Validation of pH-Metric Technique for Measurement of pKa and logPow of Ionizable Herbicides

Knowing the values of pKa and partition coefficients between 1-octanol and water (Pow) of ionizable herbicides helps us to understand the movement of these compounds in plants and soil. However, shake-flask and high-pressure liquid chromatography (HPLC) techniques. though valuable for measurement of log Pow of herbicide molecules that do not ionize in aqueous solution, are difficult to use for ionizable molecules; this is especially so for molecules that form ion pairs having appreciable log Pow values, those with multiple ionizations and for bases with high pKa values. Our aim in this study was to validate the use of the pH-metric technique (based on potentiometric titration) for measurements of pKa and log Pow of ionizable standard substances and herbicides. The values obtained show good correlation with results from other techniques, including shake-flask and HPLC. The OECD Guideline for Testing of Chemicals 117, adopted 30th March 1989, describes the use of HPLC for the measurement of log Pow. It is hoped that these studies and further testing of this technique will permit it to be included in these OECD guidelines.