Hydrofluoric Acid (HF): The Weak Acid That Dissolves Glass and Goes Through Skin

Ask a chemist to rank acids by strength and hydrofluoric acid lands near the bottom, closer to vinegar than to hydrochloric acid. Ask a hospital burn unit which acid worries them most and it lands at the top. Both answers are right, and the reason is the same property: HF is weak because it holds on to its hydrogen, and holding on to its hydrogen is what lets it slip through skin intact.

What is hydrofluoric acid?

Hydrofluoric acid is hydrogen fluoride gas dissolved in water. Its chemical formula is HF: one hydrogen atom bonded to one fluorine atom, molar mass 20.01 g/mol. The pure compound, anhydrous hydrogen fluoride, boils at 19.5 °C (67.1 °F) according to NIST and NOAA, so on a warm day it is a gas. Dissolved in water it becomes the colorless, fuming liquid sold as hydrofluoric acid, usually named by its concentration (for example 49% or 70%).

People also search for it as "HF acid." In chemistry, HF names both the molecule and the gas, and "hydrofluoric acid" names the water solution. The same bond, and the same hazards, run through both.

Is hydrofluoric acid a strong or weak acid?

In water, hydrofluoric acid is a weak acid. Its pKa is 3.17, and Goldfrank's Toxicologic Emergencies puts it plainly: HF "is approximately 1,000 times less dissociated than equimolar hydrochloric acid, a strong acid." Hydrochloric, hydrobromic and hydroiodic acid all give up their hydrogen almost completely. HF, the lightest of the four, mostly doesn't.

Two things hold it together. The H–F bond is unusually strong, and the ions that do form stay paired: in dilute solution much of the dissolved HF sits as a hydrogen-bonded pair, H₃O⁺·F⁻, instead of drifting apart as free ions. Fewer free hydrogen ions means a higher pH than the same concentration of hydrochloric acid, and a milder surface burn.

Why concentrated HF behaves like a stronger acid

As the concentration rises, a fluoride ion can grab a second HF molecule to form the bifluoride ion, HF₂⁻:

That step pulls fluoride out of the ion pairs and frees more hydrogen ions, so concentrated hydrofluoric acid is much more acidic than the pKa suggests. Push it all the way to the pure, water-free compound and the chemistry flips completely: the superacid literature (George Olah, the 1994 Nobel laureate in chemistry, and colleagues) gives anhydrous HF a Hammett acidity of about −15, stronger than pure sulfuric acid, which puts it in superacid territory. Mixed with antimony pentafluoride it forms fluoroantimonic acid, the strongest superacid ever measured. So the answer to "what is the strongest acid in the world" starts with the weak one.

The bifluoride ion also matters outside the lab. It is the anion in ammonium bifluoride (NH₄HF₂), the white flake that glass-etching creams are made from. More on that below.

Why is hydrofluoric acid so dangerous if it's a weak acid?

Because the weakness is what lets it in. A strong acid like hydrochloric is mostly ions, and charged ions do not cross skin easily; they burn the surface. HF stays largely as small, neutral molecules, and neutral molecules pass through the skin's fatty layers. Goldfrank's gives a skin permeability coefficient of 1.4 × 10⁻⁴ cm/s, which "allows HF to penetrate deeply into tissues prior to dissociating."

Once it is inside, the hydrogen matters much less than the fluoride. Fluoride ions bind calcium and magnesium, and the tissue dies as those are stripped away. The damage can reach bone. If enough fluoride gets into the blood, calcium and magnesium levels crash and potassium rises. Goldfrank's describes hypocalcemia, hypomagnesemia and, in many cases, hyperkalemia as the "preterminal events" in fatal exposures, and the heart rhythm fails.

The delayed burn

The most dangerous feature of dilute HF is that it often doesn't hurt at first. The CDC's Agency for Toxic Substances and Disease Registry (ATSDR) gives the timeline by concentration:

The University of North Carolina's HF safety sheet adds data from one study: 7% HF produced symptoms in one to several hours, 12% in under an hour, and 14.5% immediately. So a worker can splash a few percent HF on a hand, feel nothing, finish the shift and go home while the fluoride keeps moving inward.

How small a burn can kill

Goldfrank's states that if more than 2.5% of the body surface is burned with highly concentrated HF, "life-threatening systemic toxicity should be expected," and that fatalities have followed concentrated-HF burns covering less than 5% of the body. UNC's safety sheet records a death after 70% hydrofluoric acid covered 2.5% of the body, with a serum calcium of 2.2 mg/dL, far below the normal range.

The case most often retold happened in Western Australia in 1994. A 37-year-old worker splashed about 100 mL of 70% HF on his right leg, around 10% of his body surface. The leg was amputated the following week. He died in the intensive care unit of Fremantle Hospital about two weeks after the spill, and the case was written up by Gubbay and Fitzpatrick in 1997. It is sometimes repeated online as a 2.5% burn. It was about 10%; the 2.5% figure comes from a different case.

Breathing HF vapor: exposure limits and symptoms

According to ATSDR, most hydrogen fluoride exposures happen by breathing the gas or by skin contact with the acid, and concentrated hydrofluoric acid gives off fumes of its own. The gas has a sharp odor people notice at about 0.04 ppm, well below the workplace limit, so the smell is an early warning. Breathing it first irritates the nose, throat and eyes, then causes coughing and narrowed airways. In severe cases the throat swells shut. Lung injury can develop fast or be delayed 12 to 36 hours, and ATSDR notes that lung effects can follow even a splash on the skin.

Why does hydrofluoric acid dissolve glass?

HF attacks glass at room temperature, which almost no other common acid does, because fluorine bonds to silicon more strongly than oxygen does. Glass is mostly silicon dioxide, SiO₂. Hydrofluoric acid converts it to silicon tetrafluoride, a gas:

With more HF present, the silicon tetrafluoride takes on two more fluorides and stays in solution as hexafluorosilicic acid:

That is why the University of North Carolina's guidance reads "never store HF in glass containers" and lists polyethylene or fluorocarbon plastic, lead or platinum instead. It is also why the frosted look on etched glass is permanent: the surface isn't coated, a thin layer of it is gone.

The same reaction, at industrial scale

The second product, H₂SiF₆, is fluorosilicic acid (FSA), and industry makes it by the same chemistry. According to the CDC's engineering fact sheet on fluoridation additives, phosphate rock is heated with sulfuric acid to make fertilizer-grade phosphoric acid, and that process releases hydrogen fluoride and silicon tetrafluoride gases. Those are captured and condensed into a solution of about 23% fluorosilicic acid, and "approximately 95% of FSA used for water fluoridation comes from this process." It is the same pair of reactions as etching glass, run in a scrubber instead of on a window. We wrote about how that supply chain broke in The Fluoride Shortage Started in a Fertilizer Scrubber.

Glass etching cream and the other places HF hides

You can't buy hydrofluoric acid at a hardware store, but you can buy things that act like it. Glass etching cream, the kind sold in craft stores, is made from bifluoride salts, mainly ammonium bifluoride, which release HF in water. The manufacturer's safety data sheet for Armour Etch lists:

Its hazard statement is the same as a concentrated acid's: "Causes severe skin burns and eye damage." Under decomposition it warns that the product "may react with acids to liberate hydrogen fluoride," and for gloves it recommends butyl rubber, Teflon or Viton. The bifluoride ion is an HF molecule attached to a fluoride ion, and once it is in water it releases HF. Doctors treat these exposures as HF exposures: a 2017 case report in the American Journal of Emergency Medicine describes a 2-year-old who swallowed "fingers full" of an ammonium bifluoride etching cream and had low blood calcium that resisted treatment for up to 24 hours.

Other consumer and trade products carry HF itself:

• Rust stain removers. The safety data sheet for Whink Rust Stain Remover lists hydrofluoric acid at 1.5–3.5% and ships it as UN 1790, Hydrofluoric Acid. The Medical Journal of Australia published an HF burn from a household rust remover in 2001.
• Truck and car wash brighteners. A 2015 CDC MMWR report from Washington State counted 48 occupational HF burns and one death tied to car and truck washing between 2001 and 2013.

At under 20%, these products fall in the concentration band ATSDR says can burn with almost no pain for 12 to 24 hours. That is the combination to respect: a product that feels like soapy water, used for an hour without gloves.

What to do if hydrofluoric acid gets on your skin

Flush with water first, then use calcium. The CDC's instruction is to wash hydrogen fluoride off the skin "as quickly as possible" with a lot of water. Then:

1. Remove contaminated clothing, rings and watches while flushing. Anything that holds acid against the skin keeps feeding the burn.
2. Apply calcium gluconate gel and massage it into the area. ATSDR's recipe for clinicians is 2.5 g of calcium gluconate in 100 mL of water-soluble lubricant. The calcium gives the fluoride something to bind to in the skin instead of in the patient. Workplaces that use HF keep ready-made tubes on hand.
3. Get emergency care, even if nothing hurts. With dilute HF, no pain means nothing; the injury can show up the next day. Call Poison Help at 1-800-222-1222 or 911, and bring the product's safety data sheet.
4. Eyes: ATSDR's instruction is to irrigate with plain water or saline for at least 20 minutes and to remove contact lenses if they come out easily. Keep rinsing on the way to emergency care. Don't put calcium gluconate gel in the eye.

Is hydrofluoric acid in Breaking Bad accurate?

Not the way the show depicted it. In the second episode of Breaking Bad, "Cat's in the Bag...", a body is left in hydrofluoric acid in an upstairs bathtub, and the tub, the floor and the ceiling below give way. MythBusters tested it in its Breaking Bad special, episode 206, which aired August 12, 2013. At a lab at UC Berkeley, samples of steel, wood, linoleum, drywall, cast iron, ceramic tile and pork soaked in HF for 8 hours. They deteriorated to different degrees, but none were completely destroyed.

For the full-scale test, the team switched to concentrated sulfuric acid plus an undisclosed additive. Six gallons in a cast-iron tub left part of the pig carcass behind, and the tub and floor held. It took 36 gallons in a fiberglass tub to produce anything close to the scene. The verdict was "busted." The show got one thing right: HF is dangerous to handle. Its danger is systemic poisoning, not melting.

What is hydrofluoric acid used for?

Hydrofluoric acid's main industrial jobs are refinery alkylation for high-octane gasoline, etching silicon dioxide off semiconductor wafers, pickling stainless steel alongside nitric acid, and etching or polishing glass. It is also the fluorine source for refrigerants, fluoropolymers and fluoride salts, so almost all industrial fluorine chemistry starts with it.

The 2019 Philadelphia refinery explosion

The US Chemical Safety Board's report on the Philadelphia Energy Solutions refinery shows the stakes. On June 21, 2019, a corroded pipe elbow in the refinery's HF alkylation unit ruptured. The vapor cloud, about 95% propane and 2.5% HF, ignited. PES estimated that 5,239 pounds of HF were released, of which about 1,968 pounds were contained by water spray. In the largest explosion, a fragment of a hydrocarbon vessel weighing about 38,000 pounds flew across the Schuylkill River. Property damage was estimated at $750 million. Five workers had minor injuries during the incident and response, and a Philadelphia firefighter reported breathing problems two days after the fire. Among the CSB's recommendations: remotely operated emergency isolation valves on vessels containing HF.

A short history of hydrofluoric acid

Carl Wilhelm Scheele made hydrofluoric acid in 1771 by heating fluorspar (calcium fluoride) with sulfuric acid, and he noticed the fumes etched his glass retort. Many histories credit the Nuremberg glass artist Heinrich Schwanhardt with etching glass using fluorspar and acid a century earlier, around 1670. A modern re-reading of Schwanhardt's own writings found no mention of fluorspar and suggests his "very strong acid" may have been nitric acid or aqua regia, so treat that credit as traditional rather than settled.

Isolating fluorine itself from HF took more than a century and injured several of the chemists who tried. Henri Moissan finally did it on June 26, 1886, by electrolyzing a solution of potassium fluoride in anhydrous hydrogen fluoride. He received the 1906 Nobel Prize in Chemistry for it.

Ammonium bifluoride: the glass-etching flake we sell

We don't stock hydrofluoric acid. For glass etching and metal cleaning we ship ammonium bifluoride from our warehouse in Taylor, Texas, and we also carry fluorosilicic acid for metal finishing:

Both carry the fluoride hazard described above. Ammonium bifluoride in water releases HF, and both are corrosive to skin and eyes. We send the Safety Data Sheet with each shipment and ask that anyone handling them keep calcium gluconate gel within reach. If you're starting a process that uses either one, read Section 4 (first aid) and Section 8 (gloves and PPE) of the SDS before the drum arrives; our guide to reading a safety data sheet walks through both.