Double salts of Organic Acids

lead acetate-bromate (diaceto-diplumbo-bromate)

Good stuff!

According to the german reference (yes, sir! I read it, and important bits extracted here):


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Lead Bromate (Pb(BrO3)2 ) can be prepared iin several ways, obviously by mixing HBrO3 with lead carbonate, or from concentrated solutions of lead salts with a mixture of KBrO3/HBrO3, or, most importantly here, by reacting Na/KBrO3 with lead acetate in the presence of excess acetic acid.

Lead bromate on its own is a harmless substance, and decomposes at 180 deg C while producing Br2 vapours. Neither shock or friction cause it to explode.
This comes as no big surprise.

However (there always is one), when the authors made leadbromate according to the last method (KBrO3 + HAc/PbAc2), 500 g of the crystallised putative lead bromate (this is what the authors were after) exploded, killed one worker, and injured the eyes of one author severely (that's the chemistry of the good old days...).

This is the preparation:

175 g of KBrO3 are dissolved in 1.5 l hot water, and mixed with a solution of 175 g acetic acid (100%) and 260 g lead acetate hydrate in 2 l water.
The solution remains clear initially.
It is then filtered, cooled down and seeded with a few crystals of lead bromate while rubbing the side of the glass vessel with a glass rod.
Soon after, the solution becomes turbid, and yields a heavy crystalline precipitate, which does not increase anymore after 12 hours in the cold.
The supernatant is decanted or extracted, and discarded (I'd rather add some Na2CO3 to salvage the remaining lead in the form of PbCO3).
The crystalline precipitate is washed with cold water until it is free of acetic acid and Na/K acetate/bromate.
This is dried in a dessicator over H2SO4. Yield is 123 g. (not very good).


The authors note that this 'Weinland''s substance explodes extremely powerfully upon shock, but they also note that the crystallisation of either the double salt (explosive) or lead bromate on its own did not seem entirely predictable, i.e. seemingly tiny alterations in reaction conditions produces either pure lead bromate, or a mixture of the double salt with lead bromate (axt you may want to check whether there are two crystal types).

The authors also note that if one does NOT seed the mixture with lead bromate crystals, a leafy light crystal precipitate occurs upon standing overnight in the cold, which seems physically different to the heavy prisms from the lead bromate alone. This is mainly the double salt, which has this explosive nature.

At last, a mixture of lead bromate with 25% lead acetate (it doesnt say whether it is dehydrated) explodes upon on shock, but presumably with much less force as the degree of mixing is not intramolecular (i.e. similar to KClO3 & sugar or whatever).

Oh, and one more noteworthy point: They state that the explosion of 50mg (caused by shock or heat) is so powerful that a thick-walled container would shatter!
Pretty impressive, huh?

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For improvements of the above, one might consider to use much smaller volumes, so that the solution is much more concentrated - which should precipitate more (greater yield).
I'm not sure though whether double salts with any organic acid could be formed (you mentioned citric acid), as not all necessarily crystallise as a double salt rather than the mixture of the Pb-bromate and Pb-organic acid.

But then, why not try?

Also - such explosive double salts, wouldnt they, in principle, also be possible with much smaller divalent metal ions, such as calcium etc?

[Edited on 4-12-2004 by chemoleo]

Thanks for the translation

The precipitate I recieved had no real structure to its crystals ranging in size from very fine to quite large, I've no microscope but its most likely the impure mixture the ref. refers to. A greater concentration of acetic acid may help since its only going to be the bromate or the double salt since they are relatively insoluble (compared to its acetate).

As for the salts of calcium/sodium etc, isn't sodium acetate + potassium nitrate a "fulminating" powder, exploded when melted together? Ive never tried it, but I wrote it down from somewhere.

Theres also the sodium benzoate / lead nitrate salt. Check PATR for the entry "Oxybenzoic Acid, nitrated lead salt" page O55, formed from Lead nitrate and sodium benzoate. I believe this is formed under a simular mechanism. Even though its mentioned as a "nitro" compound (Pb trinitro-m-oxybenzoate), they may also be confusing itr with the salt of trinitrobenzoic acid, but not so likely, due to only one reference being given.