Phenol Coefficients and the Ferguson Principle*

It is shown that the thermodynamic activity at which many phenols produce an equal degree of antibacterial action is approximately the same. The thermodynamic activity of a dissolved phenol can be expressed in terms of the per cent saturation by setting the saturates solution as the standard state of reference. Thus it is demonstrated that the equitoxic solutions of different phenols are those in which the thermodynamic activity or the per cent saturation and not the actual concentrations are the same. It is shown that, as expected, the reported phenol coefficients of 23 different phenols examined are approximately the same as the ratios between the solubility of the unknown disinfectant and the solubility of phenol and that the deviations are interpretable.