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bearbot22
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Quote: Originally posted by dicyanin  | Beautiful work  I am tempted to build one according to your design. Is the
resistance across the shrimp breeding tube diaphragm workable? I've used flowerpots succesfully in the past, but resistance is usually high even after
soaking in dilute sulfuric acid, things heat up fast and especially the anode suffers from this. The other downside of flowerpots is their shape &
size and the fact that the setup is open to the atmosphere releasing corrosive vapors.
I have always wondered about Tyvek, which is semipermeable high density polyethylene, it allows for gas/vapour exchange but is waterproof, so I'd assume it would let current pass but not the solvent? Tyvek is said to be quite
resistant against acid and base.
Or perhaps these cylindrical shaped corundum lab crucibles?
You can buy Nafion perfluorosulfonic acid-PTFE membrane from China but even the cheaper knockoffs are $20 for a 3 x 3 cm size film. I've also seen H-type divided cells for sale but they aren't cheap either, up to $200 for 2x 100ml size with a clamp and Nafion diaphragm included.
To prepare a lead cathode, lead can be finicky regarding surface purity and give inconsistent results. It is best to pre-treat them in a divided cell
with 10% aq. H2SO4 or Na2SO4 electrolyte, use the cathode-to-be as anode (and another piece of lead as
cathode) and run a current through it until it is covered with a thick chocolate-brown layer of lead dioxide. Then switch polarity and run a current
until all oxide is reduced, leaving a gray layer of spongy lead. In this way even roofing sheet grade lead can be used as a reliable high overvoltage
cathode for electroreductions with no other preparation that a light sanding and an acetone rub to clean the surface before pre-treatment.
The only thing I would add to your design, although it is optional, is an extra hole for a reference electrode over the catholyte. I've seen Chinese
calomel and silver-silver chloride reference electrodes sold for about $30 a piece. This allows for fine-tuning the required voltage over the cathode
for a specific reaction, minimizing loss as heat and competing hydrogen evolution.
| Quote: Originally posted by Hexabromobenzene | | I know about separator from li-ion battery, but separator with enough size is rare. Plastic bag is common. They made from polyethylene. Can thin bag
use for electrolysis? |
Back in the day I've used the separator envelopes inside car batteries, which separate the lead electrodes from the lead dioxide wafer electrodes.
They consist of microporous polyethylene, and they work well as diaphragm, but in my experience they tend to be leaky and easily damaged. Also I don't
want to go through the hardship of opening an old car battery again, probably contaminated myself with enough lead for one lifetime during that
ordeal.
I don't think a plastic bag will work. The porous PE separators resemble paper more than plastic bags in how they feel and how easily they tear.
Here's some reading material on the manufacture of microporous battery separators.
[Edited on 4-4-2024 by dicyanin] |
I my case [i regular do organic reductions to amines using H2SO4(7%) in H2O/IPA mix as electrolyte] 1,5 Amperes with ~7 Volts are possible with the
shrimp clay tube. Could be more Volts and Amps but i'm limited by cooling capabilities, because the run needs to be kept below 15°C.
As far as i remember Tyvek was one of my first tries as a membrane. It's a good electric insulator and no liquid i know of would permeate it. Did not
work for me.
I didn't try corundum as they were quite expensive from local Lab supply
I totally agree on cathode surface to be a crucial factor, therefore i use them only once and prepare them with care.
mine are cut out of rolled lead sheets from roofing supply which his supposed to be 99.9% pure.
The suggested oxidize-reduce pretretment was used by me in the beginning.
Actual i get best results with this pretreatment, done right before starting the run:
cut to size piece of lead sheet is rolled on a clean hardened glass plate both sides. sanded wet both sides, cleaning the glass plate very time it's
turned over. Finally polished to a mirror-like finish using a heavy brass (polished) tube. No polishing paste, no acetone wipe.
Right after polishing the cathode is put in diluted sulfuric acid and coated with zinc using a circular anode taken from a zinc-carbon battery (20mA /
0,8V / 20min).
Reference electrode would be a next step but this would also include more sophisticated power supply etc... would love to read from anyone to
challenge this.
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Catslovechemistytoo
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| Quote: Originally posted by bearbot22 |
I my case [i regular do organic reductions to amines using H2SO4(7%) in H2O/IPA mix as electrolyte] 1,5 Amperes with ~7 Volts are possible with the
shrimp clay tube. Could be more Volts and Amps but i'm limited by cooling capabilities, because the run needs to be kept below 15°C.
I totally agree on cathode surface to be a crucial factor, therefore i use them only once and prepare them with care.
mine are cut out of rolled lead sheets from roofing supply which his supposed to be 99.9% pure.
The suggested oxidize-reduce pretretment was used by me in the beginning.
Actual i get best results with this pretreatment, done right before starting the run:
cut to size piece of lead sheet is rolled on a clean hardened glass plate both sides. sanded wet both sides, cleaning the glass plate very time it's
turned over. Finally polished to a mirror-like finish using a heavy brass (polished) tube. No polishing paste, no acetone wipe.
Right after polishing the cathode is put in diluted sulfuric acid and coated with zinc using a circular anode taken from a zinc-carbon battery (20mA /
0,8V / 20min).
Reference electrode would be a next step but this would also include more sophisticated power supply etc... would love to read from anyone to
challenge this.
|
Interesting. But how are you using zinc coating under acidic conditions? Or are you using a different electrolyte in that case? I don't understand
your logic of doing this, zinc and lead have nearly similar overpotentials, why coat it with zinc?
And in the case of organic reductions to amines, what typical yields do you get? I wonder if electrochemistry can substitute NaBH4 or Na(AcO)3BH in
reducing imines.
I've read about another way to prepare lead cathode:
A lead electrode is put into 20% sulfuric acid and anodized (another lead electrode is used as cathode). The current is set to 0.08-0.1 A/cm^2, and
electrolysis is done for 5 minutes. The oxidised lead electrode is washed with water, then the polarity of the cell is changed and the oxides are
reduced to lead sponge. This is repeated for 3 times and in the end of the process the lead electrode is anodized once again. (Source: Tafel J. Ber.,
1900, Vol. 33, Page 2224) Edit: just realised another person already wrote about this here.
[Edited on 11-8-2024 by Catslovechemistytoo]
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bearbot22
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| Quote: Originally posted by Catslovechemistytoo |
Interesting. But how are you using zinc coating under acidic conditions? Or are you using a different electrolyte in that case? I don't understand
your logic of doing this, zinc and lead have nearly similar overpotentials, why coat it with zinc?
And in the case of organic reductions to amines, what typical yields do you get? I wonder if electrochemistry can substitute NaBH4 or Na(AcO)3BH in
reducing imines.
I've read about another way to prepare lead cathode:
A lead electrode is put into 20% sulfuric acid and anodized (another lead electrode is used as cathode). The current is set to 0.08-0.1 A/cm^2, and
electrolysis is done for 5 minutes. The oxidised lead electrode is washed with water, then the polarity of the cell is changed and the oxides are
reduced to lead sponge. This is repeated for 3 times and in the end of the process the lead electrode is anodized once again. (Source: Tafel J. Ber.,
1900, Vol. 33, Page 2224) Edit: just realised another person already wrote about this here.
[Edited on 11-8-2024 by Catslovechemistytoo] |
Zinc is used on cathode-side, so it will not oxidise, even in acidic conditions.
I read about Zinc plating increases reducing potential for aromatic nitroolefins in a reference and my experiments confirm this. Unfortunately i can't
find the ref. anymore. Electrochemical organic reductions are influenced by all aspects of a cathode, not just overpotential.
>And in the case of organic reductions to amines, what typical yields do you get? I wonder if electrochemistry can substitute NaBH4 or Na(AcO)3BH
in reducing
>imines.
I get about 50 to 60% of theoretical yield, maybe more. Some % of product is lost because solvents used for extraction are not optimal.
In theory it's possible to use electrolytical reduction instead of reducing agents. Electrical current is most powerful reductor in chemistry. You can
check electrochemistry_of_organic_compounds in Sciencemadness' Library.
>I've read about another way to prepare lead cathode:
>A lead electrode is put into 20% sulfuric acid and anodized (another lead electrode is used as cathode). The current is set to 0.08-0.1 A/cm^2,
and
>electrolysis is done for 5 minutes. The oxidised lead electrode is washed with water, then the polarity of the cell is changed and the oxides are
reduced to
>lead sponge. This is repeated for 3 times and in the end of the process the lead electrode is anodized once again. (Source: Tafel J. Ber., 1900,
Vol. 33,
>Page 2224) Edit: just realised another person already wrote about this here.
The polished cathode surface is beneficial in my reductions for physical reasons, i guess. Hydrogen bubbles are separating faster from a smooth
surface. A spongy cathode is probably blocked to a certain level by Hydrogen Gas sticking to it.
I'm aware H2 bubbling could be avoided by using lower voltages, but I need to raise voltage to pump enough current through the cell.
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