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Partition coefficient

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Description of key information

Log Pow = 3.34 at 25 °C and pH 9.35(measured; Laus, 2010)
  
  
  
  
  
  
  
    
    
    
    
    

pH was measured with 9.35. The following result was obtained: Log Pow = 3.338 ± 0.046 (25 °C; pH 9.35) RSD (relative standard variation) = 1.4%

The draw-back of the study is, that Pow was not determined for each ionisation state (protonated / not protonated):

Given the pKa of tributylamine at 25 °C (10.89) at pH of determination (pH 9.35), approximately 97% will be present as tertiary ammonium ion, while ca. 3% will be in the neutral, non-protonated form.

In consequence, log Kow will be lower than 3.34 in the environmentally relevant neutral pH-range, while the log Kow value for the neutral molecule will be higher and could only be determined at very high pH values (pronouncedly higher than pKa of 10.89 at 25 °C).

This fits with the results determined in the supporting study (BASF

Key value for chemical safety assessment

Log Kow (Log Pow):
3.34
at the temperature of:
25 °C

Additional information

In the reliable (with restrictions) key study (LAUS, 2010), the log Pow was determined for the submission substance tributylamine according to OECD 123 (Slow-Stirring Method) compliant with GLP. pH was measured with 9.35. The following result was obtained:

Log Pow = 3.338 ± 0.046 (25 °C; pH 9.35)

RSD (relative standard variation) = 1.4%

The draw-back of the study is that Pow was not determined for each ionisation state of the ionisable tributylamine: given a pKa at 25°C of 10.89, at pH 9.35 as determined within the study approximately 3% of tributylamine will be present in the neutral state, while ca. 97% will be present as tertiary ammonium ion.

In conclusion, log Pow will be lower in the environmentally relevant neutral pH range around pH 7, while log Pow will be higher for the neutral, non-protonated form. Pow for the latter could only be determined at very high pH-values significantly above pKa.

This fits with the data from the supporting study (BASF, 1988): a lower log Pow of 2.4 at 25°C was determined at the pH of 9.0. Because pH was less alkaline in this study, a higher fraction of tributylamine was protonated (neutral state ca. 1.27%, i.e. less than half the fraction at pH 9.35) and thus log Pow was lower.