Spontaneous Combustion
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Background
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.
Composition
The following composition is derived from the reaction
equation.
|
Name |
Formula |
Quantity |
| 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:
- The size of the particles of potassium permanganate (the finer, the
smaller amount of time it takes before ignition occurs)
- Whether or not the glycerine has any water dissolved in it (the less
water, the smaller amount of time it takes before ignition to occurs)
- As always, the composition, ie. the ratio of the fuel to the oxidixer (the
more oxidizer, the smaller amount of time it takes before ignition occurs)
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.
Equation
"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:
CO (Incomplete combustion of of the glycerine)
KMnO2
(KMnO4 which has not fully decomposed) and others
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
| | |
H-C--C--C-H
| | |
OH OH OH
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