| |||
| Names | |||
|---|---|---|---|
| IUPAC name Ammonium hexafluorosilicate | |||
| Other names Ammonium fluorosilicate Ammonium fluosilicate | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider |
| ||
| ECHA InfoCard | 100.037.229 | ||
| EC Number |
| ||
| RTECS number |
| ||
| UNII | |||
| UN number | 2854 | ||
| |||
| |||
| Properties | |||
| (NH4)2[SiF6] | |||
| Appearance | White crystals | ||
| Density | 2.0 g cm−3 | ||
| Melting point | 100 °C (212 °F; 373 K) (decomposes)[1] | ||
| Solubility | dissolves inwater andalcohol | ||
| Related compounds | |||
Othercations | Hexafluorosilicic acid | ||
| Hazards[2][3] | |||
| GHS labelling: | |||
   | |||
| Danger | |||
| H301,H311,H315,H319,H331,H335,H372 | |||
| P260,P261,P264,P270,P271,P280,P301+P310,P302+P352,P304+P340,P305+P351+P338,P312,P314,P321,P330,P332+P313,P337+P313,P362,P403+P233,P405,P501 | |||
| NFPA 704 (fire diamond) | |||
| Safety data sheet (SDS) | ChemicalBook MSDS | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Ammonium fluorosilicate (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH4)2SiF6. It is a toxic chemical, like all salts offluorosilicic acid.[4] It is made of white crystals,[5] which have at least threepolymorphs[6] and appears in nature as rare minerals cryptohalite orbararite.
Ammonium fluorosilicate has three majorpolymorphs: α-(NH4)2[SiF6] form iscubic (space group Fm3m, No. 225) and corresponds to the mineral cryptohalite. The β form istrigonal (scalenohedral) and occurs in nature as mineral bararite.[7] A third (γ) form was discovered in 2001 and identified with thehexagonal 6mm symmetry. In all three configurations, the [SiF6]2−octahedra are arranged in layers. In the α form, these layers are perpendicular to[111] directions. In the β- and γ- forms, the layers are perpendicular to the c-axis.[6] (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.[8]) The silicon atoms of α-(NH4)2[SiF6] (alpha), havecubic close packing (CCP). The γ form hashexagonal close packing and the β-(NH4)2[SiF6] has primitive hexagonal packing.[9] In all three phases, 12fluorine atoms neighbor the (NH4)+.[6]
Although bararite was claimed to bemetastable atroom temperature,[10] it does not appear onepolymorph has ever turned into another.[6] Still, bararite is fragile enough that grinding it forspectroscopy will produce a little cryptohalite.[11] Even so, ammonium fluorosilicate assumes a trigonal form at pressures of 0.2 to 0.3GPa. The reaction is irreversible. If it is not bararite, the phase is at least very closely related.[6]
Thehydrogen bonding in (NH4)2[SiF6] allows this salt to change phases in ways that normal salts cannot. Interactions betweencations andanions are especially important in howammonium salts change phase.[6] (To learn more about the β-structure, seeBararite.)
This chemical makes rare appearances in nature.[12] It is found as a sublimation product offumaroles andcoal fires. As a mineral, it is either called cryptohalite orbararite, the two being two polymorphs of the compound.[7]
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, includinghydrogen fluoride,silicon tetrafluoride, andnitrogen oxides. It willcorrodealuminium. In water, ammonium fluorosilicate dissolves to form an acid solution.[5]
Inhaling dust can lead to pulmonary irritation, possibly death. Ingestion may also prove fatal. Irritation of the eyes comes from contact with the dust, as well as irritation or ulceration of the skin.[5]
Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.[5] It is also used to help neutralize washing machine water aslaundry sour.
