When will I get Perchlorate?
All tables showing conversion time for Chloride to Perchlorate should be ignored IMHO.
You won't get ANY Perchlorate if you run your cell for the amount of time in run time tables that assume 100% current efficiency for the electrolytic conversion of Chloride to Perchlorate.
Say you have the following conditions and assumptions.
One mole NaCl (58.5 grams) in the cell.
Ten amps going through cell.
22 minutes per gram of NaCl to convert to Perchlorate (from (100% current efficiency) run time tables).
16 minutes per gram of NaCl to convert to Chlorate (from (100% current efficiency) run time tables).
60% current efficiency (approx. what you will get with an amateur cell without pH control)
for the conversion of chloride to chlorate.
Assume perchlorate actually starts to form when NaCl is at 10% of its starting concentration.
In order to convert one molecule of chloride into one molecule of chlorate you must move 6
electrons, 8 electrons must be moved for perchlorate to form. (two electrons per oxygen).
That's 6 and 8 moles of electrons per mole Chloride for Chlorate and perchlorate.
The run time tables show conversion time from Chloride to Perchlorate assuming 100% current
efficiency, that is, every electron that enters the cell will go towards causing an oxygen atom
to do what we want it to do.(attach to a chloride)
So for our one mole of NaCl (58.5 grams) the run time table says 22 (minutes) X 58.5 = 1287
minutes to convert to perchlorate. You won't get ANY Perchlorate if you run the cell for this
amount of time.
The current efficiency will be about 60% (and indeed will be probable more like 53%, ie. worse) for the conversion of Chloride into Chlorate and
we must convert 90% of 58.5 = 52.7 grams of Chloride into Chlorate to get Perchlorate to
even start forming. (NaCl will be at 10% of its starting conc.)
This will take 52.7 X 16 X [100/60] = 1405 minutes.
This is greater than the time the run time tables suggests that Perchlorate should have
been formed in abundance.
An amateur would be inclined to think that at least SOME perchlorate should have formed
but none will have been formed.
Its even worse than this. I have assumed above that you will get 60% current efficiency
but you will only get 60% current efficiency when there is a reasonable high concentration of
Chloride present. When the Chloride concentration of the electrolyte begins to fall to 20% or below, the current efficiency of conversion of Chloride to Chlorate will get worse and worse and
the time for Chloride concentration of solution to reach 10% (and Perchlorate to start forming) will be
even worse that stated above.
It will be very difficult to predict when you will get down to 10% Chloride concentration, (when Perchlorate starts to form). A good tactic is to have some methylene blue (0.3% solution) handy and you can test a small sample of your solution with the methylene blue solution to see if Perchlorate has started to form. When it has started to form you can then estimate the run time needed to convert most of the Chlorate in the cell into Perchlorate.
All above is assuming that no additives like NaF has been added to electrolyte.
Additives like Fluoride or Persulphate are added to increase current efficiency. See US patent 3,493,478.
But even with additives, the conversion of Chloride to Perchlorate without any intermediate separation of the Chlorate, is a process that has low current efficiency, it saves labour though.
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