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Partition coefficient

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Reference
Endpoint:
partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 123 (Partition Coefficient (1-Octanol / Water), Slow-Stirring Method)
Deviations:
no
GLP compliance:
yes
Type of method:
slow-stirring method
Partition coefficient type:
octanol-water
Analytical method:
high-performance liquid chromatography
Type:
log Pow
Partition coefficient:
ca. 3.06
Temp.:
25 °C
Remarks on result:
other: no data on pH available

EVALUATION OF THE TEST RESULTS

Data handling

The logPow value is obtained by dividing the concentration of the test item in 1-octanol by its concentration in the aqueous solution, as follows:

logPow = log (c1 -octanol/cwater)

c1 -octanol: concentration of the test item in 1-octanol phase

cwater: concentration of the test item in aqueous phase

 

The average value of log POW of different experimental units (here, each of the three replicates) will be calculated as the average of the results of the individual experimental units weighted with their respective variances.

log POW,Av = (Σwi × log POW,i) × (Σwi)-1

where:

log POW,i: log POW value of the individual experimental unit i

log POW,Av: weighted average value of individual log POW determinations

wi: the statistical weight assigned to the log POW value of the experimental unit i.

The reciprocal of the variance of log POW,i is employed as wi (wi = var (log POW,i)-1)

The error of the average of log POW is estimated as the repeatability of log CO/CW determined during the equilibrium phase in the individual experimental units. It is expressed as the weighted standard deviation of log POW,Av (σlog POW,Av), which in turn is a measure of the error associated with log POW,Av. The weighted standard deviation can be computed from the weighted variance (varlog POW,Av) as follows:

varlog POW,Av= (Σwi×(log POW,i-log POW,Av)2) × (Σwi×(n-1))-1

σlog POW,Av= (varlog POW,Av)0.5

The symbol n stands for the number of experimental units, and wi is the statistical weight of the experimental unit as defined above.

Furthermore, a mass balance was determined at the end of the test, considering all concentrations measured and all volumes withdrawn from the vessels to show that the recovery rates are in a suitable range.

Validity criterion

The test guideline OECD 123 does not give clear validity criteria. However, the attainment of the chemical equilibrium between the two solvent phases should be demonstrated. To this end, the log POW is calculated for each sampling time and plotted against time. Equilibrium is attained, if the slope of the regression from log POW against time over at least four samplings yields a slope that is not significantly different from zero at a p-level of 0.05. The significance of the slope of the regression for each replicate was assessed with the Data-Analysis Tool of Excel, Microsoft Office 10.

 

RESULTS AND DISCUSSION

Determination of the log POW

The log POW of test item was determined according to TG OECD 123.

The determined log POW values ranged from 3.02 to 3.08 over the whole test period and all replicates.

For each time point and for each of the three replicates, the mass balance calculation demonstra-ted that the experiment was reliably conducted, as the recovery rates ranged from 95% to 102%. This was further confirmed by the analysis of the samples from the blank replicate, which showed no interferences at the retention time of the test item in both, the 1-octanol and the water phase.

The determined log POW value of Test Item is 3.06 ± 0.01 at about 25 °C.

Validity of the test

Attainment of the chemical equilibrium between the two solvent phases could be demonstrated for each replicate, as a linear regression of the log POW values against sampling times showed for each replicate, that the system was in equilibrium over the whole sampling period (see Table 2 and Figure 1). Thus, all data points could be used for the final calculation of the log POW value.

Further remarks

There is a considerable difference between the theoretical and the experimental log POW value of 4.17 and 3.06, respectively. The reason for that is unknown. However, the experimental log POW of 3.06 is in good agreement with the water solubility of 95 mg/l as given by the sponsor. In comparison, the theoretical log POW of 4.17 corresponds to a water solubility of only 0.94 mg/l according to KOWWIN v1.68, EPI Suite v4.11.

Conclusions:
logPow = 3.06 at 25 °C
Executive summary:

Method

The partition coefficient between 1-octanol and water (log POW) of the test item was determined by the slow-stirring method, according to TG OECD 123. Water, 1-octanol and the test item were equilibrated in a thermostat cabinet at about 25 °C and the exchange between the phases was accelerated by stirring.

Observations

Both phases of the three replicate assays were sampled after 93, 100, 116, 125 and 146 hours of stirring. Subsequently, the samples were analyzed by HPLC. The determined log POW values ranged from 3.02 to 3.08 over the whole test period and all replicates.

A linear regression of the log POW values against sampling times showed for each replicate, that the system was in equilibrium over the whole sampling period. Thus, all data points could be used for the final calculation of the log POW value.

For each time point and for each of the three replicates, the mass balance calculation demonstra-ted that the experiment was reliably conducted, as the recovery rates ranged from 95% to 102%. This was further confirmed by the analysis of the samples from the blank replicate, which showed no interferences at the retention time of the test item in both, the 1-octanol and the water phase.

Results

The determined log POW value of the substance is 3.06 ± 0.01 at about 25 °C.

Description of key information

logPow = 3.06 at 25 °C

Key value for chemical safety assessment

Log Kow (Log Pow):
3.06
at the temperature of:
25 °C

Additional information

OECD 123

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