Dangers caused by alkali metal stocks

Beware of old stocks

We reported on a series of different explosion incidents with one fatality and numerous injured caused when handling potassium in Sichere Chemiearbeit 2 (1998), p. 17, and also provided a detailed report in Nachrichten aus Chemie, Technik und Laboratorium 46 (1998), p. 16.

The analysis of the yellowy-reddish deposit of oxidised potassium fragments has since revealed that these consist of 10 - 15% potassium superoxide. This substance is an extremely strong oxidising agent which, together with the oil used to store potassium, can detonate when subjected to mechanical pressure. 80 - 90% of the deposit is made up of a potassium-hydroxide monohydrate, which loses its water when heated to 110 - 140°C and thereby causes further danger (J. Evers, T. Klapöthke, G. Oehlinger, Nachrichten aus der Chemie 50 (2002), p. 1126).

The released water also reacts explosively with the potassium and forms hydrogen. This is why potassium fragments with crusts, regardless of their type, must not be heated. As all alkali metal hydroxides can form various hydrates MOH • n H2O, one must always assume that a reaction will take place as a result of dehydration.

An explosion recently occurred when attempting to remove a sample from a 16-year-old bottle with 1.2 kg of 33% sodium dispersion in xylene using a metal spatula. A chemist was seriously injured in the process (Chemie in Labor und Biotechnik 53 issue 7 (2002) M56). Here, too, it must be assumed that sodium peroxide, which has a powerful oxidising effect, had been formed.

A corresponding level of caution should be exercised when handling all old stocks and waste of alkali metals. The most important measure is preventing a dangerous oxidation, in particular by preventing excessive ageing. Storage in sealing liquids such as paraffin oil does not fully prevent the influx of oxygen and water, and thus leads to slow oxidation. An additional inertisation of the bottles with argon or filling with sealing liquids up to the brim increases safety by further delaying oxidation, but the metals can only be kept without oxidation in glass ampoules in argon or in a tight vacuum. However, the glass ampoules must be secured against unintentional breakage, for example by placing them in metal tanks filled with expanded mica.

If the metals catch fire, suitable fire extinguishers must be used to put out the fire. Attempts to put out such a fire with water or carbon dioxide can have catastrophic results. Metal fire powder (fire class D) is recommended, or alternatively dry sodium carbonate for smothering small fires. Other chemicals can also undergo dangerous changes over the course of time, for example alkali metal amides or ethers. The guidelines for laboratories (Working Safely in Laboratories) and the Technical Rules for Hazardous Substances (TRGS) 526 therefore stipulate the annual inspection of all stocked chemicals and compounds as well as their waste under 4.9.4 and 4.16.2.

See also: Sichere Chemiearbeit 4-2003, 46

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