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Partition coefficient

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Endpoint:
partition coefficient
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
Pitch, coal tar, high-temp. (coal-tar pitch, CTPht), is an UVCB substance of variable composition. It consists of a complex mixture of polycyclic aromatic hydrocarbons (PAH), predominantly of highly condensed aromatic ring systems forming an inert matrix. In this matrix, individual PAH with a lower degree of condensation are included (mostly four- and five-membered ring aromatic PAH). Physico-chemical properties and biological effects of coal-tar pitch are characterised on the one hand by the inert matrix, on the other hand by the individual PAH that may be set free from the inert matrix.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Experimental determination of partition coefficients was performed with individual PAH included in the list of the 16 EPA PAH. Selected values are taken as characteristic for the environmental fate of the substance pitch, coal tar, high-temp. as such.

3. ANALOGUE APPROACH JUSTIFICATION
As consequence of the production process (residue of the oxygen free high-temperature distillation of tar, coal, high-temperature, CAS No. 65996-89-6), coal-tar pitch consists mainly of a highly polycondensed, to some extent carbonised inert matrix including some less condensed materials (polycyclic aromatic hydrocarbons). This is demonstrated by the fact, that only a fraction of ca. 20 % of the substance will evaporate at a distillation temperature up to 500 °C. This fraction comprises polycyclic aromatic hydrocarbons with predominantly 4- and 5-membered (up to 6) rings. CTPht is scarcely soluble in water (ca. 0.24 mg dissolved C/L at a loading of 1000 mg substance/L) and only limited soluble in organic solvents like quinoline or toluene. Most of the substance is not accessible by common analytical tools. Only about 10 % can be identified by GC. Fraction of the 16 EPA PAH in GC analysis is up to 7.5 % with up to 1.5 % of them being benzo[a]pyrene (BaP).
Under environmental conditions or during processing of the substance, environmentally available or volatile components of CTPht may be released. This will be PAH (mainly four and five ring representatives) incorporated into the inert matrix. The most relevant PAH among them under an environmental or toxic point of view is benzo[a]pyrene. This substance is besides benzo[b]fluoranthene the most abundant PAH, determined under the components of CTPht analysable by GC.
For these reasons, BaP is selected as marker substance for the substance CTPht itself. Data determined for BaP will be taken to characterise CTPht as a whole and will be used in the chemical safety assessment of CTPht.
Reason / purpose for cross-reference:
read-across source
Principles of method if other than guideline:
Read-across to preceding entry:
Source test material: Mixture of PAH (de Maagd, log Pow study)
Reference: de Maagd et al. 1998
Key result
Type:
log Pow
Partition coefficient:
6.13
Temp.:
25 °C
Remarks on result:
other: log Pow value for benzo[def]chrysene (benzo[a]pyrene)
Type:
log Pow
Partition coefficient:
>= 4.57 - <= 6.22
Temp.:
25 °C
Remarks on result:
other: range of log Pow values for selected PAH (phenanthrene to benzo[ghi]perylene)
Details on results:
These log Pow values are used to specify the octanol water partition characteristics of CTPht
Endpoint:
partition coefficient
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
The source test materials are individual polycyclic aromatic hydrocarbons. Depending on size/molecular weight and state of condensation, physico-chemical and environmental properties can be quite different.
The target substance pitch, coal tar, high-temp. is composed of a broad range of PAH differing in size and degree of condensation. Physico-chemical and environmental properties of CTPht will be characterised by the range of PAH that are components of CTPht.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Source chemicals are individual PAH. The specific analytical purity of individual chemicals is not reported. But they are obtained from commercial sources and are described as being of the highest available commercial quality. Hence, the results reported related to the individual test substances.
The target material pitch, coal tar, high-temp. is a UVCB substance obtained by high temperature distillation of tar, coal, high-temperature (CAS No. 65996-89-6). It consists of a complex mixture of polycyclic aromatic hydrocarbons, predominantly of highly condensed to some extent carbonised aromatic ring systems forming an inert matrix. In this matrix, individual PAH with a lower degree of condensation are enclosed. PAH available from the solid material consist of four and five (up to six) condensed ring systems. Fraction of the 16 EPA PAH in GC analysis is up to 7.5 % with up to 1.5 % of them being BaP.

3. ANALOGUE APPROACH JUSTIFICATION
Properties of the target substance CTPht relevant for environmental distribution will be determined by the properties of the PAH that are constituents of CTPht. Under environmental conditions or during processing of the target substance, environmentally available or volatile components of CTPht may be released. This will be PAH (mainly four and five ring representatives) incorporated into the inert matrix. The most relevant PAH among them under an environmental or toxic point of view is benzo[a]pyrene. This substance is besides benzo[b]fluoranthene the most abundant PAH, determined under the components of CTPht analysable by GC. For these reasons, BaP is selected as marker substance for the substance CTPht itself. Data determined for BaP will be taken to characterise CTPht as a whole and will be used in the chemical safety assessment of CTPht.
Reason / purpose for cross-reference:
read-across source
Principles of method if other than guideline:
Read-across to preceding entry:
Source test material: polycyclic aromatic hydrocarbons (PAH), generic mixture
Reference: Miller et al. 1985
Key result
Type:
log Pow
Partition coefficient:
5.98
Temp.:
25 °C
Remarks on result:
other: log Pow value for benzo[def]chrysene (benzo[a]pyrene)
Remarks:
the test result of the source substance is adopted as weight of evidence for the target substance pitch, coal tar, high-temp.
Key result
Type:
log Pow
Partition coefficient:
>= 4.5 - <= 6
Temp.:
25 °C
Remarks on result:
other: range of log Pow values for PAH components of pitch, coal tar, high-temp. (phenanthrene to benzo[a]pyrene)
Remarks:
the test results of the source substances are adopted as weight of evidence for the target substance pitch, coal tar, high-temp.
Details on results:
Note: The range of log Pow of approx. 4.5 and approx. 6.0 and above relates only to approx. 10 % of coal-tar pitch, i.e. the GC-analysable fraction.
Only about 27 - 29 % of the pitch mass were volatile up to a temperature of > 500 °C (simulated distillation).

Description of key information

The determination of Pow is not applicable to the substance pitch, coal tar, high-temp. itself, but only to single PAHs. Due to its physico-chemical and ecotoxic properties, benzo[a]pyrene is selected as marker substance for representing distribution properties of the UVCB substance pitch, coal tar, high-temp.

Key value for chemical safety assessment

Log Kow (Log Pow):
6.13
at the temperature of:
20 °C

Additional information

An octanol-water partition coefficient cannot be determined for the almost water-insoluble complex UVCB substance pitch, coal tar, high-temp. The given log Kow value, relates to benzo[a]pyrene (BaP) as marker substance. It is taken from de Maagd et al. 1998 and is an average of two measurements (log Kow 5.98 and 6.24) at different BaP concentrations. The lower value is identical to the log Kow value for BaP reported by Miller er al. 1985. Similar results supplemented by log Kow values of additional PAH are reported in references EU (2008) and Mackay et al. (1992).

Sources

EU (2008) Coal-Tar Pitch, high temperature - Risk Assessment. European Union Risk Assessment Report, The Netherlands (URL: https://echa.europa.eu/documents/10162/433ccfe1-f9a5-4420-9dae-bb316f898fe1)

According to EU 2008, data are presented from:

Mackay D, Shiu WY, Ma KC (1992). lllustrated Handbook of Physical-Chemical Properties and Environmental Fate of Organic Chemicals. Lewis Publishers, Boca Raton, FL.