metalresearcher
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NaClO3 => NaClO4 conversion failed ?
First I made NaClO3 by electrolyzing 180g table salt for 32 hours at 15 Amp using MMO anode for making chlorate with a tiny bit K2Cr2O7 and the
temperature was 70 C.
In theory three moles of chlorate should be formed (32h*3600*15/6)/96560 where the division by 6 is because of 6 electrons to be oxidized (-1 for Cl-
to +5 for ClO3-).
Then I put a PbO2 anode into the bath and let it electrolyze 16 hours to convert it to perchlorate at 45 C. So it should contain NaClO4.
The PbO2 anode is from ebay: https://www.ebay.com/itm/174251053308?itmmeta=01JJ2MR0QCM6PF... bkAAOSwjPJmBG-C
But the methylene blue test did not turn purple, so the test failed. What did I wrong ?
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Deathunter88
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You should not use lead dioxide electrodes in a cell that has dichromate additive since you will form a blocking layer of chromium oxide on your
electrode that ruins current efficiency. Use sodium persulfate as an additive instead.
Also, at best you will get 40-50% efficiency when using MMO, so there is mostly likely still 90 grams of NaCl remaining. NaClO4 will not form until
NaCl concentration drops below 5%.
Get rid of the chromate, pray that your electrode isn't ruined, add sodium persulfate, and just let it run for a week. PbO2 can convert NaCl all the
way to NaClO4. Then check again with the methylene blue.
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metalresearcher
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Quote: Originally posted by Deathunter88  |
Get rid of the chromate, pray that your electrode isn't ruined, add sodium persulfate, and just let it run for a week. PbO2 can convert NaCl all the
way to NaClO4. Then check again with the methylene blue. |
How can I remove the dichromate from the NaCl / NaClO3 solution mixture ?
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Deathunter88
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| Quote: Originally posted by metalresearcher | | Quote: Originally posted by Deathunter88 |
Get rid of the chromate, pray that your electrode isn't ruined, add sodium persulfate, and just let it run for a week. PbO2 can convert NaCl all the
way to NaClO4. Then check again with the methylene blue. |
How can I remove the dichromate from the NaCl / NaClO3 solution mixture ?
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Maybe add a reducing agent like sodium sulfite or sodium metabisulfite. Otherwise I would start from scratch since even small amounts of dichromate is
bad.
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Xenoid
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Was your solution warm?
The methylene blue test works best at lower temperatures, especially with low perchlorate levels.
Try cooling your solution to say, 15oC.
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metalresearcher
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Well, the first run was with K2Cr2O7 and I saw orange crud appearing at the anode. I wondered what that was. Later I realized that it can be CrO3
which is orange when wet.
In the intermediate period I used the MMO anode to finish the conversion to chlorate. When I thought is was ready (presumable it is not), I used the
PbO2 anode again, but no crud on the anode anymore and it stayed clean. so no preciptation anymore and the current (15 A constant) and the voltage (8V
@ 40 C decreasing to 6.5A at 70 C) remained the same.
In the meantime I've ordered K2S2O8 and will run again with the MMO to be sure all NaCl is converted.
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Mister Double U
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Calcium Hydroxide Diaphram
Hello metalresearcher,
To prevent reduction in a chlorate cell, you can add a little Calcium Chloride to the cell. This will create a Calcium Hydroxide diaphragm on the
cathode.
The performance should be as good as Dichromate addition.
I have done this a couple times myself. I add 1g of CaCl2 for each 5cm^2 of cathode area (calculated as one side of the electrode). The cathode I used
was titanium mesh, so the CaOH2 formed can grow into the mesh and connect both sides of the electrode. If you use smooth surface sheet metal it might
not adhere well - but that I have not tested, so is a guess.
Here a table from some German electrochemical journal I found:
The second column which says 'Stromausbeute' translates to current efficiency.
Best Greetings!
P.S.: What I forgot to mention, if you use titanium mesh, make sure it is clean. One time i did this, I used an old piece of mesh which was
dirty/oxidized, and the diaphragm did not adhere well.
You can place the cathode in ~20% HCl for half an hour for cleaning before use, that should do the trick.
Most of the diaphragm growth over the course of the 1st day of electrolysis and the whole electrode gets covered in a nice thick layer (~2mm on both
sides) of CaOH2.
Voltage-drop for the diaphragm was something ~0,5 volts if remember it correctly - so not bad.
[Edited on 24-2-2025 by Mister Double U]
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woelen
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With an MMO electrode you certainly can go further than 50% conversion of chloride to chlorate. I have done this myself. I used this with KCl with a
pinch of dichromate added. Removing this from the KClO3 is very easy, because KClO3 can be recrystallized very well. My product is snow-white. With
NaClO3 this is much more difficult. I also made NaClO3, but my stuff still is somewhat yellow, due to small amounts of dichromate in it.
You must keep in mind, that 9 electrons per chloride ion are needed to convert it to chlorate, so you must take more time for the electrolysis. This
is explained on this webpage: https://woelen.homescience.net/science/chem/exps/miniature_c...
When your goal is making perchlorate, then indeed you should not use dichromate. I would suggest to make chlorate with an MMO electrode and when you
have quite some chlorate (assume around 10 electrons per chlorate ion), then use a PbO2 anode. This is better than going all the way up from chloride
to perchlorate. In the initial stages of this electrolysis, you will have more wear of your PbO2 anode, so use MMO for those initial stages.
So, summarizing:
- if you want chlorate, then best is to use an MMO anode, and add a pinch of dichromate
- if you want perchlorate, use MMO with persulfate for the initial stage, and use PbO2 with persulfate for the final stage.
Never use PbO2, combined with dichromate.
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metalresearcher
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Thanks. woelen.
But I already did this in your way.
I started with NaCl and MMO anode and a pinch of K2S2O8. When the conversion was finished (more bubbling at the anode and no swimming pool smell), I
replaced the MMO anode by a PbO2 anode and let it run again. Afterwards I poured a concentrated KCl solution into it and KClO4 precipitated out as a
white powder. When KClO3 starts crystallizing out I decanted off and collected the white powder which I rised with cold water and dried.
It is not completely, but almost KClO3 free.
https://www.metallab.net/jwplayer/video.php?v=L2NsaXBzLy9NYW...
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woelen
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In your starting post you mention that you used K2Cr2O7 and that you have no perchlorate. Now I read you used K2S2O8 and have some KClO4. I assume
that is in a new experiment.
If I were you, I would add a little KHSO3 and some drops of H2SO4 to your KClO4 and boil off, until there is no more smell of SO2. In this way, you
are sure that you don't have any KClO3 left in your KClO4. Your boiling solution should be clear, no crystals in it. Use as little as possible of
water in your process, such that the KClO4 completely dissolves on boiling. Then add about 10% addiitonal water and keep boiling and then add the
KHSO3 and stir, till all is dissolved again. Then add drops of H2SO4. You should smell some SO2. If this is not the case, then add a little more
KHSO3. In this way all KClO3 is destroyed. On cooling down, KClO4 crystallizes and in a refrigerator you can recover nearly 100% of your perchlorate.
After one crystallization, your KClO4 may contain a little K2SO4 and KCl. After a second recrytallization from water, such that the solid just
dissolves on boiling, you'll get KClO4 without KCl, and only very little K2SO4.
A few tenths of percent of K2SO4 in your KClO4 is not a problem. A little KClO3 really is a problem. KClO3 is a highly undesirable contamination in
KClO4, both for aqueous chemistry and for experiments, where you use it as a solid oxidizer at high temperatures.
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metalresearcher
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Woelen, indeed it is another experiment after my first attempt.
The idea of KHSO3 + H2SO4 sounds good. I tried concentrated HCl which releases Cl2 gas with KClO3, but that is war worse than SO2.
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Mister Double U
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Ozone is your Friend
Hey metalresearcher,
With a Lead Dioxide anode you can run your cell until full conversion. Once Ozone smell starts you are getting close to the end. Then just test a ml
with some conc. HCl and heat to see, if still yellow ClO2 is formed. If so, let the cell run for another day and repeat this procedure until no more
reaction occurs. This way you don't have to add any additional chemicals to your cell liquor.
I have found this to be the easiest method.
On the other hand, if you can't run until full conversion (or are worried about current efficiency), then woelen's method sounds really great :-).
When I ran a 1.5 liter Sodium Perchlorate cell at 5A it usually took ~3 days from the first smell of Ozone until full conversion.
Hope that helps & friendly greetings!
@Deathunter88: Where is the info about Sodium Persulfate as a reduction inhibitor from? I have read that before somewhere but wonder if it would not
simply be reduced at the cathode to the Sulfate. On the other hand, if it works it would be great to have another substance for a reduction inhibitor
in the repertoire :-D.
[Edited on 27-2-2025 by Mister Double U]
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vertexrocketry
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be careful as you will make chloric acid
(i made some once it was not nice.............)
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Mister Double U
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Hello vertexrocketry,
In this instance no free Chloric Acid is produced as that would require additional 'free' protons to be introduced.
Here the electrode reactions:
Anode: [ClO3-] + H2O - 2*[e-]-> [ClO4-] + 2*[H+]
Cathode: 2*H2O + 2*[e-] -> H2 + 2*[OH-]
As you can see, for each proton which is generated at the anode, there is also a hydroxide ion generated at the cathode - so no free acid is formed.
If you do this in a divided cell it is a different story though.
Would you mind sharing more about your incident?
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