Hydrofluoric acid (HF) is a colorless gas that can exist as a liquid at everyday temperatures. It is a weak acid that reacts with metals, metal oxides, and silicates, and is used in the production of PTFE, semiconductor industry, uranium extraction, glass etching, and rust stain removal. HF is highly toxic and corrosive, causing severe burns and hypocalcemia. Strict safety precautions are necessary when handling HF, and medical attention is required for any contact. Calcium gluconate gel can help minimize damage and prevent hypocalcemia.
Hydrofluoric acid is a compound of hydrogen and fluorine with the chemical formula HF. Fluorine is a member of a group of elements known as halogens, all of which combine with hydrogen in a similar way to form hydrogen halides. At room temperature and normal pressure, hydrofluoric acid is a colorless gas with a boiling point of 67.1°F (19.5°C), which is much higher than those of the other hydrogen halide and allows it to exist as a liquid at everyday temperatures. In water, it dissolves to form hydrofluoric acid. Liquid HF is also known as anhydrous hydrofluoric acid – meaning free of water – and “HF” can be used to mean gas, liquid or aqueous acid.
In aqueous solution, hydrofluoric acid is a weak acid, due to the hydrogen bonding between HF and water molecules, which limits the degree of dissociation into ions. The hydrogen bonding between HF molecules accounts for the relatively high boiling point of hydrofluoric acid compared to the other hydrogen halides. The acid reacts with many metals, usually forming hydrogen gas and a metal fluoride, for example: Mg + 2HF -> MgF2 + H2. Unlike many acids, however, it also reacts readily with most metal oxides and silicates, including glass, forming soluble compounds. For this reason it cannot be stored in glass bottles.
Hydrofluoric acid can be produced by the reaction of a metal fluoride, for example calcium fluoride, with sulfuric acid: CaF2 + H2SO4 -> CaSO4 + 2HF. It is produced this way in the chemical industry, using fluorspar, a common mineral form of calcium fluoride. The main industrial uses are in the production of polytetrafluoroethylene (PTFE), in the semiconductor industry for the removal of oxide from silicon, in the extraction of uranium from its oxide ore, in glass etching and as a catalyst in industry petrochemical. It is also used to remove rust stains, as it reacts with metal oxides to form soluble fluorides. Fluorine is produced industrially by the electrolysis of liquid HF.
In the laboratory, HF in the form of aqueous hydrofluoric acid is used in mineral analysis due to its ability to dissolve silicates. It is also used in the analysis of pollen in soil samples. The soil consists mainly of organic and mineral material, with the minerals mainly consisting of carbonates and silicates. To identify any pollen present, this material must be removed, and after treatment with other reagents to remove carbonates and organic material, hydrofluoric acid is used to remove silicate minerals.
Hydrogen fluoride and hydrofluoric acid are highly toxic and very corrosive. Inhaling the gas damages the respiratory system and can cause pulmonary edema and death. Skin contact with hydrofluoric acid, even in very dilute solutions, can cause severe burns and allow fluoride ions to enter the bloodstream. The acid is absorbed very rapidly through the outer skin and kills the underlying living tissue, mainly due to the combination of the fluoride ion with calcium ions and the precipitation of insoluble calcium fluoride. Calcium is essential for cellular metabolism and for the functioning of vital organs; its removal from the system can result in a condition known as hypocalcemia, which can lead to death from cardiac arrest or multiple organ failure.
Because of these risks, hydrofluoric acid and hydrofluoric acid must be handled very carefully and strict safety precautions are normally observed where they are used. Ingestion, inhalation or skin contact with HF requires urgent medical attention, even if there are no immediate symptoms, as with diluted solutions the effects may be delayed. Spills covering 2% or more of the body surface area are considered life-threatening, due to the risk of significant amounts of fluoride ions entering the bloodstream. Applying calcium gluconate gel to the affected area provides calcium ions which bind to fluoride ions, helping to minimize damage and prevent hypocalcemia.
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