CHEMICAL DATA NOTEBOOK SERIES #62: HYDROGEN SULFIDE
HAZARDOUS MATERIALS
Hydrogen sulfide is a toxic, flammable, irritating, colorless gas with a distinctive, offensive odor said to be similar to that of “rotten eggs.” It is used to manufacture adhesives, dyes, pesticides, pharmaceuticals, pigments, and plastics as well as to produce specific chemicals and classes of chemicals such as ethylene, sodium hydrosulfide, sodium hydroxide, “heavy water” (water containing deuterium, a radioactive isotope of hydrogen, instead of stable hydrogen), sulfur, mercaptans, organosulfuric compounds, and metallic sulfides. It has many other uses, including waste water cleanup and the restoration of groundwater sources.
Hydrogen sulfide may be shipped as a compressed gas in cylinders or as a liquefied gas in tank trucks. It formerly was manufactured by hydrochloric acid action on iron sulfide or by combining sulfur and hydrogen at elevated temperatures. Currently, it is produced as a by-product of natural gas refining.
The molecular formula of hydrogen sulfide is H2O
PROPERTIES
Hydrogen sulfide is a gas in its natural state and has a flammable range of from 4.3 to 46 percent in air. Its autoignition temperature is 500° F. Hie liquefied gas has a specific gravity of 0.79, a vapor density of 1.17, and a molecular weight of 34. It freezes at — 117°F, boils at — 76.4°F, and is somewhat soluble in water.
HAZARDS
Hydrogen sulfide is a toxic gas with the insidious property of being able to overcome and hide the most obvious clue to its presence: its powerful and disagreeable odor. It can be detected at levels as low as 0.1 ppm in air. As the concentration rises to 50 ppm, however, the odor of “rotten eggs” begins to become overpowering and “sickeningly sweet.” Between 50 and 100 ppm, the effects of the gas deaden the olfactory nerve, making it appear that the powerful odor that warns of its presence has disappeared, which keeps potential victims from realizing that imminent danger exists. Death can occur within 30 minutes upon exposure to levels as low as 600 ppm. The respiratory system becomes paralyzed— causing rapid death — at levels higher than 1,500 ppm.
OSHA and the ACGIH (the American Conference of Governmental Industrial Hygienists) adopted a 10ppm time-weighted average exposure limit for exposed workers. OSHA’s limitation is called a PEL (permissible exposure limit), while ACGIH’s limitation is called TI.V/TWA (threshold limit value/time-weighted average). These limitations are over an eighthour period with the average being weighted over time. In no case, however, shall the exposure be higher than 15 ppm for periods longer than 15 minutes. Both organizations call this the STEL (short-term exposure limit). The National Institute for Occupational Safety and I lealth (NIC)SI 1) recommends an exposure limit of 10 ppm for a maximum of 10 minutes.
Exposure at 20 ppm causes irritation of the eyes and throat; deadening of the sense of smell occurs as concentrations approach 100 ppm. Headaches, dizziness, and fatigue result when levels are around 250 ppm, and rapid unconsciousness takes place when levels are greater than 500 ppm. Continued exposure at this level can cause respiratory paralysis, coma, and death. At least one reference states that a single breath at approximately 600 to BOO ppm can cause unconsciousness.
Because hydrogen sulfide is so toxic and its poisonous nature is enhanced by its ability to deaden the sense of smell, individuals who normally work with it or handle it tend to forget that it is also flammable. Its relatively low ignition temperature of 500°F is well within the energy-producing range of all common ignition sources. Its very wide flammable range of 4.3 to 46 percent in air makes it very dangerous, since it would be very difficult to find a mixture of gas and air too rich to ignite.
Hydrogen sulfide is considered a stable chemical, but it must never be stored near oxidizing agents because it is flammable. Also, it should be segregated from all corrosive materials. Since it is slightly soluble in water and is an acid, hydrosulfuric acid forms. This acid is corrosive to metal in high concentrations.
NONFIRE RELEASE
As is the ease when any hazardous material is released, authorities must be notified. Notification should be automatic as part of your community emergency plan as mandated by SARA Title III (the Superfund Amendments and Reauthorization Act of 1986), but the incident commander should double-check to make sure that notification was given These experts are a valuable resource to help the incident commander in his decision making. Also, immediately notify’ the manufacturer, shipper, and buyer of the gas.
Since hydrogen sulfide is highly toxic, consider immediate evacuation of the area around the release. Evacuation distances should be at least 1,500 feet on three sides and two to three times that distance downwind, depending on wind conditions. The gas travels farther in a gentle breeze and threatens a larger area; a strong wind disperses the gas rather quickly.
Whether the leak is gas from a compressed cylinder or gas and/or liquid from a container, always approach it from upwind. All pcrsoncl must wear SCBA. Should the wind change, personnel without respirator) protection may be overcome immediately. In addition, approaching apparatus could ignite released gas within the flammable range.
Remember that explosion occurs first when a flammable gas within its flammable range contacts an ignition source of sufficient energy. The fire associated with flammable gases usually occurs next.
Gas leaking from a compressed cylinder or a container of liquefied gas drops to the ground quickly. Its vapor density of 1.19 is measured at about 60°F, but the gas escaping from its container is considerably colder. Even if the cylinder were at 80°F or higher, the Combined Gas Law’ states that the temperature of a gas drops as its pressure drops. When a gas is released from a small-volume container to a large-volume container (the atmosphere), its pressure drops rapidly—and so does its temperature. The colder the gas, the higher its vapor density. Cold gas, therefore, drops to the ground faster than warm gas.
Leaking gas coming from a liquefied gas container is even colder—probably near — 70°F—as it escapes. The major difference is that the gas from the cylinder is under greater pressure than the gas in the liquefied gas container and escapes slightly faster (assuming that all other things are equal).
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
7783-06*4
STCC
(Standard Transportation Commodity Code)
4905410
RTECS
(Registry of Toxic Effects of Chemical Substances)
MX 1225000
UN/NA
(United Nations/North America)
1053
CHRIS
(Chemical Hazard Response Information System)
HDS
RCRA
(Resource Conservation and Recovery Act)
U135
DOT
(U.S. Department of Transportation)
Flammable gas
NFPA 704 Rating
3-4-0
1MO
(International Maritime Organization)
Flammable gas. 2.0
In any case, the gas sinks to the ground as it is released, flows along low spots in the terrain, and gathers in enclosed spaces. The accumulations of gas are deadly for anyone without respiratory protection. High winds disperse the gas as it is released but may not disturb the gas that has entered a basement through an open window.
The gas —unless dispersed—continues to flow. It seeks an ignition source, ignites, and flashes back to the source of the leak. If the gas has not spread very far, there is little wind, and the release is in a protected place or natural low spot, the ignition could be explosive.
The worst scenario is a release inside a building. Gas may build up fast and rapidly overcome exposed people. The gas seeks its way to lower floors until it finds an ignition source. The resulting explosion destroys the building, entombing within it the individuals killed by the toxic effects. Emergency responders to a hydrogen sulfide release within a building must remember that anyone exposed to the gas already may be dead, and anyone entering the building (protected by SCBA, of course) may be killed by the ensuing explosion.
Consequently, automatic gas-monitoring-alarm systems should be installed in any building where hydrogen sulfide is used, manufactured, or stored. Employees must be trained in all safety procedures, including the use of SCBAs.
Liquefied gas released from its container begins to boil as it touches the ground or other surroundings, producing massive amounts of gas. If you can contain the liquid by pushing up soil around the spill to form a containment pond, the spread and evaporation will be reduced. Be careful to avoid igniting the vapor if you use this technique. The liquid may seep into the ground, thus slowing its conversion to gas. The contaminated soil when removed still may be liberating the gas.
In any event, any gas produced by the liquid or gas that may have escaped from a cylinder can be dispersed by applying a water fog or high-pressure spray. The runoff water, however, must be contained since it now holds some dissolved hydrogen sulfide.
Any liquid entering a sewer boils away and fills the sewer with hydrogen sulfide gas. If the gas is forced out of the sewer through an opening, it quickly dilutes in air and enters its flammable range. Any nearby ignition source produces at that opening a fire that may be difficult or impossible to extinguish. Notify the sewage treatment facility immediately if the liquid enters a sewer.
Liquid reaching a waterway boils quickly as it floats on the water. Some hydrogen sulfide dissolves in the water, endangering local waterfowl and aquatic life. The gas stays dissolved in the water as it flows downstream, creating a dangerous situation for any downstream user of the water. All downstream users immediately must be notified of the danger, and the environmental authorities will monitor the water flow to determine when the water can be used again. Contaminated water taken into an industrial operation or wateror sewage-treatment facility could be deadly to exposed workers.
Warming, agitating, or aerating the water removes the hydrogen sulfide. Exposure to the released gas must be avoided. Aeration techniques can be used only if the flow of water can be controlled and diverted into a holding area where the water can be agitated, be sprayed into the air, or have air bubbled through it. If the contaminated waterway is a river or stream, natural agitation occurs if the water flows over a dam or other obstruction. Environmental authorities must test the water for safety after agitation or other mitigation techniques are used.
Portable air-monitoring equipment with tubes specifically designed to test for hydrogen sulfide may be used to test a building or other enclosed area suspected of containing the gas. Ventilation can occur naturally or be performed with fans. Remember, however, that hydrogen sulfide is highly flammable. The use of sparkproof and explosionproof equipment is mandatory.
A patching kit could be used to stop the flow of gas, but the pressure of the escaping gas may be too high. Emergency responders attempting to stop the flow of gas by plugging the leak or turning a valve must have respiratory protection or they could be overcome and die almost instantly. The manufacturer, buyer, or shipper may be able to load the liquefied gas and transfer it to a secure container.
The radical technique of deliberately igniting the gas could be used under very special conditions. Hiis approach could be considered if the release is outside, if a large and/or difficult-to-evacuate population is imminently threatened, and if there is no danger of explosion on ignition. A mitigation technique of last resort, deliberately burning the hydrogen sulfide is a way to prevent its highly toxic properties from killing exposed unprotected persons. The approach might be used if it has been determined that the previously released gas has been dispersed or is otherwise outside its flammable range.
FIRE SCENARIO
If containers, particularly cylinders, of hydrogen sulfide are exposed to the radiated heat of a fire —or worse—come in contact with flame, the container may be in immediate danger of catastrophic failure. Cylinders holding hydrogen sulfide are protected by pressure-relief devices such as fusible plugs or a combination fusible plug/frangible disk. Once heat and/or pressure activates either of these devices, the cylinder completely empties. In the unlikely event that the safety-relief device is faulty or the pressure increase is so fast that the container does not empty rapidly enough, the container may explode from overpressurization, causing deadly shrapnel to join the deadly gas release.
SYNONYMS
hydrogen sulfuric acid hydrogen sulphide hydrosulfuric acid stink damp sulfu retted hydrogen sulfur hydride sulphuretted hydrogen
The leaking gas and/or released boiling liquefied hydrogen sulfide must be prevented from reaching an ignition source. T his may be impossible, since the energy radiated from a fire easily could raise the temperature of the gas above its ignition temperature of 5()()°F. Emergency responders must be aware that ignition in this manner is possible. Do not, therefore, get caught between the containers and an approaching fire.
The containers could be cooled with water applied by unmanned appliances positioned as far away from the containers as possible. Water has little or no effect on cooling a tank car or trailer because of the insulation between the liquefied gas and the outer container. This insulation also causes the liquefied gas to heat more slowly than the compressed gas in a cylinder.
If the safety relief device has operated, chances are the gas will ignite and flash back to the source of the release in a fire scenario. The resulting flaming tongue of escaping, burning gas must not be extinguished deliberately or accidentally unless the flow of gas can be stopped immediately on extinguishment of the flame. Stopping the flow of fuel is the standard procedure for fighting a gas fire.
Plans should be in place to relight the escaping gas immediately should the flame be accidentally extinguished. If the gas is not relighted quickly, a large amount of gas may accumulate and ignite explosively, causing much more damage than the burning gas.
If the escaping gas does not ignite, deadly concentrations quickly will build up and pose a serious death threat to all unprotected exposed persons.
Allowing the gas to completely burn itself up while firefighters protect all exposures may be the safest technique in a fire scenario. The chances of the liquefied gas progressing from burning to a BLEVE are reduced with an insulated tanker, but don’t bet your life on it —there is still the chance that it could happen.
PROTECTIVE CLOTHING AND EQUIPMENT
The main hazard of hydrogen sulfide gas is its toxicity; therefore, all exposed persons must wear SCBA, which also offers protection from the irritating effects of the gas on the eyes.
Normal turnout gear plus rubber boots and gloves, splashproof chemical goggles, and a full face mask protect the skin momentarily from the effects of the cold liquefied gas. Gloves and boots might become very brittle and break, exposing the skin to serious damage from frostbite.
FIRST AID
Move victims who have inhaled the gas to fresh air and keep them calm and w arm. If 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. Call for immediate medical attention.
For eye contact with the liquefied gas. immediately flush the eyes with water for at least 15 minutes, occasionally lifting the eyelids. Immediate medical attention is required.
For skin contact with the liquefied gas. gently apply tepid (not hot) water to the affected body areas. Remove clothing carefully so that damage to frostbitten skin is not aggravated.
Ingestion of the liquefied gas is highly unlikely. Severe frostbite damage occurs to the mouth and esophagus. Immediate medical attention is i needed *
