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Introduction
Perchloric
acid (HClO4) is a water-white liquid (water soluble), it has
no odor, the boiling point at atmospheric pressure is 203C, and under
high vacuum, a 73.6% composition can be produced. The acid can be
dangerously reactive and is very corrosive. At ordinary temperatures,
72% Perchloric acid reacts as a strong non-oxidizing acid. At elevated
temperatures approximately 160C, it
is an exceedingly strong and active oxidizing agent as well as a strong
dehydrating agent.
Cold
Perchloric acid, 70% or weaker, is not considered to have significant
oxidizing power. Acid of 73%+ (which gives off fumes in even relatively
dry air) is a fairly good oxidizer at room temperature. The monohydrate
of Perchloric acid (85% acid strength and a solid) is indeed a very good
oxidizer at room temperature, as it will even react with gum rubber,
whereas the 73% acid does not. Contact with combustible material at
elevated temperatures may cause fire or explosion. It should be known
that Perchloric Acid, when in crystalline form, may be unstable,
possibly leading to combustion of the accumulation.
Safety
Considerations
Personal safety should be highly regarded, especially
when dealing with chemicals. It is recommended that one wears positive
pressure self-contained breathing apparatus (SCBA), when entering an
area where Perchloric Acid is stored or handled. Perchloric acid
use has long been associated with some violent explosions. From a
safety viewpoint, one main feature of the accidents has been the
severity of the accident and the fact that the persons involved are
often experienced workers. The basic cause of accidents involving
Perchloric acid is due to contact with organic material, or a reducing
material. For any use of the acid, a review of the MSDS from each
manufacturer is strongly recommended.
When
considering the hazards involved in the use of Perchloric acid it should
be clearly recognized that many of the reported serious laboratory
accidents involved only small quantities (<1g) of reactant. These
accidents occurred, even though experienced and qualified personnel were
handling the reactant. The CRC Handbook of Laboratory Safety cites the
Perchloric acid-Acetic Anhydride-Acetic Acid system, which, if used
improperly, can lead to explosions.
The example
shows that 1 g of the mixture instantaneously produces about 7 liters of
gas at the explosion temperature of 2400C. There are many other
illustrations and examples, but several things should be very clear to
the reader. Specifically, no one should attempt to use Perchloric acid
who is not fully conversant with the chemistry of the material, who has
not made a careful appraisal of the operating conditions and techniques,
and who exhibits any unsafe attitude about their work. Each user of the
material should also recognize that any acid digestion type usage must
occur in a designated Perchloric acid fume cupboard. Finally, each user
should recognize that dangerous secondary reactions can occur with
reactive Perchlorate compounds formed from the acid liquid or vapors in
fume cupboards, storage cabinets, and laboratory benches or floor areas.
Potential Health Effects
Inhalation: Inhalation of concentrated
vapors may cause serious damage to the lining of the nose, throat, and
lungs. Breathing difficulties may occur. Neither odor nor degree of
irritation are adequate to indicate vapor concentration.
Ingestion:
Swallowing can cause severe injury leading
to death. Symptoms include sore throat, vomiting, and diarrhea.
Ingestion of as little as 1.0 ml has resulted in perforation of the
esophagus.
Skin
Contact: Contact with concentrated
solution may cause serious damage to the skin. Effects may include
redness, pain, skin burns. High vapor concentrations may cause skin
sensitization.
Eye
Contact: Eye contact with concentrated
solutions may cause severe eye damage followed by loss of sight.
Exposure to vapor may cause intense watering and irritation to eyes.
Chronic
Exposure: Repeated or prolonged exposures
may cause darkening of the skin, erosion of exposed front teeth, and
chronic inflammation of the nose, throat, and bronchial tubes.
Aggravation of Pre-existing Conditions:
Persons with pre-existing skin disorders or eye problems, or impaired
respiratory function may be more susceptible to the effects of the
substance.
First
Aid Measures
Inhalation: Remove to fresh air. If not
breathing, give artificial respiration. If breathing is difficult, give
oxygen. Call a physician.
Ingestion:
DO NOT INDUCE VOMITING! Give large
quantities of water or milk if available. Never give anything by mouth
to an unconscious person. Get medical attention immediately.
Skin
Contact: In case of contact, immediately
flush skin with plenty of water for at least 15 minutes while removing
contaminated clothing and shoes. Wash clothing before reuse. Call a
physician.
Eye
Contact: Immediately flush eyes with
plenty of water for at least 15 minutes, lifting lower and upper eyelids
occasionally. Get medical attention immediately.
General
Considerations for equipment:
For
conventional wooden wall construction, which is [not] desirable, it is
highly recommended that a 6-inch concrete curb be provided for the walls
to rest on. In this way, acid seepage under the wall is minimized.
Concrete of course, is not resistant to acids, and thus should be
covered. Epoxy paints in general are resistant to room temperature
Perchloric acid spills; however, epoxy paint will peel off concrete if
pools of water stand for several days. Therefore, the floor should have
a gentle slope to a drain and contain no low spots.
No
equipment of any kind should ever be bolted to a floor by using bolts
that screw into the floor. Perchlorates can enter and form hazardous
metallic Perchlorates that can be initiate a detonation when the bolt is
removed. Studs, firmly and permanently set into the floor to which the
equipment can be bolted, are far safer. The nuts can then be flushed
with water and sawed off with a hacksaw under a constant water spray or
remove that equipment.
Laboratory benches:
Laboratory benches should be constructed of resistant materials and not
wood, to prevent acid absorption, especially at the bottom surface which
rests on the floor and would be subject to the greatest exposure from
acid spills. Bench tops of resistant and non-absorbent materials such as
chemical stoneware, tile, epoxy composites, Viton, tantalum,
chemically pure titanium, zirconium, niobium, Hastelloy C (slight
corrosion rate), PVC, Teflon, polyethylene, polypropylene, Kel-F,
vinylidene fluoride, Saran, epoxy resins, glass, glass-lined steel,
alumina, and Fluorolube are recommended.
Shelves and cabinets: Shelves and cabinets of epoxy-painted steel
are highly recommended over wood.
Heating source: Hot plates (electric), electrically or
steam-heated sand baths, or a steam bath are recommended for heating
Perchloric acid. Direct flame heating or oil baths should not be used.
Vacuum source: It is highly
recommended that an exhaust system, with no moving parts, be used.
The exhaust fan motor shall not be installed in the ductwork and
the pulley shall be conductive as specified by NFPA 45.
An induced draft system, located outside the laboratory, could be used.
Equip the ventilation system with a built-in water wash-down
system. Ensure that the system will adequately spray all interior
surfaces of the ducting, tack, fan, plenum, baffles, and hood. The hood
shall be washed down at least each day after use or more frequently if
specified in a safety plan. The water shall be drained to an
appropriate holding tank pending removal for treatment and disposal.
Glassware: The hazards that may ensue if an apparatus cracks or
breaks due to thermal or mechanical shock are sufficient to make it
desirable that quartz apparatus be considered, especially as it is
necessary in many experiments to chill rapidly from the boiling point.
Glass-to-glass unions, lubricated with 72% Perchloric acid, seal well
and prevent joint freezing arising from the use of silicon lubricants.
Rubber stoppers, tubes, or stopcocks should not be used with Perchloric
acid due to incompatibility.
Stirrers:
Pneumatically driven stirrers are recommended rather than the electric
motor type. Repeated exposure of the motor windings to Perchloric acid
vapor could result in a fire, unless the motor is an explosion-proof
type, which would be unlikely.
Sundry items:
The
choice of tongs for handling hot flasks and beakers containing
Perchloric acid mixtures should be given due thought. Since the use of
radioactive materials has become commonplace, much thought has been put
into the design of indirect handling equipment. The cheap, commonly used
crucible tongs are most unsuitable for picking up laboratory glassware.
If possible, tongs with a modified jaw design should be used to ensure
that a safe grip is obtained.
Spills: Perchloric acid spilled on the floor or bench top represents
a hazard. It should not be mopped up, nor should dry combustibles be
used to soak up the acid. The spilled acid should first be neutralized
and then soaked up with rags or paper towel. The contaminated rags and
paper towel must be kept wet to prevent combustion upon drying. They
should be placed in a plastic bag and sealed and then placed in a
flammable waste disposal can. If the spill can be rinsed down a
chemical drain, neutralization of the wetted area is recommended
followed by additional rinsing.
Other recommendations include wearing a
face shield and gloves while working on the spill. Cover the spill with
a weak solution of sodium thiosulfate, and then transfer the slurry into
a large container of water, where it should be neutralized with soda
ash. After neutralization, it can be drained into the sewer,
accompanied by abundant water.
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