Hydrogen fluoride (HF) is a colorless, fuming chemical compound known for its extreme corrosiveness and systemic toxicity. It exists as a gas or a liquid, boiling near room temperature at about 19.5 °C (67.1 °F). When dissolved in water, it forms an aqueous solution called hydrofluoric acid. HF is widely used across various industries, posing a serious occupational and public health risk. Its danger stems from its ability to cause severe chemical burns and quickly poison the body through a unique biological mechanism.
Defining the Chemical and Physical Characteristics
Hydrogen fluoride (HF) is a molecule composed of one hydrogen atom and one fluorine atom. In its pure, anhydrous form, it is a highly reactive compound considered a superacid, stronger than pure sulfuric acid. When dissolved in water, the resulting hydrofluoric acid is classified as a weak acid because it does not fully dissociate into hydrogen and fluoride ions.
This weakness is deceptive, as the acid is intensely corrosive toward many materials, including tissue. Unlike strong acids that cause immediate damage, HF’s small, un-dissociated molecules easily penetrate deep into tissue layers. It uniquely reacts with silicon dioxide, allowing it to dissolve glass and ceramic materials. The acid solution is clear and visually indistinguishable from water, adding to its hazard.
Essential Applications in Industry
Hydrogen fluoride is a foundational chemical in modern manufacturing, serving as the principal industrial source of fluorine. A large portion of HF is used as a precursor in the production of fluorocarbons, such as those utilized in refrigerants, aerosols, and fluoropolymers like PTFE (Teflon™). It acts as a catalyst in the petroleum industry for the alkylation process, producing high-octane gasoline components.
The microelectronics and semiconductor sectors rely on hydrofluoric acid for etching silicon wafers and cleaning electronic components. It is also used to create synthetic cryolite for aluminum production. Other applications include manufacturing pharmaceuticals, herbicides, and pickling stainless steel and other metals.
The Unique Dangers of Hydrogen Fluoride Exposure
The danger of hydrogen fluoride lies in the toxicity of the free fluoride ion (\(\text{F}^{-}\)). Upon contact, the HF molecule rapidly penetrates the skin, eyes, or respiratory tract before dissociating inside the body. This deep penetration allows the fluoride ion to leach into underlying soft tissue, nerves, and even bone.
Once inside, the fluoride ions immediately begin to bind with calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions present in the body’s tissues and bloodstream. This binding process, known as chelation, precipitates insoluble salts and leads to the destruction of cells and liquefaction necrosis. Systemic absorption of fluoride depletes the body’s calcium levels, causing a fatal condition called hypocalcemia.
This severe electrolyte imbalance can lead to ventricular fibrillation and cardiac arrest, even from small skin exposures. For low-concentration exposures, the onset of severe pain may be delayed for up to 24 hours while the fluoride slowly penetrates the skin. Inhalation of HF vapor is also hazardous, causing immediate irritation followed by pulmonary edema and systemic toxicity.
Specific Safety and Emergency Response
Exposure to hydrogen fluoride requires immediate, specialized first aid and medical intervention, as standard burn treatment is insufficient. The initial step for skin contact is rapid decontamination by flushing the area with copious amounts of water for at least 15 minutes to dilute and remove the chemical. Following this, the affected area must be treated with calcium gluconate gel.
This specialized gel works by providing external calcium ions that bind to the penetrating fluoride ions, neutralizing the toxic process. The gel must be continuously massaged into the burn site until the pain is completely relieved, indicating the fluoride has been successfully chelated. For inhalation exposure, nebulized calcium gluconate may be administered to bind fluoride ions in the respiratory tract.
Immediate transport to a hospital is mandatory for any HF exposure, regardless of the apparent severity or success of field first aid. Medical professionals will monitor the patient for systemic toxicity using an electrocardiogram (ECG) and blood tests to check for hypocalcemia, hypomagnesemia, and hyperkalemia. In severe cases, intravenous or intra-arterial injections of calcium gluconate may be necessary to correct electrolyte imbalances and prevent fatal cardiac complications.