Registration Dossier
Registration Dossier
Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.
The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.
Use of this information is subject to copyright laws and may require the permission of the owner of the information, as described in the ECHA Legal Notice.
EC number: 241-034-8 | CAS number: 16961-83-4
Endpoint summary
Administrative data
Description of key information
Additional information
Hexafluorosilicic acid is reported to be a transparent, colourless, fuming liquid (Lewis, 1999) with a sour, pungent odour (Merck Index, 2001).
Melting point/freezing point
The freezing point of a sample of 10% aqueous hexafluorosilicic acid was found to be -16.6°C using differential scanning calorimetry (The REACH Centre, 2010). The freezing point of a 60 -70% aqueous solution of hexafluorosilicic acid is reported to be 19 degrees Celsius (Hollemann et al, 1995), with the formation of the crystalline dihydrate (Merck Index). The freezing point of a 25% solution of hexafluorosilicic acid is reported to be -15.5 degrees Celsius (ATSDR, 2001).
Since concentrations above about 30 -35% are not stable, the results from the 10% and 25% solutions are more relevant and will be used for the assessment of the registered substance. The Melting point of the 10% solution was determined using the most sophisticated test design and hence the result from this test will be used as key informaiton, i.e., the melting point is -16.6°C.
Boiling point
No data are available: a waiver is proposed for this endpoint. The substance decomposes on heating, therefore measurement of the boiling point is technically not possible. Distillation of aqueous solutions of hexafluorosilicic acid produces a gas consisting of silicon tetrafluoride, hydrofluoric acid, and steam.
Density
The relative density at 17.5⁰C is reported to be (Merck Index, 2001):
1.0407 g/cm3 for a 5% solution
1.0834 g/cm3 for a 10% solution
1.1281 g/cm3 for a 15% solution 1.174 g/cm3 for a 20% solution 1.2235 g/cm3 for a 25% solution 1.2742 g/cm3 for a 30% solution
The density of a 60.97% aqueous solution of hexafluorosilicic acid is reported to be 1.4634 at 25 degrees Celsius (Hollemann et al, 1995).
Since the solutions above 30 -35% are not stable, the result of the 60.97% solution is considered to be not relevant for the assessment.
Granulometry
This data requirement is not relevant: the substance is a liquid at room temperature.
Vapour pressure
The vapour pressure of a 10% aqueous solution of hexafluorosiclic acid is reported to be 2.30 kPa at 20 degrees Celcius (REACH Centre, 2010). The vapour pressure of a 35% aqueous solution of hexafluorosilicic acid is reporetd to be 30 hPa at 20 degrees Celsius. Hence, the reported vapour pressure is considered likely to be attributable to the release of hydrogen fluoride vapour (IPCS, 2004).
The partial pressure of the Fluorine compounds vary between 0.015kPa for an acid of 32% H2SiF6 concentration and 0.003kPa for an acid of 23% H2SiF6 concentration at 293 K.
Partition coefficient
A waiver is proposed for this endpoint as the substance is inorganic
Water solubility
Hexafluorosilicic acid is known to be miscible with water up to at least 60.97%.
Surface tension
A waiver is proposed for this endpoint as the substance is not expected to be surface active on theoretical grounds.
Flash point
The data requirement can be waived on the basis that hexafluorosilicic acid is an inorganic substance.
Autoflammability
No data are available. A waiver is proposed for this endpoint as substance is non-flammable.
Flammability
Hexafluorosilicic acid is not flammable and does not support combustion
Explosivity
A waiver is proposed for this data requirement as the substance does not contain any groups associated with explosive properties.
Oxidising properties
A waiver is proposed for this endpoint, on theoretical grounds. The substance is not predicted to have any oxidising properties, based on a consideration of its chemical structure.
Stability in organic solvents
The substance is not expected to be surface active on the structure of the substance.
Dissociation constant
The anhydrous substance dissociates totally to form SiF4 and HF (Merck Index, 2001). For this reason, the substance contains HF to prevent dissociation and hydrolysis. Under dilute aqueous conditions, the substance dissociates to form SiF4 and HF; the SiF4 formed from this reaction undergoing hydrolysis to form HF and Si(OH)4; HF dissociates (under aqueous conditions) for form the fluoride and hydronium ions. Soluble silicates engage in complex behaviour and will react further to form oxo and hydroxo species (Urbansky et al, 2001). Hexafluorosilicic acid is a strong acid with respect to loss of the first proton (complete dissociation) to form HSiF6- and H+. The second dissociation is limited (Ka of between 10e-0.65 to 10e-1.83), with the formation of SiF62- and H+ (Nicholson & Czarecka, 2008).
Viscosity
The viscosity of a 10% aqeuous solution of hexafluorsilicic acid at 10 Pa was found to be 0.011Pas or 11cps (REACH Centre, 2010). The viscosity of 23% aqeuous solution of hexafluorosilicic acid is reported to be 6.5 cps at 20 degrees Celsius (ATSDR).
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.
