1-Octanol/water partition coefficient (Kow) and pKa for ionisable pesticides measured by a pH-metric method

pK(a) values for a wide range of commonly used ionisable pesticides, together with the log K-ow values of the most lipophilic form of each, have been measured using ps-metric techniques. Examples of acids, bases and multiprotic compounds from the major classes of herbicides, and a number of insecticides and fungicides that contain ionisable groups, are included. The pK(a) and log K-ow values so obtained were generally in good agreement with values taken from the literature that were measured by other methods. The lower limit of log K-ow that could be measured by the pH-metric method lay below the -0.97 obtained for amitrole, but the method could not be applied to glyphosate for which shake-flask measurements indicated log K-ow below -3. The highest log K-ow obtained in this study was 5.12 for pentachlorophenol. The pH-metric technique offers a rapid and convenient method to determine pK(a) and log K-ow for ionisable compounds, especially when utilising an automatic titration system linked to a dedicated computer.