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From: (Tom I Perigrin)
Subject: Re: Questions on storage of chemicals
Date: Fri, 26 Jul 1996 14:46:30 GMT

In article <>,
(DrD) wrote:

> Are there any maximum lengths for which chemicals can be stored?  
> I'm not talking mixtures, but chemicals, such as Al, Mg, etc.
> Do chemicals "expire" after a given length of time?  Is there a source
> for expiration periods if they do?

This is NOT an easy question...

Chemicals can deteriorate for a number of reasons:

1) unimolecular thermal decomposition
2) reaction with air
3) reaction with moisture
4) reaction with contaminants (purposeful contaminants included)
5) reaction with light

1) This form of decomposition is based on a "unimolecular reaction".  I.e.,
the individual molecules decompose without any influence from other
molecules.  Thus questions of density, air, moisture, etc... are not
important.  Many molecules undergo unimolecular thermal decomposition,
including most oxidizers and fuels.   However, the rate of the reaction is
a function of temperature, and generally depends on temperature in an
exponential fashion.  A rule of thumb is that most such reactions seem to
double their rate for every 10 deg C rise in temperature.  Thus, if
potassium perchlorate starts to decompose at an appreciable rate at 300 deg
C, then the rate of decomposition at room temp will be 27 powers of 2
slower.  In other words - negligable.   Samples of potassium perchlorate
which are decades old will be 99% pure, if no other mechanism of
decomposition has taken place.

Generally pyro chemicals are chosen so that they are stable in mixtures at
room temp.  Thus, they are almost always stable as pure samples at room
temp, at least with regard to thermal decomposition.

2) Many fuels react with air - after all, if they didn't react with oxygen,
they wouldn't be fuels.  However, once again the rate may be sufficiently
slow to consider them to be "stable".  For example, sawdust is stable under
air storage for centuries.  However, some of the more reactive fuels may
have shelf lifes.  For example, sulfur does slowly react with air.   A
sample of sulfur sealed in a bottle with air will often give off the
distinctive smell of SO2 after storage for a few months.  However, this
problem can be minimized if the sample is sealed away from fresh air.  For
example, assume a sample of 100 grams of sulfur in a 500 milliliter
(roughly a pint) jar.  The jar will contain almost 1/2 liter of air, which
is roughly 21% oxygen, or 105 ml of O2.  Since a mole of gas occupies 22.4
liters at standard temp and press, this works out to 4.8*10^-3 moles.  
That means that 4.8*10-3 moles of sulfur could be consumed and made into
SO2.  That works out to .15 grams of sulfur, or 0.15%.  As long as the jar
is sealed to prevent the ingress of more oxygen,  the loss of sulfur will
be negligible.  However, note that SO2 is acidic, and this would cause
problems with certain formulations. (anything with chlorate, for example)

Other fuels that may exhibit some air-oxidation are Aluminum, Magnesium,
Magnalium, Iron, FerroTitanium, Zinc, etc...    However, closing these in a
bag and pressing out all the extra air will reduce the amount of oxygen
degredation to a minimal amount.  As long as the bag is not air-permiable,
then the sample should be stable for decades.

3)  Many fuels and some other materials react or change form with moisture.
 The most notable fuels which do this are Magnesium, Magnalium, Zinc, and
Aluminum to a lesser extent.   With regard to storage of these materials,
the same caveats apply as to air decomposition - simply seal it in a tight
bag excluding the admission of more humidity and the loss of material will
be negligable.

The change of form due to moisture is a more pernicious problem, since only
a trivial amount of moisture may cause some fine powders to turn into large
solid lumps.  Apart from dessicating the materials though heat, vacuum, or
by storing them in a dessicator, little can be done to completely prevent
this from happening.  However, the extent can be lessened by storing the
clumpable materials away from moisture.

4)  Some materials are not pure when purchased.  Many aluminums contain
stearin or other surface coating agents,  and some oxidizers contain flow
agents, etc.   These materials were chosen with regard to long shelf life,
and should not present a problem at normal temperatures.  

5) When a photon of light is absorbed by a molecule, the molecule gets all
of the energy of the photon.  The energy must go somewhere.  Typically the
energy goes into "electronic excitation" (or vibrational excitation for
infrared light).  What happens next depends on the nature of the molecule. 
Metals just turn the energy into heat, so metallic fuels do not show
photodegradation.  Some organic fuels would be sensitive to light - shellac
and redgum amoung them.  However, the amount of light necessary to show any
serious change would roughly equal that obtained by having the sample
exposed to the sun in Arizona for a year.

Chlorates may be the most sensitive material with regard to
photodegredation.  There is some indication that chlorates undergo a
decomposition to give a gas which can catalyze more decomposition, even in
the dark.  Thus chlorates should be stored away from bright light.

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