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EC number: 236-704-1
CAS number: 13465-77-5
The hydrolysis half-life of hexachlorodisilane is approximately 5
seconds at 25˚C and pH 7; the substance will therefore undergo rapid
hydrolysis in contact with water. This half-life relates to
hydrolysis of the Si-Cl bonds to give hexahydroxydisilane and
hydrochloric acid. Further hydrolysis of the Si-Si bond is also expected
to happen rapidly, giving monosilicic acid (Si(OH)4) which
condenses at concentrations above approximately 100-150 mg/l as SiO2to
give insoluble amorphous polysilicic acid. The precise rate the
hydrolysis of the Si-Si bond is uncertain but the current best
estimate is that it is less than 12 hours at environmentally relevant
The current uncertainty does not significantly affect the outcome
of the environmental hazard assessment. Read-across data are available
from the substances trimethoxysilane and tetraethyl orthosilicate.
Trimethoxysilane, like hexachlorodisilane, hydrolyses very rapidly (t1/2≤0.3
min at pH 4, 7 and 9 and 2°C) to give silanetriol and methanol;
silanetriol then reacts further to monosilicic acid. Under neutral
conditions of the environment and buffered test media, neither chloride
nor methanol will significantly influence the hydrolysis and
condensation reactions of the silanetriol species formed by initial
hydrolysis of hexachlorodisilane and trimethoxysilane. Tetraethyl
orthosilicate hydrolyses rapidly (t1/2≤0.11 h at
pH 4, 4.4 h at pH 7 and 0.22 h at pH 9 and 25°C) to give monosilicic
acid and ethanol. Therefore, regardless of the rates of these further
reactions, the final silicon-containing products of trimethoxysilane,
tetraethyl orthosilicate and hexachlorodisilane hydrolysis are
equivalent, and produced on an equivalent timescale. Short-term toxicity
studies with fish, invertebrates and algae indicate that this substance
is not toxic to aquatic organisms at a loading rate of 100 mg/l. The low
acute aquatic toxicity, physicochemical properties and limited exposure
indicate that silanetriol is unlikely to be of concern for the
REACH guidance (ECHA 2012, R.16) states that “for substances where
hydrolytic DT50 is less than 12 hours, environmental effects
are likely to be attributed to the hydrolysis product rather than to the
parent itself”. TGD guidance, (EC 2003a) and ECHA (2016) also suggests
that when the hydrolysis half-life is less than 12 hours, the breakdown
products, rather than the parent substance, should be evaluated for
aquatic toxicity. Therefore, the environmental hazard assessment,
including sediment and soil compartments due to water and moisture being
present, is based on the properties of the silicic acid hydrolysis
product in accordance with REACH guidance. As described below,
condensation reactions of the silicic acid are possible.
In order to reduce testing, read-across is proposed to fulfil up
to REACH Annex VIII requirements for the registration substance from
substances that have similar structure and physicochemical properties.
Ecotoxicological studies are conducted in aquatic medium or in moist
environments; therefore the hydrolysis rate of the substance is
particularly important since after hydrolysis occurs the resulting
product has different physicochemical properties and structure.
In moist medium, hexachlorodisilane hydrolyses very rapidly
(half-life 5 seconds at 25°C and pH 7), with the final hydrolysis
products being polysilicic acid and hydrochloric acid. The non-silanol
hydrolysis product hydrochloric acid is not expected to contribute to
any adverse effects at the relevant dose levels. This is discussed
The registration substance and the substances used as surrogate
for read-across are part of a class of chlorosilane and alkoxysilane
compounds which hydrolyse rapidly or moderately rapidly to produce
monosilicic acid (Si(OH)4) and another non-Si hydrolysis
product. Si(OH)4 has not been isolated and only exists
in dilute aqueous solution. It readily and rapidly (within minutes)
condenses to give amorphous polysilicic acid at concentrations above
approximately 100-150 mg/l. Depending on the pH and concentration,
solutions will contain varying proportions of monosilicic acid, cyclic
and linear oligomers and polysilicic acid of three-dimensional
structure. Further details are given in PFA 2015ao.
In the following paragraphs the read-across approach for
hexachlorodisilane is assessed for each surrogate substance taking into
account structure, hydrolysis rate and physicochemical properties, presented
in Table 7.0.1.
Table 7.0.1: Physicochemical parameters and ecotoxicity data for the
registration and surrogate substances
Si hydrolysis product
(Poly)silicic acid (note, properties below refer to monomeric monosilicic acid under dilute conditions)
Molecular weight (parent)
Molecular weight (hydrolysis product)
log Kow (parent)
n/a (reacts in contact with water)
log Kow (silanol hydrolysis product)
Water sol (parent)
170 000 mg/l (QSAR)
8600 mg/l (QSAR)
Water sol (silanol hydrolysis product)
1000000 mg/l (predicted; in reality limited to 100-150 mg/l as SiO2 by condensation reactions)
Vapour pressure (parent)
310 Pa at 25°C (QSAR)
11370 Pa at 20°C (measured)
110 Pa at 25°C (QSAR)
Vapour pressure (hydrolysis product)
<1E-06 Pa (QSAR)
Hydrolysis t1/2 at pH 7 and 25°C
<5 seconds(analogue group read-across)
≤0.3 min at 2°C (measured)
4.4 hours (measured)
Hydrolysis t1/2 at pH 4 and 25°C
<5 seconds (analogue group read-across)
0.11 hours (measured)
Hydrolysis t1/2 at pH 9 and 25°C
0.22 hours (measured)
Short-term toxicity to fish (LC50)
Short-term toxicity to aquatic invertebrates (EC50)
Algal inhibition (ErC50 and NOEC)
EC50 >100 mg/l and NOEC <6.3 mg/l
72-hour EC50: >100 mg/l; NOEC: ≥100 mg/l
Long-term toxicity to fish (NOEC)
Long-term toxicity to aquatic invertebrates (NOEC)
Long-term sediment toxicity (NOEC)
Short-term terrestrial toxicity (L(E)C50)
Long-term terrestrial toxicity (NOEC)
Analogue approach justification
Silicic acid is a naturally-occurring substance which is not
harmful to aquatic organisms at relevant concentrations. Silicic 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, Soluble silicates).
Silicic acid has been shown to be beneficial in protection against
mildew formation in wheat and to be non-phytotoxic in non-standard
studies (Côte-Beaulieu et al. 2009).
Silicic acid is therefore not expected to be harmful to organisms
present in the environment. To support this view, all the available
studies with aquatic organisms report no effects at 100 mg/l nominal
loading in short-term toxicity studies (see Table 2 in PFA 2013x for key
studies). Some of the other non-Si hydrolysis products do have the
potential to cause harm at high treatment levels and therefore the
hazard assessment and Predicted No Effect Concentrations (PNECs) will be
based on the properties of these where appropriate.
Si(OH)4 has not been isolated and only exists in dilute aqueous
solution. It readily and rapidly (within minutes) condenses to give
amorphous polysilicic acid at concentrations above approximately 100-150
mg/l. Depending on the pH and concentration, solutions will contain
varying proportions of monosilicic acid, cyclic and linear oligomers and
polysilicic acid of three-dimensional structure.
Further details are given in PFA 2015ao.
Given that all substances produce silicic acid and no toxicity is
observed, it is possible to read-across freely within the analogue
group. (Reference PFA 2013x).
Read-across from trimethoxysilane to hexachlorodisilane:
Trimethoxysilane, HSi(OMe)3 (CAS 2487-90-3), is very
unstable in the presence of water. The substance contains two reactive
groups: Si-OMe and Si-H. The rate of Si-OMe hydrolysis has been measured
in a reliable study; half-lives of ≤0.3 min at pH 4, 7 and 9 and 2°C
were obtained. Methanol is produced by this reaction. If Si-OMe were
hydrolysed, but Si-H were not, silanetriol (HSi(OH)3) would
be formed. However, the Si-H bond is also expected to react, forming
monosilicic acid, Si(OH)4. The rate of this reaction has not
been measured but is expected to be fast to moderately fast. Si(OH)4
has not been isolated and only exists in dilute aqueous solution.
It readily and rapidly (within minutes) condenses to give amorphous
polysilicic acid. Depending on the pH and concentration, solutions will
contain varying proportions of monosilicic acid, cyclic and linear
oligomers and polysilicic acid of three-dimensional structure. This is
the same as the product of Si-Cl hydrolysis for hexachlorodisilane. The
co-products of hydrolysis (hydrogen chloride and methanol) are not
expected to significantly influence the further hydrolysis and
condensation reactions of silanetriol. Therefore, regardless of the
rates of these further reactions, the final silicon-containing products
of trimethoxysilane and hexachlorodisilane hydrolysis are equivalent,
and produced on an equivalent timescale. Further details are given in
Section 4.1 of the CSR and Section 5.1 of the IUCLID 5 dataset.
Trimethoxysilane is used to read-across to short-term
toxicity to fish, invertebrates and algae endpoints. E(L)C50 values
of >100 mg/l for each endpoint have been determined.
Read-across from tetraethyl orthosilicate to hexachlorodisilane:
Tetraethyl orthosilicate (CAS 78-10-4) is an alkoxysilane that
hydrolyses rapidly (t1/2 of 4.4 h at 25°C and pH 7) to
produce monosilicic acid and ethanol. Hexachlorodisilane undergoes very
rapid hydrolysis with a half-life of approximately 5 seconds at 25˚C and
pH 7, to form hydrochloric acid and hexahydroxydisilane, which reacts
further to give monosilicic acid. The effects of hydrochloric acid are
limited to effects in an unbuffered media and are assessed below.
Tetraethyl orthosilicate and hexachlorodisilane are considered part of
the same analogue group as they both react in water to produce
monosilicic acid. Si(OH)4 has not been isolated and only
exists in dilute aqueous solution. It readily and rapidly (within
minutes) condenses to give amorphous polysilicic acid at concentrations
above approximately 100-150 mg/l. Depending on the pH and concentration,
solutions will contain varying proportions of monosilicic acid, cyclic
and linear oligomers and polysilicic acid of three-dimensional
structure. The non-silicon hydrolysis products, ethanol and hydrogen
chloride respectively, do not cause effects in aquatic organisms at
relevant concentrations and under appropriately buffered conditions as
Tetraethyl orthosilicate is used to read-across to
short-term toxicity to fish, invertebrates and algae endpoints. E(L)C50
values of >245, >844 and >100 mg/l, respectively, have been
Short-term toxicity data for fish and invertebrates with
tetraethyl orthosilicate indicate that this substance is of low toxicity
to aquatic organisms.
Considerations on the non-silanol hydrolysis products:
Methanol and ethanol are well-characterised in the public domain
literature and are not hazardous at the concentrations relevant to the
studies; the short-term EC50 and LC50 values for
these substances are in excess of 1000 mg/l (OECD 2004a - SIDS for
methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5).
Chloride ions occur naturally (typically at levels 40 – 160
mg/l in environmental fresh waters). Standard test media contain
chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.
Effects on aquatic organisms arising from exposure to hydrochloric
acid are thought to result from a reduction in the pH of the ambient
environment (arising from an increase in the H+concentration)
to a level below their tolerable range. Aquatic ecosystems are
characterized by their ambient conditions, including the pH, and
resident organisms are adapted to these conditions. The pH of aquatic
habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in
well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to
natural variations in pH is well understood and has been quantified and
reported extensively in ecological publications and handbooks (e. g.
OECD SIDS for CAS No. 7647-01-0, hydrochloric acid). It is not
considered appropriate or useful to derive a single aquatic PNEC for
hydrochloric acid because any effects will not be a consequence of true
chemical toxicity and will be a function of, and dependent on, the
buffering capacity of the environment. Physical hazards related to pH
effects are considered in the risk management measures (e. g.
neutralisation) for effluents/aqueous waste.
It is not appropriate for this substance to discuss the combined
ecotoxicological potency of the silicon and non-silicon hydrolysis
Côté-Beaulieu C, Chain F, Menzies JG, Kinrade SD, Bélanger RR (2009)
Absorption of aqueous inorganic and organic silicon compounds by wheat
and their effect on growth and powdery mildew control. Environ Exp. Bot
OECD SIDS (2002) SIDS Initial Assessment Report for SIAM 15, Boston,
USA, 22-25th October 2002, Hydrochloric acid, CAS 7647-01-0.
OECD SIDS (2004) 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 Silicic acid (H2SiO3), disodium salt;
CAS 10213-79-3 Silicic acid (H2SiO3), disodium salt, pentahydrate; CAS
13517-24-3 Silicic acid (H2SiO3), disodium salt, nonahydrate; CAS
1312-76-1 Silicic acid, potassium salt.
OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin,
Germany, 18-20 October 2004, Methanol, CAS 67-56-1.
OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin,
Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.
PFA, 2013x, Peter Fisk Associates, Analogue report - Ecotoxicity of
(poly)silicic acid generating compounds , PFA.300.003.001.
PFA, 2015ao, Peter Fisk Associates, The aquatic chemistry of inorganic
silicic acid generators, PFA.404.001.001.
short-term toxicity data are read across from analogous substances (on
the basis of common hydrolysis products). Short-term
EC50 values reported in the studies used as read-across for
the silanol hydrolysis product indicate that it would not be toxic at a
loading rate of 100 mg/l. A NOEC value of <6.3 mg/l is reported in the
algal study with the surrogate substance trimethoxysilane.
registration substance rapidly hydrolyses to hydrogen chloride and
inorganic silicate moieties. The non-silanol hydrolysis product,
hydrogen chloride, has a harmonised classification in Annex VI of
Regulation No 1272/2008 and does not require classification for the
hydrolysis product, monosilicic acid, is a naturally-occurring substance
which is not harmful to aquatic organisms at relevant concentrations.
All available studies with aquatic organisms report no effects at 100
mg/l in short-term toxicity studies (reference PFA 2013x).
are consistent with the following classification under Regulation (EC)
No 1272/2008 (as amended) (CLP):
toxicity: Not classified.
toxicity: Not classified.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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