The ins and outs of perchloric acid in the laboratory

By Luke Savage, LEED Green Associate, Director of Ventilation Technology

On August 17, 2011 in Articles

Perchloric acid (HClO4) is a clear colorless liquid, useful in the laboratory as a strong oxidant. Desirable in chemical processing, perchloric acid offers the properties of a mineral acid, without introducing ions such as chloride, nitrate, or sulfate.

This corrosive chemical carries hazards typical of most acids; it is harmful if swallowed, causing digestive and respiratory tract burns, and if exposed to the outside of your body can cause eye and skin burns. Additionally, perchloric acid is also explosively unstable under some conditions. Perchloric acid is not explosive in solution, only extremely corrosive and harmful to breathe. This is justification for the use of a chemical fume hood. 

If the acid is evacuated in the same ventilation equipment that captures organics, the salt residue of the perchloric acid is saturated by the organics, and a new molecular structure is formed that is highly unstable. This occurs when the perchloric acid vapor is allowed to condense in the ductwork and then evaporate, leaving behind a salt called perchlorate.

Perchlorate crystals are explosive and can be detonated by heat, flame, friction, percussion or chemical reaction. Though something as small as the vibration of the blower motor can cause a violent reaction, normally, no additional difficulties are encountered until it is time to dismantle the system.

The danger occurs when a mechanical contractor, unaware of the hazards lying within, may attempt to dismantle or service the mechanical system and in the process dislodge crystals resulting in a catastrophic situation.

Fortunately, perchloric acid can be neutralized with water and perchlorate salts will dissolve in water. Perchloric acid applications require special equipment that includes complete washdown systems, specific materials of construction, and dedicated mechanical systems.

There may be exceptions to the above, especially if the acid is dilute, in small amounts, infrequently used, and not heated. Extreme care should be taken to avoid spills in this situation. An example of this type of work is Perchloric transfers. The issue should be addressed with the facility’s Health and Safety Officer, to determine if the specific application falls into this category.

For detailed composition, first aid measures, safe handling of an accidental release, and/or firefighting measures, a great reference is the Material Safety Data Sheet (MSDS). For information on fume hood and mechanical system design, reference a variety of reputable resources, including:

• CRC Handbook of Laboratory Safety, CRC Press, Boca Raton, FL, 1990.
• ACGIH Industrial Ventilation, A Manual of Recommended Practice
• NFPA 45-2011, Section 8.11 Perchloric Acid Hoods
• ANSI/AIHA Z9.5, 2003, 3.2.4 Perchloric Acid Laboratory Chemical Hoods