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Partition coefficient

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Reference
Endpoint:
partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Remarks:
comparable method to OECD guideline, but with some information not present.
Principles of method if other than guideline:
Monsanto Environmental Sciences method ES-79-M-15: Log Kow of the substance was measured by equilibrating aqueous solutions of radiolabelled compounds with n-octanol. The concentrations of the substance in each phase were determined by Liquid Scintillation Counting (LSC).

equilibrating aqueous solutions of radiolabelled compounds with n-octanol
GLP compliance:
no
Type of method:
other: equilibrating aqueous solutions of radiolabelled compounds with n-octanol
Partition coefficient type:
octanol-water
Key result
Type:
log Pow
Partition coefficient:
-3.57
Remarks on result:
other: No information on pH and temperature stated, 1000 mg/L concentration used
Type:
Pow
Partition coefficient:
0
Remarks on result:
other: No information on pH and temperature stated, 1000 mg/L concentration used
Key result
Type:
log Pow
Partition coefficient:
-3.5
Remarks on result:
other: No information on pH and temperature stated, 100 mg/L concentration used
Type:
Pow
Partition coefficient:
0
Remarks on result:
other: No information on pH and temperature stated, 100 mg/L concentration used

Test concentration:  Equivalent to:        Kow:
100 mg/l            50 mg active acid/l    3.16 x 10-4
1000 mg/l           500 mg active acid/l   2.7 x 10-4

Conclusions:
log Kow values of -3.5 and -3.57 at water concentrations of 100 ppm and 1000 ppm respectively were reported for ATMP-H in a reliable study conducted according to generally accepted scientific principles. The result is considered to be reliable.

Description of key information

log Kow [ATMP-xNH4]: <-3.5

log Kow [ammonia]: not relevant for an inorganic substance

Key value for chemical safety assessment

Log Kow (Log Pow):
-3.5

Additional information

Log Kow values of -3.50 and -3.57 at 100 ppm and 1000 ppm respectively were reported for the parent acid, ATMP-H in a reliable study conducted according to generally accepted scientific principles comparable to an appropriate test guideline. The method has been adequately described in the report. The result is considered to be reliable and has been assigned as key study.

 

ATMP-xNH4 is freely soluble in water and, therefore, the ATMP anion is fully dissociated from its potassium cations when in solution. The ATMP species has six P-OH groups that can be ionised. They lose a hydrogen to form a negatively charged group (P-O-). As the pH increases, the number of ionised groups increases. Under any given conditions, the degree of ionisation of the ATMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was ATMP-xNH4, ATMP-xNa, ATMP-H or another salt of ATMP.

 

Therefore, the log Kow measured for ATMP-H is directly relevant for ATMP-xNH4. The log Kow for an ionising substance will vary with pH as the ionised species will have a greater hydrophilicity and lower log Kow than the neutral species. The pH was not measured in the study with ATMP-H, however, a solution of ATMP-H in unbuffered water will have a very low pH. Therefore, the log Kow value relevant for the chemical safety assessment of ATMP-xNH4 has been reported as <-3.5.

 

For further details of the speciation of ATMP see PFA 2020.

 

This value is supported by a predicted value of -4 for ATMP-H using a relevant estimation method.

Log Kow is not applicable for ammonia, because it is an inorganic substance. However, an indicative value of 0.23 at 20°C has been predicted using the software KOWWIN (KOWWIN v1.68, US EPA, 2010) and this value is used in the exposure modelling. The value applies to unionised ammonia, NH3.

 

Reference

PFA 2020: ATMP speciation at pH relevant to Ecotoxicology and Toxicology, Reference: PFA.224.607.001, Nov. 2020.