The farmer had purchased some potassium permanganate. Exactly why he needed it isn鈥檛 clear but using it as a disinfectant to clean the udders and teats of cows before and after milking is a possibility. The compound can also be used to remove iron and hydrogen sulphide from well water. Potassium permanganate is a purple crystalline material that produces a very pretty purple color when dissolved in water. But when dissolved in some other liquids, the story can be quite different, as our unfortunate farmer discovered.
For storage, he poured the permanganate he had purchased into a plastic jug that had held antifreeze. Supposedly it had been rinsed out, but as it turns out, not quite well enough. The small amount of ethylene glycol that remained in the container reacted with the permanganate in a highly exothermic reaction. This reaction produces so much heat that the chemicals can burst into flame. And that is just what happened here. The farmer had put the container in the kitchen and went on to do other chores. Within a few minutes the fire was out of control and the farmhouse burned to the ground. He learned the hard way about the oxidation potential of permanganate.
Under the right conditions, this reaction can be useful. It is actually used to create backfires to stop forest fires from spreading. Small spheres, about the size of ping pong balls that contain potassium permanganate are injected with glycerol and are dropped from airplanes or helicopters. As the balls fall, the chemicals mix and catch fire by the time they hit the ground where they then set fire to anything combustible. The aim is to produce a controlled burn that clears an area of potential fuel that a forest fire needs in order to spread.
The rate of combustion depends on the particle size of the permanganate. When it is ground to a fine powder it burns very quickly. Bigger chunks doused with glycerol take longer to begin burning. This has been called the 鈥渋nsurance reaction,鈥� because it has been used to burn down 鈥渦nprofitable businesses.鈥� If the pieces of permanganate are big enough, the criminal has enough time to get far away before the fire starts.
I鈥檓 not in the habit of starting fires, but I do use potassium permanganate for teaching purposes. It provides a visual way to determine if a molecule contains a carbon-carbon double bond. If such a bond is present, adding an alkaline solution of potassium permanganate results in the colour changing from purple to brown. The purple permanganate donates oxygen to the double bond leaving behind brown manganese dioxide. This reaction has another interesting application, extending the life of stored bananas.
The ripening of a banana is due to the fruit producing ethylene gas. Some of this gas is released, hastening the ripening of nearby bananas. This is a problem when bananas are shipped on mass. However, if they are packed in plastic bags together with potassium permanganate, the ethylene gas which contains a double bond reacts with the permanganate and is removed. This can extend the shelf life of the bananas for weeks without needing refrigeration.
There is yet another interesting historical use of permanganate. It seems that during World War I, Canadian soldiers were equipped with vials of permanganate to mix with an ointment that they would then apply to their privates. There was no evidence that this would have any effect. Except for one. The soldiers ended up with violet-coloured genitals.
