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Partition coefficient

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Reference
Endpoint:
partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Study period:
16 Nov 2001 to 4 Oct 2002
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
comparable to guideline study
Qualifier:
according to guideline
Guideline:
other: Draft OECD Guideline 122 (Partition Coefficient (n-octanol/water), pH-Metric Method for Ionizable Substances
Version / remarks:
November 2000
Deviations:
yes
Remarks:
pH range used was 3 to 12, instead of 2 to 12.
GLP compliance:
not specified
Type of method:
other: pH-metric titration
Partition coefficient type:
octanol-water
Key result
Type:
log Pow
Partition coefficient:
4.276
Temp.:
25 °C
Remarks on result:
other: Log Pow of un-ionised species.

Conclusion

The calculated dissociation constant (pKa) at zero-percent organic content was 9.59.

The partition coefficient (log Pow) of the un-ionised species was evaluated by pH- metric titration to be 4.276 at nominally 25°C.

Discussion

The lipophilicity profile demonstrates that the log D changed little over the environmentally significant pH range of pH 4 to pH 9 (see table of Log D at Specific pH, below).  In the pH region where the test substance is fully un-ionised, log D is equivalent to log P, and the substance is maximally partitioning in the organic phase. Above pH 9, (pKa value of 9.59), the test substance becomes ionised. This in turn causes its redistribution between the two phases, with more substance shifting to the aqueous phase and the log D value falls to ca 2 at pH 12. The Distribution of the Species plot indicates that in acidic to neutral conditions (pH 2 to 7) the test substance is un-ionised and will partition in the organic phase. As the conditions become basic (pH 8 to 12) the presence of un-ionised test substance reduces with the dominating species of ionised test substance tending to partition in the organic phase.

Indvidual Raw Results

pKa Value

Titration data sets of pH against volume of titrant were manipulated electronically using pKaLOGP v5.1. Six apparent pKa values in the presence of co-solvent were evaluated. Using the same software a “Multi-set” of the six data sets was formed and a Yasuda-Shedlovsky extrapolation performed to calculate the pKa value at zero percent organic content. Two of the six points were excluded from the Yasuda- Shedlovsky plot due to their poor fit with the line.The pKa was evaluated to be 9.59.

Apparent pKa values are presented in Table below.

Methanol Content  (% w/v)

Apparent pKa

49.75

10.415

42.30

10.351

31.99

10.038

57.14

10.551

46.73

10.425

36.84

10.143

Log P Value

Titration data sets of pH against volume of titrant were manipulated electronically using pKaLOGP v5.1 software. Initially the multi-titration containing runs 1, 2 and 3 were performed. Only the first titration containing 0.1 mL of octanol gave a refineable result. Three further individual titrations using 0.2, 0.5 and 0.3 mL of octanol respectively were then performed. The higher volumes of octanol caused problems during the titration, and only those with octanol volumes of 0.1 and 0.2 mL gave refinable results. These two data sets were combined to form a “multi-set” to give a set of averaged results.

 

The calculated log P of the un-ionised species was determined as 4.276, with a goodness of fit (GOF) of data of 2.53.

 

The table below shows the log D value at a specific pH over the environmentally significant pH range of 4 to 9.

Table of Log D at Specific pH:

pH

4

5

6

7

8

9

 

log D

 

4.276

 

4.276

 

4.276

 

4.275

 

4.265

 

4.175

Description of key information

Log Kow is 4.276 at 25 ºC (OECD 122, pH-metric titration in the presence of octanol).

Key value for chemical safety assessment

Log Kow (Log Pow):
4.276
at the temperature of:
25 °C

Additional information

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