UN 2508 — Molybdenum pentachloride
Placard: Corrosive. ERG Guide 156. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 2508 is Molybdenum pentachloride, a corrosive moisture-reactive metal chloride assigned to ERG Guide 156. Water or humid air releases HCl fumes and heat.
Hazard overview: CORROSIVE moisture-reactive metal chloride; dust or vapor can burn eyes, skin and respiratory tissue. Reacts with water or moist air, releasing hydrogen chloride fumes and heat. Fumes in humid air and may form acidic corrosive runoff.
Response guidance: For UN 2508, isolate the spill or leak, stay upwind and use SCBA with chemical protection. Control moisture contact, contain acidic runoff and use compatible dry media or protected cylinder cooling under ERG 156.
Firefighter training notes: Training for UN 2508 should emphasize water-reactive corrosive fuming, dry-agent selection, SCBA/Level A decisions, decontamination and acidic runoff control. Use ERG 156, SDS and local SOP.
Regulatory context: Molybdenum pentachloride is regulated as a hazardous material for transport and emergency response. Storage, exposure, spill reporting, waste and fire-code duties depend on quantity, concentration and jurisdiction; verify shipping papers, SDS and local authority requirements.
Storage & handling: Molybdenum pentachloride should be stored dry in tightly closed compatible containers away from water, moisture, bases, oxidizers/reducing agents where incompatible, heat and unauthorized access. Provide secondary containment.
UN 2508 Quick Details
Common Hazards of UN 2508
- CORROSIVE moisture-reactive metal chloride; dust or vapor can burn eyes, skin and respiratory tissue.
- Reacts with water or moist air, releasing hydrogen chloride fumes and heat.
- Fumes in humid air and may form acidic corrosive runoff.
- Combustible behavior is limited, but heating may produce toxic/corrosive chloride fumes.
- Contact with metals may generate flammable hydrogen under acidic conditions.
- Containers may rupture when heated or contaminated with water.
- Avoid dust generation and direct water contact with released material.
Chemical Identity & Physical Properties
Dark green to black crystalline solid with a pungent, irritating odor. Hygroscopic and fumes in moist air.
| Also known as | Molybdenum(V) chlorideMolybdenum chloridePentachloromolybdenum |
| CAS Number | 10241-05-1 |
| Appearance | Dark green to black crystalline solid with a pungent, irritating odor. Hygroscopic and fumes in moist air. |
| Flash Point | Not applicable (non-flammable solid, but combustible) |
| Boiling Point | 268C (514F) with decomposition |
| Vapor Density | Not applicable (solid, but vapors heavier than air when heated) |
| Water Reactivity | Reacts violently with water, releasing corrosive and toxic hydrogen chloride gas and heat |
Fireground Response Guidance — UN 2508
Extinguishing Media
PPE Requirements
Use positive-pressure SCBA and chemical-resistant protective clothing. Level A may be needed for heavy fuming, splash risk, liquefied gas contact or unknown concentrations.
Isolation & Evacuation
First Actions for a UN 2508 Incident
- Call 911 and the emergency response number on the shipping paper, if available.
- Keep unauthorized personnel away and establish incident command.
- Stay upwind, uphill and upstream.
- Avoid breathing vapor, dust, mist, smoke or fumes and avoid skin or eye contact.
- Keep water contact controlled because moisture can increase heat release or corrosive fuming.
- Do not touch damaged containers or spilled material without proper training and PPE.
- Ventilate confined spaces only after monitoring and only if properly trained and equipped.
- Use ERG Guide 156, SDS, shipping papers and monitoring to set isolation, evacuation and entry decisions.
📋 Copy & Share Field Card
UN 2508 — Molybdenum pentachlorideUse for: Quick radio or face-to-face size-up. Short, structured, field-ready.
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
Use for: Quick text to command or incoming units. Fits in a single SMS.