Hydrochloric acid is made by dissolving hydrogen chloride gas in water. The most accessible method, known as the Mannheim process, reacts common table salt (sodium chloride) with sulfuric acid to produce the gas, which is then absorbed into water to form the liquid acid. Commercially, concentrated hydrochloric acid contains about 36 to 38% hydrogen chloride by weight, though lower concentrations (around 31%) are more commonly sold for general use.
The Salt and Sulfuric Acid Method
The oldest and most widely understood route to hydrochloric acid uses two ingredients: sodium chloride and sulfuric acid. When heated together, they react in two stages. The first reaction begins around 150°C (300°F) and produces hydrogen chloride gas along with sodium bisulfate as a solid byproduct:
NaCl + H₂SO₄ → NaHSO₄ + HCl (gas)
At higher temperatures, a second reaction kicks in. The sodium bisulfate reacts with additional salt to produce more hydrogen chloride gas and sodium sulfate (also called salt cake, used in glass and paper manufacturing):
NaCl + NaHSO₄ → Na₂SO₄ + HCl (gas)
The overall equation simplifies to: 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl. In industrial settings, this takes place inside a rotary kiln where hot combustion gases flow upward while the solid reactants move downward. The hydrogen chloride gas escapes through a vent at the top and is channeled into an absorber filled with water.
How the Gas Becomes Liquid Acid
Hydrogen chloride on its own is a colorless gas at room temperature. It becomes hydrochloric acid only when dissolved in water. Industrial facilities use a falling-film absorber, where water flows down a surface while the gas passes over it, allowing continuous absorption. The process generates heat, so cooling is necessary to keep the concentration high.
At standard temperature and pressure, HCl dissolved in water forms a constant-boiling mixture (azeotrope) at about 20.2% concentration. Pushing beyond that requires careful temperature and pressure control. The fuming, concentrated acid you can buy commercially tops out at roughly 38%. Common industrial grades are sold at 31% or 35% by weight.
Industrial Direct Synthesis
Large chemical plants often skip the salt route entirely and instead burn hydrogen gas directly in chlorine gas. This reaction is extremely energetic, reaching temperatures above 2,000°C (and sometimes up to 2,500°C) inside specialized graphite-lined furnaces. Graphite is used because it resists corrosion from the acid and conducts heat well enough to keep the furnace walls from overheating.
The resulting hydrogen chloride gas is then cooled and fed into the same type of falling-film water absorber to produce strong hydrochloric acid. This method yields very pure acid because there are no salt or sulfate byproducts, making it the preferred route for pharmaceutical-grade and food-grade HCl.
Why Most People Should Buy It Instead
Hydrochloric acid is widely available at hardware stores under the name muriatic acid, typically at concentrations around 31%. For pool maintenance, concrete etching, metal cleaning, and pH adjustment, store-bought muriatic acid is cheaper and safer than anything you could produce at home.
Making HCl yourself involves heating concentrated sulfuric acid, which is dangerous. The hydrogen chloride gas produced is heavier than air, meaning it pools in low-lying or enclosed spaces and can cause severe respiratory damage. Even brief exposure to high concentrations irritates the throat, lungs, and eyes. The reaction also requires glassware or other corrosion-resistant equipment, since the gas and acid attack most metals.
Regulatory Status
Hydrochloric acid (including anhydrous hydrogen chloride gas) is classified as a DEA List II regulated chemical. This means purchases above certain thresholds may require identification and record-keeping by the seller. The regulation exists because HCl can be used in the manufacture of controlled substances. Buying muriatic acid at a hardware store for household quantities is legal and straightforward, but bulk purchases or unusual buying patterns can trigger reporting requirements.
Handling and Storage
If you’re working with hydrochloric acid in any concentration, a few precautions matter. The acid corrodes most metals, including steel and aluminum, and produces flammable hydrogen gas when it does. Suitable storage containers include high-density polyethylene (HDPE) and glass. Never store it in metal containers.
Keep it in a well-ventilated area away from bases like sodium hydroxide. Mixing strong acids with strong bases produces intense heat and can cause liquid to splash violently. The EPA groups hydrochloric acid with other acids for storage purposes and recommends keeping it completely separate from bases, oxidizers, and dry chemicals.
For personal protection, rubber or chemical-resistant gloves, eye protection, and a rubber apron are the minimum when handling the liquid. If you’re working in an enclosed area or with concentrated acid, a respirator rated for acid gases is necessary, since the fumes alone are harmful. The CDC recommends positive-pressure, self-contained breathing apparatus for any situation involving potentially unsafe vapor levels.
Cleaning Up Spills
Small spills of dilute hydrochloric acid (around 5 to 10%) can be neutralized with sodium bicarbonate, ordinary baking soda. Sprinkle it over the spill gradually. The reaction produces carbon dioxide gas, water, and sodium chloride, so expect fizzing. Add baking soda until the fizzing stops, then check the pH. Once the liquid is neutral, it can be safely rinsed away with water.
For concentrated acid spills, the neutralization reaction generates significant heat. Add the baking soda slowly and in small amounts to avoid splattering. Larger spills in poorly ventilated areas are genuinely dangerous because the vapor is heavier than air and accumulates at floor level, exactly where you’d be kneeling to clean it up. Open windows and doors first, and leave the area if you feel any throat or eye irritation.