Fluoroboric acid
[CAS# 16872-11-0]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Metal halides and halides >> Metal fluorides and salts
• Name: Fluoroboric acid
• Synonyms: Fluoboric acid; Tetrafluoroboric acid; Hydrogen tetrafluoroborate
• Molecular Formula: HBF₄
• Molecular Weight: 87.81
• CAS Registry Number: 16872-11-0
• EC Number: 240-898-3
• SMILES: [H+].[B-](F)(F)(F)F

Properties

• Density: 1.41 g/mL
• Melting point: -90 °C
• Boiling point: 130 °C (dec.)
• Water solubility: MISCIBLE

Safety Data

• Hazard Symbols: GHS05 Danger
• Risk Statements: H290-H314-H318
• Safety Statements: P234-P260-P264-P264+P265-P280-P301+P330+P331-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P321-P363-P390-P405-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Serious eye damage	Eye Dam.	1	H318
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Acute toxicity	Acute Tox.	3	H301
Skin corrosion	Skin Corr.	1A	H314

• Transport Information: UN 1775

Discovery and Applications

Fluoroboric acid, also known as tetrafluoroboric acid, is an inorganic compound with the chemical formula HBF₄. It is a strong acid that typically exists in aqueous solution and is commonly encountered as a colorless, fuming liquid. The acid is composed of hydrogen ions (H⁺) and tetrafluoroborate anions (BF₄⁻), with the latter consisting of a boron atom coordinated tetrahedrally to four fluoride atoms. In pure form or in concentrated solutions, fluoroboric acid is highly corrosive and reacts vigorously with water-releasing hydrogen fluoride (HF), a hazardous byproduct.

Fluoroboric acid was first synthesized through the reaction of boric acid or boron trifluoride (BF₃) with hydrofluoric acid (HF). This reaction produces HBF₄ as a stable acid in aqueous medium. The acid does not exist as a solid in pure form under standard conditions but is handled as a concentrated aqueous solution. The discovery and characterization of fluoroboric acid were part of early 20th-century investigations into complex fluoride chemistry, driven by the broader development of inorganic fluorine compounds.

One of the main applications of fluoroboric acid is in the formation of tetrafluoroborate salts, which are used widely in industrial and laboratory settings. These salts are often formed by reacting fluoroboric acid with metal oxides, hydroxides, or carbonates. The resulting tetrafluoroborate salts, such as sodium tetrafluoroborate (NaBF₄) or potassium tetrafluoroborate (KBF₄), are used in electroplating, particularly for plating tin, zinc, and lead. The use of fluoroborate-based electrolytes provides enhanced metal deposition efficiency, smoother coatings, and better conductivity compared to traditional systems.

Fluoroboric acid is also employed in the preparation of ionic liquids and organometallic complexes. Its anion, BF₄⁻, is relatively weakly coordinating, making it useful for stabilizing reactive cationic species in solution. This property has been utilized in the synthesis of stable catalysts and intermediates in organic and inorganic reactions. In analytical chemistry, fluoroboric acid is used as a reagent for precipitating basic organic compounds, particularly amines, by forming their tetrafluoroborate salts, which often exhibit low solubility and good crystallinity.

Another application is found in the textile industry, where fluoroboric acid is used for wool finishing and dyeing. It assists in the mordanting process by interacting with the fiber and the dye, enhancing color fastness. Additionally, it has been applied in cleaning and etching metal surfaces, including aluminum and stainless steel. Fluoroboric acid's ability to dissolve metal oxides makes it useful in surface preparation for industrial coatings and bonding.

In the field of materials science, fluoroboric acid has been utilized in the synthesis of specialty glasses and ceramics. The presence of boron and fluorine imparts unique thermal and optical properties to the resulting materials. These properties are exploited in the fabrication of optical fibers, specialty lenses, and chemically resistant containers.

Due to its strong acidity and corrosiveness, handling fluoroboric acid requires appropriate safety measures. It can cause severe burns on contact with skin or mucous membranes, and inhalation of vapors can damage the respiratory system. In aqueous solutions, the equilibrium between HBF₄ and HF means that exposure to the acid may involve risk from free hydrofluoric acid, which can penetrate tissues deeply and cause systemic toxicity. For this reason, work with fluoroboric acid is typically conducted in fume hoods with appropriate protective equipment, including acid-resistant gloves, eye protection, and chemical-resistant clothing.

Fluoroboric acid is considered a stable compound under controlled storage conditions. However, it must be kept in tightly sealed containers made of compatible materials, such as Teflon or certain grades of polyethylene, since it is corrosive to glass, many metals, and most common plastics. Storage away from moisture and heat is critical to prevent decomposition and the release of HF fumes.

The compound does not undergo significant oxidation or reduction reactions under standard laboratory conditions but can decompose at high temperatures or in the presence of strong bases. Decomposition leads to the formation of toxic fluorine-containing products, necessitating careful disposal according to hazardous waste regulations. Fluoroboric acid, with its well-documented behavior and defined role in both industrial and research applications, continues to be an important reagent in the chemistry of fluorine and boron.

References

1928. Die Trennung von Hafnium und Zirkonium durch Fällung der Phosphate aus schwefelsaurer Lösung (Schwefelsäuremethode). Zeitschrift für analytische Chemie, 74(10).
DOI: 10.1007/bf01470035

2024. Synthesis and Study of the Physicochemical Properties of Composite Solid Electrolytes (C4H9)3CH3NBF4-Cnanodiamonds. Russian Journal of Electrochemistry, 60(1).
DOI: 10.1134/s1023193524010105

2024. Enhanced performance of organic-inorganic carbon-based stable perovskite photovoltaic cells using pseudohalide additives. Journal of Materials Science: Materials in Electronics, 35(22).
DOI: 10.1007/s10854-024-13384-9