Phosgene, also known scientifically as carbon oxy dichloride, carbonyl chloride or carbon oxy chloride, is the dichloride of carbonic acid. Its molecular formula is COCl2. At normal temperatures, phosgene is gaseous and extremely toxic. Primarily used as a lethal chemical warfare agent in World War I, the substance has long been outlawed by the International Chemical Weapons Convention. In industry, phosgene is only used in closed circuits due to its high hazard potential.
History of phosgene
Phosgene was discovered in 1812 by the English physician, chemist and zoologist John Davy (1790 to 1868), the younger brother of the English chemist Sir Humphry Davy (1778 to 1829). The latter is best known for discovering the analgesic effect of nitrous oxide.
The name phosgene (Greek: produced by light), can be traced back to the photo-induced addition of chlorine gas to carbon monoxide carried out by Davy.
Properties of phosgene
Phosgene is a highly toxic colorless gas that is much heavier than air. Its sweetish, foul odor, reminiscent of rotting banana skin or damp hay, is very characteristic and can be quickly identified. However, it is not perceptible below toxic concentrations, with perceptibility diminishing rapidly with exposure.
Phosgene is well soluble in organic solvents such as benzene, chlorobenzenes, and toluene. In water, it gradually decomposes to carbon dioxide and hydrochloric acid. Therefore, anhydrous organic solvents must be used for its synthesis or conversion. Under the influence of short-wave UV light, phosgene decomposes into carbon monoxide and chlorine.
The molar mass of phosgene is 98.916 g/mol. The melting point is -128 °C, the boiling point 7.6 °C. At 25 °C, the gaseous substance has a density of 4.03 g/l. As a liquid at the boiling point it is 1.4 g/cm3.
Production of phosgene
Phosgene is produced from carbon monoxide and chlorine using a catalyst. The highly exothermic reaction requires complex cooling, as the temperatures in the “hotspot” rise to as high as 600 °C. The reaction is then carried out at high temperatures. To ensure complete conversion of the chlorine and avoid a chlorine-iron fire (exothermic reaction of chlorine and iron at more than 170 °C), carbon monoxide is added to the reaction in excess. To prevent the formation of chlorine oxyhydrogen gas, it is essential that the carbon monoxide is free of hydrogen.
Carbonyl chloride can also be formed when chlorine-containing plastics (e.g. PVC) are burned in the presence of metal and coal, or when the refrigerant R22, (which has been banned since January 2010), is burned (e.g. when old copper pipes in refrigeration systems filled with it are soldered). On a laboratory scale, it can also be produced from tetrachloromethane and fuming sulfuric acid (oleum).
Health hazards of phosgene
Because of its poor solubility in water, phosgene passes through the bronchi into the alveoli after inhalation without significant decomposition. It damages the alveoli because it destroys the thin membrane which normally controls the equilibrium of carbon dioxide and oxygen in the blood.
The amount of damage depends on the phosgene concentration and the inhalation dose (product of concentration and inhalation time). Both may contribute to the development of pulmonary edema (water accumulation in the area of the alveoli). The extent of the damage depends primarily on the inhaled dose. Another important factor is the intensity of breathing. Physical exertion increases the toxic effect.
For this reason, the “phosgene indicator badges” worn for safety reasons by workers in phosgene-producing and -processing plants show the exposure dose instead of the exposure concentration, so that adequate therapy can be initiated immediately in an emergency.
In the case of low inhalation doses, the affected persons generally remain under medical observation for a few hours and receive precautionary therapy. Permanent damage is not to be expected in these cases. At higher installation doses (from about 150 ppm – min), pulmonary edema may form within two to three hours, the extent of which depends on the dose. Breathing is severely impaired as a result. Oxygen uptake from the breathing air into the blood decreases. Without treatment, death by suffocation usually follows. In some cases, two to three days pass before this happens. Very high doses can cause death within seconds to minutes if the tissue is so severely damaged that there is no gas exchange at all.
The health hazard posed by phosgene is based less on its toxicity, but on the nature of its effects. Even after exposed to a lethal dose, there may be no symptoms for hours. Therefore indicator badges are mandatory in order not take medical countermeasures in due time. Only the badge reliably shows the attending physician the absorbed dose.
With adequate treatment, pulmonary edema caused by phosgene intoxication heals in the medium and long term without further consequences. Only in the first few weeks after contact with the toxic substance are there restrictions in lung function, which gradually subside.
Fatal accidents involving phosgene
Several fatal accidents occurred during the industrial production and processing of phosgene. The most serious occurred on May 20, 1928, in Germany. On the premises of the Stoltzenberg chemical factory (Hamburg), a valve on a tank car had burst off for unexplained reasons, whereupon the phosgene contained in it changed to a gaseous state at outside temperatures of 20 °C. The tank car contained around 10,400 liters of phosgene, forming a toxic gas cloud moving towards the Müggenburg Canal in direction to Wilhelmsburg and entered residential areas there. At least ten people died and another 300 were injured.
The last major accident in Europe occurred in 2008 in the Bavarian capital. Fortunately, there were no fatalities here. On March 14, a small hose of a test facility in a laboratory of the Technical University of Munich in Garching came loose. This caused a phosgene leak. Two people were taken to the intensive care unit in order to avoid pulmonary edema, and another 38 were observed in the hospital as a precaution.
The last fatal accidents involving phosgene occurred in the USA in 2010 and in South Korea in 2016. On Jan. 23, 2010, the sudden rupture of a steel hose connected to a phosgene tank at the DuPont Chemical Facility located in Belle, West Virginia, led to the release of the toxic gas. One employee died a day later. On May 27, 2016, a phosgene spill occurred at a BASF TDI plant in South Korea. In this incident, contractor working at the plant inhaled phosgene and died as a result on June 9, 2016.
Between the accident in Hamburg in 1928 and the one in Dupont in 2010, there were other fatal accidents in the second half of the 20th century, which were generally made public and investigated by the respective government authorities.
Areas of application for phosgene
The main area of application for this substance is the large-scale production of aromatic diisocyanates such as toluene diisocyanate (TDI) and methylene diphenyl isocyanate (MDI). These are used, for example, in the manufacture of polyurethane-based foams and the production of polycarbonate (PC). The latter is required for the production of high-quality plastics. More than 90 percent of the phosgene produced worldwide (over three million metric tons per year) is used for this purpose.
Other uses include the production of “aliphatic” diisocyanates such as hexamethylene diisocyanate (HDI) and isophorone diisocyanate (HPDI), which are used to make high-quality coatings, e.g. automotive paints, and the production of monoisocyanates such as methyl isocyanate, which are used to manufacture crop protection products.
Of secondary importance in terms of volume and economy is the use for the production of carboxylic acid chlorides and pharmaceuticals. The same applies to the production and use on a laboratory scale. The largest plants are located in Saudi Arabia, the USA, Germany, China, Japan and South Korea. Major producers include DOW, Covesto and BASF.
Because of its high toxicity, more than 99 percent of phosgene is produced on site where it is processed. In this way there is no hazardous transportation on public transport routes needed. Interim storage is also avoided, since comparatively harmless downstream products are immediately manufactured from the carbonyl chloride produced.
Excess phosgene is used for further processing. Unused surplus quantities are recovered and fed back into the production process. Residual quantities are absorbed by activated carbon and destroyed by hydrolysis with water. Alternatively, liquid diphosgene (trichloromethyl chloroformate) and solid triphosgene (bistrichloromethyl carbonate) are available in the laboratory sector. These have similar reactivity they are said to be less hazardous to use and less problematic to store, but even solid Triphosgene has a very high vapor pressure and the same effects on health as Phosgene. Therefore the term “safe Phosgene” used for Triphosgene is misleading.
Safety measures when handling phosgene
Due to its health hazards it presents, phosgene must be labeled with the hazard symbol T+ (very toxic). In addition, the labels R 26 (very toxic by inhalation) and R 34 (causes burns) are obligatory. According to TRGS 900, the occupational exposure limit is 0.082 mg/m³ or 0.1 ml/m³.
The following protective measures must be observed when handling phosgene manually:
- good ventilation of the work area
- Extraction of vapors
- prohibition of smoking and welding
- Do not open valves by force
- when changing cylinders, check valves of filled and empty cylinders for leaks
- Avoid contact with skin, eyes and clothing at all costs
- Observe personal hygiene
- Wear chemical protective gloves
- use gas filter B (gray) for respiratory protection
- Carry respirators at all times so that you can escape quickly in the event of a phosgene leak.
When filling phosgene, at least one trained person extra must be present. Supervision may also be provided by monitor from a permanently manned control room, provided that rapid intervention is assured in the event of an emergency.
If phosgene has been inhaled, it is important to first ensure that the person concerned stays calm. Then a cortisone-containing spray shall be applied at the respiratory tract as soon as possible. In addition, it is essential to call in trained physicians who will take further medical measures as far as required.
Gas detectors minimize risks when handling phosgene
The insidious thing about phosgene is that it does not immediately cause symptoms when inhaled in small doses. In addition, the characteristic odor of the gas is generally only noticeable at toxic concentrations and the sense of smell will quickly be lost. Therefore, it is important to continuously monitor hazardous areas by gas detection technology in order to enhance plant safety and avoid health risks.
For this purpose, Compur Monitors has developed both stationary and portable electrochemical sensors and colorimetric systems that reliably warn of phosgene and other hazardous gases in the ambient air as soon as the specified limit value is exceeded.
The product range includes, for example, the Tracer mobile leak detector, which can detect phosgene at levels as low as a few ppb. In addition, Compur offers the Statox product family with various models tailored to specific applications. Of particular note is the Statox 560, which can check for good operation even with phosgene generated on board.