Xylene
Properties
| State | Liquid |
| Color | Colorless |
| Solubility | Practically insoluble in water (about 0.18 g/L at 20 °C); miscible with most organic solvents |
| Melting Point | -47.4 °C (mixed isomers) |
| Boiling Point | 138-144 °C (depending on isomer) |
About Xylene
Xylene is the C8H10 dimethylbenzene that the petrochemical industry produces by the tens of millions of tonnes a year, almost entirely so that the para isomer can be oxidized into the terephthalic acid that feeds PET production. The name covers three structural isomers — ortho, meta, and para-xylene — and commercial 'mixed xylenes' is normally a blend of all three plus a meaningful slug of ethylbenzene, separated by superfractionation, adsorption, or selective crystallization depending on which isomer the buyer wants. Para-xylene is the prize. The PX route runs through liquid-phase oxidation in acetic acid with Co/Mn/Br catalysis at about 200 °C and 15 atm to give purified terephthalic acid (PTA), which condenses with ethylene glycol to make PET — every soda bottle, every polyester shirt, every food-grade plastic clamshell. Ortho-xylene goes mostly to phthalic anhydride (vapor-phase oxidation over V2O5 catalyst), the precursor for plasticizers and alkyd resins. Meta-xylene goes to isophthalic acid for specialty polyesters and thermoset resins. Outside polymer feedstock, xylene shows up as the standard clearing agent in histology, where it pulls residual ethanol out of fixed tissue before paraffin embedding, and as a thinner in oil-based paints, varnishes, and printing inks.
Where you'll encounter it
If you've ever opened a bottle of xylene in a histology lab to clear paraffin off tissue slides, or watched a building's exterior get repainted with an oil-based coating in winter, you've smelled the sweet aromatic odor of mixed xylenes drifting on the air. In a pathology lab, slide racks move through sequential 100% ethanol then xylene baths in a drawer-style automatic processor before the tissue gets infiltrated with molten paraffin — every histology technician on the planet has a story about the day a fume hood went down and the room briefly turned into an OSHA case study. In a refinery's aromatics complex, xylenes leave the reformer along with benzene, toluene, and ethylbenzene as the BTX cut, then get separated by superfractionation and Parex-type adsorption to pull para-xylene out at 99.9% purity for downstream PTA plants. Auto-body shops still use mixed xylenes as a brush-cleaning solvent for two-component urethane paints, though most newer shops have moved to lower-VOC alternatives.
Common Uses
- Para-xylene oxidation to purified terephthalic acid for PET bottle resin and polyester fiber
- Histology and pathology tissue clearing agent for paraffin embedding and microscopy slide preparation
- Thinner and cleanup solvent for oil-based paints, alkyd varnishes, and lithographic inks
- Ortho-xylene oxidation to phthalic anhydride for plasticizer and alkyd resin manufacture
- Industrial degreasing solvent for metal parts, electronic components, and automotive assemblies
- Aviation gasoline octane booster blended into specialty fuel formulations
Safety Information
Mixed xylenes are flammable, neurotoxic on inhalation, and the standard cause of ventilation citations in older histology labs. GHS classifications: H226 (flammable liquid Category 3), H304 (aspiration hazard Category 1), H312 (harmful in skin contact), H332 (harmful if inhaled), H315 (skin irritation), H319 (eye irritation), H335 (respiratory irritation), H373 (organ toxicity from repeated exposure to nervous system and ear). Occupational exposure limits are set as an eight-hour time-weighted average with a modestly higher short-term ceiling. Flash point sits near 27 °C closed cup, so always use bonded and grounded transfer with electrical equipment classified for Class I Div 2. Use local exhaust ventilation, viton or PVA gloves (not nitrile, which swells), splash goggles, and biological monitoring (urinary methylhippuric acid) for chronic exposure assessment.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.