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Phosphorous acid is a diprotic acid of phosphorus that plays a crucial role as an intermediate in chemical synthesis and as a reducing agent in industrial chemistry.
Despite its simple molecular structure, it exhibits complex behavior due to tautomeric forms and strong reducing capabilities.
Synonyms: Orthophosphorous acid, Phosphonic acid, Hydrogen phosphonate, Phosphite acid
CAS Number: 10294-56-1
Structural Characteristics
Phosphorous acid (H₃PO₃) exists primarily in the tautomeric form HPO(OH)₂, rather than the trihydroxyl form P(OH)₃, which is thermodynamically less stable.
The structure includes:
A phosphorus atom bonded to one =O double bond, one –H, and two –OH groups.
The molecule exhibits sp³ hybridization around phosphorus.
Bond lengths:
P=O: ~1.50 Å
P–OH: ~1.60 Å
P–H: ~1.43 Å
This structure explains its diprotic nature (two ionizable hydrogens) since only the hydrogen atoms on the –OH groups dissociate in aqueous solution.
Physical and Chemical Properties
Property Description
Molecular formula H₃PO₃
Molar mass 82.00 g/mol
Appearance White crystalline solid
Odor Odorless
Melting point 73–74 °C
Boiling point Decomposes before boiling (~200 °C)
Density 1.65 g/cm³
Solubility Freely soluble in water, alcohol, and ether
pKa₁ 1.3
pKa₂6.7
Hygroscopic Yes
Oxidation state of P+3
Phosphorous acid is thermally unstable, decomposing to phosphine (PH₃) and phosphoric acid (H₃PO₄) upon heating.
Preparation and Industrial Production
Laboratory Synthesis
Phosphorous acid is commonly produced via hydrolysis of phosphorus trichloride (PCl₃):
This reaction proceeds in two steps:
Formation of phosphorous acid dichloride (HPOCl₂) intermediate.
Hydrolysis to yield phosphorous acid and hydrogen chloride.
Industrial Routes
In industry, phosphorous acid is produced by controlled hydrolysis of phosphorus trichloride or via reaction of elemental phosphorus with steam under catalytic conditions.
Another industrial method involves oxidation of phosphine gas in water at moderate temperatures.
Chemical Behavior and Reactivity
Phosphorous acid is a strong reducing agent. It readily reduces silver nitrate, gold chloride, and other metal salts to their elemental forms.
Redox Reactions
This disproportionation shows simultaneous oxidation (+3 → +5) and reduction (+3 → –3) of phosphorus.
Reaction with Bases
Phosphorous acid forms phosphite salts with metals:
Reaction with Halogens
When treated with halogens, phosphorous acid undergoes oxidation to yield phosphoric acid and halogen acids.
Esterification
It reacts with alcohols to form phosphite esters:
These esters serve as intermediates in organophosphorus chemistry.
Spectroscopic Characteristics
IR Spectrum: Broad absorption bands at 3200–3400 cm⁻¹ (O–H stretch), 1250–1150 cm⁻¹ (P=O stretch).
¹H NMR: Signals for –OH and P–H hydrogens; coupling constant (J_PH) ≈ 600 Hz.
³¹P NMR: Single resonance around +3 to +5 ppm.
UV–Vis: No significant absorption in visible region.
Thermodynamic Properties
Property Value
Standard enthalpy of formation (ΔH°f) –1272 kJ/mol
Standard entropy (S°) 125 J·mol⁻¹·K⁻¹
Heat capacity (Cp) 100 J·mol⁻¹·K⁻¹
Phosphorous acid is exothermic upon dissolution in water due to hydrogen bonding and ionization.
Acid–Base Behavior
Phosphorous acid is diprotic, releasing two protons in aqueous solution:
The resulting phosphite ions are stable and form complexes with various metal cations (e.g., Na⁺, Ca²⁺, Zn²⁺).
Coordination Chemistry
Phosphite ions act as bidentate ligands via oxygen atoms or occasionally phosphorus.
Transition metal phosphite complexes (e.g., Ni, Co, Cu phosphites) exhibit catalytic, optical, and magnetic properties.
These coordination compounds have applications in:
Homogeneous catalysis
Flame-retardant materials
Functional ceramics
SAFETY INFORMATION ABOUT PHOSPHOROUS ACID
First aid measures:
Description of first aid measures:
General advice:
Consult a physician.
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
If inhaled:
If breathed in, move person into fresh air.
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately.
Wash off with soap and plenty of water.
Consult a physician.
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
If swallowed:
Do NOT induce vomiting.
Never give anything by mouth to an unconscious person.
Rinse mouth with water.
Consult a physician.
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment.
Avoid breathing vapours, mist or gas.
Evacuate personnel to safe areas.
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste.
Keep in suitable, closed containers for disposal.
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place.
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles.
Faceshield (8-inch minimum).
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
Skin protection:
Handle with gloves.
Gloves must be inspected prior to use.
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product.
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls.
If the respirator is the sole means of protection, use a full-face supplied air respirator.
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions.
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company.
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product
