Tetraethyl orthosilicate

Endpoint summary

Additional information

On contact with water, tetraethyl orthosilicate (TEOS, CAS 78-10-4; EC No. 201-083-8) rapidly hydrolyses (half-life approximately 4.4 hours at 25°C and pH 7) to monosilicic acid (CAS 10193-36-9; EC No. 233-477-0) and ethanol (CAS 64-17-5; EC No. 200-578-6). Due to the rapid hydrolysis of the substance, the chemical safety assessment (CSA) is based on the silicon containing hydrolysis product monosilicic acid. The consideration of ethanol is discussed in IUCLID Section 6, CSR Section 7.0. 

No terrestrial toxicity data are available with the registered substance or monosilicic acid, on which the CSA is based. Testing for long-term toxicity to terrestrial species is not considered necessary because: 

Log K_ow and log K_oc are not relevant for inorganic compounds such as monosilicic acid. However, on the basis of structure, monosilicic acid has a high affinity for water and low affinity for lipids and organic carbon (refer to IUCLID Section 5.4.1 for further information). Monosilicic acid is therefore not expected to partition to the terrestrial compartment.

Monosilicic acid is inorganic, water-soluble and has low potential for bioaccumulation (refer to IUCLID Section 5.3 for further information). Due to the substance being inorganic, PBT assessment does not apply and the substance cannot be classed as PBT or vPvB. Monosilicic acid and its condensation products are unlikely to diffuse across membranes or accumulate in fatty tissues, and are expected to be effectively eliminated in faeces and via the kidneys in urine (refer to IUCLID Section 7.1 for further information).There is therefore no evidence to suggest that this substance will accumulate in the terrestrial compartment or in the body.

Monosilicic acid exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100 - 150 mg/l as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble polysilicic acid. 

(Poly)silicic acid is a naturally occurring substance, ubiquitous to the natural environment, which is not harmful to aquatic organisms at relevant environmental concentrations. Monosilicic acid is the major bioavailable form of silicon for aquatic organisms and plays an important role in the biogeochemical cycle of silicon (Si). Most living organisms contain at least trace quantities of silicon. For some species Si is an essential element that is actively taken up. For example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures (OECD SIDS 2004, Synthetic amorphous silica and silicates; PFA, 2013x). Monosilicic acid, oligomeric and polysilicic acids are therefore not expected to be harmful to organisms present in the environment, as confirmed by the available data. 

Further discussion on the ecotoxicity of silicic acid producers can be found in the attached report “PFA, 2013x Analogue report Ecotoxicity of (poly)silicic acid producers_20130516”.

As inorganic substances, soluble silicates are not amenable to photo- or biodegradation (OECD SIDS 2004, Synthetic amorphous silica and silicates), therefore will not give rise to toxic degradation products.

 

The bioavailable forms of silica are dissolved silica [Si(OH)₄] almost all of which is of natural origin (OECD SIDS 2004, Synthetic amorphous silica and silicates). Anthropogenic sources of silicic acid will rapidly give rise to molecular species indistinguishable from natural dissolved silica (OECD SIDS 2004, soluble silicates). The potential releases of inorganic silicon resulting from use of tetraethyl orthosilicate are negligible compared to the natural flux of silica in the environment (refer to IUCLID Section 5 endpoint summary for further information).

Because silicic acid occurs naturally in the environment and organisms are naturally adapted to utilise silica, and because monosilicic acid, oligomeric and polysilicic acids from anthropogenic sources will enter the natural biogeochemical cycles and will be indistinguishable from naturally occurring silica, the registered substance, which rapidly hydrolyses to silicic acid, is not expected to be harmful to organisms present in the environment; hence toxicity to terrestrial organisms is not expected.

Short-term aquatic toxicity tests with fish, Daphnia and algae have been carried out with substances that rapidly hydrolyse to monosilicic acid. No effects were observed in the tests. The substances in the silicic acid producers analogue group are silicon compounds with no carbon attached and which contain no structural features indicative of a specific mode of toxic action for ecotoxicity. Therefore, the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely. 

Overall, it is concluded that no hazard is identified and further testing is not considered necessary.

Details on how the substance is evaluated for risk can be found in IUCLID Section 6.0, and Chapters 7, 9 and 10 of the Chemical Safety Report.

 

References

OECD SIDS 2004, soluble silicates. SIDS Initial Assessment Report for SIAM 18, Paris, France, 20-23 April, 2004, Soluble Silicates, CAS 1344-09-8 Silicic acid, sodium salt; CAS 6834-92-0.

OECD SIDS 2004, Synthetic amorphous silica and silicates. SIDS Initial Assessment Report for SIAM 19. Berlin, Germany, 19-22 October 2004.

PFA 2013x: Peter Fisk Associates, Analogue grouping report: Ecotoxicity of (poly)silicic acid generating compounds. PFA.300.003.001.