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Dissociation constant

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dissociation constant
Data waiving:
study technically not feasible
Justification for data waiving:
Justification for type of information:
The substance is UVCB and includes: glycerides; free fatty acids; partly degraded glycerides; all of the above sulfated (carrying one or more -OSO3(-) groups) and salified by ammonia (ammonium salt); some quantity of mineral compounds (ammonium sulfate); water. The substance has a very low water solubility (see water solubility endpoint, section 4.8) and emulsifies in water (see analytical information).

Regarding the type of molecules included in the substance, glycerides and fatty acids are exempted from registration according to Regulation (EC) No. 1907/2006, Annex IV and V, and gathering information on such molecules is not necessary.
What is not in the form of a salt or is not included in glycerides and fatty acids is however represented by organic compounds containing weak acid functional groups. Therefore, according to Annex XI, section 1, and ECHA documentation "Waiving information requirements", provided with the Webinar on Information requirements, 10 December 2009, the testing does not appear scientifically necessary.

Further, according to REACH Regulation (EC) No. 1907/2006, Annex IX, column 2, the study does not need to be conducted as:
— it is scientifically not possible to perform the test.
Actually, ECHA Guidance on information requirements and chemical safety assessment, R.7a, section R., mentions OECD TG 112 as reference for the test methods to determine the pKa of a substance, while Regulation (EC) No. 440/2008 does not mention any method for this endpoint.
In OECD test guideline 112, three methods are described: the titration method, the spectrophotometric method and the conductometric method.
All the methods require a measure of the concentrations of the dissociated and undissociated forms of the substance and, from a knowledge of the stoichiometry of the dissociation reaction, the appropriate constant can be determined. Being the substance UVCB, it is quite difficult, if not impossible, to significantly predict the species involved, their respective dissociation reactions and to determine their concentrations.
Further, the titration method is not suitable for low solubility compounds and the substance shows a very low solubility. Even if the method allows to use a small amount of water-miscible solvent, in order to aid dissolution of sparingly soluble substances, the method requires to check for the presence of emulsions, letting understand that it is not suitable for substances which emulsify; as the substance emulsifies, this method has been considered not suitable.
Also the spectrophotometric method has been considered not suitable, inter alia because it requires that the compounds have appreciably different UV/VIS-absorbtion spectra for the dissociated and undissociated forms. Looking at the analitycal information section, this is not the case and the method cannot be applied.
Finally, also the applicability of the conductometric method and Onsager equation to the substance seems not reliable, because the substance emulsifies, the species involved are not significantly known, the interations between different species may interfere on the results and it is not feasible to determine the parameters required in the equation.
Reason / purpose:
data waiving: supporting information

Description of key information

Dissociation constant: not applicable

Key value for chemical safety assessment

Additional information

The study is regarded as technically not feasible:

- The standard methods cannot be applied for the substance and according to REACH Regulation (EC) No. 1907/2006, Annex IX, column 2, the study does not need to be conducted as it is scientifically not possible to perform the test. - The substance results to be very slightly soluble and contains only weak acid functional groups and according to Annex XI, section 1, and ECHA documentation on waiving information requirements, the testing does not appear scientifically necessary.