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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:
DTPMP (1-3Na) is a member of the DTPMP category, which is comprised of the acid form and any simple salts of the same acid.

The category hypothesis is that all the members are various ionised forms of the same parent acid. This category covers various sodium, potassium and ammonium salts of diethylene triamine penta(methylene phosphonic acid). The different salts are prepared by neutralising the acid to a specific pH and accordingly the proportions of the constituents and their degree of ionisation are comparable between substances under similar conditions (in vivo and in the environment). All category members are based on the DTPMP structure. Data are available for the acid form and some salts.

The read-across is therefore based on the presence of common functional groups in both the acid and the salt. The main assumption is that sodium is not significant in respect of all the properties under consideration. The sodium salt is expected to decompose before boiling and have a high melting temperature.
Reason / purpose for cross-reference:
read-across source
Type:
log Pow
Partition coefficient:
-3.4
Remarks on result:
other: Temperature and pH not stated
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
Principles of method if other than guideline:
Method: other (measured): 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).
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:
-3.4
Remarks on result:
other: Temperature and pH not stated

Test concentration       Kow
50 mg/L             4.46 x 10-4
500 mg/L             3.49 x 10-4

Test concentrations are expressed as active acid.

Conclusions:
log Kow value of -3.4 was obtained for DTPMP-H using a relevant method. The result is considered to be reliable.

Description of key information

log Kow [DTPMP (1-3Na)]: <-3.4

Key value for chemical safety assessment

Log Kow (Log Pow):
-3.4

Additional information

Log Kow values of -3.4 and -3.5 at 50 mg/L and 500 mg/L test concentrations respectively were reported for the parent acid, DTPMP-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.

DTPMP (1-3Na) is freely soluble in water and, therefore, the DTPMP anion is fully dissociated from its sodium cations when in solution. The DTPMP anion has ten P-OH groups that can be ionised. They lose 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 DTPMP 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 DTPMP (4-8K), DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-H, DTPMP (xNH4) or another salt of DTPMP.

Therefore, the log Kow measured for DTPMP-H is directly relevant for DTPMP (1-3Na). 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 DTPMP-H, however, a solution of DTPMP-H in unbuffered water will have a very low pH. Therefore, the log Kow for DTPMP (1-3Na) is reported as <-3.4.

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

The key study is supported by a calculated log Kow value of -2.8 for DTPMP (5-7Na) using a relevant estimation method. Also, DTPMP-2Na has a reported log Kow value of -8.2; in a study using HPLC method, log Kow value of <0 was obtained for DTPMP-2Na.

Reference

PFA 2020: DTPMP speciation at pH relevant to Ecotoxicology and Toxicology, Reference: PFA.224.606.001, May 2020.