Registration Dossier

Physical & Chemical properties

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).

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 17.5C 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).

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. However the reported vapour pressure is considered likely to be attributable to the release of hydrogen fluoride vapour (IPCS, 2004).

Partition coefficient

A waiver is proposed for this endpoint as the substance is inorganic

Water solubility

A waiver is proposed for this endpoint as hexafluorosilicic acid is known to be miscible with water.

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

A waiver is proposed for this data requirement as hexafluorosilicic acid is an inorganic 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).