Oxidizing Materials
Oxidizers are compounds which are capable of reacting with
and oxidizing (i.e., giving off oxygen) other materials. A well known
example of oxidation is the process we know as corrosion, where the metal
reacts with air to form the metal oxides referred to as "rust".
The primary hazard associated with this class of compounds
lies in their ability to act as an oxygen source, and thus to readily
stimulate the combustion of organic materials. As you may recall, it takes
three components for a fire to happen - a fuel source (usually an organic
compound, such as paper), an ignition source (such as a flame, a spark,
friction, etc.) and an oxygen source (in this case - a cylinder of OXYGEN,
a strong oxidizer).
Identifying Oxidizing Materials
-
1. Chemical structures which should serve as a warning sign
Because of their chemical structure, these materials have excess oxygen which may be liberated, especially at higher temperatures. This capacity to provide excess oxygen at elevated temperatures makes these chemicals a fire and explosion hazard when they come in contact with all forms of combustibles (wood, paper, textiles, plastics, etc.). In addition, mixtures of oxidizers and combustibles can be ignited by a heat energy originating from a weak ignition source such as friction, physical impact or static electricity. Particular caution is required when working with or storing strong oxidizers (see below classifications) because these compounds are capable of reacting with combustibles at room temperature. The result of any of the above contacts can be fire and/or explosion. Typical oxidizers include chemicals with the oxygen-containing groups listed in the following table.
TABLE 1 - Common Oxidizing Groups Chemical Group Chemical Formula peroxides O2-2 nitrates NO3- nitrites NO2- perchlorates ClO4- chlorates ClO3- chlorites ClO2- hypochlorites ClO- dichromates Cr2O7-2 permanganates MnO4- persulfates S2O8-2
2. Using chemical labels as an aid
As an aid in identifying the chemicals which pose a hazard in the laboratory, all chemical manufacturers are required to include relevant information on the chemical label. One of the most common grading systems is that developed by the National Fire Protection Association (NFPA). In this system, chemicals are rated from 0 (no hazard) to 4 (extreme hazard). It is important for all laboratory personnel to recognize and become familiar with the NFPA diamond and understand the grading levels established by the NFPA for hazardous materials.
The white portion of the diamond gives an indication of hazardous properties of the material other than those associated with toxicity (blue portion), flammability (red portion) and reactivity (yellow portion). Commonly displayed warnings in this portion of the diamond include an alert as to corrosivity, radioactivity, water reactivity and strong oxidizing strength.
Another warning system that indicates to the user that a material is an oxidizer is that devised by the Department of Transportation (DOT). The "burning O" is the standard symbol required to be used on all packages offered for transport over public highways, airways or by sea, which carry materials classified by the DOT as strong oxidizers.
3. Oxidizing agents other than oxygen
The table below (Table 3) shows examples of materials that all have one structural feature in common -- they all contain oxygen. This, however, is not a requirement for a material to be classified as an oxidant. By definition, all that is necessary is for the chemical substance to be capable of accepting electrons. In the case of oxygen, this involves existing as O-2, which is the preferred state for oxygen containing compounds. This requirement is met by other chemicals in the class of compounds known as the halogens, and by materials that can act as halogenating agents. Examples of this includes fluorine and chlorine in the gaseous state, and bromine in the liquid state. Therefore, it is important to remember the oxidizing properties of these materials whenever work is being performed with them.
Examples of Oxidizing Materials
Oxidizers may be grouped into 4 classes based on their ability to affect
the burning rate of combustible materials or undergo self-sustained decomposition.
This classification system was established by the National Fire Protection
Association (NFPA 43A, 1980) as a means to provide information on safe
storage of oxidizing materials. These materials are classified according
to the following guidelines shown in Table 2 below. Also shown (Table
3) are typical examples of each class of oxidizing material.
| TABLE 2 - Classification System for Oxidizing
Materials |
|
|---|---|
| Class Rating | Hazard Description |
| Class 1 | An oxidizing material whose primary hazard is that it may increase
the burning rate of combustible material with which it comes in contact. |
| Class 2 | An oxidizing material that will moderately increase the burning
rate or which may cause spontaneous ignition of combustible material with which it comes in contact. |
| Class 3 | An oxidizing material that will cause a severe increase in the burning rate of combustible material with which it comes in contact or which will undergo vigorous self-sustained decomposition when catalyzed or exposed to heat. |
| Class 4 | An oxidizing material that can undergo an explosive reaction
when catalyzed or exposed to heat, shock or friction. |
-
TABLE 3 - Oxidizing Materials
(as Classified by the NFPA)
Class Rating Examples Class 1 aluminum nitrate
potassium dichromate
ammonium persulfate
potassium nitrate
barium chlorate
potassium persulfate
barium nitrate
silver nitrate
barium peroxide
sodium carbonate peroxide
calcium chlorate
sodium dichloro-s-triazinetrione
calcium nitrate
sodium dichromate
calcium peroxide
sodium nitrate
cupric nitrate
sodium nitrite
hydrogen peroxide (8-27.5%)
sodium perborate
lead nitrate
sodium perborate tetrahydrate
lithium hypochlorite
sodium perchlorate monohydrate
lithium peroxide
sodium persulfate
magnesium nitrate
strontium chlorate
magnesium perchlorate
strontium nitrate
magnesium peroxide
strontium peroxide
nickel nitrate
zinc chlorate
nitric acid (<70% conc.)
zinc peroxide
perchloric acid (<60% concen.)
Class 2 calcium hypochlorite (<50% wgt)
potassium permanganate
chromium trioxide (chromic acid)
sodium chlorite (<40% wgt.)
halane
sodium peroxide
hydrogen peroxide (27.5-52% conc.)
sodium permanganate
nitric acid (>70% conc.)
trichloro-s-triazinetrione
Class 3 ammonium dichromate
potassium chlorate
hydrogen peroxide (52-91% conc.)
potassium dichloroisocyanurate
calcium hypochlorite (>50% wgt.)
sodium chlorate
perchloric acid (60-72.5% conc.)
sodium chlorite (>40% wgt.)
potassium bromate
sodium dichloro-s-triazinetrione
Class 4 ammonium perchlorate
ammonium permanganate
guanidine nitrate
hydrogen peroxide (>91% conc.)
perchloric acid (>72.5%)
potassium superoxide
Proper Storage and Handling of Oxidizing Materials
The primary consideration in the storage of these materials is that they
must be isolated from all flammable or combustible material. No other
special requirements are necessary for storing small quantities (less
than 200 pounds) of Class 1, 2 or 3 oxidizers. Stringent storage requirements
are levied against Class 4 oxidizers in quantities greater in 10 pounds
(combined, all Class 4 materials). These materials may not be stored,
for quantities above 10 pounds, in the laboratory or within the building.
Use of Perchloric Acid
Special consideration will be given to the storage and/or use of perchloric acid. The dangers of working with this chemical are well documented and the literature has many examples of situations where fires, explosions, injuries and even death have occurred because of careless or uneducated use of perchloric acid in the laboratory. Perhaps the most disturbing features regarding accidents involving perchloric acid are:
- the severity of the incidents and
- that the persons involved are generally experienced in the use of the acid.
Because of this, it is important that no one should attempt to use perchloric acid who is not fully informed of its chemistry and until safe working conditions are assured.
Perchloric acid is a powerful oxidizing agent that may react explosively with any organic compound or reducing agent. If you consider how many materials that encompasses, you quickly realize the magnitude of the hazard that this acid presents. It reacts with alcohols and certain organics to form very unstable (to shock or heat) perchlorate esters, which have the same shattering power as nitroglycerin.
Anhydrous perchloric acid is exceedingly corrosive to the skin and mucous membranes and explosively unstable. It may explode spontaneously. It should never be prepared except for special needs and not stored unless at very low temperature. The dangerously explosive anhydrous acid may also be formed when a drying agent is inadvertently added to diluted concentrations of perchloric acid. This may happen if a material such as hot, concentrated sulfuric acid or phosphorous pentoxide is added.
Aqueous perchloric acid at concentrations of less than 85% is stable under ordinary storage situations. The concentration that is normally supplied commercially is 60-72%. Though stable under ordinary circumstances, it must only be stored on ceramic or glass trays never on wood shelves or in wood cabinets. Grave danger exists where perchloric acid evaporates and condenses on the walls and shelves of the storage area. It must not be stored in contact with other chemicals, especially organic solvents. It should not be stored with other acids, since it is capable of oxidizing and reacting with almost all other chemicals including other acids.
Explosive reactions may occur when working with hot and concentrated (60 - 72%) perchloric acid. It is particularly critical to keep organics and combustibles away from hot perchloric since these reactions are particularly explosive.
Organic matter (paper, wood, grease, plastic or cloth) that has come in contact with perchloric acid fumes must be regarded as a fire and explosion hazard until it has been thoroughly washed down and tested for flammability.
If using perchloric acid in the decomposition of organic material (a common use), first treat the material with nitric acid or a mixture of perchloric and nitric acid. This allows the more reactive materials to be oxidized at a low temperature by the nitric acid, thus decreasing the hazard.
Perchloric acid must not be used in fume hoods where other materials (organics, flammables) are used because of its capacity to form explosive perchlorates on the hood walls. This necessitates the use of a specially designed perchloric acid hood constructed throughout of stainless steel and equipped with water wash-down capabilities. The perchloric acid hood is designed specifically for use with materials that can deposit shock-sensitive crystalline materials in the hood and duct system. These materials become pyrophoric or explosive when they dry out. The mixture of a perchlorate and any organic material may result in explosion that may be touched off by friction such as in simply adjusting the panels and baffles in the hood. The water spray should be used whenever perchloric acid is heated in the hood. The exhaust from perchloric acid hoods should not be manifolded with any other type of hood.
If a laboratory hood has been exposed to heated perchloric acid, tests must be conducted to determine if explosive perchlorates have formed on the hood walls and duct system. This must be completed before any inspection, cleaning, maintenance, or any other work is done on any part of the hood interior or exhaust system.
Some accidents involving perchloric acid:
- A violent explosion took place in an exhaust duct from a laboratory hood in which perchloric acid solution was being fumed over a gas plate. It blew out the windows, bulged the exterior walls, lifted the roof, and extensively damaged equipment and supplies. Some time prior to the explosion, the hood had been used for the analysis of miscellaneous materials. The explosion apparently originated in deposits of perchloric acid and organic material in the hood and duct.
- A chemist was drying alcohol off a small anode over a Bunsen burner in a hood reserved for tests involving perchloric acid. An explosion tore the exhaust duct from the hood, bent a portion of the ductwork near the fan, and blew out many window panes.
- A 7-lb bottle of perchloric acid solution broke while an employee was unpacking a case containing three bottles. The spilled acid instantly set the wood floor on fire.
- A stone table of a fume hood was patched with a glycerin cement and several years later, when the hood was being removed, the table exploded when a worker struck the stone with a chisel. The hood had been used for digestions with perchloric acid and, presumably, acid spills had not been properly cleaned up.
- During routine maintenance involving partial dismantling of the exhaust blower on a perchloric acid ventilating system, a detonation followed a light blow with a hammer on a chisel held against the fan at or near the seal between the rear cover plate and the fan casing. The intensity of the explosion was such that it was heard 4 miles away. Of the 3 employees in the vicinity, one sustained face lacerations and slight eye injury; the second suffered loss of 4 fingers and possible loss of sight in one eye; the third was fatally injured when the 6 inch chisel entered below his left nostril and embedded in his brain.
- A 6-lb bottle of perchloric acid broke and ran over a fairly large area of wooden floor. It was cleaned up but some ran down over the joists. Several years later a bottle of sulfuric acid was spilled in the same location and a fire broke out immediately in the floor and joists.
Based on these special needs, which are not provided for in most laboratories, this chemical must not be used or stored in any laboratory that has not been especially equipped to address the associated hazards.
Remember These Things About Oxidizing Agents
- the primary hazard is the ability to act as an oxygen source, especially hazardous during fire situation.
- these materials present a fire and explosion hazard when in contact with organic or combustible materials. All contact with organic or combustible material must be avoided.
- they are generally corrosive.
- the hazards associated with use of perchloric acid
are particularly severe.
PERCHLORIC ACID MAY NOT BE USED IN ANY HOOD EXCEPT THOSE SPECIALLY DESIGNED FOR PERCHLORIC ACID USE
- strong oxidizing agents, such as chromic acid, should be stored and used in glass or other inert, and preferably unbreakable, containers.
- corks or rubber stoppers must never be used.
- reaction vessels containing appreciable amounts of oxidizing materials should never be heated in oil baths, but rather on a heating mantle or sand baths.
Created and maintained by Nancy Magnussen
last revised 2 Aug 1997
nancy@isc.tamu.edu
Copyright © 1996 by College of Science, Texas A&M University