Phosphonic Acid, Phosphorous Acid, Phosphorus Trihydroxide, Orthophosphorus Acid, Dihydroxyphosphine Oxide, Trihydroxyphosphine, 13598-36-2

PHOSPHONIC ACID (PHOSPHOROUS ACID, H₃PO₃)

1. Chemical Identity and Material Classification

• Chemical Name: Phosphonic Acid, Phosphorous Acid, Orthophosphorous Acid

• Synonyms: Phosphorus Trihydroxide, Dihydroxyphosphine Oxide, Trihydroxyphosphine, Hydrogen Phosphonate

• CAS Number: 13598-36-2

• Molecular Formula: H₃PO₃

• Molecular Weight: 82.00 g/mol

• EC Number: 237-066-7

• MDL Number: MFCD00137263

• RTECS Number: SZ6400000

• UN Number: 2834 (Phosphorous acid, solid)

2. Physical Properties

2.1 General Physical Properties

2.2 Thermal Properties

3. Chemical Structure and Acid-Base Properties

3.1 Molecular Structure

• Structure type: Phosphorus oxoacid with P–H bond (unlike phosphoric acid)

• Molecular geometry: Tetrahedral (P center with OH, OH, H, and O)

• Actual formula: HP(O)(OH)₂ (contains one P–H bond and two P–OH groups)

• Tautomerism: Exists predominantly as HP(O)(OH)₂ (not P(OH)₃)

• P–H bond: Characteristic band at ~2300–2400 cm⁻¹ in IR spectrum

3.2 Acid Dissociation Constants (pKa, 25°C)

• Acidity comparison: Stronger than phosphoric acid (H₃PO₄ pKa₁ = 2.14)

• Dibasic nature: Despite formula H₃PO₃, only two acidic hydrogens (third hydrogen is directly bonded to P, non-acidic)

4. Solubility Behavior

5. Chemical Properties and Reactivity

5.1 Oxidation (Air)

5.2 Thermal Decomposition

5.3 Reducing Agent Properties (Strong Reducer)

5.4 Reactions with Bases and Amines

5.5 Reactions with Metal Salts

6. Production Methods

6.1 Phosphorus Trichloride Hydrolysis (Primary Industrial Method)

• Reaction: PCl₃ + 3H₂O → H₃PO₃ + 3HCl (exothermic, vigorous)

• Process: PCl₃ added slowly to water with stirring and cooling → hydrolysis reaction → hydrogen chloride (HCl) released as by-product → HCl recovered as hydrochloric acid → solution refined → cooled → crystallized → decolorized → dried

• Yield: 90–95%

• Purity achieved: 98–99.5%

• By-product: HCl (valuable – sold as hydrochloric acid)

6.2 Phosphite Method (Alternative)

• Process: From phosphite salts using mineral acid → yields phosphorous acid

• Less common: Used for specialty grades

6.3 Purification

• Recrystallization from water or ethanol

• Activated carbon treatment: For color removal (decolorization)

7. Quality Specifications

7.1 Technical Grade

7.2 Reagent Grade

8. Industrial Applications

8.1 Plastic Stabilizers (Largest Use)

8.2 Water Treatment and Corrosion Control

8.3 Agriculture (Pesticide Intermediate)

8.4 Chemical Industry (Reducing Agent)

8.5 Analytical Chemistry

9. Chemical Reactions of Industrial Importance

9.1 Synthesis of Phosphites (Esters)

9.2 Synthesis of Glyphosate (Herbicide)

• Route: H₃PO₃ + formaldehyde + glycine → oxidation → glyphosate (N-(phosphonomethyl)glycine)

• Reaction steps:

  1. H₃PO₃ + CH₂O → HOCH₂P(O)(OH)₂ (hydroxymethylphosphonic acid)

  2. Reaction with glycine → iminodiacetic acid derivative

  3. Oxidation → glyphosate

• Global importance: One of the largest volume herbicides worldwide

9.3 Synthesis of Stabilizers

• Dibasic lead phosphite: 2PbO + PbHPO₃ → (2PbO·PbHPO₃) – PVC heat stabilizer

• Calcium/zinc phosphites: Emerging non-toxic stabilizer alternatives

10. Safety and Toxicology

11. Safety Precautions and Personal Protective Equipment (PPE)

• Hazards:

  • Corrosive – causes severe skin burns and eye damage (R35)

  • Harmful if swallowed (R22)

  • Decomposes above 180°C to release toxic, flammable phosphine (PH₃)

  • Hygroscopic – absorbs moisture from air

• Reactivity:

  • Incompatible with strong bases (neutralization – exothermic)

  • Incompatible with oxidizing agents (reaction can be violent)

  • Reacts with metals (may release hydrogen gas)

  • Slow oxidation in air to H₃PO₄

• PPE (mandatory):

  • Chemical-resistant gloves (nitrile, neoprene, or butyl rubber, EN 374)

  • Chemical splash goggles (EN 166) or full face shield

  • Protective clothing (acid-resistant apron or suit)

  • Respiratory protection (P2/P3 filter for dust; SCBA if heating)

• Engineering controls:

  • Local exhaust ventilation (LEV) for powder handling

  • Eyewash stations and safety showers within immediate vicinity

• First aid:

  • Inhalation: Remove to fresh air; if breathing difficulty, administer oxygen; seek medical attention

  • Skin contact: Remove contaminated clothing; wash with copious water for at least 15 minutes; seek medical attention

  • Eye contact: Rinse with water for 15–20 minutes, lifting eyelids; seek immediate medical attention

  • Ingestion: Rinse mouth; do NOT induce vomiting; drink milk or water; seek immediate medical attention

12. Environmental Fate and Disposal

13. Storage and Shelf Life

• Storage conditions:

  • Cool, dry, well-ventilated area (15–30°C)

  • Keep tightly closed in original packaging (hygroscopic)

  • Store away from strong bases, oxidizing agents, and metals

  • Avoid high temperatures (>150°C) – decomposition to PH₃

  • Protect from moisture (prevents caking and oxidation)

  • Use corrosion-resistant storage (plastic, glass-lined, or stainless steel)

• Shelf life:

  • Sealed container (dry): 24–36 months

  • Opened container: 6–12 months (if properly resealed, stored under inert gas)

• Degradation indicator:

  • Caking/hardening (moisture absorption)

  • Yellow discoloration (oxidation to H₃PO₄)

  • Loss of reducing power (analytical test)

14. Transport Information

15. Analytical Methods

15.1 Assay Determination (Alkaline Titration)

15.2 Phosphite vs. Phosphate Determination

15.3 Reducing Power Test (Silver Nitrate)

16. Synonyms and Common Names

• English: Phosphonic acid, Phosphorous acid, Orthophosphorous acid, Phosphorus trihydroxide, Dihydroxyphosphine oxide, Trihydroxyphosphine

• French: Acide phosphoreux

• German: Phosphonsäure, Phosphorige Säure

• Spanish: Ácido fosforoso

• Italian: Acido fosforoso

• Turkish: Fosfonik asit, Fosforlu asit, Ortofosforlu asit

17. Standards Compliance

18. Why Choose Phosphonic Acid? (Technical Summary)

19. Limitations and Precautions

20. Sectoral Suitability Summary Table

This TDS is prepared in compliance with ISO 11014-1 format and is intended for chemical engineers, water treatment specialists, plastics compounders, agrochemical manufacturers, analytical chemists, and procurement professionals. Certificates of Analysis (CoA), Safety Data Sheets (SDS), reducing power test reports, and sample validation reports are available upon request.