A solvent is a substance capable of dissolving another substance to form a solution. Examples range from water, a universal solvent for polar compounds, to organic chemicals like acetone or mineral spirits used in industrial and household products. Flammability is determined by the specific physical properties of the compound, not just its ability to dissolve other materials. While many organic solvents are highly flammable, others, such as water or certain chlorinated solvents, are not.
Understanding Solvents and Ignition
For any material to burn, three elements must be present: fuel, an oxidizing agent, and an ignition source. For organic solvents, the chemical acts as the fuel, combining with the oxidizing agent, which is nearly always oxygen in the air. The ignition source provides the heat necessary to start the combustion reaction.
It is the invisible vapor rising from the liquid’s surface that ignites, not the liquid solvent itself. The rate at which a solvent produces this ignitable vapor is dictated by its volatility, or tendency to vaporize. Volatile solvents, such as gasoline or ether, evaporate rapidly and create a high concentration of fuel vapor, increasing the fire risk even at room temperature. Less volatile solvents, like certain oils, must be heated significantly before they release enough vapor to pose a fire hazard.
The Mechanism of Flammability: Flash Point
The scientific measurement that quantifies a liquid’s flammability risk is the Flash Point. The flash point is the lowest temperature at which a liquid emits sufficient vapor to form a combustible mixture with air near the surface. Below this temperature, the vapor concentration is too low to sustain a flash of fire, even if an ignition source is introduced. Liquids with a flash point below 100°F (37.8°C) are classified as flammable, while those above are considered combustible.
This measurement is distinct from the Autoignition Temperature, which is the minimum temperature required to spontaneously ignite the vapor without an external spark or flame. For most solvents, the autoignition temperature is significantly higher than the flash point. For example, acetone has a flash point of about 0°F (-18°C) but an autoignition temperature over 869°F (465°C). The flash point indicates the temperature where a liquid becomes a fire risk in the presence of a spark, while the autoignition temperature relates to ignition from contact with a hot surface.
Classification Systems and Risk Levels
International organizations translate flash point data into standardized hazard categories for public safety and emergency response. The Globally Harmonized System of Classification and Labelling of Chemicals (GHS) uses a “Flammable Liquid” hazard class divided into four categories based on flash and boiling points. Category 1 represents the most severe hazard, applying to liquids like diethyl ether with a flash point below 73°F (23°C) and a boiling point below 95°F (35°C). Category 3 solvents, such as xylene, have a lower risk, with a flash point between 73°F (23°C) and 140°F (60°C).
The National Fire Protection Association (NFPA) 704 standard uses a color-coded diamond placard to communicate hazards to emergency responders. The red quadrant indicates flammability using a numerical rating from 0 to 4, where 4 signifies the highest degree of flammability. A rating of 2 is assigned to liquids that must be moderately heated before ignition can occur, corresponding to flash points between 100°F and 200°F (37.8°C and 93.3°C). These classifications ensure that anyone handling or storing a solvent can quickly assess the associated fire risk.
Safe Handling and Storage Practices
Minimizing fire risk requires strict adherence to control measures. Proper ventilation is a primary control, ensuring that solvent vapors are continuously diluted and removed from the workspace to prevent ignitable concentrations. This often involves using certified local exhaust ventilation, such as a fume hood, to capture vapors at the source.
Preventing an ignition source is essential, including the elimination of open flames, hot plates, and spark-generating equipment like electrical contacts. Static electricity can build up during solvent transfer and discharge a spark capable of igniting vapors. To counteract this, bonding connects containers with a conductive wire to equalize electrical potential, and grounding connects the system to the earth to safely dissipate charge buildup.
All flammable liquids must be stored in approved safety cabinets. These cabinets are typically constructed with double-walled, 18-gauge steel and a two-inch raised sill to contain spills. They are designed to limit the internal temperature to a safe level for a specific period if a fire occurs outside the cabinet.