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Partition coefficient

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Description of key information

The determination of the Log Pow is technically not feasible.

Key value for chemical safety assessment

Additional information

Sepisol Fast Red SN is an amphoteric compound made of an organic cationic part and an organic anionic part. Therefore a non-ionized form will not be observed. Dissociation is unlikely to occur unless extreme pH is used. It also shows some surfactant properties.

All the testing methods (experimental and in silico) have been tried or envisaged.

Results are neither reliable nor fully pertinent because of the following reasons:

1- Determination by the HPLC method according the guideline OECD 117.

This method is an estimate and is based on the dependence of partition coefficient to the retention time when measured by HPLC.

When measuring in systems buffered at pH7, the chromatogram of Sepisol Fast Red SN shows several peaks. Thus, the application of this method is not possible.

2 -Determination by the shake flasks method according the guideline OECD 107.

According the guideline "measurement should be made on ionizable substances only in their non-ionized form". This guidance also states that "the shake flask method is impossible to use with surface-active materials (for these, a POW can be calculated from indiviual solubilities in water and n-octanol)". However, the Sepisol Fast Red SN is an amphoteric compound and non-ionized form will not be observed. Furthermore, it shows some surfactant properties. Thus, the application of this method to determine the partition coefficient is not technically feasible.

3 -Determination by the comparison of measured solubilities in octanol and in water separately

This method is advised in the chapter R.7.a ECHA REACH guideline. Due to the high hydrophobicity of the molecule, it leads to a non-reliable and a non-realistic value. The hydrosolubility of the molecule is almost zero (less than 0.1 mg/L). Therefore in some cases the results tend towards the infinity or to non-realistic value. The partition coefficient value obtained with this method was not reliable and makes no sense.

4 -Determination by calculation of the partition coefficient with the software ACD 10.0 or Kowwin

It is not possible to calculate the partition coefficient using the ACD software nor Kowwin for the chemical structures of concern.

5 -Determination by the slow-stirring method inspirated from the guideline OECD 123.

Although it is the most suitable method for the direct determination of log P of highly hydrophobic compound, the substance displays a significant interfacial activity (presence of the Dowfax 2A1) and such substances are out of the applicability of the test. Besides the solubility calculated in water with the wavelength of basic red 1 shows results where a hydrosolubility is observed while it should not. Indeed Sepisol Fast Red SN is insoluble in water and BR1 + residual represents a very small proportion and it has already been shown to be less than 3.3 mg/L. It means a phase exchange took place (microdroplet of octanol in water) and thus the results are biased (the presence of too many artefacts which distorts the results of the hydrosolubiltiy of the molecule). It is not possible to use this method to determine the partition coefficient.


It has been concluded that it was technically not possible to determine a log Pow for this molecule. The results obtained from the different methods show that the log Pow of the molecule should be comprised between 4 and the infinity since the substance is considered to be almost insoluble in water and soluble in octanol.

The log Pow measured thanks to the slow-stirring method concerns solely the cationic part (so the basic red 1) since it is the only part of the molecule which is detected in water. BR1 is present in very few amounts and its measured log Pow is about 3.6. In no circumstances, this value is representative of the Sepisol Fast Red SN.

To conclude, it is not technically possible to measure the coefficient partition for the test item. However the value is most likely expected to be > 4.