Octylamine

Partition coefficient

Description of key information

The log Pow was estimated as 3.46. This represents the average (geomentric mean value) of three reliable experimental results (weight of evidence).

Key value for chemical safety assessment

Log Kow (Log Pow):

3.46

at the temperature of:

20 °C

Additional information

n-Octylamine is a difficult substance with regard to the determination of the partitioning coefficient n-octanol water (Pow): It is a strong base, such that considerable ionisation (primary ammonium ion) upon solubilisation in water occurs, and the primary ammonium derivative is surface active.

According to EU method A.9 (Partition Coefficient), neither of the two alternative methods outlined do fully apply:

The shake-flask method is not applicable to surface active materials: In conclusion, it could only be applied to a highly alkaline solution (pH 13 or higher), where less than 1% is present as primary ammonium ion and surface activity is thus minimal.

The HPLC method is neither applicable to surface active materials nor to strong bases. However, ionisable compounds may be tested in buffered solutions. However, as the column properties do not allow for pH-values above 8, only the primary ammonium derivative can be tested, which is surface active.

In conclusion, either the shake-flask method could possibly be applied if a bufferd solution at very high pH is used, or - according to EU method A.9 - Pow should be calculated from the solubilities in water and n-octanol.

In the WOE study by Clariant (2010; RL 2), the latter approach was followed:

Solubility in water: The critical micell concentration (CMC) was determined as the most meaningful value for surface active compounds: 320 mg/L. From the pKa of 10.6 and the concentration at the cmc in pure water (0.32 g/L), the approximation of pH by calculation yields pH of 10.9. This corrosponds with a protonated fraction of n-octylamine of 32.9% according to Henderson-Hasselbach equation.

Solubility in n-octanol: n-octylamine turned out to be fully miscible with n-octanol. Thus, as an upper limit value for the solubility in octanol, the density of n-octylamine is used: 782.6 g/L (20°C).

From both solubilities, a log Pow of 3.39 was estimated by calculation.

In the second WOE-study (Gagnaire at al., 1993) the shake flask method was applied to determine the Pow (OECD 107). The pH of the aqueous phase was set at pH 13 to prevent ionization. As such, surface active properties may expected to be minimal. With this setting, a log Pow of 3.3 was determined. The study is reliable with restrictions (RL 2, publication), the most important drawback is that the applied concentration of n-octylamine is not reported and results for different octanol - water ratios and possible concentration dependence are not given. According to OECD 107, a maxium concentration of test item in either phase of 0.01 mol should not be exceeded.

The third WOE-study (AQura, 2010; reliability category 1) applied the HPLC-method: the partition coefficient n-octanol / water (Pow) of the test item was determined according to OECD guideline no. 117 [adopted on 13 April 2004] and EU test method A.8 [Directive 92/69/EEC, Official Journal L 383 A 1992]. A partition coefficient n-octanol / water (Pow) of Pow = 5012 / log Pow = 3.7 resulted from this method at a buffer pH of 7.5.

Due to the substance properties or deficits in reporting, respectively, all three studies have their drawbacks as outlined above. In principle, log Pow for n-octanol should be highest in the non-protonated state, i.e. according to the set-up of the shake flask study applying a pH of 13. However, this value is the lowest (log Pow 3.3), while the value determined by the HPLC method is the highest one (log Pow 3.7) in spite of that full protonation may be assumed at this pH. Due to the fact that all three values are rather close to one another, the geometric mean value is used as key value for risk assessment.