Isopropyl alcohol (IPA) at 91% concentration is a solvent and common household disinfectant found in many medicine cabinets. Chemically identified as propan-2-ol, it is often used for cleaning electronics and removing sticky residues. The 91% Isopropyl Alcohol is highly flammable. It is classified as a Category 2 flammable liquid, meaning it presents a significant fire hazard that requires careful handling.
The Science of Isopropyl Alcohol Flammability
The flammability of 91% IPA is not related to the liquid itself burning, but rather the ease with which its vapor ignites. Isopropyl alcohol is highly volatile, meaning it evaporates quickly, releasing flammable vapor into the surrounding air. The danger is defined by the substance’s flash point, which is the lowest temperature at which the liquid creates enough vapor to form an ignitable mixture with air.
The flash point for the 91% solution is very near room temperature, often cited in the range of 55°F to 72°F (13°C to 22°C). This means that at a typical indoor temperature, the liquid is constantly producing enough flammable vapor to ignite if it encounters a spark or flame.
The resulting vapor is approximately 2.1 times heavier than air. Because of this density, the vapor flows along the ground or floor, accumulating in low-lying, poorly ventilated areas. This invisible vapor trail can travel a considerable distance to an ignition source, such as a pilot light, and flash back to the liquid source. Once ignited, the alcohol-air mixture is explosive within a concentration range of 2.0% to 12.0% in the air.
How Concentration Affects Ignition Risk
The percentage of water mixed with the isopropyl alcohol directly determines the flammability risk. Water acts as a non-flammable diluent, effectively raising the solution’s flash point. Since 91% IPA contains only 9% water, it is considerably more hazardous than the more common 70% concentration.
The 91% solution produces flammable vapor at nearly all ambient temperatures because the small amount of water does not significantly suppress evaporation. By contrast, the 70% solution contains 30% water, which increases its flash point to approximately 77°F (25°C). This higher flash point means the 70% solution is less likely to produce an ignitable vapor concentration at standard room temperature.
The higher concentration translates to a much easier ignition, as less heat is required to reach its flash point. This difference is why 91% IPA is often preferred for applications requiring fast evaporation, but it also elevates the fire hazard significantly compared to the 70% mixture.
Safe Handling and Storage Guidelines
Due to the high flammability of the 91% solution, users must prioritize safety when handling and storing it. Good ventilation is necessary when using 91% IPA to prevent the accumulation of heavy, explosive vapors. Containers should be kept tightly closed when not in use to minimize vapor release and evaporation.
Storage and Safety Precautions
For long-term storage, containers should be stored in a cool, dry, and dark location.
Key precautions include:
- Keep the product away from all sources of ignition, including open flames, sparks, hot surfaces, and electrical equipment.
- Be aware that pilot lights on stoves or water heaters can ignite the heavy, traveling vapors.
- Keep the alcohol separate from strong oxidizing agents, which can react dangerously.