CAS 13598-36-2 · Inorganic Acid / Reducing Agent
Phosphorous Acid
H₃PO₃ / Orthophosphorous Acid · MW 82.00 g/mol · Diprotic
Phosphorous acid (H₃PO₃) is a diprotic inorganic acid and strong reducing agent with phosphorus in the +3 oxidation state. Unlike phosphoric acid, it contains a direct P–H bond that makes it non-ionizable at that position, giving it unique reactivity as a reducing agent and phosphonate synthesis feedstock. Texas-Chem supplies phosphorous acid in industrial, technical, ACS reagent, and food-grade equivalents in drums, totes, and bulk.
Formula
H₃PO₃
Melting Point
73.6 °C (164.5 °F)
Solubility
Very soluble in water
Pack Sizes
Drums · Totes · Bulk
Structural Formula
Diprotic acid with a direct P–H bond; only the two O–H groups ionize in solution
Safety & Regulatory
- NFPA Health
- 3 — Serious hazard
- NFPA Flammability
- 0 — Will not burn
- NFPA Reactivity
- 1 — Slight hazard
- OSHA PEL (TWA)
- Not established (handle as corrosive)
- ACGIH TLV
- Not established
- DOT Hazard Class
- 8 — Corrosive
- UN Number
- UN 2834
- Packing Group
- III
- Special Hazard
- Strong reducing agent; avoid oxidizers
SDS available upon request. Corrosive — causes skin and eye burns. Strong reducing agent; keep away from oxidizers, peroxides, and strong bases. Use acid-resistant PPE and handle in ventilated areas.
Grades & Packaging
Available Grades
Pack Sizes
Physical & Chemical Properties
| Molecular Formula | H₃PO₃ |
| Molecular Weight | 82.00 g/mol |
| CAS Number | 13598-36-2 |
| Appearance | White to off-white crystalline solid or solution |
| Melting Point | 73.6 °C (164.5 °F) |
| Boiling Point | Decomposes ~200 °C |
| Density (solid) | 1.651 g/cm³ at 20 °C |
| Solubility in Water | Very soluble — ~310 g/100 mL at 20 °C |
| pKa₁ | 1.3 (diprotic acid) |
| pKa₂ | 6.7 |
| Oxidation State of P | +3 (reducing agent) |
| Hygroscopicity | Highly hygroscopic |
Industrial Applications
Phosphonate & Organophosphorus Synthesis
Primary feedstock for manufacturing phosphonate chelating agents (HEDP, ATMP, DTPMP) and organophosphorus flame retardants. Phosphorous acid's P(+3) oxidation state makes it a key reducing intermediate in the Michaelis–Arbuzov and related reactions used to build phosphonate backbones for scale inhibitors, corrosion inhibitors, and specialty chemicals.
Antioxidant & Stabilizer in Polymers
Used as a processing stabilizer and antioxidant in polyolefins, PVC, and engineering plastics. Phosphorous acid and its esters (e.g., triphenyl phosphite) scavenge peroxides and hydroperoxides during melt processing, preventing oxidative degradation, discoloration, and molecular weight loss in polymer manufacturing.
Metal Surface Treatment & Pickling
Applied in metal cleaning, pickling, and rust-conversion formulations. Reacts with iron oxides to form stable iron phosphate passivation layers, providing corrosion protection on steel and iron surfaces. Used in pre-treatment baths prior to painting, coating, or galvanizing operations.
Agricultural Fungicides (Phosphonate Salts)
Neutralized with potassium or ammonium hydroxide to produce potassium phosphonate (mono- and di-potassium salts), which are systemic fungicides effective against Phytophthora, Pythium, and downy mildew. Widely used in avocado, citrus, ornamental, and turf applications as a plant defense activator.
Water Treatment & Scale Inhibition
Precursor to phosphonate-based scale and corrosion inhibitors used in cooling water systems, boiler water treatment, and oilfield water injection. Phosphonate derivatives derived from phosphorous acid provide superior calcium carbonate, calcium sulfate, and barium sulfate scale inhibition at low dosage rates.
Reducing Agent in Chemical Synthesis
Mild reducing agent in organic and inorganic synthesis. Used to reduce diazonium salts, convert aryl halides, and prepare hypophosphorous acid derivatives. Its selective reducing power and water solubility make it useful in pharmaceutical intermediate synthesis and specialty chemical manufacturing.
Why Source Phosphorous Acid from Texas-Chem?
- 35+ years supplying industrial acids and specialty chemicals to Texas manufacturers
- Industrial, technical, ACS reagent, and food-grade equivalents available
- Flexible packaging from 55-gal drums to bulk tanker delivery
- Competitive pricing backed by top-tier producer relationships
- Technical support and SDS documentation with every order
- Same-day quotes for standard quantities