Hydrochloric Acid (HCl): Chemical Name, Strengths, Uses and Handling

The short answer

HCl is hydrogen chloride. Dissolved in water it is hydrochloric acid, known in trade as muriatic acid. CAS 7647-01-0, molar mass 36.46 g/mol. It is one of the strongest common acids, completely dissociated in water, and it is sold as a solution in a narrow band of strengths — typically 30 to 33 % in India, up to about 37 % as concentrated acid.

Two facts govern everything practical about it. It fumes: concentrated acid gives off hydrogen chloride gas that forms a visible white mist with moisture in the air. And it destroys most metals, stainless steel included, so the material of construction is not a detail but the first decision.

Chemical name, formula and identifiers

A note on the long tail of names you will see on product lists: a compound written as pyridoxine HCl or tris HCl is not hydrochloric acid. It is the hydrochloride salt of that substance — the base reacted with one equivalent of HCl to give a stable, water-soluble crystalline solid. Forming hydrochloride salts is one of the largest uses of the acid in pharmaceutical manufacture.

Hydrogen chloride and hydrochloric acid

Pure hydrogen chloride is a colourless gas that boils at −85 °C. It is enormously soluble in water — roughly 700 volumes of gas dissolve in one volume of water at room temperature — and dissolving it gives hydrochloric acid.

The distinction matters in three practical ways:

• Different dangerous goods entries. Anhydrous hydrogen chloride is a liquefied toxic and corrosive gas, UN 1050, Class 2.3. The aqueous acid is UN 1789, Class 8. They are shipped, stored and permitted completely differently.
• Different uses. The dry gas is used in gas-phase organic chemistry and in some semiconductor processes; almost everything else uses the solution.
• Fuming. Concentrated solutions continuously release HCl gas from the surface, which is why an open drum smokes and why a store room needs low-level extraction and a scrubber.

Strengths, Baume and the azeotrope

Commercial acid is quoted by weight percent HCl, and sometimes by degrees Baumé, an old density scale still used in construction and pool supply.

Why the ceiling at around 37 %? Because that is close to the practical limit of what water will hold at ambient temperature and pressure, and because above roughly 20.2 % HCl the solution fumes increasingly hard, losing strength in storage. That 20.2 % figure is the azeotrope: a mixture of 20.2 % HCl and water that boils unchanged at 108.6 °C. Distil any hydrochloric acid and it converges on 20.2 %, which is why you cannot concentrate the acid by boiling, and why 20.2 % is the strength a reclaimed or regenerated acid tends to arrive at.

Physical properties

That last line is worth a specification clause. A water-white acid and a straw-yellow acid of the same assay are not the same product. If the acid feeds a process where iron matters — ion exchange regeneration, food, pharma — put an iron limit and a colour limit in the purchase specification, not just the HCl percentage.

Synthesis acid and by-product acid

Two routes, and the difference shows up in the impurity profile.

• Direct synthesis. Hydrogen and chlorine from a chlor-alkali plant are burned together and the gas absorbed in water. This gives a clean, water-white acid with very low iron and no organics. It is the basis of food, pharmacopoeial and electronic grades.
• By-product acid. A great many organic processes — chlorination, isocyanate manufacture, PVC chemistry, fluorocarbon production — produce hydrogen chloride as a co-product, which is absorbed and sold. It is cheaper, and it can carry traces of the organics from the process it came from, along with iron from the plant.

For steel pickling or effluent neutralisation, by-product acid is usually fine and is the sensible purchase. For anything where a trace organic would end up in your product, or where the acid contacts food or a pharmaceutical intermediate, specify synthesis acid and ask for the total organic carbon figure on the certificate.

What hydrochloric acid is used for

• Steel pickling. The single largest use. Mill scale and rust are dissolved off hot-rolled coil, wire and tube before galvanising, cold rolling, drawing or coating. The spent liquor, rich in ferrous chloride, is regenerated or sold on for ferric chloride manufacture.
• Chloride chemicals. Feedstock for ferric chloride, poly aluminium chloride, calcium chloride, zinc chloride and a long list of metal chlorides.
• Water and effluent treatment. pH correction of alkaline effluent, and regeneration of the cation resin in demineralisation plants.
• Oil and gas. Matrix acidising and fracture acidising of carbonate reservoirs, typically at 15 or 28 % with corrosion inhibitor.
• Food processing. Starch hydrolysis to glucose syrups, gelatin manufacture, and pH adjustment — all on food grade acid against the FCC or an equivalent monograph.
• Pharmaceutical. Formation of hydrochloride salts, pH adjustment, and hydrolysis steps.
• Leather, textile and construction. Pickling of hides, neutralisation after alkaline treatment, and removal of mortar, efflorescence and scale from masonry.

Scapex Exports supplies hydrochloric acid 33 % technical, hydrochloric acid 30 to 33 % for water treatment, food grade muriatic acid and hydrochloric acid AR grade.

Materials of construction: what it destroys

This is where hydrochloric acid installations go wrong, so it is worth stating plainly.

Two further rules. Hydrochloric acid attacking a metal releases hydrogen, so a closed vessel holding acid over steel or scrap can build a flammable atmosphere. And acid vapour will corrode steelwork, electrical panels and instruments some distance from the tank — site the store where the fumes cannot drift onto the plant.

Safety, handling and export documents

• Classification. UN 1789, Class 8, packing group II. Causes severe skin burns and eye damage; the vapour causes respiratory irritation and, at concentration, damage.
• Never mix with hypochlorite. Hydrochloric acid and bleach react to release chlorine gas. This is one of the most common serious accidents in cleaning and water treatment, and it belongs on the tank label, not just in the safety data sheet.
• Segregate from alkalis, oxidisers including nitric acid and permanganate, cyanides and sulphides, and from active metals.
• PPE. Face shield with goggles, PVC or neoprene gloves and apron, and boots. Eyewash and safety shower within reach of the transfer point.
• Dilution. Always add acid to water, never water to acid, and slowly — dilution is exothermic.
• Storage. Bunded, well ventilated, shaded, away from direct sun. Vent through a water or caustic scrubber. Keep the store cool: fuming and pressure both rise with temperature.
• Packing. 35 kg HDPE carboys, 250 kg HDPE drums, 1000 litre rigid IBCs, rubber-lined road tankers and ISO tanks.

An export consignment should carry a certificate of analysis stating HCl assay, specific gravity, iron, free chlorine, sulphate, heavy metals, residue on ignition and colour; a 16-section GHS safety data sheet; a dangerous goods declaration with UN 1789, Class 8, PG II and the correct packing; commercial invoice and packing list; certificate of origin; and for food or pharmacopoeial grade the FCC or monograph reference with any Halal, Kosher or food-contact declarations required.

Enquire with a specification

Hydrochloric acid is supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, in technical, water treatment, food and analytical grades at 30 to 33 % and concentrated strengths, with certificate of analysis, safety data sheet and full export and dangerous goods documentation.

Send the strength, grade, iron and organic limits if they matter to your process, packing, quantity and destination port, and you will get a quotation written against that specification rather than a generic one.

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