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Dissociation constant

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Description of key information

According to Column 2 of REACh Annex IX, measurement of dissociation constant is not required where it is technically not feasible to do so.  The substance is UVCB, with several components possessing one or more ionizable functional groups.  Measurement of pKa would result in several overlapping pKa values, which could not be attributed to specific components of the mixture.

Key value for chemical safety assessment

Additional information

According to Column 2 of REACh Annex IX, measurement of dissociation constant is not required where it is technically not feasible to do so. The substance is UVCB, with several components possessing one or more ionizable functional groups. Measurement of pKa would result in several overlapping pKa values, which could not be attributed to specific components of the mixture. According to ECHA Chapter 7a (2008) guidance, the assessment of pKa values for UVCB substances which include ionizable components should be fulfilled by estimating the individual components’ pKa values, if appropriate. Thus, the ACD Labs pKa DB software (v ) was used to estimate apparent pKa values of the four primary components at 25 °C and 0 ionic strength:

CASRN 24599 -21 -1:

pKa1 = 1.78 ± 0.10

pKa2 = 6.27 ± 0.30

CASRN 32435 -46 -4:

pKa = 0.57 ± 0.50

CASRN 7664 -38 -2:

pKa1 = 1.97

pKa2 = 6.42

pKa3= 12.5

As the pKa1 values for these key components of the UVCB substances are </= 1.97, these components will occur as fully-ionized species over the pH range which is relevant to environmental and physiological conditions (pH 2 - 9)