Bis(2-ethylhexyl) hydrogen phosphate

Adsorption / desorption

Description of key information

The adsorption coefficient of the dissociated form of bis(2-ethylhexyl) hydrogen phosphate was determined by HPLC was log Koc = 2.4 at 25°C. This value indicates a low sorption to soil and sediment.

Key value for chemical safety assessment

Koc at 20 °C:

251

Additional information

Based upon its pKa, bis(2-ethylhexyl) hydrogen phosphate exits predominantly in its dissociated form under relevant environmental pH 5 -9. The non-dissociated form of bis(2-ethylhexyl) phosphate exists at pH values <0.5.

The determination of the adsorption coefficient of the non-dissociated form of bis(2-ethylhexyl) hydrogen phosphate could not be performed at pH <0.5 because the material of the HPLC column is not stable at pH values <2. For this reason the determination of the adsorption coefficient of bis(2-ethylhexyl) hydrogen phosphate was carried out at pH 6 with the dissociated form.

The adsorption coefficient (Koc) of bis(2-ethylhexyl) hydrogen phosphate was determined according to the HPLC method mentioned in EU-method C.19 and OECD TG 121 at 40 °C. In order to comply with the guideline OECD 121 and considering environmental relevance, pH 6 was chosen as pH value for the buffer. The reference Koc values in the OECD-guideline presumably refer to 25 °C. Even though the column temperature was 40 °C, this temperature is without influence on the result of the determination. The calibration was done referring to Koc values of reference substances determined at 25 °C. For adsorption coefficient of the dissociated form of bis(2-ethylhexyl) hydrogen phosphate determined by HPLC was log Koc = 2.4.

In addition, the organic carbon partition coefficient (Koc) for the dissociated and non-dissociated form of bis(2-ethylhexyl) hydrogen phosphate was predicted using the QSAR calculation of the Estimation Program Interface (EPI) Suite v 4.11. For bis(2-ethylhexyl) hydrogen phosphate KOCWIN estimated the log Koc using two separate estimation methodologies. The estimations are based on the first-order Molecular Connectivity Index (MCI) and the log Kow (internally calculated by KOCWIN), respectively.

The log Koc was estimated to be 4.39 (log Kow method) and 4.31 (MCI method) for the nondissociated form of bis(2-ethylhexyl) hydrogen phosphate. In order to estimate the organic carbon partition coefficient (log Koc) for the dissociated form of bis(2-ethylhexyl) hydrogen phosphate, the corresponding sodium salt of bis(2-ethylhexyl) hydrogen phosphate (CAS No. 141-65-1) was chosen as a surrogate for calculation, as generally estimation by the program apply to compounds that are predominantly in a non-ionized/non-dissociated form. Estimation of the dissociated form of a substance via MCI-method is not considered by the program. The log Koc for the dissociated form of bis(2-ethylhexyl) hydrogen phosphate was estimated to be 2.62 at 25°C. The result for the non-dissociated form and dissociated form of bis(2-ethylhexyl) hydrogen phosphate falls within the applicability domain described above and the applied estimation rules appear appropriate and the predicted value can be considered reliable.

It is shown, that the estimated log Koc value for the dissociated form of bis(2-ethylhexyl) hydrogen phosphateis in good correlation with the experimental determined value. As bis(2-ethylhexyl) hydrogen phosphate is mainly present in the dissociated form at environmental relevant pH, the Koc value for the dissociated form of bis(2-ethylhexyl) hydrogen phosphate is taken into account for assessment.

[LogKoc: 2.4]