SLOW-STIRRING METHOD FOR DETERMINING THE n-OCTANOL/WATER PARTITION COEFFICIENT (Pow) FOR HIGHLY HYDROPHOBIC CHEMICALS: PERFORMANCE EVALUATION IN A RING TEST

Abstract

The n-octanol/water partition coefficient (Pow) is one of the most important parameters employed for estimating a chemical's environmental fate and toxicity. The currently adopted test guidelines for its determination do not allow for reliable determination of log Pow greater than 5. The slow-stirring experiment, in contrast, has been demonstrated to provide reliable log Pow data up to log Pow of 8.3. To validate this method and to obtain its approval as an official Organization for Economic Cooperation and Development (Paris, France) test guideline, a ring test was performed to evaluate the accuracy and precision of the slow-stirring experiment for determination of log Pow, particularly for highly hydrophobic compounds. Up to 15 volunteer laboratories tested 1,2,3,4-tetrachlorobenzene, hexachlorobenzene. 2,2',3,3',5,5',6,6'-octachlorobiphenyl, and decachlorobiphenyl (4.5 < log Pow < 8.2). The ring-test results for the respective chemicals were 4.62, 5.50, 7.39, and 8.18. The results deviated by less than 0.1 log Pow units from the reference log Pow. In addition, the relative standard deviations of log Pow were less than 2%. Using the protocol of the ring test, log Pow of p,p'-DDT was determined to be 6.24 +/- 0.05 (mean +/- standard deviation). In combination, these results indicate that the slow-stirring method is precise and accurate and, thus, allows for reliable determination of log Pow of highly hydrophobic chemicals.

Comparative evaluation of three high‐performance liquid chromatography–based Kow estimation methods for highly hydrophobic organic compounds: Polybrominated diphenyl ethers and hexabromocyclododecane

“…Estimation of K ow values with RP-HPLC is based on the premise of the stationary and mobile phases acting as a surrogate for octanol and water. Relationships between K ow and kЈ usually take the form B K ϭ AkЈ ow (4) or, in logarithmic form, ln K ϭ B ln kЈ ϩ constant ow (5) indicating that a straight line with slope B should be observed on a plot of ln K ow versus ln kЈ. Only if the stationary and mobile phases were perfect surrogates of water-saturated octanol and octanol-saturated water would K SM and K ow be identical and B ϭ 1.…”

“…log kЈ ϭ slope· ϩ intercept (6) No general agreement exists regarding which eluent composition is best suited for the estimation of K ow in equations such as Equation 5. Many studies have used the intercept of Equation 6, which is the logarithm of the theoretical capacity factor at 100% aqueous solvent (log ).…”

“…Many experimental methods for determining K ow rely on direct measurements of concentration after equilibration in an octanol‐water system. These methods are important for providing empirical, physically derived K ow values, but they often are laborious, difficult, and inherently subject to large uncertainties [4,5]. Direct determination by the shake‐flask and generator‐column methods is prone to octanol droplets in the aqueous phase, which contain relatively large amounts of analytes when dealing with hydrophobic chemicals.…”

Uncertainty in Octanol−Water Partition Coefficient: Implications for Risk Assessment and Remedial Costs

“…As a result, the shake-flask method is not reliable for use when a substance's log K ow is larger than 4 or 5. Nevertheless, if an appropriate method is used, the variability between Kow measurements is low; for example, recently Tolls et al (32) coordinated a ring test involving interlaboratory comparison of Kow values measured using the slow-stirring method. The entire range of values reported by 15 participating laboratories spanned less than one log unit for each chemical.…”

The octanol–water partition coefficient: Strengths and limitations

“…Later on, de Bruijn et al [15] at Utrecht University applied this method to a large number of compounds including hydrophobic polychlorinated biphenyls and several classical chlorinated insecticides such as DDT. While it was soon clear that this method was more reliable, only after 20 years was an OECD protocol developed [16] following validation via a ring test [17].…”