metalresearcher
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When is chlorate generation finished ?
Currently I am running an electrolysis of 1 Mole (60 g) NaCl to make NaClO3.
I see bubbles (H2) at the anode but barely anything at the (MMO) anode which means that oxidation indeed takes place.
It has been running for 20 hours now at constant 15A.
On a certain moment, all Cl- is oxidized to ClO3-, as the MMO anode does not support further oxidation to ClO4-.
I think that finally O2 will bubble off the anode, but in that true ?
And will the voltage increase (due to poorer conductivity ?)
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woelen
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You indeed will see formation of O2 at the anode. and voltage goes up (provided you keep the current constant).
I myself computed the total amount of charge, passing through the cell. In a home-made cell of reasonable efficiency you can assume that for
conversion of 1 chloride ion to one chlorate ion you need 9 electrons (theoretically you can do with 6 electrons, but the reaction pathway in a
home-cell without pH-control is such that 9 electrons are needed). For ease of calculation take a ratio of 1 to 10.
If you know how many moles of NaCl (or KCl) you have in your solution, and you know the current, then you can compute how many moles per second of
electrons you are producing for the reaction (one Coulomb of charge is just a little over 0.00001 mole of electrons). With 15A current, you produce 15
Coulomb of charge every second.
Using the above computations, I had satisfactory results. You don't want to continue the process longer than needed, because at the end of the
process, when only oxygen is formed, you have much more wear of your anode.
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yobbo II
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Here is an app (modern name for a program) for android that computes run times for cells.
It does nothing that you cannot do with a spread sheet.
It is cool to have a phone app for chlorate making to impress girls (including your mother!)
Yob
Attachment: app-debug.apk (3.6MB)
This file has been downloaded 94 times
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Mister Double U
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Faraday Equation
Might also be nice to have the Faraday Equation in this threat:
I * t = n * Z * F
I: Current [A]
t: Time [s]
n: Molar Amount [mol]
Z: Number of electrons needed for the reaction [e-]
F: Faraday Constant 96485[C/mol]
If you divide the equation by 'I' you get:
t = (n * Z * F) / I
Now just plug in the numbers:
n = 1[mol]
Z = 6
F = 96485[C/mol]
I = 15[A]
t = 38594 seconds = 10.72 hours
As woelen mentioned conversion might not be 100% efficient, because in a home setup there can bet the Oxygen evolution side reaction.
For a Z = 9 electrons this would change to t = 16.08 hours.
Additionally, there can be reduction happening at your cathode, which fill further increase the time needed. You can add ~1g/l of Potassium Dichromate
to prevent most of the reduction. Alternatively, if you do not like carcinogenic Dichromate you can add some CaCl2 to your electrolyte. This will
cover the cathode with a nice diaphragm of Ca(OH)2 and works nicely as well (as mentioned in the 'NaClO3 => NaClO4 conversion failed ?' threat).
[Edited on 25-5-2025 by Mister Double U]
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MrDoctor
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i found it didnt really matter, directly producing KClO3, since chlorate would pretty aggressively begin crystalizing on the anode, which isnt great
for it. it wouldnt do that until close enough to done. because of the damage you can do, depending on the MMO, by letting it run through to begin
trying to make perchlorate, this is the sort of thing best just let run through a wattmeter, to give you total WHr or AHr for you to convert to
faradays and just do the theoretical and deal with an incomplete mixture that at very least will never overshoot. youll get chlorate proportional to
the power put through so your time is better spent just dealing with that while the next batch runs. the longer it runs at full power the more
chlorate you are producing over any given amount of time.
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woelen
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It indeed is good to measure the number of Ah which you passed through your cell. Measuring wattage is not the best way to do that, just measure
current and multiply with time. Measure current through the cell.
I do not agree with the statement that the amount of chlorate is proportional to the power put through the solution. First, you need to distinguish
between power (amount of energy per unit of time, and total energy). But even then, at a certain moment in time, production of chlorate stops, and you
just get oxygen at the anode, and excessive wear of the anode. Also, it is not true that for longer runs you get more chlorate.
Some types of MMO (e.g. PbO2-based) allow production of perchlorate, other types of MMO (mixed Ir/Ru-oxide) do not allow production of perchlorate. If
no production of perchlorate is possible with a certain type of MMO, then you will see increased production of oxygen at the anode, increase of
voltage over the cell (for the same current) and stronger wear of the anode. Then it is time to stop.
Formation of some crystals around the anode is not really harmful, but it reduces the available surface area on the anode, and makes it harder to
maintain a certain current.
The liquid, left over from one run can be reused for a next run, when new KCl is added. With this, you already have some chlorate in the liquid, and
you get a next crop of chlorate somewhat sooner.
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MrDoctor
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sorry, to correct myself, i indeed meant to say current where i said watts or power. current (Ah, or better just call it Faradays) is what is doing
the work here specifically.
also, when i said longer, i meant in terms of total uptime not the individual batches. you dont lose anything by stopping early, so its better to just
apply a fixed number of faradays to reliably get close-ish to done knowing for sure you couldnt have gone over, especially if you are going to be
doing more batches.
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yobbo II
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A problem when discussing run times is:
What do you mean by finish?
The 'finish' means different things to different people with the beginner not really having a clear idea of what exactly they mean by 'finish'.
In the app the finish is when the salt is at 100 grams per litre.
All may not agree with this finish line.
Yob
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MrDoctor
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-im not familiar with any app in this conversation, but what i meant by finish was the reaction, of turning chloride to chlorate. the ideal end point
you are aiming for. But thats because in this context, thats the stated objective, as opposed to reaching a specific density.
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yobbo II
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The 'ideal end point' varies from person to person.
The app. is a few posts above.
The app. has an 'ideal end point' of 100 grams per liter of starting chloride, with the rest having been converted to chlorate.
The 100 grams per liter point is where the anode starts (approx) to get damaged. The erosion will get progressively worse as chloride gets lower and
lower.
Y
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