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Dissociation constant

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Description of key information

Dissociation constant of ethyl 4-hydroxybenzoate (corresponding acid of notification substance):
pKa = 8.18 (25°C, potentiometry)
pKa = 8.9 (HPLC)
pKa = 8.34 (database)
pKa = 8.22 (review article)

Key value for chemical safety assessment

pKa at 20°C:

Additional information

Sodium ethylparaben is the sodium salt of the corresponding acid ethyl 4-hydroxybenzoate. Depending on the pH both species are convertable into each other. The pKa is only defined for the corresponding acid and is related to the dissociation of the hydroxylic function. Thus the dissociation constant needs to be reported for the corresponding acid ethyl 4-hydroxybenzoate only.

Several reliable sources reporting consistently similar values for the pKa of Ethylparaben in the range of 8.18 to 8.9 determined by different methods. Considering all available values in a weight-of-evidence approach results in an average value of pKa = 8.4 for the dissociation constant of the hydroxylic function of Ethylparaben. This value was used as key value for chemical safety assessment.