An Explosive Nitric Acid-Ethanol Mixture

A postgraduate student in a local university prepared 450 mL of an electro-polishing solution by mixing concentrated nitric acid and ethanol in a 1:2 ratio as indicated in a published research paper. The student observed "no significant temperature rise" over 3 hours, he then capped the Winchester bottle and went home. About 4 hours later, the bottle exploded, scattering glass fragments to a distance of 12 meters and with enough force to break another Winchester bottle containing oil 1.75 meters away, and a plastic shield 4 meters away. Fortunately there was no one in the laboratory at the time, otherwise the powerful explosion could have caused serious injuries.

According to Bretherick's Handbook of Reactive Chemical Hazards, such mixture is "best described as an unstable... rocket fuel mixture"! In a correspondence between the safety personnel of the tertiary institute . and the author of this well-known reference, Mr Bretherick further commented that:

• It is important for any description of experimental conditions to specify whether the proportions are by weight or volume, the strength of the acid, and the grade of alcohol to be used. Different grades of ethanol can contain up to 5% or more of denaturants, of which some (methanol or isopropanol) could adversely affect the reactivity of ethanol and nitric acid.

• It has been known for 50 years that mixtures of ethanol with above 1 0% by weight of concentrated nitric acid are unstable, and that mixtures above 5% wt concentration should not be stored in closed containers, particularly after use as metal etchants, when the dissolved metal may catalyze the decomposition. It has also become clear during that period that nitric acid is the common reagent most often involved in violent chemical reactions, decompositions or explosions. This is a consequence of the tact that nitric acid, unlike other oxidants, still functions as such when cold and dilute, and oxidation is always accompanied by gas evolution, sometimes very slow but progressive. It is also able to form explosive ethyl nitrate by slow acid catalyzed esteritication.

• While mixtures of alcohols with perchloric acid (such mixtures are also used for electro-polishing) are largely free of these gas generation problems, they have their own potential problems. Perchloric acid is an excellent catalyst for esterification reactions. So its mixtures with alcohols will develop explosive alkyl perchlorates. Ethanol denatured with methanol would give both ethyl and methyl Perchlorate, the latter being less stable. Through long standing of the open mixture significant evaporation might occur and a dangerous concentration of Perchlorate might arise.

• It is important to establish whether etching solutions should be retained after use, where heavy metals (particularly catalyticallyactive transition metals) may have been dissolved. There are two distinct reasons forthis, the first being that such metals mayadversely affect the stability of the contaminated solution by accelerating the decomposition rate. Secondly, explosive fulminate salts of heavy metals could form in and separate from the used etching solution. Disposal of the used solutions, either from nitric acid or perchloric acid-based solutions should be considered an integral part of the whole operation.

These are valuable reminders for all material scientists who may need to use such etchant solutions, and in fact for all other researchers who use nitric acid for various purposes.

(This article first appeared on the June,1996 issue of SafetyWise)