Phthalic anhydride is an organic compound with the chemical formula C₈H₄O₃. It is the anhydride of phthalic acid.
It appears as white, lustrous crystalline needles and has a pungent, choking odour. It is:
Soluble in: hot water, benzene, carbon disulfide, and alcohol
Slightly soluble in: water and ether
Phthalic anhydride is made through catalytic oxidation of ortho-xylene or naphthalene. It can also be produced from phthalic acid. During separation from by-products like o-xylene and maleic anhydride, a series of “switch condensers” is used.
What is Phthalic Anhydride used for?
Phthalic anhydride is a key intermediate in the plastics industry and is widely used in the production of:
Plasticisers (via phthalate esters)
Polyester and alkyd resins
Phthaleins
Benzoic acid
Synthetic indigo
Artificial and synthetic resins
Synthetic fibres
Dyes and pigments
Pharmaceuticals
Chlorinated products
Other uses and derivatives include:
Monomer for synthetic resins like glyptal and alkyds
Precursor to dyes such as anthraquinone, rhodamine, fluorescein, and xanthene
Phthalic anhydride is widely used in the production of pharmaceuticals.
In the Environment
Phthalic anhydride is released primarily by chemical plants, especially those involved in plastic and resin manufacturing.
Sources of release:
Process off-gases
Industrial effluents
Modern catalytic oxidation techniques help reduce air emissions.
Environmental fate:
Detected (but not quantified) in U.S. drinking water and in baked Idaho potatoes
No specific data on transport in soil or the broader environment
In moist soil, it hydrolyses to phthalic acid
Significant leaching is not expected unless there is a large spill
Not expected to bioaccumulate in aquatic organisms
In a microecosystem study, plants and animals exposed to DEHP (a related compound) contained but did not accumulate phthalic anhydride
Sources and Routes of Exposure
Sources of Exposure
Exposure may occur during the manufacture of phthalate-derived products.
It may also result from leaching of phthalate plasticisers in certain medical plastics, including:
Blood bags
Plastic syringes
Plastic tubing
Phthalate esters, derived from phthalic anhydride, are known environmental pollutants.
Routes of Exposure
Likely routes of exposure to phthalic anhydride include:
Inhalation
Ingestion
Skin and/or eye contact
Health Effects
Acute Effects
Irritating to the eyes, skin, and respiratory tract in humans
No permanent injury observed
It affects wet skin but not dry skin — suggesting the irritant is actually phthalic acid, which forms when phthalic anhydride comes into contact with water
In animal tests (rats), phthalic anhydride has moderate acute toxicity
Chronic Effects
In workers, long-term exposure has led to
Conjunctivitis, rhinitis, rhinoconjunctivitis, and bronchitis
Skin and mucous membrane irritation
Occasional bloody sputum, emphysema, lower blood pressure, and mild central nervous system (CNS) excitation
In animals:
Exposure to heated phthalic anhydride caused lung congestion, irritation, and injury
In guinea pigs, exposure to phthalic anhydride dust caused:
Bronchoconstriction
Temporary increase in breathing rate
Elevated IgG antibodies
In mice exposed through diet:
Decreased body weight
Lung and kidney lymphocytosis
Bile duct inflammation
Adrenal atrophy
Mineralisation of the thalamus
EPA Reference Values:
Provisional Reference Concentration (RfC): 0.12 mg/m³ (based on respiratory effects in humans)
Reference Dose (RfD): 2.0 mg/kg/day (based on lung and kidney effects in mice)
Reproductive & Developmental Effects
No human studies available
In mice, intraperitoneal injection caused teratogenic effects
In one rat study, inhalation exposure led to decreased sperm motility time
Cancer Risk
No human data on carcinogenicity
In a bioassay, rats and mice were fed phthalic anhydride; no increase in tumour incidence was observed
EPA has not classified phthalic anhydride regarding carcinogenic potential
Safety
First Aid Measures
Eye Contact: Remove contact lenses if present. Immediately flush eyes with water for at least 15 minutes using WARM water. Seek medical attention immediately.
Skin Contact: Flush skin with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Use an emollient on irritated skin. Wash clothing and clean shoes before reuse. Seek medical attention.
Serious Skin Contact: Wash with disinfectant soap and apply an antibacterial cream. Seek immediate medical help.
Inhalation: Move to fresh air. If not breathing, give artificial respiration. If breathing is difficult, administer oxygen. Get medical attention immediately.
Serious Inhalation: Evacuate to a safe area. Loosen tight clothing. Provide oxygen if breathing is difficult. Do not perform mouth-to-mouth without proper protection. Seek medical help urgently.
Ingestion: Do not induce vomiting unless directed by medical personnel. Never give anything by mouth to an unconscious person. Loosen tight clothing and seek immediate medical assistance.
Fire Information
Combustibility: Phthalic anhydride may burn at high temperatures.
Auto-ignition Temperature: 570°C
Flammability: Slightly flammable in the presence of heat; non-flammable in the presence of shocks.
Small Fires: Use dry chemical powder.
Large Fires: Use water spray, fog, or foam — do not use a water jet.
Exposure Control & Personal Protection
Engineering Controls
Use process enclosures, local exhaust, or other controls to maintain airborne levels below exposure limits.
For operations generating dust, fumes, or mist, ensure proper ventilation.
Personal Protective Equipment (PPE)
For regular handling:
Splash goggles
Synthetic apron
Vapour and dust respirator (certified/approved)
Gloves
For large spills:
Splash goggles
Full protective suit
Vapour and dust respirator
Boots
Gloves
Self-contained breathing apparatus (SCBA) is recommended to prevent inhalation.
Suggested PPE might not be enough in all cases — always consult a safety specialist before handling phthalic anhydride.
Regulation
United States
NIOSH: The National Institute for Occupational Safety and Health has set a recommended exposure limit (REL) for phthalic anhydride of 6 mg/m3 and 1 ppm TWA
OSHA: The Occupational Safety & Health Administration has set a permissible exposure limit (PEL) for phthalic anhydride of 12 mg/m3 and 2 ppm TWA
Australia
Safe Work Australia: Safe Work Australia has established a time weighted average concentration (TWA) for phthalic anhydride of 6.1 mg/m3 and 1 ppm for a 40-hour work week.
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