It is not safe to store flammable liquids in domestic refrigerators or freezers. Under normal operating conditions the defrost timer, thermostat, switch, and/or interior lighting units in these fridges and freezers can cause electrical sparks. In the presence of chemical vapors, this can lead to ignition and powerful explosions.
To avoid explosions and other accidents you should use a flammable material storage unit or an explosion proof refrigerator or freezer.
Flammable Material Storage Refrigerators and Freezers
Flammable material storage refrigerators and freezers have a spark-free interior, meaning that they contain no internal sparking devices, relays, switches, or thermostats. They may also contain special design features such as self-closing doors, magnetic door gaskets, and special inner shell material that limits damage if an accident were to happen.
If it meets the requirements, a flammable material storage unit will have a UL listing that confirms as such.
Explosion-Proof Refrigerators and Freezers
Explosion-proof refrigerators and freezers (also known as hazardous location refrigerators and freezers) are similar to their flammable storage counterparts. The main difference is, in addition to the spark-free interior, they have a spark-free exterior, which prevents ignition from flammable vapors or gases that may be present outside the refrigerator or freezer. By default, the interior has to be spark-free because it will be exposed to the environment when the door is opened.
UL's definition for a hazardous location is: "Location where explosion or fire hazards exist due to the presence of flammable gases, flammable or combustible liquid-produced vapors, combustible dusts, or ignitable fibers or flyings."
For a spark-free exterior, all electrical components and wiring are contained in a housing that is sealed after connection. Furthermore, they don’t have a power cord and must be hard-wired into the power source, thus excluding all electrical exposure.
In a standard lab, flammable material storage refrigerators and freezers are often used as there are other items, such as phones, computers, and outlets, that can cause sparks, cancelling out the extra protection of an explosion-proof refrigerator or freezer. Explosion-proof refrigerators and freezers are use in locations where a flammable atmosphere (class I division 2 or similar classification) may develop.
Note that if a unit has a UL listing for hazardous locations, it is given that it will conform to flammable material storage standards. This means the unit will not bear a separate UL listing for flammable material storage. This is handy to know in case you are looking for a unit with both certifications, as this is something you will likely never find.