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Environmental fate & pathways

Henry's Law constant

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Description of key information

The key value of the Henry's Law constant for trifluoroacetic acid was calculated by the average of two measured values reported in two valid peer reviewed studies. In cloud and fog, CF3COOH will partition entirely into the aqueous phase. For atmospheric aerosols, at temperatures less than 15°C, significant proportions of CF3COOH may reside in both phases and partitioning will be sensitive to the composition of the aerosol and concentration of the dissolved solutes.

Key value for chemical safety assessment

Henry's law constant (H) (in Pa m³/mol):
0.009
at the temperature of:
298.15 K

Additional information

Two reliable studies on Henry's Law constant measurement published in peer reviewed jounals are available. In the first study, an experimental determination of the Henry's law constant of trifluoroacetic acid is described, and the implications for its partitioning and removal from the atmosphere are discussed. Equilibrium partial pressures CF3COOH over aquous solutions containing the dissolved acid were measured at a temperature of 298.15 K (25°C) using the pH changes of the solution and the results of separate calibration experiments which relate change in pH to added acid concentrations . KH was 8950 +/- 100 mol/kg/atm for pKa = 0.47 corresponding to 1.1E-02 Pa.m3.mol-1. In cloud and fog, it is clear that CF3COOH will partition entirely into the aqueous phase. For atmospheric aerosols, the situation is more complex. At temperatures less than 15°C, significant proportions of CF3COOH may reside in both phases and partitioning will be sensitive to the composition of the aerosol and concentration of the dissolved solutes.

In the second study, equilibrium partial pressures of trifluoroacetic acid over aqueous trifluoroacetic acid test solutions were determined at 278.15, 288.15 and 298.15 K. The concentration of undissociated trifluoroacetic acid in each test solution was determined by means of attenuated total reflection IR spectroscopy and window factor analysis. The Henry's law constant of trifluoroacetic acid (KH) at 298.15 K was determined to be 5800 +/- 700 mol/dm3/atm corresponding to 7.13E-03 Pa.m3.mol-1. The KH value at 298.15 K was 0.65 times the reported value by Bowden et al. (1996) for pKa = 0.47 and it was equal to that for pKa = 0.2.

The key value of the Henry's Law constant for trifluoroacetic acid was calculated by the average of the two valid measured values corresponding to 0.009 Pa.m3.mol-1.