Run time for Sodium Perchlorate cell

The current efficiency for a Perchlorate cell is not pH dependent and the electrolyte will drift alkaline. Efficiency increases as the current density on the anode increases and decreases as the amount of Chlorate in the solution decreases. You will generally get higher current efficiency's for a Perchlorate cell than for a Chlorate cell. The current efficiency will be of the order of about 80%, especially if you use an additive like Fluorine and run your cell at a high anode current density of about 350mA per cm squared. This, together with the fact that it only takes two moles (as opposed to 6 moles for Chloride to Chlorate) of electrons to make one mole of Perchlorate from one mole of Chlorate, gives Perchlorate cells (pleasantly) surprising short run times.
In order to make one Chlorate molecule into one Perchlorate molecule you must move one oxygen atom from water to the Chlorate. This takes two electrons. Another way of saying this, is that it takes two moles of electrons (26.8 Ah * 2) to make one mole of Perchlorate out of one mole of Chlorate.
The overall reaction equation is: NaClO3 + H2O + two electrons ===> NaClO4 + H2 gas
The actual mechanisms are not fully agreed and can be further investigated in J. of Applied ElectroChem. 17 (1987) 33 - 48 and also Comprehensive Treatise of Electrochemistry Vol. 2 (see further reading section).

The Chlorate concentration at which the current efficiency starts to decline is not fully agreed in the literature. There are a lot of variables that effect the point at where the low Chlorate concentration effects current efficiency. I have assumed that it is effected at about 100g/l Chlorate which is roughly where it happens in a fairly typical cell.
1 Mole Na Chlorate = 106.45g (+16 grams for Na Perchlorate)

Run time in minutes per gram of NaClO3 for 50% and 80% current efficiency's
(no Chloride present in the cell at start, additives used, F or Persulphate)
Amps 50% current efficiency
Chlorate level below about 5g/l
(at the end of the run)
80% current efficiency
Chlorate levels above 100g/l
(at the end of the run)
2 30.19 16.23
4 15.09 8.11
8 7.54 4.05
10 6.03 3.24
15 4.02 2.16
20 3.01 1.62
30 2.01 1.08
50 1.20 0.649
75 0.805 0.432
100 0.603 0.324
150 0.402 0.216

The table shows recommended run times for a Perchlorate cell running at an overall current efficiency of 50% and 80%.


To put the table into perspective, @ 80% current efficiency you will convert 1g of NaClO3 into 1.15 grams Perchlorate ever 32.5 minutes per amp.
That's 106.5g(one mole) NaClO3 converted into 122.5g Perchlorate every 57.3 hours

The table above assumes that you have not used Chlorate solution to top up your cell if it needed topping up, it assumes you used water. If you use Chlorate solution you can take a similar approach to calculating run time as used in the Chlorate run time section.
When the cell is run for a length of time shown in the 50% coulomb there will be very little chlorate left in the cell.
The current efficiency will be about 80% when the Chlorate levels are above about 100g/l but as the Chlorate level drops below the 100g/l mark (as more and more Perchlorate is formed) the current efficiency drops and you get an overall current efficiency of 50% if you run your cell to the point where there is very little Chlorate left (ie. the run time in the 50% coulomb). This saves labour and you can take out nearly pure Perchlorate out of the cell by simply evaporating off all the water. You may wish to destroy the small amount of Chlorate left in the cell first by chemical means.
If you want to run your cell and stay in the high current efficiency region (>100g/l Chlorate) then you can stop running your cell at the recommended time in the 80% coulmb and take out a crop of Perchlorate. You should be able to get a crop of Perchlorate out by evaporating off some water and also by adding some highly concentrated solution of Chlorate which will help to ppt out the Perchlorate. Adding a small amount of solid Perchlorate might help to start precipitation of the Perchlorate load. This method of extracting Perchlorate from a cell will be very difficult for the amateur to do successfully. I would recommend running your cell for long enough (50% coulomb) for to convert nearly all of the Chlorate into Perchlorate and then destroying residual Chlorate by Chemicals.
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 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 and use an additive to stop cathodic reduction (F or Persulphate).