimation data from this laboratory and elsewhere for formic (HCOOH) and acetic (CH3COOH) acids. The comparison is good. Having shown that the method described at right is a robust technique for measuring vapor pressures and heats of sublimation, we applied it to solid carbonic acid. We first synthesized H2CO3 by layering KHCO3 and HBr solutions onto a cold KBr window at 10 K and warming to promote reaction.

Once this method of synthesis was established, it was easy to spectroscopically measure the rate of H2CO3 loss at specific temperatures. The loss rate was then used to calculate vapor pressures and ΔHs of carbonic acid, values that were previously unknown.

ACKNOWLEDGMENTS

The authors acknowledge support from the Goddard Center for Astrobiology. ASL additionally thanks NASA for a summer astrobiology internship. PDC acknowledges support from a NASA Postdoctoral Fellowship. MHM and RLH are supported for this work by NASA’s PG&G, Planetary Atmospheres, and Outer Planets Program, and the Goddard Center for Astrobiology.

− ΔHs = slope × 8.314 J / K•mol− ΔHs = − 7977.6 K−1 × 8.314 J / K•mol

ΔHs = 66.3 kJ / mol

CONCLUSIONS:

1. Vapor pressures and ∆Hs values for sublimating ices can be determined from spectral changes. Our results compare well with published measurements on formic and acetic acids.

2. Carbonic acid can be synthesized via thermal annealing of bicarbonate and acid ices.

3. Carbonic acid has ∆Hs = 66.3 ± 6.6 kJ/mol, based on our measurements.

4. Martian temperatures are 180 – 270 K. As the upper end of this range, carbonic acid will undergo significant sublimation.

5. At temperatures below ~200 K, solid-phase H2CO3 will be resistant to sublimation and decomposition.

HBr KHCO3 H2CO3

Changes in H2CO3 IR Transmission Spectra

WHY CARBONIC ACID?

Carbonic acid, H2CO3, exists in aqueous solutions at room temperature, but only at concentration of ~10−5 molar, readily decomposing into H2O and CO2. For this reason, H2CO3 was long thought to be too unstable for either spectroscopic or thermodynamic measurements. The first IR spectral measurements were only made in 1991 (Moore and Khanna), and even today almost no data are available to describe this molecule’s bulk properties.

T ~250 K