Perchloric Hoods

Perchloric acid (68-72 percent) acts as a strong acid at room temperature. Perchloric acid is an exceedingly strong and active oxidizing agent as well as a strong dehydrating agent at elevated temperatures (approximately 160°C, or 320°F). Anhydrous perchloric acid (highly unstable) is usually formed when perchloric acid is mixed with concentrated sulphuric acid or phosphorus pentoxide.

Dry perchloric acid crystals are unstable, and can explode when subject to shock or vibration. (An analogous compound to perchlorates is nitroglycerine.) The problem of exhausting perchloric acid vapors is that the vapors condense and then crystallize in the exhaust system. The crystallized or dried perchloric acid is very unstable, so much so that vibrations or shock waves to the duct and exhaust fans system can result in an explosion. This vibration, caused by fan operation or general maintenance may cause the crystals within the system to dislodge and thus also causing the explosion. However, perchloric acid can be neutralized with water and flushed from the exhaust system. In addition, perchloric acid is extremely corrosive, and it is recommended that exhaust system components (ductwork, fans, etc.) be fabricated with not less than 316 SS or fiberglass.

The exhaust system should be smooth with no crevices, corners or edges, so it can collect perchloric crystals with less difficulty and flush the lodged crystals with water. As it is understood, the greatest collection device of particulate (or in this case, perchlorate particles) would be the exhaust fan. Mentioned below are the recommendations for perchloric acid exhaust systems:

  • Perchloric acid fume hoods are single use hoods and should only be used for perchloric acid digestions.
  • Each perchloric acid fume hood shall have its own exhaust system and should not be manifolded together.
  • These fume hoods use 316 stainless steel, unplasticized PVC or fiberglass duct and exhaust system components for corrosion resistance.
  • Induction exhaust system where the blower is out of the exhaust air stream should be preferred; or centrifugal blower with MOC compatible for perchloric application has to be used. This is to prevent perchloric vapors passing through the blower.
  • Preferably, install only vertical duct runs without elbows (install perchloric fume hoods as close as possible to the exhaust system).
  • Install auxiliary wash rings in vertical ductwork at every 10’-12’.
  • If horizontal duct runs must be used, install spray rings every 4’-5’ feet, with spray rings installed, up and down stream, for each elbow.

Procedures for using perchloric acid fume hoods:

  • Ensure that the fume hood’s exhaust fan is ON. The black turn-switch should be located on the left of the sash and has to be ON. Ensure that the green ‘Normal’ light is illuminated on the air flow monitor unit. The face velocity should be between 100 -120 FPM; if the velocity is below this specified safety limit, DO NOT USE THE FUMEHOOD. Call your facility or safety team for help.
  • Conduct an experiment
  • Remove all materials and equipments from the fume hood.
  • ENSURE THE SASH IS PULLED ALL THE WAY DOWN before the wash-down procedure is started. Press and hold the ‘red’ activation button on the right hand control panel. The orange light will indicate that the system is ON. The wash-down procedure will be activated and will shut down at the end of the cycle. Ensure that the water is draining away at the back of the hood.
  • The fume hood is NOT a storage area. If you use the fume hood for more than one day, remove everything from the fume hood and wash the fume hood.
  • Follow other general fume hood procedures and perchloric acid waste disposal procedures.
  • Please contact your HSE or facilities team if you have any questions or concerns.


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