Commercial hydrochloric acid is an aqueous solution of hydrogen chloride. It is ordered as technical acid, food-grade acid, or high-purity acid, and the supplier’s FOB price is only one component of the delivered cost. Two quotations cannot be evaluated until they are normalized to the same concentration basis, the same production route, and the same freight mode. A low FOB quotation for by-product 20°Bé material may not remain low after freight is allocated across the water content and trace contamination risk is priced for the receiving process.

Production Route Determines Trace Profile and Downstream Suitability

Direct synthetic hydrochloric acid is produced by continuous combustion of chlorine and hydrogen in a graphite or fused silica burner. The flame temperature is high, and the burner is cooled to protect the construction material. The resulting hydrogen chloride gas is cooled and absorbed in a falling-film absorber or packed column. Because the feed gases can be sourced from membrane-cell chlorine and high-purity hydrogen, the synthetic product tends to have low iron, low organic carbon, and low free chlorine. High-purity synthetic acid is used in pharmaceutical processes under the USP Hydrochloric Acid monograph and in food applications where 21 CFR 182.1057 applies. The FOB price of synthetic acid reflects the cost of chlorine, hydrogen, energy, and analytical release testing.

By-product hydrochloric acid is generated in chlorination and oxychlorination processes. In ethylene dichloride production, ethylene is chlorinated and the intermediate is thermally cracked to vinyl chloride monomer, releasing hydrogen chloride. In chloromethane and fluorochemical operations, additional hydrogen chloride is generated as a by-product of chlorine substitution. These streams are quenched, condensed, and absorbed, but they can retain chlorinated organic compounds, dissolved chlorine, color bodies, and metals. Some by-product acid requires dilution, stripping, adsorption, or oxidative post-treatment before it can be used in sensitive applications. Certificates of analysis from the specific production unit should control; published data for a specific by-product configuration is limited.

The cost difference between synthetic and by-product acid is not solely a quality premium. Synthetic acid typically has narrower batch-to-batch variation, which reduces dosing changes in continuous neutralization and cleaning systems. By-product acid can vary with upstream furnace severity, quench temperature, and maintenance cycles. A buyer that accepts by-product material may need to increase titration frequency, install activated carbon polish or air stripping, or restrict the use to less-sensitive neutralization duties. Food-grade hydrochloric acid in the United States must meet the GRAS conditions in 21 CFR 182.1057 and is generally manufactured to the USP monograph when pharmaceutical use is intended. The production route does not automatically disqualify by-product acid, but the analytical burden is higher because by-product streams may contain trace chlorinated organic compounds that are not present in synthetic acid.

Why Two HCl Quotes Differ: Production Route, Concentration and Freight

The commercial concentration of hydrochloric acid is quoted either as wt% HCl or as Baumé gravity. Baumé is not a concentration; it is a hydrometer scale tied to density. At 20 °C, the common 20°Bé grade corresponds to approximately 31.45 wt% HCl, the 22°Bé grade to approximately 35.21 wt% HCl, and the 23°Bé grade to approximately 37.14 wt% HCl. Baumé values are temperature-dependent. A value stated at 60 °F differs from one stated at 20 °C. The standard hydrometer and assay methods under ASTM E224-16 provide a consistent basis; quotations lacking a temperature basis cannot be audited.

The azeotrope of aqueous HCl at atmospheric pressure is approximately 20.2 wt% HCl, with a boiling point near 108.6 °C. Concentrated grades above the azeotrope are therefore produced by absorption of HCl gas into water rather than by simple fractional distillation. The commercial significance is that concentration is controlled at the absorber and is quoted on acid content, not on water removal. The difference in concentration changes the number of wet tonnes that must be transported per dry tonne HCl. The multiplier is the inverse of the HCl weight fraction. Table 1 normalizes three standard commercial concentrations.

The same freight rate per wet tonne therefore produces a materially different delivered cost per dry tonne. For a constant wet-tonne freight rate F, the delivered freight burden per dry tonne HCl is F divided by the HCl weight fraction. A 20°Bé shipment carries approximately 3.179 wet tonnes per dry tonne HCl, while a 23°Bé shipment carries approximately 2.692 wet tonnes per dry tonne HCl. This is not a minor adjustment: the 20°Bé grade imposes about 18 % more solution mass than the 23°Bé grade for the same HCl mass.

The FOB comparison must include the delivered dry-tonne basis. If the delivered wet-tonne price is P, the delivered dry-tonne price is P divided by the HCl weight fraction. For two delivered wet-tonne quotes at the same location, with one at 20°Bé and 110 per wet tonne and the other at 22°Bé and 120 per wet tonne, the dry HCl basis is 349.8 and 340.8 per dry tonne, respectively. The higher wet-tonne quote is lower on the basis that matters at the receiving tank.

When Freight Charges Distort FOB Price Comparisons

Hydrochloric acid is regulated in the United States as a corrosive liquid under 49 CFR §172.101. The proper shipping name is UN1789 Hydrochloric acid solution, with hazard class 8. Packing group assignment changes with acid concentration: commercial grades above 25 wt% HCl are typically offered as Packing Group II, while dilute acid from 10 wt% to 25 wt% HCl is offered as Packing Group III. Packing group affects allowable bulk packaging, tank specification, driver training, emergency response information, limited quantity exceptions, placarding, and employee training under 49 CFR §172.101. Table 2 summarizes the two most relevant commercial categories.

Freight charges are not proportional to acid content because the truck, rail car, or ISO tank moves the entire solution mass. Bulk shipments of concentrated acid require dedicated return cleaning and lined equipment. Rubber-lined carbon steel, fiberglass-reinforced plastic, and high-density polyethylene are common materials of construction; unlined carbon steel, aluminum, and 300-series stainless steel are not suitable for concentrated HCl storage. Rail and barge modes reduce the unit cost per wet tonne but require larger minimum volumes, longer lead times, and demurrage or wharfage charges. A quote based on truck freight for a partial load may be higher per dry tonne than a rail quote for the same full-tonnage HCl volume, even if the FOB price is lower.

The freight contribution per dry tonne HCl is calculated by dividing the wet-tonne freight rate by the HCl weight fraction. For a constant wet-tonne freight rate, a 20°Bé shipment carries 3.179 times the freight rate per dry tonne; a 23°Bé shipment carries 2.692 times. The water burden is therefore not a small fraction but a primary driver. Freight side costs also differ by production route. By-product acid sometimes moves from a continuous chlorination plant to a terminal, while synthetic acid may be shipped from chlor-alkali complexes. The difference in origin and equipment availability affects backhaul, cleaning, and asset repositioning charges. A supplier quote that appears low may exclude cleaning, demurrage, or terminal transfer. The comparison must be made on the delivered cost normalized to 100 wt% HCl at the receiving tank flange, not on the supplier’s FOB gate price.

At ambient temperature, concentrated hydrochloric acid above 30 wt% has a significant hydrogen chloride partial pressure. The OSHA ceiling limit is 5 ppm under 29 CFR 1910.1000. Receiving systems for concentrated acid require closed-loop vapor return or scrubbing. Tank trailers and storage tanks are typically vented through scrubbers to comply with permitted air emissions. These controls add capital and operating cost that a quote may exclude if the supplier does not operate the receiving station.

Hydrochloric acid is incompatible with sodium hypochlorite, cyanide-bearing solutions, strong alkaline oxidizers, and reactive metals. Contact with sodium hypochlorite releases chlorine gas; contact with cyanide-bearing solutions releases hydrogen cyanide. Storage areas should be separated, diking should resist acid attack, and transfer hoses should be rated for acid service. Unlined carbon steel, aluminum, and 300-series stainless steel are not acceptable for concentrated hydrochloric acid storage; lined carbon steel, FRP, polypropylene, and high-density polyethylene are common materials of construction. These operational boundaries can change the total cost of a quotation even before the first batch is unloaded.