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Partition coefficient

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Description of key information

Partition coefficient: 2.9

Key value for chemical safety assessment

Log Kow (Log Pow):
2.9
at the temperature of:
20 °C

Additional information

The Key Study is a guideline study.

·        The following items support the applicability of this study for the evaluation of the partition coefficient of this mixture subsequent applicability for use in the evaluation of bioaccumulation of the registered substance:

1.      This submission is intended to cover a product with tert-alkyl amines (R-NH2) (R is a tert alkyl with total carbon = C10-C14). It is a mixture with approximately 6%, 19%, 70.5%, 3.75% and 0.75% of the C10, C11, C12, C13 and C14, respectively. The logKOWvalue was calculated based on the total areas under the broad envelops of peaks for both water and octanol phases in the GC-NPD analysis, or the total titratable basicity in the titration method. Therefore, the logKOWvalues should be viewed as the average logKOWvalue for the product mixture. Note that this value will not be comparable with any logKOWvalue for non-ionic organic compound (see point #3 below). 

2.      In the case of the registered substance, it is a mixture with mostly C10-C12 (together ~95.5%) and only < 5% C13-C14. The separation and analysis of each compounds without changing the distribution of species (neutral vs ionisable) under environmentally relevant conditions are difficult, if not impossible. Theoretically, logKOWvalues of the minor components C13 and C14 will be higher and C10 and C11 will be lower than those of the major component C12. The measured average logKOWvalue will mostly likely to correspond to C12, although it may be affected by other species. According to EPI Suite, for neutral species of those compounds, each addition of one –CH2- into the R will increase logKOWby 0.49 log unit. If the log KOW of 2.9 is assumed for C12, the estimated logKOWwill be 1.92, 2.41, 3.39 and 3.88 for C10, C11, C13 and C14, respectively, assuming the chain length effect on logKOWfor all species will be the same. In general, the longer chain length isomers are a minor component of the overall product and will contribute significantly less to the potential of the product to bioaccumulate. The product average logKOWof 2.9 is also likely a conservative estimate of the bioaccumulation potential of the product as a whole. Thus, it can be concluded that the bioaccumulation potential of the product is low consistent with the criteria in Annex XI, Section 9.3.2 (e.g., logKow < 3). In addition, the consideration of the product as a “mixture” from a bioaccumulation perspective is also consistent with the evaluation of the ecotoxicity and environmental fate of the product as a mixture rather than individual isomers.

3.      The pKa range of the primary amine compounds is estimated as 10.56 to 10.80 +/- 0.25 for the source substance. However, these base compounds will exist mostly as positive ions (RNH3+) at pH < pKa due to their “association” with protons and then become mostly neutral (RNH2) at pH > pKa:

RNH2 + H2O«RNH3+ + OH-

Kb = [RNH3+][OH-]/[RNH2]

[RNH3+]/[RNH2] = 10^(pKa – pH)

Therefore, the amine groups do not “dissociate” within the environmental pH range.

Measuring the logKOWvalues at some arbitrarily given high pH (such as 11.56) will be not useful in describing the partition behaviors of these ionisable compounds. This is because the compounds will undergo acid/base equilibrium in the environment besides partitioning to natural organic matter or lipids. Therefore, accurate partition coefficients should be based on the sum of neutral and ionized amine concentrations in octanol versus those in water. Using the cited pKa values, it is easy to show that >97% (at pH 9) to >99.999 % (at pH 5) of these compounds will exist in water as protonated positive ions, RNH3+, within the environmental pH range of 5 to 9, as ECHA specified. For that reason, the logKOWvalue for the neutral species RNH2 obtained at e.g., pH 11.56 has no relevance to environmental behavior of these compounds. On the other hand, the reported “logKOW” value was obtained using a method lumping together both ionized and neutral amine species and should be more representative and more environmentally relevant.