4.3: Henry's law constants

In this example Henry's law constants for 17 different compounds are calculated and compared to experiment.

The Henry's law constants depend on the density of the solvent, Water in this case. If one does not supply a density of the solvent in the input the program calculates the density of the solvent by dividing the mass of a molecule with its COSMO volume. The density of Water at 20 °C (293.15 K) is approximately 0.998 kg/L. Note that the calculated activity coefficients do not depend on the density of Water.

Remark: To only add the compounds needed in this example, one can also open a new COSMO-RS GUI window (File → New), copy the $ADFHOME/examples/crs/Tutorial4/tutorial4.3.compoundlist to the directory where the COSMO-RS database is downloaded, and select this file with File → Add Compound(s). The tutorial4.3.compoundlist is a file with a list of compounds that is limited to the coumpounds needed in this example.

Select Properties → Activity coefficients
Select 'Water' for the first component in Solvent
Enter '293.15' for 'Temperature Kelvin'
Click the checkbox 'Use input density solvent (kg/L)'
Enter '0.998' in the 'Use input density solvent (kg/L)' field
Click the 'None' button next to 'Compounds'
Click the search button next to 'Compounds' to add some compounds:
'Acetaldehyde', 'Acetone', 'Acetonitrile', 'Benzene', 'Chloromethane',
'Cyclopentane', 'Dimethyl sulfide', 'Ethanol', 'Formaldehyde',
'Methanol', 'Methyl acetate', 'Methyl bromide', 'Methyl fluoride',
'Methyl iodide', 'Pyridine', 'Thiophene', and 'Toluene'
Select File → Run
Click 'Yes' when asked to 'Save changes (required to run)?'

The result of the calculation (may take a few seconds, depending on the number of compounds selected) is given in the form of a table.

The Henry's law constants also depend on the vapor pressure of the pure compounds in the gas phase. In the compounds window one can also set these vapor pressures of the pure compounds at a given temperature, or set the Antoine parameters. If these values are not specified (if they are zero) then the pure compound vapor pressure will be approximated using the COSMO-RS method. Best is to include the experimental vapor pressure for a pure compound at the used temperature, thus in this case at 293.15 K.

Select View → Compounds
Click on the left side 'Acetaldehyde'
Enter '0.968' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Acetone'
Enter '0.246' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Acetonitrile'
Enter '0.095' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Benzene'
Enter '0.100' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Chloromethane'
Enter '4.94' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Cyclopentane'
Enter '0.346' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Dimethyl sulfide'
Enter '0.530' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Ethanol'
Enter '0.059' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Formaldehyde'
Enter '4.47' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Methanol'
Enter '0.129' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Methyl acetate'
Enter '0.230' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Methyl bromide'
Enter '1.83' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Methyl fluoride'
Enter '33.7' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Methyl iodide'
Enter '0.443' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Pyridine'
Enter '0.021' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Thiophene'
Enter '0.082' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Toluene'
Enter '0.029' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Click on the left side 'Water'
Enter '0.02536' in the 'Pure compound vapor pressure:' field
Enter '293.15' in the 'at temperature:' field
Select Properties → Activity coefficients
Select File → Run
Click 'Yes' when asked to 'Save changes (required to run)?'

For some of the compounds the Henry's law constants differ quite substantially.

Experimental determined Henry's law constants might, for example, be found at http://www.henrys-law.org, where a 'compilation of Henry's Law constants for inorganic and organic species of potential importance in environmental chemistry' were listed by R.Sander, and where also an explanation can be found of the many different definitions and units for Henry's law constants.

The calculated Henry's law constants will be compared to experimental values in the next graph and table. The experimental numbers were taken from Ref. [431], where the experimentally determined dimensionless Henry's law constant H^cc is the inverse of the dimensionless Henry's law constant k_H^cc, that is used in the COSMO-RS module.

H^cc = k_{H, inv}^cc = 1/(k_H^cc)

 

	Solute	experimental Hcc@20°C [431]	calculated Hcc@20°C
1	Methyl_bromide	2.01 10-1	2.05 10-1
2	Chloromethane	3.05 10-1	3.80 10-1
3	Methyl_fluoride	6.04 10-1	9.86 10-1
4	Methyl_iodide	1.70 10-1	1.04 10-1
5	Cyclopentane	5.25 100	3.27 100
6	Benzene	1.91 10-1	1.65 10-1
7	Toluene	2.09 10-1	1.89 10-1
8	Methanol	1.37 10-4	2.43 10-4
9	Ethanol	1.48 10-4	3.70 10-4
10	Formaldehyde	8.61 10-6	2.22 10-2
11	Acetaldehyde	2.21 10-3	3.72 10-3
12	Acetrone	1.10 10-3	0.78 10-3
13	Methyl_acetate	4.02 10-3	4.89 10-3
14	Acetonitrile	6.35 10-4	6.35 10-4
15	Pyridine	1.14 10-2	2.03 10-4
16	Dimethyl_sulfide	6.35 10-2	1.82 10-1
17	Thiophene	7.46 10-2	6.23 10-2

In most cases the calculated Henry's law constants are quite close to the experimental ones, except for Formaldehyde and Pyridine.

The Henry's law constant of Formaldehyde is more than a factor of 10³ wrong. The origin of this error is that in Water solution the hydration of Formaldehyde leads to Methanediol, and Methanediol is even the dominant form if one dissolves Formaldehyde in Water. This is not taken into account in the calculation. In Ref. [432] a distinction is made between the apparent and intrinsic Henry's law constants, which differ from each other by approximately a factor of 10³ for Formaldehyde, and a factor of approximately 2.4 for Acetaldehyde. In Ref. [432] the intrinsic Henry's law constant for Formaldehyde was determined to be 2.5 mol/(L atm) at 25 °C, which is close to the value of 1.8 mol/(L atm) which was calculated with COSMO-RS, although at a different temperature of 20 °C.

The calculated Henry's law constant of Pyridine is approximately a factor of 10² different than the experimental value in Ref. [431]. However, the experimental values for Pyridine taken from Ref. [433] are 1.1 10² mol/(L atm) and 9.0 10¹ mol/(L atm), which are not very different from the calculated value with COSMO-RS of 2.0 10² mol/(L atm). Also the experimental values for Pyridine reported in Ref. [434] are in much better agreement with the value calculated with COSMO-RS.

References

[431] J. Staudinger and P.V. Roberts, A critical compilation of Henry's law constant temperature dependence relations for organic compounds in dilute aqueous solutions, Chemosphere 44 (2001), 561

[432] E.A. Betterton and M.R. Hoffmann, Henry's law constants of some environmentally important aldehydes, Environmental Science & Technology 22 (1988), 1415

[433] R. Sander (1999), Compilation of Henry's Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry (Version 3), http://www.henrys-law.org

[434] M. Bernauera and V. Dohnal, Temperature dependences of limiting activity coefficients and Henry's law constants for N-methylpyrrolidone, pyridine, and piperidine in water, Fluid Phase Equilibria 282 (2009), 100

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