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khlor
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[*] posted on 29-1-2023 at 07:45
Electrodialysis


Hello, I've been on this forum for a while, and I am yet to see a post dedicated to electrodialysis(ED) so.. I decided to start something here, and with some reports and observations from my own experiments, but I wish to know if someone else has done something on the subject and also why is it not so popular? I mean, is it too expense compared to other methods of obtaining chemicals? is it a niche subject? or so it just happens that I am not looking it right?

Anyhow the rundown is the following some of my runs were crude and the data collection not too scientific but it could shed some light or give someone some insights(at least when it comes to a practical capacity) note that these runs were done some time ago. and all of the tests involve in some way sulphate ions and sulphuric acid, which is one of the products and was not titrated, but qualitative tests were made to confirm. such as the formation of CuSO4, Al2(SO4)3, pH and reacting Ca(H3C-CO2)2 to form CaSO4 precipitate.

Summary

With the success of the first full run on the latest model of electrolytic
reactors with dual chamber that allows for collection electrolytes and ceramic
filter. Now, same setup but different configuration and conditions are being
introduced. test run is expected to be finished within 3.5 to 4 weeks
time(full cycle 7 days) yielding relatively clean dilute sulfuric acid.

Test Conditions

Roughly 500ml volume on each chamber(cathode and anode)
Voltage(max) 18.5v allowable for variations, mean expected 16v
Upon warm up measured current: 10mA
Anode chamber electrolyte: unknown concentration mixture of Na2SO4,
NaOH(traces), Ca(OH)2(traces), CaSO4(traces)
Cathode chamber electrolyte: Unknown concentration of Ca(OH)2(mostly
precipitate), Na2SO4, CaSO4(mostly precipitate - expected to be leached by
evolution of NaOH oh cathode)
Anode: Graphite
Cathode: Stainless Steel, Copper covered graphite rod
Ceramic Filter diaphragm
Anode chamber pH after 2 hours: measured between 3-4

Expectations

The leftover CaSO4 from the last experiment on the cathodes will be leached by
the evolving NaOH, for which Na+ ions will come from the anode mixture
migrated to the cathode chamber leaving soft Ca(OH)2 precipitate at the end.
acidity on the anode will evolve and lower pH to below 2.

Anode chamber: Cation ions migrate to the cathode chamber(Ca^2+ ans Na^+)
leaving SO4^-2 to accumulate sulfuric acid.

Cathode chamber: Na^+ ions will form NaOH that will react with CaSO4 leaving
Ca(OH)2 and Na2SO4 to and increasing the mobility of SO4^-2 ions to the anode
chamber.

expectations and estimates on amounts and yield cannot be made due to the
unknown concentration on the source of the ions.

--EOR

Report #

Measured parameters:
Voltage raised to 18.5v the previous day
Current significantly lowered to less than 50mA
pH around 1

Experiment was prematurely ended since the expected results were reached way
before the given deadline. like the last experiment, anode solution was
collected to be graded in the future, flame test revealed little presence of
Na^+ ions and given the Na^+ ions color overpower any other ions it is safe to
assume that the majority of the sodium ions has migrated over to the cathode
chamber as expected and since acidity of the anode sharply increased it is
safe to assume that sulfuric acid has evolved as expected however, since the
diaphragm has not been properly cleansed it is also expected to have some
copper contamination, traces perhaps, further testing with electrowining
setups may reveal the extent of the contamination, since it did not appeared
on the flame test.

As expected the solution darkened due to suspended fine graphite particles
from anode erosion. for now solutiuon has been transfered to a holding
container filtered with a simple gravity filtration setup for further
processing and testing. titration may reveal acid concentration or at least
give rough estimations. throughout experiment it was observed volume level
shifting towards the cathode, as expected since sodium ions solutions are
hygroscopic and do pull water through osmosis however sulfate ions migration
is not expected.

From the above observations it is correct to assume that in order to produce
acids in this manner it is best to use sodium salts probably due to the small size of
sodium ions, organic acids can not be used due to the same effect produced by
kolbe electrolysis cells and since the diaphragm used is ceramic, it is best
to run the cell with a slightly basic solution on the cathode chamber to
increase conductivity and help to jumpstart the operation in a fast manner and
the salt solution to be kept at the anode to promote cationic migration over
the cathode. it seems that the process is far more efficient than electrolysis
in on itself. future tests have to be conducted with known concentration salt
solutions in order to gauge energetic efficiency and to get hard evidence that
this is faster and more efficient electrolysis, with a few drawbacks, the
major one being that the salt solution has to be of extreme purity or else the
resulting acid solution on the anode will be contaminated with whatever
contamination the salt might have i.e. large cations such as copper, iron or
other anions forming acidic mixtures and other non ionic contaminants.




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blogfast25
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[*] posted on 29-1-2023 at 12:00


Can you please summarise the objective of your experiments? Thanks.



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[*] posted on 29-1-2023 at 12:28


Basically, the my main objective with this is study the possibility of making acids and bases from salts. how feasible it is. from my experience, with my current setup and knowledge based on what I done, it may be possible to "farm" acid/base "separators" though the solution will be very dilute and depending on the anode (graphite on my case) dirty but it may be usable for something and since it uses low wattage, it could be used over a long time. an equipment that stands on the background making acid/base overtime. I don't know... it may be "esoteric" and a waste of time, but I guess I went far enough on my own with this and having external input might help to define what direction should I take with this if any at all... I know that there are plenty of ways to get things like sulphuric acid, from CuSO4, battery water, drain cleaner(for those on US, seems the rest of the world uses NaOH) but I do like to explore new ways(wasteful as though these ways may be) and somewhere along the way find something new.

I hope that shed some light on your question... for I'm not sure that "objectives" means what I aim to achieve or what my motivations are. so I tried to approach both angles. sorry for the long text. I do tend to get carried away.




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[*] posted on 30-1-2023 at 07:12


Quote: Originally posted by khlor  

I hope that shed some light on your question... for I'm not sure that "objectives" means what I aim to achieve or what my motivations are. so I tried to approach both angles. sorry for the long text. I do tend to get carried away.


So you're trying to separate (say) Na+ and (say) Cl- by a form of electrophoresis, so that:

NaCl + H2O --> HCl | NaOH

... across an ion-permeable membrane.

How are going to stop migrating the H3O+ and OH- from migrating across that membrane?

What membrane/diaphragm are you using?

How do you prevent electrolysis?

[Edited on 30-1-2023 by blogfast25]




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[*] posted on 30-1-2023 at 16:48


Did not had results with NaCl, not on the Acid part... it turns to chlorine . I guess that hologen salts do not work. But oxyacids do, except nitric. As for the diaphragm, I use a ceramic/clay filter white clay. Though as per demonstration by a YouTube channel(Scrap Science) claypots do work as well... I should try some membrane recipies from mysteriusbhoice. But I didn't got around that yet. So far my success is limited by sodium sulphate. But yeah it is as you described, but instead of chloride you put sulphate in there.



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[*] posted on 30-1-2023 at 19:31


when using a diaphraghm and with the anode chamber containing Na ions you will get a mix of H2SO4 and NaHSO4.
If you want pure sulfuric from Na2SO4 then you need to use an AEM or feed your salts cathodically and have the anode chamber as pure water or dilute H2SO4
Anode recommended for this process is MMO Ir-Ta or PbO2 as they tend to be stable in O2 evolution conditions and graphite will errode badly here.
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[*] posted on 31-1-2023 at 01:56


Quote: Originally posted by mysteriusbhoice  
when using a diaphraghm and with the anode chamber containing Na ions you will get a mix of H2SO4 and NaHSO4.
If you want pure sulfuric from Na2SO4 then you need to use an AEM or feed your salts cathodically and have the anode chamber as pure water or dilute H2SO4
Anode recommended for this process is MMO Ir-Ta or PbO2 as they tend to be stable in O2 evolution conditions and graphite will errode badly here.


What's an AEM?

What concentrations of, say, H2SO4 can you achieve?

Do you have any lit. references for this process?

[Edited on 31-1-2023 by blogfast25]




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[*] posted on 31-1-2023 at 03:54


Quote: Originally posted by blogfast25  


What's an AEM?

What concentrations of, say, H2SO4 can you achieve?

Do you have any lit. references for this process?

[Edited on 31-1-2023 by blogfast25]


That is an anion exchange membrane which is a polymer containing quaternary amine functionality which allows the transfer of sulfate anions through the polymer along with other negatively charged ions in solution while blocking positively charged sodium potassium etc..

https://www.researchgate.net/publication/350945585_Electrodi...
This reference talks about a process in which an AEM is used and a concentration of 180g/l is achieved but that is a continuos flow cell while the amateur using a standing cell can expect up to 360g/l concentrations.
Flow cells need to use both CEM and AEM but the standing cell only needs an AEM and cathodic feed of desired soluble sulfate salt Na2SO4 MgSO4 CaSO4(with Na2SO4 catalyst) (NH4)2SO4etc...
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[*] posted on 31-1-2023 at 04:38


Quote: Originally posted by mysteriusbhoice  

Anode recommended for this process is MMO Ir-Ta or PbO2 as they tend to be stable in O2 evolution conditions and graphite will errode badly here.


That is good and fine but with sulfate electrolyte, you can use lead metal as anode and pbO2 will form. Lead anodes are used to electrolyze sulfuric acid from copper sulfate




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[*] posted on 31-1-2023 at 08:07


Quote: Originally posted by mysteriusbhoice  
Quote: Originally posted by blogfast25  


What's an AEM?

What concentrations of, say, H2SO4 can you achieve?

Do you have any lit. references for this process?

[Edited on 31-1-2023 by blogfast25]


That is an anion exchange membrane which is a polymer containing quaternary amine functionality which allows the transfer of sulfate anions through the polymer along with other negatively charged ions in solution while blocking positively charged sodium potassium etc..


This is all rather promising, what with the draconian restrictions on SA and NA in the EU and UK.

But the method doesn't work for HNO3?




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[*] posted on 31-1-2023 at 08:20


@blogfast- I’m pretty sure nitrate is reduced to ammonium ion at certain cathodes. Now graphite and GSLD anodes should be fine for nitrate since one would use lead nitrate with copper nitrate catalyst is preparation of GSLD anodes. Just anticipate lower yields based on cathode reduction



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[*] posted on 31-1-2023 at 11:00


Quote: Originally posted by chloric1  
Now graphite and GSLD anodes [...]


You keep throwing acronyms at me. "GSLD"?




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[*] posted on 31-1-2023 at 12:47


Sorry

Graphite
Substrate
Lead
Dioxide

Anodes





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[*] posted on 31-1-2023 at 13:06


Quote: Originally posted by chloric1  
Sorry

Graphite
Substrate
Lead
Dioxide

Anodes



So what does that look like, when it's home?




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[*] posted on 31-1-2023 at 18:56


Quote: Originally posted by blogfast25  

This is all rather promising, what with the draconian restrictions on SA and NA in the EU and UK.

But the method doesn't work for HNO3?


The nitrate anions would oxidize at the anode to N2 and O2 so sadly it wll not work effectively past an equilibrium concentration without a means of continuos removal.

There is one way and that is using a copper/iron sacrificial anode where the nitrate forms copper/iron nitrate which can b oxidized to iron 3 and copper 2 nitrate which when dry distilled yields RFNA which you can dilute into regular nitric acid at 68%.

[Edited on 1-2-2023 by mysteriusbhoice]
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[*] posted on 31-1-2023 at 18:59


Quote: Originally posted by chloric1  
Sorry

Graphite
Substrate
Lead
Dioxide

Anodes



GSLD anodes suck badly because at the interface while running CO2 is produced at high current densities due to OH radical attack and interface oxidiation which limits their working current density to below 100ma/cm^2.

plain lead tin solder 60/40 cast into a plate is fairly resistant to H2SO4 while more so than regular lead due to tin oxide being notoriously hard to dissolve in acids.

MMO electrodes are still preffered for such an application as this you can buy Ir-Ta in aliexpress and make sure its Ir-Ta because Ir-Ru wont work as its only for saltwater.
https://www.aliexpress.com/item/1005002682948243.html
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[*] posted on 1-2-2023 at 06:40


Thanks for all that info so far!

For SA, wouldn't sodium bisulphate be a good starting point? Cheap as chips and unregulated.

So if I wanted to run an electrodialysis experiment as a beginner, what conditions would you recommend, in term of diaphragm, electrodes, voltage etc etc?

Quote: Originally posted by mysteriusbhoice  



plain lead tin solder 60/40 cast into a plate is fairly resistant to H2SO4 while more so than regular lead due to tin oxide being notoriously hard to dissolve in acids.



Yes, but tin is quite soluble in non-oxidising acids but not NA. Lead is mainly soluble in NA and peroxide/glacial acetic acid.


[Edited on 1-2-2023 by blogfast25]




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[*] posted on 1-2-2023 at 14:32


while tin is soluble when oxidized to tin 2 oxide and polarized anodically it is insoluble in acids just like some surprising metal oxides including Fe3O4 which normally is soluble in acids but when polarized will not dissolve in conditions where it would.
BTW magnetite wont work here because eventually at high concentrations it will be attacked and your best bet is either lead or tin lead alloy for cheap anodes.
Bisulfate can be used because its quite easy and you can feed it into the cathode chamber yes but you must use an AEM which you can buy from aliexpress. because otherwise it having a high concentration of both anions and cations in one chamber will make it so that shit like donnan effect will make it hard to seperate them and best you will end up with is a mix of sulfuric and bisulfate slush as your final product.
I recommend getting a commercial AEM when trying to seperate out monovalent salts because if you had used MgSO4 then you can keep using a terracotta/fiberglass as it precipitates Mg(OH)2 into the cathode chamber while NaHSO4 will first form Na2SO4 but you will get Na ions leeching into the aode chamber.
https://www.aliexpress.com/item/4000838462090.html
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[*] posted on 1-2-2023 at 14:54


Quote: Originally posted by chloric1  

That is good and fine but with sulfate electrolyte, you can use lead metal as anode and pbO2 will form.

My experience with this has been poor. The oxide layer flakes and cracks.
I have had success taking this brittle material and melting it into a solid anode
Nice red and yellow colors. Very toxic process. Propane torch will work
After a little use, or long exposure to air it turns black and rough but stays together

Final product is very easy to break. Needs a substrate




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[*] posted on 1-2-2023 at 16:44


for electrodes honestly its either lead or MMO (Ir-Ta) if you bought Ir-Ta for chlorate production then u can simply use those for this too.
PbO2 anodes work.
doped SnO2 electrodes also work.
MnO2 anodes work.
Fe3O4 works but below 10% acid concentration.
Other types of custom MMO also works like Ta2O5 or TiO2 with a dopant to make it conductive like antimony/cerium.

video on preparing homemade PbO2 entirely from scratch skip to part 3 if you already have an MMO electrode of any type to use as a substrate.

Pt3
https://www.youtube.com/watch?v=xECCZYFDK30

Pt2
https://www.youtube.com/watch?v=cXH1qT3k-9k

Pt1
https://www.youtube.com/watch?v=HG6ZLxmd4MU

[Edited on 2-2-2023 by mysteriusbhoice]
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[*] posted on 2-2-2023 at 02:27


Quote: Originally posted by mysteriusbhoice  
while tin is soluble when oxidized to tin 2 oxide and polarized anodically it is insoluble in acids just like some surprising metal oxides including Fe3O4 which normally is soluble in acids but when polarized will not dissolve in conditions where it would.


Thanks again for the info.

How does, for H2SO4, does electrodialysis compare to ordinary electrolysis of CuSO4?

CuSO4 + H2O --> H2SO4 + Cu + 1/2 O2

[Edited on 2-2-2023 by blogfast25]




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[*] posted on 2-2-2023 at 03:14


they both work on the same principle but see copper goes from its +2 to 0 oxidation state while Na+ or Mg2+ do not change their oxidation state and therefore the only way to seperate the ions and form sulfuric acid is to use an ion selective polymer membrane to seperate the 2 bulk solutions from eachother but allow ions to pass through that thin piece of plastic.

This catboy below can explain it.
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[*] posted on 3-2-2023 at 19:36


So, lead anodes would work with sulphate without dissolving? that is good to know. what concerns me is using lead and lead compounds at home, even insoluble compounds do concern me. I mean, in a proper lab or a proper work place it would be okay... but doing stuff like this at home give me some concerning levels on anxiety.

to all talk about HNO3, if there is a way, it is pretty complicated. I did some digging and well... HNO3 or NO3- ions do weird stuff...

speak of mysteriusbhoice... and he appears... seriously that is amazing!

it is all good everyone is excited with anodes and ion exchange membranes... still, I do have some information that may help those who lack access and still use the simple stuff(ceramic filters diaphragm, sorry but it is simple, affordable and not too much work to get my hands on it) instead of using ion exchange membranes and move ions from cathode chamber to anode chamber and make acids, I did discover, back on my experiments, you can use a small cation like Na+ and move it from the anode chamber to the cathode chamber. true, the acid solution might be a little contaminated and yet it works. so I put Na2SO4 solution on anode chamber and move the Na+ ions out of there. But, some of the points that I'd like to touch here is... how useful this process is for people like us and is it as energy intensive as electrolysis or can I save watts? I ran the numbers but I still don't believe them.I do not want to trick myself into being biased.




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