Sodium Perchlorate is not used in
pyrotechnics due to the fact that it is hygroscopic. It has three hydrates
containing 0, 1 and 2 water molecules. It is used to make Ammonium or Potassium
Perchlorate and other Perchlorate's by double decomposition.
There are two routes you can take when making Sodium Perchlorate.
When mild steel and Nickel is used, Chromate's are desirable to stop the Cathode from being
attacked by the Hypochlorous acid in the cell electrolyte. (0.5 to 5 grams/Liter). Chromates also increase
current efficiency by stopping reduction at the cathode (the conversion of
Chlorate and Hypochlorite to Chloride).
Chromate's are not compatible with Lead Dioxide. Use Titanium as a cathode if you can get it.
Sodium Fluoride or a Persulphate may also be used to stop Chlorate or Hypochlorite being converted (reduced) back into Chloride at the Cathode.
When using Platinum as an Anode it's wear rate will increase as the Chlorate
concentration decreases, and below 50g/l it may be excessive, high temperatures also
increase it's wear rate. Wear rates from manufacturers have been reported
as 3 to 6 grams Pt per ton Sodium Perchlorate.
The concentration of Chlorate should be kept above 100g/l if a high current efficiency is desirable. (The Perchlorates. Schumacher J.C. P86)
The temperature of the cell is usually maintained at about 30 to 60oC. The current density on the Anode is usually much greater than with Sodium Chlorate manufacture, and high current density's do not decrease current efficiency. Typical Anode current densities are from 150 to 400mA per square cm for Lead Dioxide anodes and 200 to 500mA per square cm for Platinum. Most commercial plants use water circulation through coils in the cell for to remove heat generated in the cell to stop it from overheating. The amateur can keep the cell cool by putting it sitting in a large tub of water or by keeping the current per volume low at about one amp per 100ml of solution.
The voltage across a Perchlorate cell is higher than a Chlorate cell and will be in the range of about 4 to 6.5 volts.
The cost of electrical power for to make one Kg of Na Perchlorate from Na Chlorate is about two KWh (two "unit's" of power) which is approx. 18 US cents.
To start your cell about 700grams Na Chlorate are dissolved in some water and
more water added to make one liter (A solution of Sodium Chlorate containing
700g/l has a density of 1.428 and is 49%(wt) Sodium Chlorate, see Graphs and
table's in the Sodium Chlorate section).
Dichromate, Fluoride (Na, K, or Hydrofluoric acid) or some Persulphate (Na, or K) or both F and the Persulphate is added to the cell for to improve the current efficiency. About 2 g/l Sodium fluoride or about 2g/l of K Persulphate is OK. If using Dichromate use 2 to 4 grams per liter.
The current is turned on and the cell is run for the appropriate time as outlined in the run time section. The product is then either turned into K or Ammonium Perchlorate (see relevant section) or solid Sodium Perchlorate is extracted and the mother liquor is returned to the cell for the next run of the cell.
All mother liquor should always be returned to the cell as it will be rich in Chlorate and Perchlorate because of their high solubility. Note that Platinum Anodes will corrode if used to reduce the Chlorate concentration to a very low value. You need Lead Dioxide if you wish to run a cell from Chloride to Chlorate to Perchlorate to Low Chlorate concentration, without stopping.
There is a good description of a Perchlorate cell using a Platinum wire anode at the bottom
of this page by GarageChemist. In his cell you must use Dichromates to stop Chloride from forming in the cell and you must
start the cell with zero (thats 0.00%), Chloride in the electrolyte to avoid Pt anode erosion.
Since Sodium Perchlorate has a number of hydrates this will need to be noted if yields are being calculated. Reduce all product to the anhydrous state. Which hydrate forms depends on the solution concentration and temperature as your product crystallizes.
Run time for a Sodium Perchlorate cell
Separating out Sodium Perchlorate
GarageChemist Perchlorate cell using Pt and Dichromate additive
US patent No 3,493,478. Making Perchlorate from Chloride using Fluorine additive.
US patent No 2,813,825. Making Perchlorate from Chlorate using Persulphate additive.
JAE 1971. Large scale production of Perchlorates directly from Sodium Chloride
Article from Encyclopedia of electro chemistry regarding Perchlorate manufacture.
Why run time tables for Chloride to Perchlorate are not a good idea
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