Hydrofluoric Acid
Properties
| State | Liquid (aqueous solution) at room temperature |
| Color | Colorless |
| Solubility | Miscible with water |
| Melting Point | -35°C (40% solution) |
| Boiling Point | 106°C (40% solution) |
About Hydrofluoric Acid
Hydrofluoric acid (HF in water, 20.006 g/mol) is one of those compounds where the textbook answer ('weak acid, pKa 3.17') and the lab reality ('arguably the most dangerous acid you'll ever handle') diverge completely. It's a weak acid in the formal sense — the H-F bond energy is so high (570 kJ/mol, the strongest of the H-X bonds) that dissociation in water is incomplete. But the chemistry of fluoride ion is what kills people. F- is small, penetrates intact skin without immediate pain, then chelates Ca(2+) and Mg(2+) ions in tissue and serum. A spill of 50% HF on as little as 2.5% of body surface area can cause fatal hypocalcemia within hours, even after the initial burn appears minor. The treatment is calcium gluconate gel rubbed into the contact site, calcium gluconate IV for systemic exposure, and sometimes intra-arterial calcium for digital exposures — every HF user should have a tube of 2.5% Ca-gluconate gel within reach before opening the bottle. Despite this, HF is irreplaceable for several jobs. It's the only common acid that dissolves SiO2 (glass): SiO2 + 6 HF → H2SiF6 + 2 H2O. That's the basis of glass etching, semiconductor wafer cleaning (the standard 'BOE' buffered oxide etch is HF/NH4F mixture), and silicon device fabrication. It's the feedstock for fluoropolymers (PTFE, PVDF) and fluorocarbon refrigerants. In petroleum refining, HF catalyzes alkylation of isobutane with light olefins to make high-octane gasoline blendstock — though most new alkylation units use H2SO4 to avoid HF's hazards.
Where you'll encounter it
If you've ever frosted glass, used a smartphone (whose silicon chip was etched with BOE solutions), driven a car burning alkylate gasoline, or even cooked on a Teflon pan — the chemistry started with HF. Every silicon wafer fabricated at TSMC, Intel, or Samsung passes through dozens of buffered oxide etch steps where HF/NH4F mixtures selectively strip SiO2 layers off the wafer surface — without HF, there is no integrated circuit. Stained-glass restoration shops use dilute HF to frost or shape the borders of replacement pieces because no other acid touches glass. In refining, HF alkylation units at older Phillips 66 and Marathon refineries combine isobutane with light olefins to produce the high-octane blendstock that boosts pump-grade gasoline above 91 octane. The PTFE coating on every nonstick pan in your kitchen traces back to TFE monomer made from HF + chloroform.
Common Uses
- Buffered oxide etch (BOE) for SiO2 in semiconductor fabrication
- Glass etching, frosting, and chemical strengthening of optical components
- Feedstock for HF-based fluoropolymers including PTFE, PVDF, and FKM
- Production of fluorocarbon refrigerants (HFCs, HFOs replacing CFCs)
- Alkylation catalyst in petroleum refining for high-octane gasoline blendstock
- Conversion of UO2 to UF4 then UF6 for nuclear fuel enrichment
- Stainless steel pickling at low concentration mixed with HNO3
Safety Information
EXTREMELY HAZARDOUS — fundamentally different risk profile from other acids. Penetrates intact skin without immediate sensation, then F- chelates serum and tissue Ca(2+)/Mg(2+) causing local necrosis and systemic hypocalcemia. Even dilute HF (under 7%) can be lethal if exposure covers more than ~25 in2 of skin. OSHA PEL is 3 ppm (8-hr TWA, as F); ACGIH TLV is 0.5 ppm. GHS: H300+H310+H330 (fatal by all routes), H314 (severe burns), H410 (toxic to aquatic life). Mandatory protocol: dedicated training, calcium gluconate 2.5% gel within arm's reach, double nitrile or butyl-rubber gloves, full face shield, splash apron, fume hood with sash down, never alone, posted emergency response plan with hospital contact for IV calcium gluconate. Many academic labs simply prohibit HF and substitute NH4F or buffered alternatives wherever possible.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.