Industrial Acids Selection Guide — Which Acid for Which Job
Direct answer
Choose an industrial acid by the anion you can live with, not by strength. Sulfuric acid is the cheapest source of hydrogen ions per tonne and the default for bulk pH depression, ion-exchange regeneration and effluent neutralisation — but it leaves sulfate, which precipitates calcium as gypsum scale. Hydrochloric acid dissolves carbonate and iron scale fastest and leaves a soluble chloride, which is exactly why it must never be used on stainless steel that will stay wet. Phosphoric acid is the slow, low-corrosion choice for food-contact plant and light rust conversion. Sulfamic acid is a dry, shippable, low-fume acid for descaling heat exchangers and cooling systems where chloride is banned. Oxalic acid removes iron staining from stone, timber and radiators where mineral acids would etch. Formic acid is a reducing organic acid for leather, textile and biogas duty. Sodium bisulfate is a dry granular acid used where handling liquid acid on site is not acceptable.
The five questions that decide the acid
- What are you dissolving? Carbonate scale yields to almost anything. Iron oxide needs hydrochloric or a chelating organic acid. Sulfate scale (gypsum, barite) does not dissolve in acid at all — that is a conversion or mechanical job.
- What metal is the vessel? Chloride and austenitic stainless steel are a stress-corrosion-cracking pair. Carbon steel tolerates inhibited HCl for short cleans. Sulfuric above 70 % passivates carbon steel; dilute sulfuric attacks it aggressively — dilution is the dangerous state, not concentration.
- What anion enters your effluent? Sulfate loads gypsum scale downstream and is capped in many municipal discharge permits. Chloride raises conductivity and corrodes concrete rebar. Phosphate triggers nutrient limits.
- Can the site handle a liquid acid? Dry acids — sulfamic, oxalic, sodium bisulfate — remove bunding, fume scrubbing and tanker unloading from the scope. They cost more per equivalent of acidity and buy back safety.
- What is the temperature? Sulfamic hydrolyses to sulfuric above roughly 60 °C, losing the low-corrosion behaviour you bought it for. Formic and oxalic decompose in hot oxidising systems.
Practical selection table
| Duty | First choice | Avoid |
|---|---|---|
| Bulk pH depression, effluent | Sulfuric acid | Hydrochloric (chloride load) |
| Carbonate + iron scale in carbon steel | Inhibited hydrochloric acid | Sulfuric (gypsum) |
| Cooling tower / heat exchanger descale, stainless | Sulfamic acid | Any chloride acid |
| Food and beverage plant CIP acid step | Phosphoric acid | Oxalic (toxicity) |
| Rust and iron staining on stone, wood, radiators | Oxalic acid | Hydrochloric (etching) |
| Leather, textile, silage, biogas pH | Formic acid | Mineral acids |
| Pool and process pH without liquid handling | Sodium bisulfate | Bulk sulfuric |
Cost is per equivalent, not per litre
Compare acids on cost per unit of acidity delivered, adjusted for concentration and for the downstream cost of neutralising the resulting salt. A 98 % sulfuric at a given price is usually several times cheaper per equivalent than 31–33 % hydrochloric before freight, and freight then matters more than unit price because every dilute acid ships water. Whether a site can take a concentrated delivery at all is an engineering and permitting question, not a purchasing preference — settle it with the SDS, the manufacturer and your own site procedures before the concentration is written into an RFQ.
To put two offers on the same footing, use the Chemical Strength and Active-Basis Comparator with the assay and price values from your own quotations, and the Dilution Arithmetic Calculator when you need the quantity relationship between a delivered strength and a working strength. Both perform arithmetic on values you enter and verify yourself; neither describes how to prepare anything physically.
Concentration basis is where acid comparisons go wrong
Acid offers arrive on several different bases and they are not interchangeable: percent by weight, percent by volume, degrees Baumé, molarity and normality all describe the same drum differently. A sulfuric quoted at 96 % w/w and one quoted by density are only comparable once you know the basis and, where the conversion needs it, a verified density from the supplier's own specification. Never carry a number across bases with an assumed density — ask for the value in writing or leave the comparison unmade.
Equal acidity is also not equal suitability. Grade, iron and heavy-metal limits, inhibitor packages and form all change whether an acid is acceptable for a duty, even when the strength matches.
What to state on the RFQ
- Acid and concentration (e.g. sulfuric acid 96–98 %, HCl 31–33 %)
- Grade: technical, food, reagent, or electronic
- Iron and heavy-metal limits if the acid touches product or a catalyst
- Inhibited or uninhibited, if the duty is metal cleaning
- Packaging: IBC, drum, 25 kg bag, or bulk tanker with unloading arrangement
- Delivery point restrictions: hose length, pump on truck, bund capacity
- SDS, TDS and a recent certificate of analysis
Questions to ask before you buy
- What is the maximum chloride the downstream plant can accept?
- Is the storage tank material compatible at the delivered concentration and at the diluted working concentration?
- Who neutralises the rinse water, and with what?
- Does the site need a dry acid to avoid a bunded liquid installation?
Specification and documentation before the offer is compared
The acceptance basis belongs in the specification, the lot values in the COA, the hazard information in the SDS, and the property description in the TDS — one does not substitute for another. The Industrial Chemical Specification and Documentation Guide sets out what each document does and does not prove before you treat two quotations as comparable.
Where the decision is which alkali rather than which acid, the counterpart framework is the Industrial Alkali Selection and Form Comparison Guide.
Handling stays with the SDS
This guide is a selection and procurement framework. It does not give addition sequences, dilution procedures, neutralisation procedures, working concentrations or emergency response. Concentrated acids generate heat, attack materials of construction and react with incompatible chemicals; any physical preparation, transfer or disposal must follow the current SDS, the manufacturer's instructions, your site procedures and qualified technical or engineering review.