February 2009
An experiment was being performed which required the use of Lithium Aluminium Hydride, a Class 4.3 dangerous good (Flammable Solid, Dangerous when wet).This hazardous substance has been used previously in the same and similar experiments, the MSDS was consulted and a risk assessment had been completed. An unopened bottle was being used in this instance, however it had been stored for a period of time and the contents had settled as a result, increasing the likelihood of hydration and in turn volatility. While using a clean dry spatula to remove a few granules, an exothermic reaction resulted from the associated friction, the bottle heated and pieces of LAH were ejected as the compound readily releases hydrogen. The ejected LAH caused damage to the fume cupboard and charred the floor. Due to quick thinking by the scientist, the jar was placed back into a desiccator, thereby stopping any further reaction. The area was promptly evacuated, isolated and decontaminated using self contained breathing apparatus (SCBA) and a universal chemical binder.
Following the incident it was noted that the substance before use appeared light grey instead of white; indicating that hydration may have occurred.
For further information please contact your OHS consultant or OHS on extension 51016.
Notes from a senior chemist who has spent a lot of time working and purifying Lithium Aluminium Hydride (LAH)
While reducing a diester compound that also had an acid labile acetal, there was a need to use an alkaline isolation method1 which required the amount of LAH to be accurately known. As a result the procedure described in Vogel2 (5th Ed, p 446) to purify it and to iodometrically titrate was used. It was found that routinely a freshly opened "10 g packet" of LAH sealed in a tin contained well in excess of 10 g, presumably due to decomposition with moisture. For example a "10 g packet" weighed 14.62 g was dissolved in diethyl ether by refluxing under nitrogen, with stirring, for 3 hours. The mixture was then cooled to ambient temperature, filter under nitrogen (Schlenck tube) and the filtrate titrated iodometrically as described in Vogel.2 1 ml of this filtrate was determined to be 1.13 M which equates to 6.43 g of LAH in 150 ml. The residue from filtration is best dealt with by prior dilution with sodium dried diethyl ether. This can then be followed by cautious addition of moist diethyl ether or dropwise addition of ethyl acetate, the partially decomposed ether insoluble LAH is probably more reactive than pure LAH due to its partial oxygenation.3
Pure Lithium Aluminium Hydride (LAH) is white and LAH has been scraped with a spatula under nitrogen without incident. There are also reports from postdocs who have worked in the USA that pure LAH is white, when they worked closely to the site of its production. LAH reacts with any moisture to produce hydrogen which seems to be the source of explosions, described on the web in other incident reports.
A possible method to the safer use of solid LAH is to work under nitrogen in a dry box when weighing it. Alternatively it can be purified as described in Vogel and titrated2 or purified as described in Amarego and Perrin (Extracted with diethyl ether, and, after filtering, the solvent removed under vacuum. The residue dried at 60° for 3 hours, under high vacuum). Care should be exercised to exclude exposure to atmospheric moisture at all times. Another alternative is to purchase solution of LAH in ether but the possibility of a build of pressure in a sealed bottle must be cautioned.
Finally, consulting Vogel should be recommended in addition to the use of MSDS sheets and that impure materials are often more hazardous than pure ones.