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Potassium bifluoride
Potassium bifluoride
Potassium bifluoride
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Potassium bifluoride
Names
IUPAC name
Potassium bifluoride
Other names
Potassium hydrogen difluoride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.233 Edit this at Wikidata
EC Number
  • 232-156-2
RTECS number
  • TS6650000
UNII
UN number 1811
  • InChI=1S/F2H.K/c1-3-2;/q-1;+1 ☒N
    Key: FLCWRBFUWAZYGV-UHFFFAOYSA-N ☒N
  • [H-](F)F.[K+]
Properties
K[HF2]
Molar mass 78.103 g/mol
Appearance colourless solid
Odor slightly acidic
Density 2.37 g/cm3
Melting point 238.7 °C (461.7 °F; 511.8 K)
Boiling point decomposes
  • 24.5 g/(100 mL) (0 °C)
  • 30.1 g/(100 mL) (10 °C)
  • 39.2 g/(100 mL) (20 °C)
  • 114.0 g/(100 mL) (80 °C)
Solubility soluble in ethanol
Structure
monoclinic
Thermochemistry
45.56 J/(mol·K) [1]
−417.26 kJ/(mol·K)
Hazards
GHS labelling:[2]
GHS05: CorrosiveGHS06: Toxic
Danger
H301, H310, H314
P260, P262, P264, P270, P280, P301+P310, P301+P330+P331, P302+P350, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P322, P330, P361, P363, P405, P501
Flash point non flammable
Related compounds
Other anions
 Potassium fluoride
Other cations
 Sodium bifluoride, ammonium bifluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Potassium bifluoride is the inorganic compound with the formula K[HF2]. This colourless salt consists of the potassium cation (K+) and the bifluoride anion ([HF2]−). The salt is used as an etchant for glass. Sodium bifluoride is related and is also of commercial use as an etchant as well as in cleaning products.[3]

Synthesis and reactions

[edit]

The salt was prepared by Edmond Frémy by treating potassium carbonate or potassium hydroxide with hydrofluoric acid:

2 HF + KOH → K[HF2] + H2O

With one more equivalent of HF, K[H2F3] (CAS RN 12178-06-2, m.p. 71.7 °C[4]) is produced:

HF + K[HF2] → K[H2F3]

Thermal decomposition of K[HF2] gives hydrogen fluoride:

K[HF2] → HF + KF

Applications

[edit]

The industrial production of fluorine entails the electrolysis of molten K[HF2] and K[H2F3].[3] The electrolysis of K[HF2] was first used by Henri Moissan in 1886.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Potassium bifluoride

View on Grokipedia
from Grokipedia
Potassium bifluoride, chemically known as potassium hydrogen fluoride with the formula KHF₂, is a colorless to white crystalline solid that serves as a key inorganic compound in industrial applications. It has a molecular weight of 78.1 g/mol, a of 239°C, and decomposes upon heating without a defined , while exhibiting high in and a specific gravity of 2.37. As an acidic salt derived from the ion [HF₂]⁻, it readily hydrolyzes in aqueous solutions to release , making it corrosive to metals, , and biological tissues. This finds widespread use as a metallurgical in and processes, where it aids in removing oxides from metal surfaces to improve bonding. It is also employed as a wood to protect against fungal decay and insects, and in metal treatment for and applications. Additionally, potassium bifluoride plays a role in the electrolytic production of gas and as a chemical intermediate for synthesizing organic and inorganic fluorides, including those used in pharmaceuticals and agrochemicals. In glass manufacturing, it acts as an etchant to create frosted surfaces on decorative items and mirrors. Due to its reactivity, potassium bifluoride poses significant and risks, classified as corrosive and acutely toxic by , , or contact. Exposure can cause severe burns, respiratory irritation, and systemic , potentially leading to fluorosis with chronic overexposure; occupational limits include an OSHA of 2.5 mg/m³ as an 8-hour time-weighted average. Handling requires such as chemical-resistant gloves, full-face shields, and respirators, with spills necessitating immediate containment to prevent environmental release. It is typically produced by neutralizing with potassium hydroxide or carbonate, followed by .

Properties

Physical properties

Potassium bifluoride has the KHF₂ or K[HF₂] and a of 78.10 g/mol. It appears as a colorless to white crystalline solid, often in the form of lumps, , or . The compound is odorless. The of potassium bifluoride is 2.37 g/cm³ at 25 °C. It has a of 239 °C, above which it decomposes rather than boiling. Potassium bifluoride exhibits high solubility in , dissolving up to 39 g per 100 mL at 20 °C to form acidic solutions. It is slightly soluble in alcohols such as and but insoluble in most organic solvents. In terms of its solid-state arrangement, potassium bifluoride crystallizes in the tetragonal system with I4/mcm (No. 140), featuring a three-dimensional network where potassium cations are coordinated to eight anions and hydrogen bonds link bifluoride units.

Chemical properties

Potassium bifluoride is an ionic compound composed of the potassium cation (K⁺) and the bifluoride anion ([HF₂]⁻). The [HF₂]⁻ anion adopts a linear, centrosymmetric structure (D∞h symmetry) featuring a strong symmetric hydrogen bond between the two fluoride atoms, with an H–F bond distance of approximately 1.14 Å and an F···F separation of about 2.29 Å. This hydrogen bond is one of the strongest known, with a bond energy of around 163 kJ/mol, contributing to the compound's distinctive chemical behavior. In aqueous solutions, potassium bifluoride dissociates to provide a source of (HF), as the bifluoride anion equilibrates with HF and ions (HF₂⁻ ⇌ HF + F⁻). This results in acidic conditions, with values less than 7; for instance, a saturated aqueous solution at exhibits a of approximately 3.0. The compound demonstrates good stability in dry environments but undergoes when exposed to moist air, releasing hazardous HF gas according to the reaction KHF₂ + H₂O → KOH + 2HF. Its hygroscopic nature further exacerbates this reactivity, as it readily absorbs atmospheric to form hydrated species such as KHF₂·nH₂O, where n varies depending on and conditions.

Synthesis

Laboratory synthesis

Potassium bifluoride is commonly prepared in the laboratory by neutralizing potassium hydroxide or potassium carbonate with hydrofluoric acid, followed by evaporation of the resulting solution to induce crystallization. The reaction with potassium hydroxide proceeds as follows: \ceKOH+2HF−>KHF2+H2O\ce{KOH + 2HF -> KHF2 + H2O} A detailed procedure involves cooling 25 mL of 40% hydrofluoric acid in a platinum or nickel vessel with an ice-salt mixture, then adding the calculated amount of potassium carbonate in small portions with stirring until carbon dioxide evolution ceases and the pH reaches approximately 6 (slightly acidic). The mixture is diluted with 40 mL of distilled water, filtered through a paraffin-coated glass funnel to remove any insoluble residues, and the filtrate is evaporated on a water bath until saturation is approached. Upon cooling to 0°C, crystals form, which are collected by filtration on a Büchner funnel, dried in a desiccator over phosphorus(V) oxide, and further dehydrated under vacuum (10^{-1} mmHg) first at room temperature for 10-20 minutes, then at 150°C until constant mass is achieved. An alternative laboratory method involves reacting with , either as or gas, in a or lead vessel to form the bifluoride directly: \ceKF+HF−>KHF2\ce{KF + HF -> KHF2} Excess is gently evaporated by heating, yielding the solid product. This approach is suitable for small-scale preparations where conditions are desirable. Purification of the crude product is achieved by recrystallization from hot , leveraging the compound's of about 39.2 g/100 mL at 20°C, which increases significantly at higher temperatures, allowing impurities to remain in solution upon cooling. Alternatively, can be used as a for recrystallization due to the low of potassium bifluoride (<0.1 g/100 mL at 25°C), facilitating removal of water-soluble contaminants. All procedures must employ hydrofluoric acid-resistant apparatus, such as platinum, nickel, lead, or specially coated glassware, to prevent corrosion. These methods typically proceed at room temperature or with mild heating and afford the product in good yield under controlled conditions.

Industrial production

Potassium bifluoride is primarily produced on an industrial scale through the direct reaction of (KF) with anhydrous (HF) in controlled reactor environments. The process involves feeding anhydrous HF into a solution or suspension of KF, forming KHF₂ according to the equation KF + HF → KHF₂. This method is efficient for high-volume production and is often integrated into facilities manufacturing fluorine compounds, where KHF₂ serves as an intermediate or byproduct during the electrolysis of HF-KF mixtures for elemental fluorine generation. An alternative industrial route starts with the production of HF from fluorspar (CaF₂) via reaction with concentrated sulfuric acid, yielding gaseous or liquid HF that is then neutralized with potassium hydroxide (KOH) to produce KHF₂. This integrated approach is common in large chemical plants, leveraging the abundant supply of fluorspar as a raw material and combining HF generation with bifluoride synthesis in a single facility for economic efficiency. Global annual output of potassium bifluoride is estimated at around 300,000 metric tons, with the majority produced in Asia (particularly China) and Europe, driven by demand in fluorochemical and manufacturing sectors. Due to the highly corrosive nature of HF involved, production reactors are typically lined with polytetrafluoroethylene (PTFE, or Teflon) or constructed from specialized alloys like Hastelloy to prevent material degradation. The resulting KHF₂ slurry or solution undergoes subsequent drying in vacuum ovens or fluidized bed dryers, followed by granulation to achieve the desired particle size for commercial handling and transport.

Reactions

Reactions with metals and oxides

Potassium bifluoride (KHF₂) acts as an effective and fluxing agent primarily through its hydrolysis to release hydrogen fluoride (HF), which reacts with silica (SiO₂) in glass to form volatile silicon tetrafluoride (SiF₄) and water, or alternatively fluosilicates under certain conditions. The simplified reaction for fluosilicates is represented as \ceSiO2+4KHF2−>K2SiF6+2KF+2H2O\ce{SiO2 + 4 KHF2 -> K2SiF6 + 2 KF + 2 H2O}, enabling precise applications on surfaces. KHF₂ corrodes certain metals, including aluminum and magnesium, by generating HF that facilitates the formation of corresponding metal fluorides and gas. For aluminum, the process can be approximated as 2\ceAl+6\ceHF→2\ceAlF3+3\ceH22 \ce{Al} + 6 \ce{HF} \rightarrow 2 \ce{AlF3} + 3 \ce{H2}, where HF derives from KHF₂ dissociation, leading to surface dissolution and complex fluoride formation. Similar reactivity occurs with magnesium, binding to its ions and disrupting protective layers. In reactions with metal oxides, KHF₂ serves as a fluorinating agent, converting oxides to fluorides by displacing oxygen. For example, with iron(III) oxide (Fe₂O₃), KHF₂ reacts in a solid-state process to yield iron(III) fluoride (FeF₃) and other fluoro complexes, as observed in thermal studies where the reaction proceeds partially below 200°C and completes around that temperature; an approximate equation is \ceFe2O3+6KHF2−>2FeF3+6KF+3H2O\ce{Fe2O3 + 6 KHF2 -> 2 FeF3 + 6 KF + 3 H2O}. This fluorination is driven by the bifluoride ion's ability to deliver fluoride for oxide reduction. Additionally, KHF₂ catalyzes reactions in various systems by slowly releasing HF under mild aqueous conditions, which acts as an acid catalyst to promote further fluorination or dissolution processes.

Decomposition and thermal behavior

Potassium bifluoride exhibits thermal stability up to its of 239 °C, beyond which it begins to show signs of upon further heating. The compound undergoes primarily above 400 °C, yielding as a solid residue and gas as the volatile product, according to the reaction KHF₂ → KF + HF. This process evolves corrosive vapors, which can pose handling challenges during high-temperature operations. The is quantitative at elevated temperatures and leaves behind anhydrous , highlighting the compound's role in production processes. Reported onset temperatures for vary between 310–440 °C depending on experimental conditions, reflecting differences in techniques and sample purity. This thermal behavior is integrated into the broader of the KHF₂–KF–HF system, where the can be reversed under controlled lower-temperature conditions by reacting with to reform potassium bifluoride. Such reversibility underscores the dynamic nature of stability in fluoride-containing environments.

Applications

Industrial applications

Potassium bifluoride (KHF₂) is widely employed in industrial processes for its controlled release of , enabling applications in , fluxing, and fluorination reactions. In the and ceramics industries, it functions as an etchant to create patterned and frosted surfaces on , producing decorative elements such as and architectural finishes. It is also utilized in the manufacture of special optical , including crown and crown flint varieties, and as a flux in production to reduce melting temperatures and enhance material fusion during firing. In , potassium bifluoride serves as a for , , and , particularly with aluminum and , where it removes layers to promote clean, strong bonds. This application relies on its reactivity with metal oxides, facilitating oxide dissolution at elevated temperatures. It is further used in aluminum brightening processes to clean and polish surfaces prior to finishing. Additionally, it acts as a complexing agent in baths for metals like , , and , stabilizing solutions and improving deposit uniformity. Within chemical synthesis, potassium bifluoride is a key in cells for the industrial production of gas, where it supports the decomposition of at high temperatures. It also functions as a precursor for organofluoride compounds, enabling fluorination in the synthesis of pharmaceuticals, agrochemicals, and specialty materials through controlled transfer. In wood preservation, it is impregnated into timber as part of formulations that release ions to deter fungal decay and insect infestation, extending the service life of treated lumber. Other industrial roles include its incorporation into chemical cleaning solutions for removing scales and deposits from equipment, as well as in laundry formulations where it aids in from and mineral-based residues. Furthermore, it acts as a catalyst component in organometallic systems for and reactions, promoting efficient linking in production.

Laboratory uses

Potassium bifluoride serves as a valuable fluorination in , particularly for introducing atoms into molecules under mild conditions. In the deoxyfluorination of tertiary alcohols, it is combined with to generate in situ, enabling the efficient conversion of the alcohol to the corresponding with high yields and good tolerance. This method is advantageous in settings due to the use of inexpensive, readily available reagents and avoidance of hazardous gaseous HF. Additionally, potassium bifluoride is employed to convert various trialkoxyborates, including alkyl, aryl, pentafluorophenyl, and alkenyl derivatives, into stable perfluoroalkyltrifluoroborates, which are useful intermediates in and cross-coupling reactions. In , potassium bifluoride finds application in for , particularly in fusion techniques to dissolve materials like silicates for subsequent determination of metals such as calcium and magnesium via or atomic absorption spectrometry. The compound facilitates the breakdown of matrices by providing ions that complex with interfering elements, ensuring accurate quantification in water samples. Potassium bifluoride is widely used as a flux in materials science for the growth of high-quality fluoride single crystals, leveraging its low melting point and ability to dissolve metal fluorides at elevated temperatures. For instance, it enables the flux growth of transition metal fluorides like FeF3 and KFeF4 by forming a molten medium that promotes controlled crystallization upon cooling. Similarly, it supports the synthesis of optical fluoride crystals, such as those doped for laser applications, by reacting with precursors like MgF2 under controlled atmospheres to yield transparent materials with desirable UV and IR transmission properties. Due to its equilibrium dissociation to release controlled amounts of HF, potassium bifluoride is utilized for pH adjustment in the calibration of fluoride-selective electrodes, maintaining stable acidic conditions essential for accurate potentiometric measurements of fluoride ions in aqueous solutions. This application ensures reproducible ionic strength and minimizes hydrolysis effects during standard curve preparation.

Safety and toxicity

Health hazards

Potassium bifluoride is highly toxic upon acute exposure, primarily through or , leading to severe chemical burns in the , , and lungs, as well as systemic . can cause immediate , vomiting, and potentially fatal due to ion sequestration of calcium; the oral LD50 in rats is approximately 160 mg/kg, indicating high . of dust or fumes irritates the , causing coughing, , and in severe cases, delayed , which may develop hours after exposure and require emergency medical intervention. Skin and eye contact with potassium bifluoride results in deep tissue penetration and severe corrosive damage, often leading to and from the binding of calcium ions by released . Ocular exposure causes intense pain, , and potential permanent vision loss due to the compound's ability to etch and delicate tissues. The primary mechanism of involves the release of (HF) in moist environments, where the undissociated HF molecule readily crosses cell membranes, dissociates to ions that inhibit enzymes such as , and disrupt electrolyte balance by complexing with essential divalent cations like calcium and magnesium, leading to cardiac arrhythmias and cellular death. Chronic exposure to potassium bifluoride through repeated inhalation or skin contact can result in fluorosis, characterized by accumulation in bones and teeth, causing skeletal pain, joint stiffness, changes, and dental mottling. Prolonged respiratory exposure may also lead to chronic bronchitis and persistent lung irritation. Occupational exposure limits for potassium bifluoride are established based on content, with the OSHA (PEL) at 2.5 mg/m³ (as F) over an 8-hour time-weighted average, and the NIOSH immediately dangerous to life or health (IDLH) value at 250 mg/m³ (as F).

Handling and storage

Potassium bifluoride is a highly corrosive and toxic substance that requires stringent handling protocols to prevent exposure and accidents. Personnel must be trained on its properties prior to use, emphasizing the use of (PPE) such as chemical-resistant gloves (e.g., ), protective clothing, eye and face protection, and respiratory protection in areas with inadequate ventilation. Handling should occur in a well-ventilated or under local exhaust ventilation to minimize of dust or fumes, and contact with skin, eyes, or clothing must be avoided by immediately changing contaminated garments and washing affected areas thoroughly with water. Key handling precautions include avoiding formation of dust aerosols, never adding directly to the compound to prevent exothermic reactions or gas release, and prohibiting eating, drinking, or smoking in the work area to reduce ingestion risks. The material is hygroscopic and reacts with moisture, strong acids, bases, and silica-containing substances like , so operations must isolate it from these to avoid or hazardous gas evolution. Good industrial practices, such as handwashing after handling, are essential. For storage, potassium bifluoride should be kept in tightly closed, original containers made of compatible materials such as or other plastics resistant to fluorides, avoiding due to risks. Containers must be stored in a cool, dry, well-ventilated area designated for corrosives, locked and accessible only to trained personnel, and separated from incompatibles like strong oxidizers, acids, bases, metals, and moisture sources. Exposure to excess heat, humid air, or water should be prevented, as it can lead to decomposition or container failure.

References

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