Commercial hydrochloric acid procurement is governed by hydrogen chloride concentration, density, and the trace impurity profile that determines eligibility for food, pharmaceutical, steel, oilfield, or semiconductor use. The conventional concentration units are weight percent HCl and degrees Baumé (°Bé); a 20° Baumé acid corresponds to approximately 31.45 wt% HCl and a specific gravity of 1.16 g/cm³ at 20°C, while a 22° Baumé acid corresponds to approximately 35.2 wt% HCl and a specific gravity of 1.179 g/cm³. Concentrated commodity acid is typically supplied at 36.5–38 wt% HCl with a density of 1.18–1.19 g/cm³. Because density response is not linear across the full concentration range, incoming inspection uses ISO 905:1976 hydrometer or digital density meter measurements with temperature compensation to 20°C. Buyers also request trace metal and sulfate data because these impurities determine whether a lot can be used in food-grade, pharmaceutical, or electronics applications.

Hydrochloric acid is identified by CAS 7647-01-0 and is classified for transport under UN 1789, Class 8, Packing Group II for concentrated solutions. Dilute solutions below 25 wt% may be shipped as Packing Group III under the same UN number. Since the vapor space contains hydrogen chloride, storage vents and scrubbers must maintain workplace HCl below the OSHA PEL of 5 ppm TWA and the ACGIH ceiling of 2 ppm.

HCl Questions Buyers Ask Us Every Week (FAQ)

Recurring procurement inquiries generally fall into five groups: concentration verification, Fe and sulfate thresholds, handling and storage of fuming acid, inhibitor compatibility in steel and oilfield use, and packaging certification for high-purity grades. The following sections address these questions with test method designations and process limits.

Muriatic acid is a commercial term for impure hydrochloric acid, typically 20° Baumé or 31.45 wt%, that may contain iron and organic contaminants from steel pickling or chlorination byproduct streams. It should not be used for food or pharmaceutical applications unless refined and tested against FCC or USP monographs.

Concentration verification by density alone is insufficient for high-purity transactions. Iron is routinely determined by ISO 908:1987 using 2,2'-bipyridyl spectrophotometry after evaporation; arsenic is measured by ISO 909:1968 with silver diethyldithiocarbamate absorption. Sulfate is quantified by the turbidimetric method of ISO 2762:1973. Buyer acceptance commonly adds ICP-MS for multielement trace metal screening at the 0.1 µg/L reporting level for semiconductor-grade acid, while industrial acid may be accepted using ICP-OES at 0.01 mg/L detection limits.

What limits FeCl₂ accumulation in continuous steel pickling lines?

Hot-rolled carbon steel is pickled in recirculated HCl at 70–85°C. Fresh acid fed at 10–18 wt% HCl reacts with iron scale and base metal according to Fe + 2HCl → FeCl₂ + H₂; the liquor exits a continuous push-pickling line at roughly 6–8 wt% free HCl and 120–160 g/L FeCl₂. The practical upper FeCl₂ limit is not set by solubility; FeCl₂ remains soluble at these temperatures. The limit is set by drag-out losses, rinse water contamination, inhibitor consumption, and fuming. Rubber-lined carbon steel tanks, graphite or PTFE-lined plate heat exchangers, and FRP fume extraction ducts are used because hot HCl attacks carbon steel and 316L stainless steel. Acid regeneration spray roasters at 900–1,000°C convert spent liquor to Fe₂O₃ and HCl gas, with HCl absorption columns returning approximately 18 wt% acid to the pickling line. A common buyer question is whether 20° Baumé acid can be used directly; at 31.45 wt% HCl it is usually diluted to 10–12 wt% with rinse water before feeding the pickling tanks because concentrated acid at high temperature increases hydrogen evolution and inhibitor demand. Propargyl alcohol-based inhibitors are effective in HCl up to approximately 80°C; above 95°C they may volatilize and lose protection.

Storage of 36.5–38 wt% HCl at ambient temperature uses atmospheric FRP or HDPE tanks with vent scrubbers. Hydrogen chloride partial pressure at 25°C requires a fume abatement system sized to meet the OSHA PEL of 5 ppm TWA and the ACGIH ceiling of 2 ppm. Tanks should not be located near cyanide, sulfide, or hypochlorite systems because toxic gas release occurs on contact. Secondary containment must resist acid spall and crystalline ferric chloride accumulation.

When 22° Baumé acid is specified for carbonate reservoir stimulation

Carbonate acidizing formulations use 15–28 wt% HCl for matrix injection and fracture acidizing. The reaction with calcite, CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O, creates wormholes that bypass formation damage if injection rate and acid spending are matched. A recurring buyer question is whether 22° Baumé acid can be substituted for 20° Baumé acid at the same inhibitor loading. The substitution increases HCl strength from 31.45 wt% to 35.2 wt%, which raises uninhibited carbon steel corrosion rate nonlinearly at bottomhole temperatures above 90°C. Static weight-loss tests conducted according to NACE TM0169 or API RP 42 are used to select inhibitor packages at 2–5 gal/1,000 gal for acid exposure times up to 24 h. Iron-control agents are added when dissolved iron is expected to exceed 1,000 mg/L, and mutual solvents are included to prevent sludge with certain crude oils.

Food-grade hydrochloric acid is used as an acidity regulator and processing aid under 21 CFR 182.1057 and must conform to the FCC monograph. Buyer questions frequently concern arsenic, heavy metal, and residual chlorine limits. The FCC monograph specifies arsenic not exceeding 1 mg/kg, heavy metals not exceeding 5 mg/kg, iron not exceeding 5 mg/kg, and free chlorine not exceeding 0.003 wt%. USP Hydrochloric Acid monographs additionally require assay by acid-base titration to 36.5–38 wt% and residue on ignition limits. Purchase orders should require a lot-specific certificate of analysis that includes the reference monograph, because technical-grade acid may contain trace organics or metals that are not listed on a standard commodity certificate.

Oxidizing impurities and inhibitor compatibility in organic chlorinations

In phosgene, isocyanate, and pharmaceutical hydrochlorination processes, free chlorine and chlorinated organic impurities in HCl feed or byproduct streams generate unwanted side products. Buyers requesting HCl for these systems ask for free chlorine by iodometric titration, total chlorinated organics by purge-and-trap GC-MS, and iron because iron chloride can catalyze radical pathways. Typical acceptance for hydrochlorination feed is free chlorine below 50 mg/kg and total organic carbon below 20 mg/L. Mixing concentrated HCl with amine-containing neutralizers must be avoided because the neutralization exotherm can exceed 80°C locally and cause amine hydrochloride precipitation or violent boiling in poorly mixed vessels.

Dilution of 37 wt% HCl with water is exothermic; the acid is added to water, not water to acid, in a polypropylene or PTFE-lined mixing vessel. Bulk temperature is maintained below 60°C by controlling acid feed rate and using cooling water on the vessel jacket. A 20° Baumé solution is often prepared from concentrated acid and demineralized water; the resulting density is checked against 1.16 g/cm³ at 20°C before transfer to day tanks.

HCl is also used for pH adjustment in cooling towers, municipal water treatment, and industrial wastewater neutralization because it does not add calcium carbonate scale. The stoichiometric neutralization of alkalinity is based on the reaction 2HCl + CaCO₃ → CaCl₂ + CO₂ + H₂O; 1 kg of anhydrous HCl neutralizes 1.37 kg of CaCO₃, while 1 kg of commercial 37 wt% HCl neutralizes approximately 0.51 kg of CaCO₃. Dosing pumps for this service are specified with PTFE or PVDF wetted heads because concentrated acid degrades elastomer diaphragms.

To maintain SEMI C28 trace metal limits during repacking, validation protocols for semiconductor-grade HCl

High-purity HCl for wafer cleans and wet etch formulations is controlled under SEMI C28. The critical impurities are iron, copper, zinc, arsenic, and particulate matter. Supplier certificates typically list Fe at ≤0.1 µg/L, Cu at ≤0.1 µg/L, Zn at ≤0.1 µg/L, and As at ≤0.05 µg/L as measured by ICP-MS after sample preconcentration. Particle counts above 0.2 µm are commonly specified at fewer than 100 particles/mL. Packaging uses PFA-lined drums or bag-in-can configurations, because high-purity HCl leaches cations from borosilicate glass and fluoropolymer resin batches can vary. Packaging validation includes a 60°C leach test for 72 h with ICP-MS analysis of the container blank. Published data for this specific packaging configuration is limited; supplier validation reports are required rather than theoretical scale-up from laboratory studies.

A final recurring inquiry concerns whether a certificate of analysis from a technical-grade supplier can be used for semiconductor-grade HCl. A technical-grade COA cannot be substituted for SEMI C28 certification because the analytical methods and reporting limits differ by orders of magnitude. Technical-grade iron values of ≤0.5 mg/L are 5,000-fold greater than the 0.1 µg/L semiconductor limit, and ICP-OES cannot provide the required lower quantification limit without matrix elimination.