Diethyl phosphonate

Toxicity to aquatic algae and cyanobacteria

Description of key information

Toxicity of the substance to aquatic algae (Desmodesmus subspicatus) was determined as ErC50 of > 92.8 mg/L, a NOEC of 92.8 mg/L and a LOEC mg of > 92.8mg/L after 72 hours.

Key value for chemical safety assessment

EC50 for freshwater algae:

92.8 mg/L

EC10 or NOEC for freshwater algae:

92.8 mg/L

Additional information

The study was performed according to EU method C.3 (algal inhibition test). The test solution of six nominal concentrations (2.8, 5.6, 11.25, 22.5, 45 and 90 mg/L) and blank control were exposed under static conditions.The cell densities were measured at 24 hours intervals. Inhibition of the algal population was measured as reduction in growth rate (index r), relative to control cultures grown under identical conditions.

The test substance triethyl phosphite hydrolyzed completely within a few hours by 100% to diethyl phosphite which was analyically confirmed to be stable during the study period in seperate study. This hydrolytical product was used for analysis in the present study. In order to be sure that triethyl phosphite was completely hydrolyzed the study started 2 hours after the test item was added to the test medium.

The present study result refers to the hydrolysis calculated from the measured TOC values. 1 mg/L TOC corresponds to the present material information 2.9 mg/L of the hydrolysis product diethyl phosphite (molecular formula: C4H11O3P; molecular weight: 138 g/mole).

The arithmetic mean of the analytically determined values ​​of the hydrolysis product diethyl phosphite gave the following values ​​of ErC50 of > 92.8 mg/L, a NOEC of 92.8 mg/L and a LOEC mg of > 92.8 mg/L.

Rational for read across

REACH regulation (Corrigendum) EC 1907/2006 on page L 136/120: “Substances whose physicochemical, toxicological and ecotoxicological properties are likely to be similar or follow a regular pattern as a result of structural similarity may be considered as a group, or ‘category’ of substances. Application of the group concept requires that physicochemical properties, human health effects and environmental effects or environmental fate may be predicted from data for reference substance(s) within the group by interpolation to other substances in the group (read-across approach).”

Further, ECHA guidance document “Guidance on information requirements and chemical safety assessment – Chapter R.6: QSARs and grouping of chemicals (May, 2008)” provides information on the use of read-across (page 78) as follows: “In the read-across technique, endpoint information for one chemical is used to predict the same endpoint for another chemical, which is considered to be similar in some way (usually on the basis of structural similarity). In principle, read-across can be applied to characterise physico-chemical properties, environmental fate, human health effects and ecotoxicity.”

Triethyl posphite (CAS 122 -52 -1) and diethyl phospite (762 -04 -9) are very similar in their chemical structure. However, there is a more relevant rational for the use of studies conducted with triethyl phosphite: In contact with water triethyl phosphite quickly hydrolyses with diethyl phosphite being the hydrolysis product. This product is expected to be stable for a certain time period which should be the period of an acute toxicity study. Thus, when conducting a study with triphenyl phosphite it should be the effects of diethyl phosphite that are detected instead.The other hydrolysis product phosphate can be expected not to cause any toxicity since it is an ubiquitious and inorganic compound.

Concluding, it is highly justified to use studies with triethyl phosphite to predict effects with diethyl phosphite.