Ammonium Perchlorate from Sodium Perchlorate and Ammonium Nitrate

The following assumes that ALL traces of Chlorate have been destroyed before conversion of the Sodium Perchlorate into Ammonium Perchlorate. If there are traces of Sodium Chlorate present, Ammonium Chlorate will form, and this will give a dangerous unstable product. See Destroying Chlorate by chemical means for to eliminate all Chlorate.
The byproduct in this case is Sodium Nitrate which may be useful for other pyrotechnic jobs, you could also convert it to KNO3 as described elsewhere.

Molecular weights

The graph below shows the solubility's of the relevant salts. The 'stable pair' is Ammonium Perchlorate and Sodium Nitrate. The solubility's of theses two salts are fairly far apart and you will be able to get a crop of Ammonium Perchlorate from the solution. I do not have the mutual solubility graph of the stable pair but you will be able to work at about 0.6 moles and dissolve in the Sodium Perchlorate and Ammonium Nitrate above 80C. The Ammonium Perchlorate will come out of solution as you cool to zero or below. You should cool slowly to get large crystals of Ammonium Perchlorate which will wash easily.
So for every 100ml water dissolve in 0.6 moles (73.5g) Sodium Perchlorate and 0.6 moles (48g) of Ammonium Nitrate and dissolve in the water at 80C. Cool slowly to about -10 to obtain your Ammonium Perchlorate.

Ammonium Nitrate can be difficult to dry or it may have inert filler in it. It may be handier to work with a 50% solution and not isolate the Ammonium Nitrate as a solid. Don't forget to include the water that is in the Ammonium Nitrate solution when you are deciding how much water to use in the double decomposition reaction.

Getting a 50% solution of Ammonium Nitrate

If the Ammonium Nitrate is not pure it will have to be purified. It is difficult to recrystalize the Ammonium Nitrate as the precipitated Nitrate will be mixed with the inert filler. The best (and handiest) way to deal with the Ammonium Nitrate is to make a 50%wt solution of it and use that in the process. To make a 50% you should take about 3KG of the Ammonium Nitrate (pure or otherwise) and dissolve it in one liter of water. An Aluminium or stainless steel container is ok. It will look like an awful lot of Ammonium Nitrate but it will all dissolve. You will need to heat the water as the solution will get very cold (its uses in freezing packs after all) when the Ammonium Nitrate is dissolving and you will want to hurry it up. When all the Nitrate has dissolved (this will have happened at 50C or less) you should stop heating. The solution may be very cloudy because of the inert filler in the Ammonium Nitrate but it's ok. Let the solution settle and all inert filler will fall to the bottom. There may be some of the inert filler floating around on the top which you can lift off with a spoon. Let the solution cool slowly to one of the chosen temperatures in the table below. You should make sure that some Ammonium Nitrate has crystallize out as long pretty needles at your chosen temperature. This will happen if you have proceeded as described above.

When the solution has reached your chosen temperature pour or siphon off as much of the clear(ish) liquid as possible and put into a seperate container. Sometimes the inert filler is very fine and some will still be suspended in the solution but you will be able to get rid of it later. Don't be tempted to be greedy and contaminate your clear(ish) solution with much of the solid material that is at the bottom of the container that you are siphoning from. The solution that you have siphoned (or poured) off will be a certain percentage of Ammonium Nitrate as shown in the table. ie. if you let your solution cool to 20C it will have 65.5g Ammonium Nitrate per 100g of solution. The particular temperature you choose is up to you. The more you cool, before siphoning, the more Ammonium Nitrate will come out of solution and you may feel you are wasting Ammonium Nitrate and may wish to sipnon off at one of the higher temperatures. As stated above, make sure that Ammonium Nitrate is actually precipitating in long needles at your chosen temperature before you start to siphon.

Temperatures, %wt & amount of water per gram solution to add to get a 50%wt solution
Temperature Weight % Grams water to add per gram solution
30 70.4 0.408
25 68.2 0.364
20 65.5 0.31

Weigh the clear(ish) solution that you have obtained by siphoning (or pouring) and for every gram of solution that you have add the amount of water that it says to add in the table. For example if you have a total weight of 1670g of solution, and you let your solution cool to 20C before siphoning off the clear(ish) liquid then you should add 1670 * 0.31 = 518 grams of water to the solution to obtain 1670 + 518 = 2188g of solution. Stir and dissolve any Ammonium Nitrate that may have came out of solution. You will now have a 50% solution of Ammonium Nitrate for the next stage of the process. Put the solution in a tallish container with a lid and if there is any suspended filler still in it, it will fall to the bottom after about 24 hours. You can then siphon (or pour carefully) the now perfectly clear solution of 50% Ammonium Nitrate for the next stage of the process. Save it in a container with a lid, you don't have to use it all at once.
If you have pure Ammonium Nitrate to start with you don't have to bother with the 50% solution, you can just use as is.
The remainder of the Ammonium Nitrate that was left in the heating container can be discarded if your Ammonium Nitrate has inert filler in it, or if your Ammonium Nitrate was pure (If you are using freezer packs) you can save it for the next time you want to make up a 50% solution of Ammonium Nitrate. Don't throw in drains etc. Dilute it with plenty of water and spread it on grass.

When using a 50% Ammonium Nitrate solution work as follows:
Take 52ml water and dissolve 73.5g of Sodium Perchlorate in it. Heat the solution to about 80C. Take 96g of 50% Ammonium Nitrate solution and and heat to 80C and add the two solutions. You will now have 0.3 moles of Sodium Perchlorate and Ammonium Nitrate in 100ml of water as per the first example. Cool slowly and filter of Ammonium Perchlorate


There is a study of the Ammonium Nitrate/Sodium Perchlorate/Water system in:
Karnaukhov, A.S., Izvest Sektora Fiz-Khim. Anal., Inst. Obshchei i Neorg. Khim., Akad. Nauk S.S.S.R., 25, 334 (1954)