Henry's law coefficients of formic and acetic acids

The Henry's law constants, K-H, Of dilute aqueous formic and acetic acids were determined experimentally as a function of concentration and temperature using a new counterflow packed-column technique. K-H was found to be (8.9 +/- 1.3) x 10(3) and (4.1 +/- 0.4) x 10(3) M atm(-1) at 25 degrees C for HCOOH and CH3COOH, respectively. The reaction enthalpies, Delta H, were found to be -51 +/- 2 kJ mol(-1) and -52 +/- 1 kT mol(-1) for formic and acetic acid, respectively. These are in good agreement with calculated thermochemical values. Whereas the K-H values are in reasonably good agreement with certain other experimentally determined values, K-H (HCOOH) is two to three times higher than calculated thermochemical values while K-H (CH3COOH) is lower than the two calculated values. The 'best' experimental values appear to be (11 +/- 2) x 10(3) M atm(-1) and (7 +/- 3) x 10(3) M atm(-1) for HCOOH and CH3COOH, respectively.