How Is Hydrochloric Acid (HCl) Made?

Most of the world’s hydrochloric acid is made either by burning hydrogen gas in chlorine gas or by capturing the HCl that escapes as a byproduct when chlorine-containing organic chemicals are manufactured. These two industrial routes account for the vast majority of supply, but HCl also forms naturally inside volcanoes, in the atmospheres of other planets, and even inside your own stomach. The chemistry behind each route is straightforward, yet the engineering and biology that make it happen at scale are surprisingly varied.

Direct Synthesis From Hydrogen and Chlorine

The most conceptually simple way to make hydrochloric acid is to combine its two elements directly. Hydrogen gas and chlorine gas are fed into a combustion chamber, where they react to form hydrogen chloride gas. That gas is then dissolved in purified water to produce hydrochloric acid at whatever concentration the manufacturer needs. The reaction is highly exothermic, meaning it releases a lot of heat, and the flame temperature inside the reactor can exceed 2,000 °C.

This method produces very pure HCl because the only inputs are hydrogen, chlorine, and water. It is the preferred route when the acid will be used in food processing, pharmaceuticals, or other applications where contamination is unacceptable. However, the extreme temperatures create engineering challenges. Researchers have explored combustion inside porous inert media as a way to improve heat distribution and reduce emissions while keeping the reactor compact.¹ More recent work has focused on nozzle design to address the high thermodynamic irreversibility that plagues conventional non-premixed hydrogen-chlorine burners, where the two gases meet and combust without being blended beforehand.²

Where the Hydrogen and Chlorine Come From

The hydrogen and chlorine used in direct synthesis rarely come from standalone production. Instead, they are co-products of the chlor-alkali process, which is one of the largest electrochemical industries in the world. In chlor-alkali plants, an electric current is passed through a solution of sodium chloride (table salt dissolved in water). This splits the salt solution into three useful outputs: chlorine gas at one electrode, hydrogen gas at the other, and sodium hydroxide (caustic soda) in solution.

Not all of the hydrogen generated in chlor-alkali plants gets used for HCl synthesis. One study of a chlor-alkali facility found that only about 47% of the hydrogen produced was channeled into making hydrochloric acid, with another 10% used for pressure management and the remaining 43% simply vented into the atmosphere.³ That vented hydrogen represents both wasted energy and wasted raw material, which is why there has been growing interest in finding productive uses for it, including fuel cells and on-site power generation. Still, the chlor-alkali process remains the dominant supplier of feedstock for direct HCl synthesis around the world.

Byproduct HCl From Organic Chemical Manufacturing

A large share of the hydrochloric acid on the market was never the intended product. When chlorine is used to manufacture organic chemicals, the reactions frequently release HCl as a byproduct. One of the biggest examples is the production of vinyl chloride, the precursor to PVC plastic. In that process, ethylene is reacted with chlorine and oxygen in a catalytic step called oxychlorination, which consumes ethylene, hydrogen chloride, and oxygen to produce dichloroethane.⁴ The HCl generated in one stage of vinyl chloride production is recycled into the oxychlorination stage, creating a closed loop. But in many other chlorination reactions across the chemical industry, the byproduct HCl is captured, purified, and sold as hydrochloric acid.

This byproduct route is significant for a practical reason: it ties HCl supply to the demand for other chemicals. When the plastics industry is booming, more byproduct HCl enters the market. When it slows down, HCl can become scarcer and more expensive, even if demand for the acid itself has not changed. The economics of hydrochloric acid are therefore somewhat unusual compared to chemicals that are made on their own terms.

Laboratory-Scale Generation

In a research or analytical lab, the needs are different. You might want a small, steady stream of HCl gas rather than thousands of tons of liquid acid. One classic method is to drip sulfuric acid onto sodium chloride (common salt), which releases HCl gas. Another approach takes advantage of a property of hydrochloric acid itself: when heated, it reaches an azeotropic point, a specific concentration and temperature at which the liquid and vapor have the same composition, so the gas coming off the liquid stays at a fixed HCl content.

Researchers have developed a continuous gas generation method by heating azeotropic hydrochloric acid (about 20.6% HCl in water) to near its azeotropic temperature of roughly 108.5 °C. By passing a nitrogen carrier gas over the heated solution, they achieved a steady output of about 104 parts per million of HCl gas, with the percentage of HCl in the vapor mixture closely matching the concentration in the liquid.⁵ The flow rate of the carrier gas can be adjusted to control the concentration, making it a simple and reproducible method for calibrating gas sensors or running small-scale experiments.

How Your Stomach Makes HCl

Your body runs its own HCl production line. The parietal cells lining your stomach produce hydrochloric acid continuously, generating a gastric juice with a pH between 1 and 2, acidic enough to dissolve metal. The biological machinery is elegant: a specialized enzyme called the proton pump (H-K-ATPase) sits in the cell membrane and actively swaps hydrogen ions from inside the cell for potassium ions from outside. Those hydrogen ions then combine with chloride ions in the stomach lumen to form HCl.⁶

The process is not always running at full blast. When you eat, chemical signals trigger a cascade inside the parietal cell that raises internal calcium and a messenger molecule called cAMP. This activates a chain of events that physically moves the proton pump enzymes from storage compartments inside the cell up to the cell surface, where they start pumping hydrogen ions into the stomach.⁷ When you stop eating, those pumps get pulled back inside the cell, and acid production slows.

This is the same reaction that drugs like omeprazole target. Proton pump inhibitors, as the name suggests, block the H-K-ATPase enzyme, reducing HCl secretion and easing conditions like acid reflux. The stomach’s acid production is a useful reminder that HCl chemistry is not just an industrial concern: every human body is a small-scale acid factory.

HCl From Volcanoes

Volcanoes are the largest natural source of HCl on Earth. When magma rises toward the surface and depressurizes, dissolved chlorine escapes into the gas phase predominantly as hydrogen chloride rather than as molecular chlorine. Modeling work on volcanic gas chemistry predicts very low concentrations of molecular chlorine but much higher concentrations of HCl at near-surface pressures, consistent with field measurements showing that volcanic emissions are dominated by HCl. Applied to melt inclusions from the 2002–2003 eruption of Mt. Etna, one model successfully reproduced the measured ratios of sulfur dioxide to HCl in the volcanic plume.⁸

Large eruptions can inject HCl into the upper atmosphere, but most volcanic HCl is efficiently washed out by rain before it reaches the stratosphere. This is relevant because stratospheric HCl plays a different and more concerning role in atmospheric chemistry, as described in the next section. The HCl that stays in the lower atmosphere is corrosive to nearby ecosystems and infrastructure but dissipates relatively quickly.

HCl in the Atmosphere and Ozone Chemistry

In the stratosphere, hydrogen chloride serves as a reservoir for chlorine: it holds chlorine atoms in a relatively stable form that does not directly destroy ozone. The trouble starts when HCl encounters ice particles, specifically the polar stratospheric clouds that form during the frigid Antarctic winter. On the surface of those ice particles, HCl reacts with chlorine nitrate at a dramatically enhanced rate, releasing molecular chlorine into the gas phase within milliseconds while trapping nitric acid in the ice.⁹

This reaction does two things that accelerate ozone loss. First, the released chlorine is easily broken apart by sunlight into chlorine atoms, which then catalytically destroy ozone molecules. Second, by removing nitrogen dioxide (locked away as nitric acid in the ice), the reaction eliminates a molecule that would otherwise deactivate those chlorine atoms. The net effect is to set up conditions for rapid ozone depletion, the mechanism behind the Antarctic ozone hole. Most of the stratospheric HCl involved in this process comes not from volcanoes but from the breakdown of human-made chlorofluorocarbons (CFCs), which is why the Montreal Protocol’s restrictions on CFCs have been so important.

HCl on Mars

Hydrochloric acid is not unique to Earth. In 2021, instruments aboard the ExoMars Trace Gas Orbiter reported the first detection of a halogen gas in the Martian atmosphere: HCl, present at concentrations in the range of 1 to 4 parts per billion by volume. That is a trace amount, but it was about 20 times higher than the upper limits previously established by other missions.¹⁰ The detections were confirmed independently by two separate spectrometers on the same spacecraft.¹¹

What makes the Martian HCl interesting is the question of where it comes from. On Earth, HCl in the atmosphere points mostly to volcanoes or industrial activity. Mars has no known active volcanism and certainly no chemical plants, so the leading hypothesis involves surface chemistry: dust storms may loft chlorine-containing minerals into the atmosphere, where reactions with trace water vapor release HCl. Supporting this idea, the detected HCl appeared to track seasonal dust activity and showed a striking similarity to the vertical distribution of water vapor in the Martian atmosphere.¹² The annual, transient nature of the signal argues against a volcanic source and in favor of a surface-atmosphere interaction driven by weather.

Handling and Corrosion

Concentrated hydrochloric acid is one of the more aggressive substances handled routinely in industry. It fumes in open air, releasing HCl gas that irritates the eyes, skin, and respiratory tract. Spills are immediately corrosive to most metals and to concrete. Storage and transport typically require containers lined with materials like rubber, certain plastics, or glass, because even stainless steel alloys corrode in hot, concentrated HCl.

Corrosion management in environments where hot HCl is present often relies on chemical inhibitors. Research on carbon steel and various stainless steel grades immersed in 20% and 28% hydrochloric acid at 90 °C found that corrosion rates could be reduced to less than 1 milligram per square centimeter per hour using specific mixtures of organic inhibitors, including quaternary ammonium salts combined with trans-cinnamaldehyde and potassium iodide.¹³ The synergy between these compounds was essential; individually, they were far less effective. For anyone working with HCl in metal-rich environments like oil wells, chemical reactors, or steel-pickling baths, choosing the right combination of inhibitors can be the difference between a functional system and a corroded-through pipe.

Why the Same Acid Comes From So Many Places

Hydrochloric acid keeps showing up across wildly different contexts because its ingredients are everywhere. Hydrogen is the most abundant element in the universe, and chlorine is a common component of salts, minerals, and seawater. Whenever energy, whether from combustion, electrochemistry, biological enzymes, or volcanic heat, brings the two together under the right conditions, HCl forms. That universality is why a single molecule can be the product of a purpose-built industrial reactor, a side effect of making plastic, the working fluid in your digestive system, and a trace gas detected on another planet. The chemistry is the same in every case. What changes is the scale, the purity, and the reason it matters.