Ethylene glycol
Ethylene glycol solutions became available in 1926 and were marketed as "permanent antifreeze" since the higher boiling points provided advantages for summertime use as well as during cold weather. They are used today for a variety of applications, including automobiles, but gradually being replaced by propylene glycol due to its lower toxicity.
Poisoning
Main article: Ethylene glycol poisoning
Ethylene glycol is poisonous to humans and other animals,[3][4] and should be handled carefully and disposed of properly. Its sweet taste can lead to accidental ingestion or allow its deliberate use as a murder weapon.[5][6][7] Ethylene glycol is difficult to detect in the body, and causes symptomsincluding intoxication, severe ........, and vomitingthat can be confused with other illnesses or diseases.[3][7] Its metabolism produces calcium oxalate, which crystallizes in the brain, heart, lungs, and kidneys, damaging them; depending on the level of exposure, accumulation of the poison in the body can last weeks or months before causing death, but death by acute kidney failure can result within 72 hours if the individual does not receive appropriate medical treatment for the poisoning.[3] Some ethylene glycol antifreeze mixtures contain an embittering agent, such as denatonium, to discourage accidental or deliberate consumption.
Propylene glycol
Propylene glycol, on the other hand, is considerably less toxic than ethylene glycol and may be labeled as "non-toxic antifreeze". It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems or in water pipes in homes where incidental ingestion may be possible. As confirmation of its relative non-toxicity, the FDA allows propylene glycol to be added to a large number of processed foods, including ice cream, frozen custard, and baked goods.
Propylene glycol oxidizes when exposed to air and heat, forming lactic acid.[8][9] If not properly inhibited, this fluid can be very corrosive,[citation needed] so pH buffering agents are often added to propylene glycol, to prevent acidic corrosion of metal components.
Besides cooling system corrosion, biological fouling also occurs. Once bacterial slime starts to grow, the corrosion rate of the system increases. Maintenance of systems using glycol solution includes regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color, and biological contamination. Propylene glycol should be replaced when it turns a reddish color.[10]
Glycerol
Once used for automotive antifreeze, glycerol has the advantage of being non-toxic, withstands relatively high temperatures, and is noncorrosive.
Like ethylene glycol and propylene glycol, glycerol is a non-ionic kosmotrope that forms strong hydrogen bonds with water molecules, competing with water-water hydrogen bonds. This disrupts the crystal lattice formation of ice unless the temperature is significantly lowered. The minimum freezing point temperature is at about −36 °F / −37.8 °C corresponding to 6070% glycerol in water.[11]
Glycerol was historically used as an antifreeze for automotive applications before being replaced by ethylene glycol, which has a lower freezing point. While the minimum freezing point of a glycerol-water mixture is higher than an ethylene glycol-water mixture, glycerol is not toxic and is being reexamined for use in automotive applications.[12][13] Glycerol is mandated for use as an antifreeze in many sprinkler systems.
In the laboratory, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C due to the depression of the freezing temperature of solutions with high concentrations of glycerol. It is also used as a cryoprotectant where the glycerol is dissolved in water to reduce damage by ice crystals to laboratory organisms that are stored in frozen solutions, such as bacteria, nematodes, and mammalian embryos.
