One of the most fundamental and routine techniques used in a chemistry or pharmaceutical lab is the process of weighing chemicals. Most often, compounds being weighed on a chemical balance are in powder form, and in such cases, it is important to adapt safe and reliable weighing techniques in a protected environment, depending on the properties of the powders, the quantities needed, and the health risks they pose. Not only does this benefit the safety of the researcher and the laboratory space, but it also supports accurate measurement of the powder weight.
Health risks of powders may include sensitization, toxicity, and carcinogenicity.1 Detailed information on the health hazards of chemical powders can be found in the manufacturer’s safety data sheet or resources/guidelines made available by the Occupational Safety & Health Administration (OSHA). Respiratory problems can result from frequent inhalation of small amounts of fine dust (often not visible to the naked eye) that may be generated while handling powders. It’s important to note that one can become sensitized even to non-toxic powders, by exposure over a long period of time.
Apart from this, certain powders can also pose a physical hazard (e.g. flammability), or react with air and/or moisture, affecting the accuracy of measurement and compromising its purity. Fine metal powders of aluminum, iron, magnesium, and titanium are highly pyrophoric (flammable) even in small quantities.2 Some powders (e.g., those of metal azides) are sensitive to friction and can cause an explosion when they come in contact with metal surfaces or other materials.3
If not contained properly, these powders and the dust generated can disperse throughout the work area, leading to an increased risk of respiratory exposure, contamination of other lab processes, and potential fires/explosions. Thus, it is imperative to prevent dispersal of such hazardous powders when they are being weighed in the lab.
To achieve this, researchers can employ the use of a powder containment enclosure that has particulate filtration capabilities. Such enclosures are constructed with durable material for the safe handling of powders, and deliver a well-controlled, horizontal airflow across the work surface to protect the operator and the lab, while also providing balance stability.1
There are three main types of powder containment enclosures that are useful for the chemistry lab:
Type A enclosures
Type A enclosures provide a safe, compact, and reliable station for weighing toxic compounds and/or liquid chemicals. They are engineered to capture particulates through HEPA filtration while reinforcing balance stability. The airflow in Type A enclosures is precisely controlled through the use of a safety controller that automatically maintains the face velocity of air, preset by the operator. Also, the hoods are ductless and portable, allowing easy installation on any countertop.
When in operation, room air enters the enclosure in a laminar flow, with no turbulence. Once inside, air moves evenly through the hood and harmful particulates are removed by the HEPA filters before the air is dispensed back into the room. A schematic diagram of the air flow and filtration, as well as dimensions of Type A enclosures, can be found here.
These enclosures are laboriously leak-tested, quality assured to industry standards (UL 61010-1, CSA C22.2 No. 61010-1, and EN 61326-1), and are normally fully assembled before shipping. They come with a variety of safety features such as strategically located electronics and filters, a filter safety lockout to ensure correct filter replacement, a post-filter blower that is brushless and sparkless, and a deep base to effectively contain accidental spills. The hoods are also fitted with real-time monitoring of filtration beds and alarms to indicate low air flow
Type B enclosures
This is the most suitable enclosure for extremely accurate weighing of powders. Inside the hood, researchers can achieve exact measurement of the weight, up to seven decimal places. A precise air flow can be managed via the automatic safety controller that supports an even laminar flow airstream for accurate weighing.
Particularly, these enclosures come equipped with HEPASafe™ filter change technology that isolates the pre-filter and main HEPA filter during filter change-out, ensuring increased safety during routine maintenance.
In Type B hoods, room air is drawn in an even, horizontal laminar fashion toward the back of the hood while being mixed with contaminated air, which then enters a pre-filter followed by the primary HEPA filter. The air thus cleaned finally enters the permanent safety HEPA filter and is exhausted into the room. A schematic diagram showing the air flow, multistage filtration, and dimensions of the enclosure is available here.
In addition to the HEPASafe technology, Type B hoods contain all the safety features found in Type A above.
Type C enclosures
Type C enclosures are designed as a “universal” solution to a wide range of powder and liquid weighing applications—combining the capabilities of both Type A and Type B, while also incorporating additional user-friendly features. Of note, these hoods are constructed with thermally fused polypropylene for high strength and chemical resistance. As such, they are compatible with hazardous powder and solvent manipulation, can bear heavy weighing equipment, and remain rust-free. Moreover, the entire rear wall of the enclosure is a filtration zone, allowing a clean sweep of all particulates and gases. Such design features make Type C the best option for weighing the lightest powders or the heaviest solvents.
These enclosures operate similarly to Type B—laminar flow room air mixes with contaminated air, contaminants are removed via electrostatic pre-filtration and HEPA filtration, and then particulate-free air moves upward into the HEPASafe filtration area. At this point, however, in Type C hoods, the air also passes into a bonded carbon filtration bed to remove chemical fumes and vapors. Finally, clean air is recirculated into the room. A schematic diagram of the air flow and filtration in Type C, as well as dimensions of the hoods, is shown here.
Along with the automatic safety controller, HEPASafe filter technology, and all other safety features found in Types A and B, Type C enclosures are also fitted with electronic gas detection capabilities for real-time gas phase filter monitoring.
More information on all three types of powder containment enclosures is available at https://www.aircleansystems.com
References
- Environment, Health & Safety Dept., University of Wisconsin-Madison, Weighing Hazardous Powders in the Laboratory.
- Angelo, P.C.; Subramanian, R. Powders of aluminium, iron and magnesium are highly pyrophoric in nature. Powder Metallurgy: Science, Technology and Applications, PHI Learning Pvt. Ltd., 2008, 48.
- Archibald, T. Strategies for safely handling industrial azide reactions: the three traps. Managing Hazardous Reactions and Compounds in Process Chemistry, ACS Symposium Series, 2014, 1181, 87–109.
Additional sources
- https://www.aircleansystems.com/lab-safety/powder-containment
- https://www.osha.gov/dts/osta/otm/otm_v/otm_v_3.html