Dangerous Potassium Hydroxide Monohydrate Crust

Jürgen Evers, Thomas M. Klapötke, and Gilbert Oehlinger
Munich, Germany

Chemical & Engineering News (16 Sept 2002) Vol. 80, No. 37, pp. 2, 4

We have recently been asked by the bavarian high court to investigate the nature of commercial potassium, the handling and manipulation of which caused a fatal accident several years ago. The potassium metal was supplied vacuum packed in polyethylene bags (high-density, 0.2-mm thickness) and not under oil as usual.

The accident (severe explosion) happened when the potassium was put into petroleum ether (Merck: "petroleum benzene," bp = 100–140 ºC, density = 0.73 g per cm3, aromatic hydrocarbons < 0.05%, cat. No. 101770) and was melted at about 90 ºC under an argon atmosphere. Since the potassium metal was covered with a millimeter-thick yellowish-white crust (10 to 70% weight per weight) it had always been assumed that peroxides had been present, which caused an explosive reaction with the organic petroleum benzene. However, our thorough investigations by chemical analysis (titration and inductively coupled plasma), IR and Raman spectroscopy, mass spectrometry, and differential thermal analysis as well as X-ray powder diffraction clearly showed that, surprisingly, the crust consisted of approximately 80 to 90% potassium hydroxide monohydrate (KOHH2O) and approximately 10 to 15% of potassium superoxide (KO2). The amount of potassium peroxide (K2O2) was below detection limit.

Therefore, we conclude that, as has already been reported in the literature, potassium--if stored in an inadequate way--can form a dangerous peroxide crust. However, even if peroxides are absent, potassium metal may be covered with a very hazardous crust of either KO2 or KOHH2O. Whereas KO2 is also a reasonably powerful oxidizing agent and may react with organic solvents (for example, petroleum benzene) the KOHH2O definitely releases water upon melting, which can then certainly react (explosively) with the molten potassium metal.

From this study the following conclusions can be drawn:

1. Potassium metal sealed in polyethylene bags under vacuum is not a safe or recommended method for the storage and transportation of this material.

2. In addition to dangerous peroxide crusts, potassium may also form a hazardous crust of both KO2 and/or KOHH2O that may cause very hazardous reactions at elevated temperatures with or without contact to organic solvents.

3. Don't package potassium in polyethylene bags!

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