Kd is defined as the ratio of the equilibrium concentrations of a substance adsorbed onto a solid sorbent to that dissolved in a liquid phase. The property is moderately temperature-dependent and typically measured between 20 and 25°C.
KOC can be defined as the product of Kd and fraction of organic content (fOC) of sorbent by the following equation:
Appendix C describes the test methods for measuring Kd or KOC. The behaviour of a chemical in the test systems provides a means for assessing sorption/desorption within a standardised test. The value derived from the standardised test should thus be reproducible between different laboratories. In practice, variations in test conditions, such as the origin and amounts of organic and inorganic material used, can lead to variation of test results. Furthermore, standardised test conditions do not necessarily extrapolate well to real-world environments. Consequently, it is typical to find significant variability in KOC values for a specific compound, which in some instances can extend over several orders of magnitude. Caution is thus warranted when interpreting data obtained for KOC of ionisable organic compounds, particularly with respect to assessing exposure and bioavailability. This is because KOC typically assesses the sorption/desorption behaviour of an organic chemical with the organic carbon content present in soil, sediment, and sludge, as seen in Eq. 16 above, but for ionisable organic compounds it may be that electronic interactions with inorganic material may be more important.