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Partition coefficient

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Description of key information

Using the slow -stirring method according to OECD TG 123 the log Pow was determined to be 4.4 at 25 °C.

Key value for chemical safety assessment

Log Kow (Log Pow):
at the temperature of:
25 °C

Additional information

Slow stirring method

A study was conducted according to OECD TG 123 and Regulation (EC) 440/2008 part A A.28 to determine the log Pow of the test item. In a preliminary test the length of the equilibration period was assessed. Using a HPLC method at time points 24, 29, 48 and 72 hours after adding the test substance, the concentration of the test substance was determined in both the 1-octanol and water phase. The logarithmic values of partition coefficient octanol/water (log Pow), calculated using the final concentrations of the test substance at four consecutive time points, showed no significant difference over the several time points. A stirring time of 24 hours was chosen to be applied in the definitive test. Three reaction vessels were filled with 980 mL of octanol saturated water and 20 mL of test item dissolved in water saturated octanol and thermostated. The test was performed in the dark and at a temperature of 25 ± 1°C. At 24 hours after adding the test substance, stirring was stopped and samples were taken from the octanol as well as the water phase. The log Pow values of the replicates were calculated using the final concentrations. The average log Pow value for the test substance was calculated to be 4.4 at 25 °C.


HPLC method

The partition coefficient Pow of tert-Butylperoxy-3,5,5-trimethylhexanoat was determined using HPLC. The test item was dissolved in acetonitrile. The concentration of this solution was 1 mg/mL. It was diluted with eluent to 100μg/mL. The partition coefficient Pow was determined based on a calibration curves using nine reference substances. The capacity factors k were calculated from the dead time and retention times of the reference substances. The log k data of the reference substances were plotted against their log Pow values. The log Pow was determined to be 5.16 ± 0.002 (with 95 % confidence interval). The estimated Pow is 142923 ± 609 (with 95 % confidence interval) at 25 °C. Quality criteria were met, as values of log P derived from individual measurements fell within a range of 0.1 log units.



The HPLC method according to OECD TG 107 was conducted to estimate the logPow of the test item. Using the HPLC method the log Pow was not measured directly but from a correlation between log k (capacity factor) and log Pow or a series of reference substances. It therefore depends on the choice of reference substances. The log Pow was determined to be 5.16 at 20 °C. The log Pow value was close to the threshold value of 5.00 used in chemical safety assessment. Thus, for a more accurate determination of the log Pow a second study was conducted in accordance with OECD TG 123. Three independent trials were conducted. The concentration of the test item in each phase was determined using a HPLC-UV method. The average logPow of the three trails was calculated to be 4.4. The logPow value determined using slow-stirring method was regarded to be more reliable as the concentration of the test item in each phase was measured directly after the equilibrium was reached.

Therefore, a logPow of 4.4 was chosen as key value for chemical safety assessment.