Chlorate
Chlorates and perchlorates are among the most often used oxidisers in pyrotechnics. Especially potassium perchlorate, KClO4, potassium chlorate KClO3, ammonium perchlorate, NH4ClO4, and barium chlorate BaClO3 are used extensively. Chlorates and perchlorates are chemically related but from a pyrotechnical standpoint perchlorates are generally safer to use. Chlorates require special handling.
Mixtures made with tend to be more sensitive than similar mixtures based on perchlorates and some may ignite spontaneously. Potassium chlorate is extensively used commercially since it is cheaper than potassium perchlorate, and many accidents have happened that might have been prevented if the corresponding perchlorate was used instead. The use of chlorates is therefore not recommended. If you do decide to use them, make sure you really understand what you are getting involved with.
The amateur pyrotechnist can prepare chlorates and perchlorates at home with some effort. Homemade chemicals are usually more expensive than commercially produced chemicals and it is ofcourse laborous, but taking shipping costs into account, the fact that perchlorates and chlorates are sometimes quite hard to obtain in most countries and their usefullness in pyrotechnics it may pay to try to make your own. Chlorates are the easiest to prepare, but again, the use of chlorates is not recommended for amateur use.
There are some basic rules you must follow when using Lead Dioxide anodes.
You cannot use Chromates in your Chlorate cell to increase current efficiency and to protect some types of cathodes (as is done when graphite anodes are used), as Chromates are not compatible with Lead Dioxide anodes, they drastically reduce the current efficiency. This in turn means that you cannot use some types of cathodes with Lead Dioxide (like Nickle or Mild Steel) as they will get corroded without the presence of Chromates to protect them.
There is also the problem of using certain types of Stainless Steel cathodes along with Lead Dioxide anodes. Some types of SS will corrode ever so slightly when used in a Chlorate or Perchlorate cell. Some Chromium will be released into the cell electrolite and will lower the current efficiency of the Lead Dioxide anode. See the section on cathodes for types of SS that are ok. Ti is perfect if you can get it, but it has a problem that it becomes brittle after some time in a cell. Graphite (cathode) is good too but if it is porous you will get salts being conducted up the cathode, out of the cell where it may corrode connections. You may have to give it linseed oil treatment.
You can use (the element)Carbon to make Chlorate but it will not make Perchlorate without excessive erosion. Carbon (as far as making Chlorate is concerned) comes on two forms,
- carbon, porous and not very dense
- graphite, more dense than carbon but may be porous depending on processing.
These are two allotropic forms of the element Carbon. Graphite is obtained
from the less dense carbon by heating it to a very high temperature
where it becomes more dense. Carbon is inferior to graphite when
it comes to making Chlorate as it erodes much more. Carbon can be
made to last much longer by impregnating it with linseed oil so
as to help stop the carbon from shredding when it is in a Chlorate
cell. It is very advantageous to treat carbon with linseed oil as
it will shred relatively quickly in a Chlorate cell. See Mike Browns
page for information regarding treating carbons with linseed oil.
Graphite, if it is porous, should be treated with linseed oil to
make it last longer too. Graphite/carbon will get rapidly eaten
away if used in a Chlorate cell that is more than 40C or has a low
concentration of Chloride in it (below 100g/l). Also, current density
must be kept below 43mA/cm2 on the graphite/carbon anode or it will
get eroded excessively, between 33 and 43mA/cm2 is used in industry.
The electrolysis is carried out in a diaphragmless cell, containing
a solution of a chloride. Several chlorides may be used, but the
use of sodium chloride has many advantages. Sodium chlorate is easily
converted to a number of other chlorates by metathesis reactions.
The most commonly used chlorates in pyrotechnics, potassium and
barium chlorate, can both be made in this manner. Potassium chloride
and barium chloride may also be used to obtain the respective chlorates
directly, but this has many disadvantages as will be discussed below.
Only sodium chlorate can be used in the manufacture of perchlorates,
due to its high solubility.
Ammonium chloride should never be used, and should in fact not even
be present in the cells in trace amounts. It could result in the
formation of two dangerously sensitive and explosive compounds,
nitrogen trichloride (NCl3) and ammonium chlorate (NH4ClO3).
The formation of both of these compounds should be avoided at all
times. Not only can they explode by themselves when present in significant
quantities, they can also lead to spontaneous ignition of pyrotechnic
mixtures contaminated with even small amounts.
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