107: Hydrofluoric Acid and Fluorides

HISTORY AND EPIDEMIOLOGY

Hydrofluoric acid (HF) has been known for centuries for its ability to dissolve silica. The Nuremberg artist Schwanhard is given credit for the first attempt to use HF vapors to etch glass in 1670.⁴⁷ Today, HF has multiple applications and is widely used throughout industry. In addition to glass etching, HF is used in brick cleaning, etching microchips in the semiconductor industry, electroplating, leather tanning, rust removal, and the cleaning of porcelain.⁴⁷ From 2009 to 2011, the American Association of Poison Control Centers reported more than 1000 exposures to HF and at least 3 deaths (Chap. 136). The hands are the commonest part of the body injured. Exposures to HF often occur as an unintentional occupational hazard. The actual number of work-related poisonings from HF appears difficult to quantitate because of limitations in International Classification of Diseases (ICD) medical coding and the lack of notification of regional poison centers by worksites.¹⁰

HF is also the commonest cause of fluoride poisoning, although other forms of fluoride, including sodium fluoride (NaF), ammonium bifluoride (NH₄HF₂), and sodium or zinc fluorosilicate, may also produce significant toxicity. Historically, NaF has been used as an insecticide, rodenticide, an antihelminthic for swine, and a delousing powder for poultry and cattle. NH₄HF₂ is mainly used in industrial inorganic chemistry, especially in the processing of alloys and in glass etching. Other fluoride salts are widely used in the steel industry, drinking water, toothpaste additives, electroplating, lumber treatment, and the glass and enamel industries.

The widespread use of HF and fluoride containing compounds has resulted in significant toxicity. In 1988, an oil refinery in Texas released a cloud of hydrogen fluoride gas that resulted in 36 people requiring hospital treatment.³⁸ The petroleum industry has since been plagued by similar HF incidents.⁹⁸ NaF was responsible for the poisoning of 263 people and 47 fatalities when it was mistaken for powdered milk and unintentionally combined with scrambled eggs.⁵⁵ Following ingestion, fluoride salts can be converted to HF in vivo, resulting in significant fluoride toxicity.

CHEMISTRY

HF is synthesized as the product of gaseous sulfuric acid and calcium fluoride, which is subsequently cooled to a liquid.⁵⁷ Aqueous HF is a weak acid, with a pK_a of approximately 3.2; as such, it is approximately 1000 times less dissociated than an equimolar strong acid such as hydrochloric acid. HF is generally available in concentrations from 3% to 40%, for use in both industry and the home. Anhydrous HF is highly concentrated (>70%) and used almost exclusively for industrial purposes. HF has unique properties that can cause life-threatening complications following seemingly trivial exposure.

Sodium fluoride is commonly synthesized by the reaction of sodium hydroxide (NaOH) with HF, with subsequent purification by recrystallization. NaF is highly soluble in water and readily dissociates.^...