Monday, November 10, 2014

Minnesota professors respond on the 5 gram azide limit

I e-mailed Professor William Tolman, the chair of the University of Minnesota's chemistry department for further explanation of the 5 gram limit on azide reactions within the department. Here was a portion of his response: 
We estimated how big an explosion would occur if the azide prep was done on various scales, and what mitigation is readily available (blast shields, capabilities of hoods, etc.), and from this we estimated that a prep done on a 5 gram scale (or less) could be handled safely and with minimal risk. That is, an explosion of a 5 gram prep, if properly mitigated by a standard blast shield, would be unlikely to cause harm. Obviously, we’re guessing here, but in a somewhat educated way (we hope).
Also, via Twitter, Minnesota professor Ian Tonks responds:
Basic idea is that 5 g could be contained w engineering protocols in our labs (fume hood, safety sash, blast shield)  
I think the important point is to emphasize that there are limits for all sorts of rxn risks (tox, explosion, fire, etc) that when reached should trigger discussions w/ PI/peers/safety committee wrt how to (or can you?) safely carry them out. 
I don't think a hard cap is necessary in many cases because all labs are differently equipped, but in this case it made sense. 
Caps can engender a sense of complacency if you're operating under the limit, so open discussion on hazards is still critical. 
There's probably nits to pick with this policy, but overall, it seems reasonable. 


  1. Interesting estimate given that hoods, sashes and "blast shields" are not rated for primary explosives at all. Sodium azide is a primary explosive (a high explosive), i.e. can detonate (explodes faster than the speed of sound) with little initiation energy.

    The "blast shields" found in lab equipment catalogs are often sold as "weighted safety shields" and are manufactured to ANSI Z87.1-2010, the standard for eye protection from high velocity impacts. Take a look at Elvex site for an idea what these are.

    These safety shields are treated by manufacturers as large remote glasses and are not meant to contain detonations. Properly used they can provide protection from violent but sub-explosion events that propel glass shards to significantly sub-sonic speeds.

    If a process safety review finds that a primary explosive is involved the lab needs a stack of SOPs to follow: transporting, storage, tracing, PPE, training, limitations on handling of dry solids, weather reporting, use of portable ballistic shielding, process stream (aka "waste") destruction, validated analytical methods, and more. All these require training and practice.

    Portable ballistic shields are indeed used in this kind of work. The sample weight limit of about 2 g is sometimes used since above that amount the shield itself becomes a projectile.

  2. I'm happy they are having the conversation and at least taking a look at it. The hard deck of 5-grams appears arbitrary and I would encourage them to do the thermodynamics to find just how energetic each individual synthesis is, then make limits based on the thermodynamics.

    If the synthetic chemists are having some challenges doing the thermo - enlist a 3rd year undergraduate chemical engineering student. They live for thermo calculations.

  3. The thermodynamic calculations are certainly a good first pass tool to look at the desired reaction. However, the accident involved a secondary, undesired reaction. Calculations of the secondary reactions are much less reliable than for the primary (desired) reactions for three reasons:

    - both the thermodynamics and the kinetics need to be calculated;
    - the calculations must assume specific products which are most often unknown;
    - the phases in which the reactions are calculated are uncertain - solids dissolve, gases evolve, solvents evaporate etc.

    A reasonable stepwise approach to getting a better idea of possible hazards could be:

    - looking at structures of all components - s.m., reagents, solvents, products, by-products. Any bonds N-X and O-X (single or multiple) where X is other than C or H are suspicious. Metal containing compounds need to be treated separately.
    - reviewing basic literature for incidents, incompatibilities, etc. - SDS, Bretherick's 7Ed, Patnaik 3Ed, Sax's 11Ed are a good start.

    If the paper review finds something objectionable than a decision could be made to drop the project or to investigate. A DSC run for questionable materials can inform subsequent decisions.

  4. All great methods and thoughts, SJ. By way of shameless DCHAS promotion, the Division of Chemical Health and Safety offers a reactive chemical hazards recognition and control workshop at each National ACS meeting where we cover both chemistry and process. A screening tool is introduced and used to analyze a number of scenarios from planned reactions to waste management. Details of the course can be found at the Division website,

  5. In my experience, a typical industry chemist attempting to run a novel multigram azide reaction would require a dedicated lab space and sign off from higher ups that a safety audit/review was performed. Anyone proposing that 5 grams of azide "would be unlikely to cause harm" or "could be handled safely and with minimal risk" would be torn to shreds.

  6. I think that many folks are missing the point--the MN profs said that 5 g is the MAX that anyone would be allowed to do, and commented that these imposed limits should stir dialogue even for reactions at lower scales. Does that mean that every azide reaction they're doing is going to start out at 5 g? Most unlikely! In the academic setting, 5 g would be a huge reaction. Most of the time people are running with > 0.1 g for new reactions, and often even less than that.