Silicon Tetrafluoride

Chemical properties

A colorless, nonflammable, corrosive and toxic gas. Sharp, pungent suffocating odor, similar to hydrogen chloride. Vapor is heavier than air; density53.6.

Chemical properties

Silicon tetrafluoride exhibits a very pungent odor. Silicon tetrachloride has a suffocating odor and is moisture sensitive.

Physical properties

Colorless gas; very pungent odor; fumes heavily in moist air; density of the gas 4.69 g/L; heavier than air, density in air 3.5 (air = 1); sublimes at -95.7°C; solidifies at -90.2°C (under pressure); critical pressure 50atm; decomposes in water forming silicic acid and hydrofluoric acid.

The Uses of Silicon Tetrafluoride

Manufacture of fluosilicic acid, intermediate in manufacture of pure silicon, to seal water out of oil wells during drilling.

Preparation

Silicon tetrafluoride is prepared by heating silica with dilute hydrofluoric acid at high temperatures:
SiO₂ + 4HF → SiF₄ + 2H₂O
Also, the tetrafluoride may be obtained by heating the elements:
Si + 2F₂ → SiF₄.

General Description

A colorless, nonflammable, corrosive and toxic gas with a pungent odor similar to that of hydrochloric acid. Very toxic by inhalation. Vapor is heavier than air. Under prolonged exposure to heat the containers may rupture violently and rocket.

Air & Water Reactions

Fumes in air. Decomposed exothermically by water or moisture in the air to hydrofluoric acid and silicic acid [Merck 11th ed. 1989; Handling Chemicals Safely 1980 p. 821].

Reactivity Profile

When heated to decomposition SILICON TETRAFLUORIDE emits toxic fluoride fumes [Lewis, 3rd ed., 1993, p. 1138]. Reacts violently with alcohols to form HF. Attacks many metals in the presence of moisture. Reacts violently to explosively with lithium nitride [Porritt, L. J., Chem. Brit., 1979, 15, p. 282]. Mixtures with sodium are shock-sensitive explosives [Leleu, Cahiers, 1975, (79), p. 270].

Hazard

Toxic by inhalation, strong irritant to mucous membranes.

Health Hazard

TOXIC; may be fatal if inhaled, ingested or absorbed through skin. Vapors are extremely irritating and corrosive. Contact with gas or liquefied gas may cause burns, severe injury and/or frostbite. Fire will produce irritating, corrosive and/or toxic gases. Runoff from fire control may cause pollution.

Fire Hazard

Some may burn but none ignite readily. Vapors from liquefied gas are initially heavier than air and spread along ground. Some of these materials may react violently with water. Cylinders exposed to fire may vent and release toxic and/or corrosive gas through pressure relief devices. Containers may explode when heated. Ruptured cylinders may rocket.

Safety Profile

A poison. Moderately toxic by inhalation. A corrosive irritant to skin, eyes, and mucous membranes. When heated to decomposition it emits toxic fumes of F-. See also FLUORIDES and HYDROFLUORIC ACID

Potential Exposure

Silicon tetrafluoride is not used in industry but may be a discharge byproduct of certain processes, including ore refining and smelting operations. It has been used in the manufacture of silane and fluosilicic acid, and pure electronic silicon.

Shipping

UN1859 Silicon tetrafluoride, Hazard Class: 2.3; Labels 2.3-Poisonous gas; 8-Corrosive material; Inhalation Hazard Zone B. Cylinders must be transported in a secure upright position, in a well-ventilated truck. Protect cylinder and labels from physical damage. The owner of the compressed gas cylinder is the only entity allowed by federal law (49CFR) to transport and refill them. It is a violation of transportation regulations to refill compressed gas cylinders without the express written permission of the owner.

Incompatibilities

Water and air reactive Fluorides form explosive gases on contact with strong acids, acid fumes, and sodium. Water and air reactive. Corrosive. Fumes in air. Decomposed exothermically by water, alcohols, or moisture in the air to hydrofluoric acid and silicic acid, forming flammable and potentially explosive hydrogen gas. Attacks many metals in the presence of moisture.

Waste Disposal

For laboratory quantities, transfer into an evaporating dish containing sodium bicarbonate, spray with ammonia (6M NH4OH)/6 M-ammonium hydroxide/while stirring and then spread with crushed ice. Continue spraying with ammonia until the smoke of ammonium chloride partly subsides and add iced water while stirring. Neutralize and slowly transfer the mixture into a drain with running water.