The flash point of a particular chemical refers to the lowest temperature at which that chemical, in liquid form, will produce combustible vapors that will ignite with the introduction of an ignition source. This temperature can vary, for a given chemical, based on factors including pressure, the quantity of the chemical, and the location of the ignition source in relation to the chemical. The flash point for a particular chemical is, therefore, somewhat unreliable and should be used as a general guideline, rather than an absolute value.
There are two basic ways in which the flash point for a particular chemical can be determined: open cup or closed cup experiments. Both types of experiments are based on the fact that a liquid that is combustible, such as gasoline, releases vapors that are potentially ignitable. As the temperature of the liquid increases, the amount of these vapors also increases. This means that at a certain temperature, the vapor concentration is high enough that it becomes ignitable. Chemicals with a fairly low flash point are referred to as flammable, while chemicals with higher flash points are usually referred to as combustible.
An open cup experiment involves a quantity of a particular chemical placed in a container that is open. The chemical is slowly heated and an ignition source, such as a small flame, is introduced above the chemical at various intervals. This process continues until a temperature is reached at which the vapors from the chemicals ignite, which establishes the flash point for that chemical. Once ignition occurs, the ignition source is removed and the vapors should then stop burning; if they continue to burn without the source, however, then the fire point for a chemical has been reached.
In a closed cup experiment, the process is similar but the container for the chemical is lidded and the ignition source is introduced through the lid. A closed cup experiment allows for the flash point of a particular chemical to be determined when that chemical is under increased pressure due to a closed system. This is important for determining safe handling conditions for various chemicals stored or transported in closed containers.
When the flash point for a particular chemical is represented, it is typically accompanied by information that indicates in what conditions that point is accurate, such as details on pressure and the position of the ignition source. The fire point for a chemical is usually slightly higher than that chemical’s flash point, and the autoignition point is higher still. A chemical’s autoignition point is the temperature at which a particular chemical or substance will ignite on its own, without an outside ignition source.