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EC number: 252-488-1 | CAS number: 35285-69-9
Adsorption / desorption
Administrative data
Link to relevant study record(s)
Description of key information
Propyl 4-hydroxybenzoate (= propylparaben) (CAS 94-13-3): log Koc = 2.72 (Koc = 519.9554) at pH 7 (QSAR; Franco, Fu & Trapp 2009 for ionisable substances)
Key value for chemical safety assessment
- Koc at 20 °C:
- 519.955
Additional information
The log Pow of the substance sodium propylparaben (CAS 35285-69-9) and its corresponding acid form propyl 4-hydroxybenzoate (= propylparaben) (CAS 94-13-3) were experimentally determined to be 0.27 (at 23°C, pH 10.8) and 3.00 (at 22°C, pH 7.5). Additionally the log Pow of propyl 4-hydroxybenzoate (= propylparaben) (CAS 94-13-3) was calculated by QSAR (EPISuite) and corroborates the measured result with a log Pow value of 2.98.
In accordance with Regulation (EC) No 1907/2006, Column 2 of Annex VIII, section 9.3.1, studies on adsorption/desorption do not have to be conducted as sodium propylparaben (CAS 35285-69-9) has a low potential for adsorption based on the log Kow of < 3 and thus the endpoint is waived. However, for the purpose of the chemical safety assessment further information on the adsorption behavior was collected and is provided here using an appropriate calculation method. Sodium propylparaben (CAS 35285-69-9) is the sodium salt of the corresponding acid propyl 4-hydroxybenzoate hydroxybenzoate (= propylparaben) (CAS 94-13-3). The pKa is defined for the corresponding acid and is related to the dissociation of the hydroxylic group.
Considering the ionization properties of the substance, the adsorption potential has been calculated using Franco, Fu and Trapp‘s expressions (2009) on estimation of the organic carbon-water partitioning coefficient of ionisable organic substances. The log Kow for the neutral molecule, the pH as well as the pKa values were used as input parameters. Measured pKa values are available from the literature (see section 4.21) from which a mean pKa of 8.46 was calculated. This value was used as an input parameter for the calculation of the Koc coefficient.
The following equation was used for Koc calculation as suggested by the authors:
Koc = (10 ^ (0.54*logPn+1.11) / (1+10 ^ (pH-0.6-pKa))) + (10 ^ (0.11*logPn+1.54) / (1+10 ^ (pKa-pH+0.6)))
Where:
Pn (Kow of the neutral molecule): 2.98
pKa: 8.46
pH: 4, 7, and 9
The log Koc was calculated to be 2.72 at pH 4 and 7 and 2.5 at pH 9. These values indicate a low to moderate adsorption potential of the substance in soils and sediments. According to the Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.7.1.8.5 the log D value for the ionized molecule determined around the default pH of 7 is considered more relevant for chemical safety assessment. On this basis, the calculated log Koc of 2.72 (Koc = 519.9554) at pH 7 has been used as key value for the chemical safety assessment.
Reference:
Franco A., Fu W., and Trapp S. (2009).Influence of soil pH on the sorption of ionizable chemicals: modeling advances. Environmental Toxicology and Chemistry 3: 458-464, doi: 10.1897/08-178.1
[LogKoc: 2.72]
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