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 (8.91 ± 1.3) x 10³ and (4.1 ± 0.4) x 10³ M atm-¹ at 25°C for HCOOH and CH₃COOH, respectively. The reaction enthalpies, ΔH, were found to be -51 ± 2 kJ mol^-1 and -52 ± 1 kJ mol^-1 formic and acetic acid, respectively. These are in good agreement for 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 10³ M atm^-1 and (7 ± 3) x 10³ M atm^-1 for HCOOH and CH₃COOH, respectively.