CHEMICAL DATA NOTEBOOK SERIES #85: ETHYL MERCAPTAN
Ethyl mercaptan is a flammable, toxic, irritating, highly volatile, yellow liquid with an overpowering, offensive odor. It is used as an odorant for natural gas and LPG (liquid petroleum gas). It also is used as an adhesive stabilizer and as an intermediate raw material in the manufacture of antioxidants, defoliants, insecticides, pharmaceuticals, and some types of plastics. The name ethyl mercaptan is slowly being replaced by the chemical’s proper chemical name, ethanethiol. Thiol is the newer designation for all chemicals previously called mercaptans.
Ethyl mercaptan is an extremely flammable liquid with a flash point of — 38°F (reported variously as 80°F, 0°F, — 4°F, and — 18°F, a spread of 118°F), a flammable range of 2.8 to 18.2 percent in air, and an ignition temperature of 570°F. It has a specific gravity of 0.839, a molecular weight of 62, and a vapor density of 2.14. It has a boiling point of 93 9°F, has a freezing point of — 232.6°F, and is slightly soluble in water. Its molecular formula is C2HsSH.
With such a low flash point, wide flammable range, low ignition temperature, and high specific gravity, one would expect flammability to be ethyl mercaptan’s greatest hazard. However, terrorists and vandals have discovered a greater hazard —its odor.
Ethyl mercaptan’s odor is so powerful that it can be detected at a level as low as one ppb (part per billion) of air. For a reference point, one part per billion is analogous to one penny in 10 million dollars. This property of the chemical, along with the offensiveness of the odor (ethyl mercaptan is related to the chemical in the working end of a skunk), makes it ideal for use as an odorant; it is added to natural gas and liquid petroleum gases to alert users in the case of a leak. It is added at levels considerably lower than its lower flammable limit but at which its odor still makes the presence of explosive fuel gases apparent.
This common use has made it possible for unscrupulous persons to spread terror in public places. The release of a very small amount of ethyl mercaptan will evaporate rapidly and spread, carrying with it the characteristic odor of a leaking fuel gas. Anyone detecting the ethyl mercaptan by odor will assume the area is saturated with natural gas or another explosive gas and logically will conclude that an ignition source will cause a terrible explosion. The odor of ethyl merc aptan and the thought of being in the center of such an impending explosion can cause panic in a crowd and result in injuries to people caught up in the attempt to get out of the area as quickly as possible.
Ethyl mercaptan’s flammability is its real number one hazard. Its flash point of – 38°F means that any liquid ethyl mercaptan exposed to the open atmosphere, as in the case of an accidental spill or release, will generate vapors sufficient to form an ignitable mixture with the air at almost all ambient temperatures. Its very wide flammable range (2.8 to 18.2 percent) means that once the ethyl mercaptan vapors have surpassed the lower flammable limit (LEE), it will be almost impossible for them to become too rich (by surpassing the upper flammable limit, or UFL). And its relatively low ignition temperature means that the vapors, once they are in the flammable range, can be ignited by any common ignition source.
Ethyl mercaptan is a toxic material with a TLV-TWA (threshold limit value-time weighted average) of 0.5 ppm (parts per million of air). It STEL (short-term exposure limit) is two ppm, and the PEL (permissible exposure limit) is 10 ppm. The TLV-TWA and STEL values are provided by the ACGIH (American Conference of Governmental and Industrial Hygienists), while the PEL is set by OSHA (the Occupational Safety and Health Administration). It is unusual that the two values (TLV-TWA and PEL) are so far apart. In recent years, more and more hazardous chemicals are being assigned the same TLV-TWA and PEL.
In lower concentrations, ethyl mercaptan is an irritant to the eyes, nose, throat, and respiratory tract; but it produces more serious symptoms at higher concentrations (usually those above 100 ppm), including cyanosis, dizziness, headache, incoordination, signs of intoxication, rapid breathing, central nervous system depression, unconsciousness, respiratory paralysis, and death. The LD50 for mice is less than 2,800 ppm over a four-hour exposure.
Skin contact with liquid ethyl mercaptan can cause severe irritation, while prolonged or chronic exposure to very high concentrations of the vapor will cause skin irritation. Eye contact with the vapors can cause moderate to severe irritation. If ingested, ethyl mercaptan will cause many of the same symptoms as inhalation of high concentrations of vapor. Aspiration of ethyl mercaptan into the lungs can cause pulmonary edema, sometimes on a delayed basis.
Ethyl mercaptan is considered to be a stable chemical, but it will react violently with alkaline earth metals, epoxides, nitrides, peroxides, strong oxidizing agents (calcium hypochlorite is specifically reactive with ethyl mercaptan), and strong reducing agents. Contact with acids or acid vapors may evolve toxic levels of sulfur dioxide (S()2) and sulfur trioxide (S03). Toxic combustion products of ethyl mercaptan include carbon monoxide, sulfur dioxide, and sulfur trioxide.
A release of even a small amount of ethyl mercaptan can produce a tremendous amount of vapor, which, as noted, will be recognized as the smell of a natural gas or LPG release. Small releases in open air pose no serious fire hazard (just as in the case of a release of a small amount of any other flammable liquid), but panic can be produced if entire neighborhoods think that a gas main has ruptured. All residents downwind must be apprised of the situation at once, so that no one is injured by operating in a state of extreme fear.
I^irge releases of ethyl mercaptan are extremely dangerous due to the explosiveness of the evolved vapors. The problem of evacuation downwind (which should be for at least two miles, and from a radius of 2,000 feet around the spill) will be very difficult once the evacuees smell what they think is a gas leak. It is entirely possible that the odor of ethyl mercaptan will be detected for many more miles downwind because the odor is so powerful and can be detected at such low concentrations. The odor may linger for several days, even after the last contaminated soil is removed. This can cause many serious problems for emergency responders and the local government, since residents of the area will continue to report the smell of natural gas or LPG, as long as the contamination remains.
Treat ethyl mercaptan like any other flammable liquid. Approach from upwind and uphill and be prepared to move when the wind shifts. Do not allow your apparatus or any of your mitigation techniques to be the ignition source tor the vapors. Also, personnel working anywhere near the “hot” zone will need approved respiratory, eye, and skin protection.
Contain all releases of ethyl mercaptan to avoid the spread of contamination. Form dikes by pushing up soil, sand, or other materials around the spilled product to create a containment pond to hold the liquid until it can be salvaged. A containment pit. if one can be dug safely, also may be used to contain the liquid, but this action may cause the product to more deeply penetrate the earth. This might severely complicate the removal of all contaminated soil.
IDENTIFICATION NUMBERS AND RATINGS
(Chemical Abstract Services)
(Standard Transportation Commodity Code)
(Registry of Toxic Effects of Chemical Substances)
(United Nations/North America)
(Chemical Hazard Response Information System)
(U.S. Department of Transportation)
Class 3, flammable liquid
NFPA 704 Rating
(International Maritime Organization)
3.1, flammable liquid
If the ethyl mercaptan is suctioned from a containment pond or pit, any remaining product may be absorbed by the use of dry sorbents such as cement powder, clay, diatomaceous earth, fly ash. Fuller’s earth, peat, perlite, sand, saw dust, soil, straw, vermiculite, or other commercial sorbents. Dispose of these contaminated sorbents in the same manner as you do the soil contaminated by the original product.
Ethyl mercaptan’s high vapor density means that its vapors will hang together for some time, flowing downhill and over lowr spots in the terrain, often for surprisingly long distances, unless disturbed by the wind. Any accumulation of vapors in low areas or confined spaces will be both an explosive threat and a toxic danger to anyone in or near the vapors. The odor may be so overpowering that the sense of smell may “go dead” after just a few whiffs at high concentration, and the exposed person may think that the danger has gone away since he/she no longer can smell ethyl mercaptan.
If the vapors traveling from the original release contact an ignition source (and you can be sure that if the vapors are moving, they eventually will find an ignition source), they will ignite and “flash back” to the source, igniting the whole “pool” of released ethyl mercaptan. To prevent vapors from moving downw ind or over low spots, disperse them at the source.
Use high-pressure water spray or fog to “sweep” the air in the path of the moving vapors to effectively disperse them. Contain all runoff water. Do not allow the water used to disperse vapors to contact the accumulated liquid.
Tools and equipment used in any mitigation techniques involving ethyl mercaptan, like those used in conjunction with any flammable liquid, must be sparkproof. Ethyl mercaptan’s very low ignition temperature makes even the smallest and most insignificant spark a potential ignition source. Remember, when the vapors of a flammable or combustible liquid are ignited, the first event that occurs is an explosion, the severity of which depends on the amount of vapors present and the size of the area over which the vapors have spread.
Block all sewer openings to prevent ethyl mercaptan from entering the sewer system, and immediately warn all sewage treatment plants that could be affected by the presence of ethyl mercaptan in the system. Ethyl mercaptan that enters a storm sewer system will create a problem throughout the district served by the sewer system. The generation of explosive vapors within the sewer will threaten a very wide area with the possibility of an explosion originating at any opening in the system downstream from the original point of entry, plus a few openings directly above that entry point. The relatively high upper flammable limit of ethyl mercaptan means that explosions can occur within as well as immediately outside the sewer system.
In some older sewer systems, the storm sewer system and the sanitary sewer system may have some crossover or may share a common line. If ethyl mercaptan vapors enter the sanitary sewer system, it is possible that some residents may report what they believe to be a leak of natural gas. Even with sufficient traps, ethyl mercaptan vapors can enter the home by being forced up the vent stack and overflowing down the outside of the building. Once again, almost anyone can detect ethyl mercaptan because of its extremely low detection level of one part per billion.
If ethyl mercaptan enters a waterway, it w ill float on the surface, dissolving into the water very slowly. It can be confined on the surface and skimmed off into secure containers much like spilled petroleum and other hydrocarbon products, using floating booms and skimmers. Straw or peat moss also may be used to absorb the product surrounded by the floating booms.
If ethyl mercaptan enters a river or stream, use diversion techniques to direct the contaminated water into low-lying areas. Once the contaminated water is isolated and confined, the product may be salvaged by pumping or suctioning it into secure containers. The product also may be removed with aeration techniques, or by being adsorbed by the addition of activated carbon and gentle agitation. Treat any sorbents used to absorb or adsorb as you would the original product; dispose of them in accordance with federal, state, and local regulations. In slower-moving rivers or streams, a temporary water bypass dam may be installed to stop the movement of ethyl mercaptan on the surface while allowing uncontaminated water below the surface to continue flowing. The product then can be pumped out or absorbed by straw or peat. In all cases, alert downstream users of the water even before you carry out diversion or other mitigation techniques.
In fast-moving rivers and streams, it may not be possible to contain the ethyl mercaptan on the surface, especially if there is turbulence in the water The faster the water moves, and/or the more mixing action is occurring, the faster the product (even very slightly water-soluble materials like ethyl mercaptan) will dissolve in the water. It is imperative that the environmental authorities who were called when the incident began monitor all contaminated water and make the final determination as to when the water is safe again.
Protect containers of ethyl mercaptan from the radiated heat of a fire or the direct heat of impinging flames, f ailure to do so may cause the liquid’s internal vapor pressure to rise faster than the action of pressure-relief devices can regulate it. If this occurs, the design strength of the container will be surpassed, and a BLEVE (boilingliquid, expanding-vapor explosion) will occur. Cool all containers of ethyl mercaptan that are exposed to heat sources emanating from a fire with water delivered by unmanned appliances from as far away as possible. Under no circumstances should firefighters be within the area that would be devastated by the explosion of an overheated container. This area will be determined by the product present. the amount present, and the size of the containers in which the product is confined.
Extinguish pools of burning ethyl mercaptan with carbon dioxide, drychemical, foam, halon, and water spray. The choice of fire extinguishment agent will depend on where the product has been released, how much is involved in the fire, wind conditions, and the topography surrounding the spill area. Water might be ineffective as an extinguishing agent, but it will be useful to cool down the fire. With such a low flash point and ignition temperature, extinguished ethyl mercaptan may reignite if the released vapor comes in contact with an object that was heated by the fire to a temperature at or above the product’s ignition temperature. This is a very likely possibility.
If vapors escaping from a container of ethyl mercaptan are burning, do not attempt to extinguish the flame unless the flow of vapors can be stopped immediately after extinguishment. If the vapors accidentally are extinguished, consider immediate, deliberate reignition to avoid the explosion that will occur as soon as the vapors enter the flammable range and reach an ignition source.
Toxic combustion products released by burning ethyl mercaptan include carbon monoxide, hydrogen sulfide, sulfur dioxide, and sulfur trioxide. Carbon monoxide and hydrogen sulfide, two deadly gases, are both flammable, and most of what is produced will be consumed in the fire. However, it is imperative that all firefighters wear proper respiratory protection when fighting fires involving ethyl mercaptan.
PROTECTIVE CLOTHING AND EQUIPMENT
Protective clothing and equipment must be worn to prevent any contact of the ethyl mercaptan with the eyes or skin. Rubber gloves, aprons, and boots offer skin protection, and splashproof chemical goggles should be worn to protect the eyes. Use positive-pressure, self-contained breathing apparatus for respiratory protection. Manufacturers of total encapsulating suits claim that suits made of butyl rubber, neoprene, and polyvinyl chloride (PVC) are compatible with ethyl mercaptan. Contact individual manufacturers of total encapsulating suits to determine the degree of safety each recommended material offers and consult ethyl mercaptan manufacturers for their recommendations.
IJPG Ethyl Mercaptan 1010
Inhalation. Move the victim to fresh air and keep him or her calm and warm. If the victim’s breathing has stopped or becomes labored, administer artificial respiration, being aware that such action might expose the first-aid giver to the material in the victim’s lungs and/or vomit. Seek immediate medical attention.
Eye contact. Immediately flush the eyes with water for at least 15 minutes, lifting the eyelids occasionally. Be careful not to direct the wash water onto the unaffected eye. Seek immediate medical attention.
Skin contact. Wash any affected areas of the body with large amounts of soap and water. If irritation continues after washing, seek immediate medical attention.
Ingestion. Authorities disagree on procedures. One resource advises that a conscious victim should drink a strong saltwater solution immediately, and induce vomiting. Other resources advise administering large amounts of water but do not address inducing vomiting in the victim. At least one manufacturer of ethyl mercaptan states vomiting should not be induced. If you are a first-aid giver and ethyl mercaptan is involved, immediately get medical advice.
In any event, never try to make an unconscious person drink anything or try to make him/her vomit.