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Two basic groups of chemical compounds are formed from elements: salts and non-salts. Within each group are families of compounds that have particular hazards associated with each. By understanding these family relationships, emergency responders can determine the general hazards of materials by identifying the family to which each belongs.
Photo by Robert Burke
Cyanide salts in contact with acids produce hydrogen cyanide gas, which is used in gas chambers and could be used in chemical terrorism.
This "rule of thumb" information can help first responders identify hazards of particular materials upon arrival at an incident but it does not eliminate the need to research the chemicals further before mitigation actions are taken.
The first group of compounds is known as salts. Most everyone is familiar with at least one salt that is found in almost every home and eating establishment table salt, known chemically as sodium chloride and which is used to season and cure foods.
Sodium chloride occurs naturally in the crust of the earth and in the oceans. It is made up of chlorine, which by itself is very toxic and an oxidizer, and sodium metal, an element that is air and water reactive. Once these elements are combined, a compound is formed that is neither water reactive nor very toxic. The molecular formula for the compound sodium chloride is NaCl. Sodium chloride is a major source of chlorine production, as chlorine does not exist in nature by itself.
In general, salts are water-soluble solids; most do not burn but they can be oxidizers and support combustion. Some salts are toxic and some may be water reactive. A rule when dealing with salts is "Don't touch, and keep them dry." The hazards present in a given salt family, other than the binaries, is a result of reactivity with water. Salts are made up of a metal and a non-metal element. For example, looking at table salt again, chlorine is a non-metal and sodium is a metal. This bonding process that creates a salt is referred to as an "ionic" bond.
Photo by Robert Burke
Some water-reactive salts produce toxic vapors when wet that would require emergency responders to don Level A protection.
Metals usually do not bond together. Metals that are combined are melted and mixed together to form an alloy. For example, copper and zinc are melted and mixed together to make brass. Brass is not an element and does not have a molecular formula. There is no actual chemical bond involved but rather it is just a mixture of zinc and copper.
The second major group of compounds are non-metal elements that could be referred to as the non-salts. Non-salts are solids, liquids and gases. Many non-salt compounds are flammable. For example, the non-metal carbon combines with the non-metal hydrogen to form a hydrocarbon compound. A typical hydrocarbon is methane, with the molecular formula CH4. Methane is also known as natural gas and is very flammable. Hydrocarbons and other non-salt families and compounds will be discussed in a later column.
Just as the Periodic Table of the Elements is organized into families of elements, compounds can also be placed into families. A family of materials has particular hazards associated with it. If you can recognize the family to which a material belongs from its name or formula, you should be able to determine the hazard even if you don't know any thing else about the specific chemical.
Salts have particular hazards depending on which salt family they belong to. Salt families can be divided into groups: binary salts and binary or metal oxides.
The first family of salt compounds we will discuss are the binary salts. Binary (meaning two) salts are made up of two elements: a metal and any non-metal except oxygen. Their chemical names end in "ide," such as potassium chloride. When a compound is encountered with a metal that ends in "ide," responders would know that it needs to be looked up to determine actual hazards.
Binary salts are the one type of family in which the family effect of common hazards does not apply, except for a few compounds. Binary salts as a group have varying hazards. They may be water reactive and toxic, and in contact with water may form a corrosive liquid and release heat. (This is also referred to as an exothermic reaction.)
The hazard of an individual binary salt cannot be determined by the family, other than four types of binary salts, so they must be researched in reference materials. This varying hazard applies to all binary salts except for nitrides, carbides, hydrides and phosphides. One way that may be helpful to remember these four binary salts is by using the first letters of the elements' names to form the initials NCHP, which can be remembered as "North Carolina Highway Patrol." These are compounds in which the metal has been bonded with a non-metal nitrogen, carbon, phosphorus or hydrogen.
These compounds are all water reactive and have particular hazards associated with them when they are in contact with water: nitrides give off ammonia, carbides produce acetylene, phosphides give off phosphine gas and hydrides form hydrogen gas. These gases produced in the reaction with water each have their own specific hazards:
- Phosphine is a colorless gas with a disagreeable, garlic-like odor. It is spontaneously flammable, toxic by inhalation and a strong irritant. Phosphine has a threshold limit value (TLV) of 0.3 ppm in air.
- Ammonia is a colorless gas with a intensely sharp irritating odor. While not considered a flammable gas by the U.S. Department of Transpor-tation (DOT), it is flammable under certain conditions, particularly inside buildings and in confined spaces. Furthermore, ammonia is toxic by inhalation with a TLV of 25 ppm.
- Hydrogen is a highly flammable gas which burns with an almost invisible flame and produces little if any smoke. Hydrogen has a wide flammable range from 4% to 75%.
- Acetylene gas is highly flammable; it has a flammable range of 2.5% to 80%.
Photo by Robert Burke
Chlorine is produced from the salt sodium chloride.
In addition to the gases released when these salts contact water, a corrosive base is formed. The corrosive base will be the hydroxide of the metal that is attached to the non-metal. For example, calcium carbide in contact with water will produce acetylene gas and the corrosive liquid calcium hydroxide. Acetylene gas is a common welding and cutting gas. It is so unstable that it is not shipped in tank car or truck quantity. Instead, calcium carbide is shipped to a facility where acetylene is generated and placed in the familiar small welding tanks. Acetylene cannot be pressurized, so it is dissolved into acetone in the tank to stabilize it.
Calcium hydroxide is a strong corrosive base with a pH of 12.4. It is a skin irritant and inhalation hazard. The TLV is 5 mg/m3 of air. When responders encounter carbides and other water reactive salts in a spill, they should be careful not to apply water unless absolutely necessary. If rain is forecast, acetylene gas would be generated when the rain contacted the calcium carbide in a spill. Care would have to be taken to cover the material to keep it dry.
With the remaining binary salts, responders must look them up to determine their hazards. For example, when lithium metal is combined with chlorine, the resulting compound has a metal and a non-metal other than oxygen and the name ends in "ide." Therefore, it fits the definition of a binary salt. If lithium chloride is researched in reference sources, it is found to be soluble in water. It is not water reactive; in fact, lithium chloride doesn't present any significant hazard in a spill. The DOT does not list lithium chloride on its hazardous materials tables.
The next example combines the metal calcium and the non-metal phosphorous, resulting in the compound calcium phosphide. It is a dangerous fire risk. The compound name ends in "ide"; therefore, it also is a binary salt.
Binary salts have varying hazards. Note the following examples:
- Lithium fluoride is a strong irritant to the eyes and skin.
- Potassium bromide is toxic by ingestion and inhalation.
- Sodium chloride is table salt; a medical concern when ingested in excess but certainly of no significant hazard to emergency responders. However, if sodium chloride is washed into a farmer's field during an incident, the farmer may not be able to grow crops in that field for many years!
Binary Or Metal Oxides
The next family of salt compounds is known as the binary or metal oxides. Each is made up of two elements a metal and a non-metal but the non-metal can only be oxygen. They end in "oxide," such as aluminum oxide. As a group, they are water reactive and when in contact with water almost always produce heat and form a corrosive liquid. However, they do not give off oxygen because there is not an excess of oxygen.
Peroxides are salt compounds, each composed of a metal and a non-metal peroxide radical "O2-2." The prefix "per" in front of a compound or element name means that the material is "loaded" with atoms of a particular element. In the case of the peroxides, they are loaded with oxygen.
When a peroxide comes in contact with water, heat is produced, a corrosive liquid is formed and oxygen is released. This makes peroxides particularly dangerous in the presence of fire. Peroxides release oxygen because, unlike the oxide salts, there is an excess of oxygen present. The heat produced in the reaction with water can be enough to ignite nearby combustibles. The excess oxygen present can then accelerate the combustion. When in contact with water, sodium peroxide is a dangerous fire and explosion risk and it is also a strong oxidizing agent.
Hydroxide salts each are made up with a metal and the non-metal hydroxide radical -OH-1. The name ends with the word "hydroxide." They are water reactive and, when in contact with water, release heat and form a corrosive liquid. If calcium metal is combined with the hydroxide radical, the resulting compound is calcium hydroxide, a hydroxide salt.
The oxysalts are another family of salt compounds. They are made of a metal and an "oxy" radical. Their names end in "ate" or "ite" and may have the prefixes "per" or "hypo." Generally, they do not react with water; they dissolve in water. Some of the "hypo-ites" technically do react with water to release chlorine but the reactions are mild. Oxysalts are oxidizers as a family which means they will release oxygen that will accelerate combustion if fire is present.
Another hazard occurs when an oxysalt dissolves in water and the water soaks into another material, such as packaging or firefighter turnouts. The water will evaporate, leaving the oxysalt. If the material is then exposed to heat or fire, it will burn very rapidly because of the oxysalt in the material accelerating the combustion.
Nine oxysalt radicals will be presented with this group. These are not the only "oxy" radicals, but the ones chosen are considered most important to emergency response personnel. The "oxy" radicals are: FO3 fluorate, ClO3 chlorate, BrO3 bromate, IO3 iodate, NO3 nitrate, MnO3 manganate, CO3 carbonate, SO4 sulfate and PO4 phosphate. All of the radicals listed are considered to be in their base state. (The base state has to do with the number of oxygen atoms that are present in the radical. The base state is the "normal" number oxygen atoms present in that oxy radical.)
When a metal is added to any oxy radical in the base state, the compound ends in "ate," such as sodium phosphate. For example, when the metal potassium is combined with the oxy radical carbonate, the resulting compound is potassium carbonate. The compound does not have a prefix on the oxy radical and it ends in "ate"; therefore, it is the base state of the compound.
Oxy radicals may be found with varying numbers of oxygen atoms. There may be more oxygen atoms than the base state or fewer atoms than the base state. When naming the compounds with an additional oxygen atom, the prefix "per" is used to indicate excess oxygen over the base state but the ending is still "ate." An example would be sodium persulfate. When potassium metal is combined with the oxy radical perchlorate, the resulting compound is potassium perchlorate. The level of oxygen is one above the base state. Potassium perchlorate is a fire risk in contact with organic materials, a strong oxidizer, and a strong irritant.
When the number of oxygen atoms is one less than the base state of an oxy radical, the ending of the oxy radical name will be "ite." An example would be magnesium sulfite. If the metal sodium is combined with the oxy radical phosphite, there is now one less oxygen than the base state. In addition to being an oxidizer, sodium phosphite is also an antidote in mercuric chloride poisoning.
Photo by Robert Burke
A railroad tank car carrying sodium chlorate solution, an oxysalt. If this solution soaks a firefighter's turnout gear, the water would evaporate and leave the oxysalts on the gear. If the material is then exposed to heat or fire, it will burn very rapidly because the oxysalt will accelerate combustion.
Finally, an oxy radical can have two less oxygen atoms then the base state. The oxy radical name will now have a prefix "hypo" and will end in "ite." An example would be aluminum hypophosphite. Or, if calcium metal is combined with the oxy radical hypochlorite, the resulting compound is calcium hypochlorite, a common swimming pool chlorinator. Calcium hypochlorite is an oxidizer and a fire risk when in contact with organic materials.
Cyanide salts each are made up of a metal and the cyanide radical "CN." The name of the resulting compound ends in the word "cyanide." An example would be potassium cyanide. Cyanide salts are toxic materials that dissolve in water to form a hydrogen cyanide solution. Hydroxide ions are produced which will make the solution a base, which is corrosive.
Cyanide salts react with acids to produce hydrogen cyanide gas which has an almond-like odor. It is hydrogen cyanide gas that is used in gas chambers. Hydrogen cyanide is also known as a blood agent that could potentially be used in a chemical terrorist attack. The cyanides are deadly poisons and can be found as a salt or in solution and may produce a toxic gas when heated.
When the cyanide ion enters the body, it forms a complex ion with the copper ion located in the cells of the body. These ions of copper are essential for the enzyme that is vital in allowing the cell to use oxygen from the blood. This enzyme is deactivated by the binding of the cyanide ion and copper ions.
Chemistry topics to be discussed in the next article will include non-salt compounds, hydrocarbon and hydrocarbon derivative families, and hazards.
Robert Burke, a Firehouse® contributing editor, is a fire protection/hazardous materials specialist with the University of Maryland and has served on state and county hazmat response teams. Burke has 16 years' experience in career and volunteer fire departments. He holds a bachelor's degree in fire science and is an adjunct instructor at the National Fire Academy and Maryland Fire and Rescue Institute. Burke's book, Hazardous Materials Chemistry For Emergency Responders, was published in January 1997. He can be reached on the Internet at firstname.lastname@example.org.