Spontaneous Combustion


There are many compositions which ignite as soon as all the components come into contact with each other. Some of these include glycerine and potassium permanganate, concentrated acids and liquid fuels, sodium/potassium and water. There are many others which I have heard of but know very little about, and from what I have heard, they are very dangerous. The only one I have sufficient information about at the moment is glycerine and potassium permanganate.

Glycerine and Potassium Permanganate

In this reaction, after the two reagents come together, the time before there are observable changes varies greatly depending on a number of factors (outlined later).  This reaction is relatively safe, the only thing special which needs to be looked out for is potassium permanganate's tendancy to stain skin and clothing brown.


The following composition is derived from the reaction equation.
potassium permanganate KMnO4 75%
glycerine C3H5(OH)3 25%

Time Variations Between Mixing and Ignition

Some of the factors influencing time variations between mixing and ignition are: When conducting the experiment myself I have seen huge variations.  When adding a relatively small amount of potassium permanganate in the form of large crystals to the glycerine, the reaction has taken as much as ten minutes to commence, and then only barely observably.  However, when the potassium permanganate is powdered finely in a mortar and pestle, and mixed in the proportions shown above and mixed immediately so that a mixture of uniform density is formed, the reaction commences almost immediately (within five seconds) and begins to smoke.

You can speed up the time till the reaction takes place by adding a little sulfur.  This has a lower ignition temperature and so lowers the ignition temperature of the mixture as a whole.


"Chemical Demonstrations: A source book for teachers." by LR Summerlin and JL Early states that the equation for this reaction is:

14KMnO4 + 4C3H5(OH)3 ---> 7K2CO3 + 7Mn2O3 + 5CO2 + 16H2O

This reaction, however, would probably vary slightly under different conditions giving other by products such as:

Other Information

Although glycerine is normally refered to as just that, glycerine (or glycerol), its systematic name is propan-1,2,3-triol.  It is used in the cosmetic industry as well as as an explosive (nitro glycerine).  It's structural formula is:
  H  H  H
  |  |  |
  |  |  |

It can be formed by the saponification of a fat, or triglyceride:

  H   H   H
  |   |   |
H-C---C---C-H   +   3NaOH ---> 3Na      +        H  H  H
  |   |   |                   |                        |  |  |
  O   O   O                   O                     H-C--C--C-H
  |   |   |                   |                       |  |  |
O=C O=C O=C                 O=C                       OH OH OH
  |   |   |                   |
  R1  R2  R3                   R1/2/3