Background

Lets say that you have a quantity of Chlorate. Maybe it was produced with your chlorate cell or maybe you bought it online. So what do you do with it? You can’t really use it in any fireworks because they almost always have sulfur. Chlorate spontaneously ignites in the presence of sulfur. Unless you have lots of money to spend, you can’t make a perchlorate cell with a platinum anode. So… what to do?

edit This method is flawed. Heating to at least \(510^{\circ}\mathrm{C}\) is required for thermal decomposition according to (Ravanbod et al. 2016).

Decompose it of course! When a chlorate is exposed to enough heat it will decompose into perchlorate and chloride. Inevitable you lose some of your product (about 15% for potassium chlorate). Be careful while heating though. Too much and the perchlorate will decompose as well. The trick is getting the temperature just right. I will be decomposing potassium chlorate.

\(4 KClO_{3} \rightarrow 3 KClO_{4} + KCl\)

In the presence of a catalyst such as manganese dioxide, oxygen and chloride is generated. High school chemistry labs use this reaction as a source of pure oxygen since it can be cheaper than getting it in bottles.

Melt it Down

Even without a thermometer we can tell when our reaction is completed. Melt the chlorate until it turns solid. If it turns back to liquid again you have gone to far. That seems simple enough. Let’s try it.

Step 1: Pure Chlorate

We are going to be heating chlorate up to a very high temperature. High purity (>99.9%) chlorate is needed. Any stray combustibles could ignite the mixture and possibly explode. Recrystallization is a great method of purifying especially for potassium chlorate because the solubility increases quickly with temperature. If you only have less than 100 grams of potassium chlorate I would suggest cold filtering (filtering with room temperature water). Its easier to do but requires a large amount of water that has to be boiled off later. If you have much more than 100 grams, hot filtering is the way to go.

Step 2: Heating

We have to have a heat source than can provide a lot of heat. Most hotplates can’t get to that temperature so we have to use an open flame. An alcohol lamp might do the trick but it can only be used for batches less than 25 grams. Since this is more of a test, we will start with an alcohol lamp and 10 grams of potassium chlorate.

First stage is the melting on chlorate:

Chloride and perchlorate solidify upon production because their melting points are higher than chlorate. You can see the crystals formed here:

Drive the reaction forward until the whole thing becomes solid. Then take the vessel off heating and let cool. Add a small amount of cold water to the mixture to dissolve the unreacted chlorate and produced chloride. To remove potassium chloride add about 1 mL for every 30 grams. Excess is better. Cool the mixture with ice water then filter.

Now you have potassium perchlorate! It probably has some unreacted potassium chlorate left so its best to destroy it with chemicals. You can do a quick test of purity by mixing at a 5:2 ratio of perchlorate to sugar and dripping sulfuric acid on the pile. If the mixture is generally free of chlorate nothing will happen. If you still have some chlorate left, it will ignite when exposed to sulfuric acid.

Potassium burns with a nice purple flame as you can see. The first pictures shows 9.33 grams of products after dissolving out the chloride and drying. Theoretically it should only have produced 8.48 grams of perchlorate and 1.32 grams of chloride. There wasn’t enough water to filter out so most of the products stayed in the solution. It would have been better to put the products in a filter and then pour ice cold water over it. Oh well, this was just a test run after all.

Next Steps

First of all I need to get a product that actually works. The first batch made in the stainless steel condiment cup was not driven to completion. That is why it ignited when exposed to sulfur. I will probably put a few grams in a test tube and heat it over the stove.

Larger batch: I have 1 Kg of Chlorate to decompose and more is coming. Maybe a large stainless steel pot. The original stainless steel cup turned a copperish color. I don’t know what this could be. Maybe it is a chloride or oxide of chromium. Or maybe the cup had some sort of film over it. It didn’t seem to affect the coloring of the burn. Probably a recrystallization is in order to remove any impurities.

Success!

A second try at decomposition proved a success. I used the same test as in the first video and this time nothing happened! That means not enough chlorate was present to ingite. It makes for a pretty boring video though. At least now I know the process works. A small whiff of steam is produced by the dehydration of sugar by sulfuric acid.

Bibliography