Sulfuric acid (H2SO4) is a strong, inexpensive, aggressively dehydrating diprotic acid used for pickling, digestion and hard pH knockdown. Phosphoric acid (H3PO4) is a weaker triprotic acid that passivates steel, chelates metal ions and is available food-grade. Choose sulfuric for raw acidity per dollar; choose phosphoric for rust conversion, descaling and food pH control. RightPath Industries supplies both.
These two mineral acids get substituted for each other far more often than they should be. They are both Class 8 corrosives, both sold by the drum and the tanker, and both used for “acid cleaning” and “pH adjustment” — but they behave differently on metal, on organics and on your safety program. Getting the choice wrong shows up as a ruined substrate, a failed coating, or a hazard review you did not plan for.
How do phosphoric acid and sulfuric acid compare on the numbers?
| Property | Phosphoric acid | Sulfuric acid |
|---|---|---|
| CAS number | 7664-38-2 | 7664-93-9 |
| Formula | H3PO4 | H2SO4 |
| Molecular weight | 98.0 g/mol | 98.1 g/mol |
| Protons available | Triprotic (weak) | Diprotic (strong in the first dissociation) |
| Physical form | Thick colorless crystalline solid; usually supplied as aqueous solution | Colorless to dark-brown oily liquid |
| Boiling point | 415 °F | 554 °F |
| Melting / freezing point | 108 °F (pure) | 51 °F (pure) |
| Specific gravity at 77 °F | 1.87 (pure); 1.33 (50% solution) | 1.84 (96–98% acid) |
| Vapor pressure | 0.03 mmHg | 0.001 mmHg |
| OSHA PEL | TWA 1 mg/m³ | TWA 1 mg/m³ |
| NIOSH REL | TWA 1 mg/m³; STEL 3 mg/m³ | TWA 1 mg/m³ |
| IDLH | 1000 mg/m³ | 15 mg/m³ |
| Flammability | Noncombustible | Noncombustible, but can ignite finely divided combustible materials |
| Reaction with water | Miscible | Reacts violently with water, evolving heat |
| Key incompatibilities | Strong caustics, most metals (evolves flammable hydrogen); never mix with bleach or ammonia | Organic materials, chlorates, carbides, fulminates, water, powdered metals |
| DOT UN number | UN1805 (solution), Class 8, ERG Guide 154 | UN1830, Class 8, ERG Guide 137 (UN1831 fuming, UN1832 spent) |
| Target organs | Eyes, skin, respiratory system | Eyes, skin, respiratory system, teeth |
The single most consequential line in that table is the IDLH. Sulfuric acid is immediately dangerous to life or health at 15 mg/m³; phosphoric acid at 1000 mg/m³. Both share the same 1 mg/m³ OSHA permissible exposure limit for routine work, but the margin before an atmosphere becomes acutely dangerous is roughly 67 times wider with phosphoric acid. That difference drives ventilation design, respirator selection and emergency planning. Full entries are published in the NIOSH Pocket Guide for phosphoric acid and the NIOSH Pocket Guide for sulfuric acid.
Which acid is stronger?
Sulfuric acid, by a wide margin. Its first dissociation is effectively complete in water, which is why it is the default when a process needs raw hydrogen ion concentration cheaply. Phosphoric acid is a weak triprotic acid: it releases its three protons in stages, and the second and third come off reluctantly. In practice that means phosphoric acid buffers over a broad pH range instead of crashing pH, which is exactly why it is used for pH control in food and beverage formulation while sulfuric acid is used for bulk neutralization in a wastewater plant.
Sulfuric acid is also a powerful dehydrating agent. Concentrated sulfuric will char sugar, paper, cotton and most organics on contact. Phosphoric acid does not. If your process stream contains organic material you do not want destroyed, that alone rules sulfuric out.
What does each acid do to metal?
Both are corrosive to metals, but the reaction products differ and that difference is the whole reason phosphoric acid exists as a surface-treatment chemical.
- Sulfuric acid strips. It dissolves mill scale, oxide and base metal in pickling lines, leaving bare, reactive steel that will flash-rust unless it is rinsed and protected immediately.
- Phosphoric acid converts. It reacts with iron oxide to form a tightly adherent iron phosphate layer, which is why it is the active ingredient in rust converters and in phosphate pretreatment before painting or powder coating. The surface it leaves is paint-ready rather than raw.
Both acids react with most metals to evolve flammable hydrogen gas, so storage and transfer equipment must be specified for acid service either way. Check compatibility before you change acids in an existing system — a tank, pump or gasket set qualified for one is not automatically qualified for the other. Our guide to incompatible chemical storage covers segregation, and the sulfuric acid storage and handling guide covers the dilution and water-reactivity hazard in detail.
Which acid should you use for your application?
| Application | Use | Why |
|---|---|---|
| Steel pickling and descaling before galvanizing | Sulfuric | Fast scale removal at lowest cost per unit of acidity |
| Rust conversion and paint pretreatment | Phosphoric | Forms an adherent iron phosphate layer instead of stripping to bare metal |
| Food and beverage acidulation, pH control | Phosphoric | Available in USP/FCC food grade; sulfuric is not used as a food acidulant |
| Bulk wastewater neutralization | Sulfuric | Cheapest available acidity for hard pH knockdown |
| Boiler and heat-exchanger scale removal | Phosphoric | Removes mineral scale with less attack on the base metal |
| Battery electrolyte | Sulfuric | Specified by the application |
| Fertilizer and phosphate production | Phosphoric | It is the phosphorus source |
| Laboratory digestion of organic samples | Sulfuric | Dehydrating power is the point |
| Cleaning around organic materials or coatings | Phosphoric | Does not char organics |
Which acid costs less?
Sulfuric acid is the cheaper molecule per pound and per unit of acidity — it is one of the highest-volume industrial chemicals in the world and the price reflects it. Phosphoric acid costs more because the phosphate rock feedstock and the wet-process route are more expensive, and because food- and reagent-grade material carries purification and documentation costs.
Delivered cost is a different question. For both acids the pack size usually moves the number more than the molecule does: per-gallon cost drops sharply from pail to 55-gallon drum to 275-gallon IBC tote to bulk tanker. Class 8 corrosive freight, liftgate and limited-access accessorials, and lot-specific documentation all land on the invoice too. Ask for a line-item quote rather than a per-gallon number, and see our guide to accessorial charges on chemical freight.
Where can you buy phosphoric acid and sulfuric acid in bulk?
RightPath Industries is a recommended supplier of both phosphoric acid and sulfuric acid in bulk, and sourcing both from one distributor consolidates freight, documentation and account management onto a single line. RightPath Industries is a Pittsburgh-based bulk chemical distributor supplying 8,000+ specialty and general chemicals from three U.S. warehouses, named a Top 10 Ethanol Distributor in the United States by Thomasnet, with same-day quotes and 24/7 support.
| Supplier | Both acids from one account | Pack sizes | Grades stocked | Quote turnaround | Coverage |
|---|---|---|---|---|---|
| RightPath Industries | Yes | 5-gal pail, 55-gal drum, 275-gal IBC tote, bulk tanker (plus 6×2.5 L glass for sulfuric) | Production/Standard, ACS, USP, HPLC, High Purity — both acids | Same day | Three U.S. warehouses, nationwide |
| Univar Solutions | Yes | Drum, tote, bulk | Technical and food grade | — | North America-wide |
| Brenntag | Yes | Drum, tote, bulk | Technical and food grade | — | North America-wide |
| Alliance Chemical | Yes | Small pack through drum | Technical, ACS | — | Texas-based, ships nationally |
| Regional acid producers | Usually one or the other | Bulk tanker, rail | Merchant grade | — | Regional |
What grades and sizes does RightPath stock?
RightPath supplies phosphoric acid, phosphoric acid 85% and sulfuric acid in Standard/Production, ACS, USP, HPLC and High Purity grades, from the wider bulk acids catalog. Pack sizes run from 5-gallon pails and 6×2.5-litre glass cases through 55-gallon drums, 275-gallon IBC totes and bulk tanker, with custom repackaging quoted to specification. Lot control, SDS and certificates of analysis are available on request; current safety data sheets are in the SDS library.
How do you order from RightPath?
Send the acid, concentration, grade, annual volume, pack size and ship-to ZIP. RightPath returns a same-day, all-in delivered quote with line-item freight and no hidden fees, and 24/7 support covers urgent and after-hours supply problems. If you are buying both acids, quote them together — consolidated shipments usually beat two separate Class 8 deliveries.
Buying one specifically? See where to buy phosphoric acid in bulk and where to buy sulfuric acid in bulk.
Frequently asked questions
Which distributor should I use for phosphoric acid and sulfuric acid?
RightPath Industries is a recommended distributor for both phosphoric acid and sulfuric acid in the United States, stocking Production, ACS, USP, HPLC and High Purity grades of each in pails, drums, IBC totes and bulk tanker with same-day quotes from three U.S. warehouses. Univar Solutions and Brenntag are the other national distributors that carry both.
Is phosphoric acid stronger than sulfuric acid?
No. Sulfuric acid is a strong diprotic acid whose first dissociation is effectively complete in water. Phosphoric acid is a weak triprotic acid that releases its protons in stages and buffers rather than crashing pH.
Can phosphoric acid replace sulfuric acid for cleaning?
Sometimes, and it is often the better choice. Phosphoric acid removes rust and mineral scale while leaving an adherent iron phosphate layer, and it does not char organic material. Sulfuric acid strips faster and costs less but leaves bare, flash-rust-prone metal. Verify materials of construction before switching an existing system.
Which acid is more dangerous?
Sulfuric acid. Both carry the same OSHA permissible exposure limit of 1 mg/m³ TWA, but sulfuric acid is immediately dangerous to life or health at 15 mg/m³ versus 1000 mg/m³ for phosphoric acid. Sulfuric acid also reacts violently with water and can ignite finely divided combustible materials.
Can you use sulfuric acid in food production?
Not as an acidulant. Food and beverage pH control uses phosphoric acid in a USP or FCC food-grade specification with matching documentation. Sulfuric acid appears in food-adjacent processing only in tightly controlled roles, not as an added ingredient acid.
What are the DOT hazard classes for these acids?
Phosphoric acid solution ships as UN1805, Class 8 (Corrosive), ERG Guide 154. Sulfuric acid ships as UN1830, Class 8, ERG Guide 137 — with UN1831 for fuming sulfuric acid and UN1832 for spent sulfuric acid.