Tetrapropyl orthosilicate

Endpoint summary

Additional information

There are no reliable short-term data for toxicity of the registered substance to fish, invertebrates or algae. However data are available for two structurally-analogous read-across substances; tetrakis(2-butoxyethyl) orthosilicate (CAS 18765 -38 -3) and tetraethyl orthosilicate (CAS 78 -10 -4).

• A 96 hour LC50 value of >201 mg/l and NOEC of ≥201 mg/l have been determined for the effects of tetrakis(2-butoxyethyl) orthosilicate on mortality of Brachydanio rerio based on measured concentrations of dissolved organic carbon (DOC).
• A 96-hour LC50 of >245mg/l and a NOEC of ≥245 mg/l have been determined for the effects of tetraethyl orthosilicate on mortality of Brachydanio rerio based on measured concentrations of dissolved organic carbon (DOC).
• A 96-hour LC50 of >245mg/l and a NOEC of ≥245 mg/l have been determined for the effects of tetraethyl orthosilicate on mortality of Brachydanio rerio based on measured concentrations of dissolved organic carbon (DOC).
• A 48-hour EC50 value of >90 mg/l and NOEC of ≥90 mg/l have been determined for the effects of tetrakis(2-butoxyethyl) orthosilicate on mobility of Daphnia magna based on nominal concentrations of the test substance.
• A 48-hour LC50 of >75mg/l and a NOEC of ≥75 mg/l have been determined for the effects of tetraethyl orthosilicate on mobility of Daphnia magna based on measured concentrations of ethyl silicate. 
• A 72-hour EC50 value of >161 mg/l and a NOEC of approximately 110 mg/l have been determined for the effects of tetrakis(2 -butoxyethyl) orthosilicate on growth rate and biomass of Scenedesmus subspicatus based on initial measured concentration.
• A 72-hour EC50 value of >22 mg/l and NOEC of ≥22 mg/l have been determined for the effects of tetraethyl orthosilicate on growth rate and biomass of Pseudokirchnerella subcapitata based on geometric mean measured concentrations (>100 mg/L and ≥100 mg/l respectively based on nominal concentration).

Both of these substances hydrolyse in water and it is therefore likely that the test organisms were primarily exposed to their hydrolysis products.

The studies were conducted according to an appropriate OECD test guideline, in compliance with GLP. Read across to the registered substance is considered scientifically justified; the read across is considered to be reliability 2.

There are no long-term toxicity data for fish or aquatic invertebrates. However, in accordance with Column 2 of REACH Annex IX, the long-term aquatic toxicity to fish and invertebrates studies do not need to be conducted as the chemical safety assessment according to Annex I indicates that these are not necessary.

The assessment is based on the assumption that exposure will be to hydrolysis products of the registered substance; silicic acid and propanol.

Tetrapropyl orhosilicate, tetrakis(2 -butoxyethyl) orthosilicate and tetraethyl orthosilicate are members of a group of substances that hydrolyse rapidly to form silicid acid, a bioavailable form of silicate. It is therefore considered appropriate to read-across from tetraethyl- and tetrakis(2-butoxyethyl)- orthosilicate to tetramethyl orthosilicate.

As described in section 1, silicic acid may condense to insoluble silica (SiO₂) at high enough concentrations. No information is available on the concentration at which significant condensation occurs however no observations of condensation products were reported in the read-across studies.

Additional information is given in a supporting report (Peter Fisk Associates, 2012) attached in Section 13 of the REACH technical dossier.

Read-across justification

The registered substance and the structurally-analogous read-across substances hydrolyse in water to form silic acid and another by-product. The registered substance hydrolyses to form silicic acid and propanol with a hydrolysis half life of 6.7 h at pH 7 and 25°C. Tetraethyl orthosilicate (CAS 78-10-4) hydrolyses to produce silicic acid and ethanol with a half-life of 4.4 h at pH 7 and at 25°C. The hydrolysis half-life of tetrakis(2-butoxyethyl) orthosilicate is not known, but is thought to be between 12 and 24 hours, resulting in the formation of silicic acid (silicate) and 2-butoxyethanol.

Silicate is a naturally occurring substance which is not harmful to aquatic organisms. It is the major bioavailable form of silica for aquatic organisms and plays an important role in the biogeochemical cycle of Si. Most living organisms contain at least trace quantities of silicon. For some species Si is essential element taken up actively, for example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures. Therefore, it is not expected to be harmful to the environment nor the organisms that will be exposed to it.

Studies available for the structurally-analogous read-across substance do not report any effects below 100 mg/l. Aquatic toxicity data for propanol have been reviewed as part of an EU risk assessment (EU, 2008) and show the low toxicity of the substance to aquatic organisms. The lowest available effect concentration identified in reviewed studies covering fish, invertebrates and algae was a 48-hour EC50 value of 1,000 mg/l for effects on mortality of the invertebrate species Gammarus pulex.

EU (2008). European Union Risk Assessment Report, PROPAN-1-OL, CAS No: 71-23-8, EINECS No: 200-746-9. RISK ASSESSMENT -Volume 82, Part I Environment (publication EUR 22159 EN). ISSN 1018-5593. Luxembourg: Office for Official Publications of the European Communities.