☣️ UN 3421 • CLASS 8
UN 3421 — Potassium hydrogen difluoride, solution
Placard: Corrosive. ERG Guide 154. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
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⚠️ Verification required:
Broad or variable material category; verify exact product, SDS and shipping papers.
⚠️ This page is a quick-reference aid. For real incidents: stage upwind, isolate, deny entry, request Hazmat early, and consult the current ERG + SOP/SOG.
UN 3421 is Potassium hydrogen difluoride, solution, a corrosive fluoride solution. It can produce serious skin, eye and systemic fluoride injury and should be handled as a toxic corrosive liquid.
Hazard overview: UN 3421 may cause deep chemical burns and systemic fluoride toxicity after skin or eye contact. Heating or reaction with incompatible materials can release toxic, corrosive fluoride fumes. Dilution can generate heat and contaminated runoff remains hazardous.
Response guidance: For UN 3421, isolate the spill, avoid contact with liquid or mist and keep runoff out of drains. Use water spray from a distance to reduce vapors or cool containers, but avoid directing a strong stream into the solution. Follow fluoride exposure medical protocols.
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UN 3421 Quick Details
UN 3421
Product name: Potassium hydrogen difluoride, solution
DOT Class: 8
Placard type: Corrosive
ERG Guide: 154 (check current ERG)
Initial isolation: ERG 154: isolate spill 25-50m all directions; for large spill isolate 100m; evacuate immediate area if fire involves containers
Common Hazards of UN 3421
- TOXIC and/or CORROSIVE; inhalation, ingestion or skin contact with material may cause severe injury
- Contact with molten substance may cause severe burns to skin and eyes.
- Avoid any skin contact.
- Fire may produce irritating, corrosive and/or toxic gases.
- Runoff from fire control or dilution water may be corrosive and/or toxic and cause environmental
- Non-combustible, substance itself does not burn but may decompose upon heating to produce corrosive
- Some are oxidizers and may ignite combustibles (wood, paper, oil, clothing, etc.).
- Corrosives in contact with metals may evolve flammable hydrogen gas.
Chemical Identity & Physical Properties
UN 3421 is a clear to slightly hazy aqueous corrosive liquid with little or slight acidic odor. It is already in solution and may heat during dilution.
| Also known as | Potassium bifluoride solutionPotassium acid fluoride solutionKHF2 solutionPotassium hydrogen fluoride solution |
| CAS Number | 7789-29-9 |
| Appearance | Clear to slightly hazy aqueous solution. Odorless or slight acidic odor. Corrosive liquid at room temperature. |
| Flash Point | Not applicable (aqueous solution, non-flammable) |
| Boiling Point | Approximately 100-110C (212-230F) depending on concentration |
| Vapor Density | Not applicable (aqueous solution) |
| Water Reactivity | Already in aqueous solution; dilution with water generates heat but generally controlled reaction |
Chemical details are general reference only. Always verify with current SDS, ERG, and SOP/SOG.
Fireground Response Guidance — UN 3421
Extinguishing Media
Water spray to reduce vapors, do not direct stream into solution; dry chemical for surrounding fires
PPE Requirements
⚠️ Level B minimum with acid-resistant suit; full face SCBA required; butyl rubber or neoprene gloves; face shield and eye protection mandatory
Use SCBA and acid/fluoride-resistant chemical protective clothing with face shield, goggles and compatible gloves such as neoprene or butyl rubber. Level B is generally the minimum for spill response.
Isolation & Evacuation
ERG 154: isolate spill 25-50m all directions; for large spill isolate 100m; evacuate immediate area if fire involves containers
Always confirm protective actions with the current edition of the Emergency Response Guidebook (ERG Guide 154).
First Actions for a UN 3421 Incident
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Ventilate closed spaces before entering, but only if properly trained and equipped.
- Isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters
- For highlighted materials: see Table 1 - Initial Isolation and Protective Action Distances.
- For non-highlighted materials: increase the immediate precautionary measure distance, in the downwind
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📋 Copy & Share Field Card
UN 3421 — Potassium hydrogen difluoride, solutionHAZMAT RADIO NOTE — UN 3421
Product: Potassium hydrogen difluoride, solution
Class 8 / Corrosive / ERG 154
PPE: Level B minimum with acid-resistant suit; full face SCBA required; butyl rubber or neoprene gloves; face shield and eye protection mandatory
ISOLATION: ERG 154: isolate spill 25-50m all directions; for large spill isolate 100m; evacuate immediate area if fire involves containers
ACTION: Stage upwind · Isolate · Deny entry · Request HazmatRADIO
Use for: Quick radio or face-to-face size-up. Short, structured, field-ready.
=== IC HAZMAT BRIEFING ===
UN 3421 — Potassium hydrogen difluoride, solution
Class: 8 | Placard: Corrosive | ERG Guide: 154
Appearance: Clear to slightly hazy aqueous solution. Odorless or slight acidic odor. Corrosive liquid at room temperature.
Water Reactivity: Already in aqueous solution; dilution with water generates heat but generally controlled reaction
Extinguishing: Water spray to reduce vapors, do not direct stream into solution; dry chemical for surrounding fires
PPE: Level B minimum with acid-resistant suit; full face SCBA required; butyl rubber or neoprene gloves; face shield and eye protection mandatory
Isolation: ERG 154: isolate spill 25-50m all directions; for large spill isolate 100m; evacuate immediate area if fire involves containers
— Key Hazards —
• TOXIC and/or CORROSIVE; inhalation, ingestion or skin contact with material may cause severe injury
• Contact with molten substance may cause severe burns to skin and eyes.
• Avoid any skin contact.
— First Actions —
• Keep unauthorized personnel away.
• Stay upwind, uphill and/or upstream.
• Ventilate closed spaces before entering, but only if properly trained and equipped.
• Isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters
SOURCE: allfirefighter.com/hazmat/un/potassium-hydrogen-difluoride-solution-un-3421 | Always use current ERG + SOP/SOGIC BRIEFING
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
UN3421 Potassium hydrogen difluoride, solution Cls8 ERG154 | allfirefighter.com/hazmat/un/potassium-hydrogen-difluoride-solution-un-3421SMS / 160 CHAR
Use for: Quick text to command or incoming units. Fits in a single SMS.
⚠️ Quick-reference only. Always use current ERG + SOP/SOG for incident-specific actions.
Page: https://allfirefighter.com/hazmat/un/potassium-hydrogen-difluoride-solution-un-3421
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Frequently Asked Questions about UN 3421
UN 3421 is Potassium hydrogen difluoride, solution. It should be identified from shipping papers, markings and the SDS before responders work near the release.
ERG Guide 154 applies to UN 3421 for initial isolation, protective actions and first-response guidance.
UN 3421 is not primarily classified as flammable, but fire may produce toxic or corrosive gases and containers may become dangerous when heated.
Use SCBA and acid/fluoride-resistant chemical protective clothing with face shield, goggles and compatible gloves such as neoprene or butyl rubber. Level B is generally the minimum for spill response.
UN 3421 may cause deep chemical burns and systemic fluoride toxicity after skin or eye contact.
Sources (high level): DOT/PHMSA marking & class concepts + ERG usage principles.
This page does not reproduce ERG guide text—always consult the current ERG for incident-specific protective actions.