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gravityzero
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Porous Pot for Voltaic Cell
Would like to start looking into electrolysis cells a bit further.
Found a patent example that calls for a porous pot basically 65mm x 190mm
Examples have been given using a terra cotta flower pot as a suitable vessel.
Attempting to keep the diagram as true to form as possible is causing some issues.
Other than actually making the vessel, nothing within the dimensions, purchasable, has been found.
The closest item found is 50mm x 150mm, which is fairly far off.
Any ideas on how or where to get such an item is much appreciated.
Thanks to all in advance. 
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CRUSTY
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Do you mean 190 mm wide or tall?
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Melgar
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Crush up all the terracotta. You don't need it anyway.
Crush it to powder then mix it with water. Mix it until it forms a workable clay.
Mold it into the shape you want. Make 3-4 in case something bad happens.
Let it dry. Like, completely.
Fire it. Kids have done it for school projects, so how hard could it be? http://www.homeschooling-ideas.com/smoke-firing.html
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unionised
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Quote: Originally posted by Melgar  | Crush up all the terracotta. You don't need it anyway.
Crush it to powder then mix it with water. Mix it until it forms a workable clay.
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I rather doubt that will work. The terra has already been cotta
Chemical changes take place during the first firing, so a second firing won't work properly.
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gravityzero
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Thanks for all the suggestions. It is appreciated. The width is 65mm x height of 190mm.
I agree with Melgar, except I would just use terra cotta clay that is new, not previously fired.
It appears that this can be done easier than originally thought.
New clay would be shaped, dried, then fired.
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Sulaiman
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For my very disappointing experiments in electrochemical voltaic cells I used two of these pots
http://www.edulab.com/prod/daniel-cell-porous-pot-150x50mm
which, judging by dimensions alone, are probably similar to the pots that you originally mentioned.
They are MUCH more porous than a teracotta flower pot.
Although it is best to follow a working example as closely as you can,
in this case just a few considerations will allow scaling with no detrimental effects;
- scale currents in proportion to areas of electrodes to achieve constant current density (A.m-2),
which is, I believe, what has a great effect on the electrochemistry at the electrodes
- maintain molar concentrations of electrolytes (overall cell e.m.f.) rather than quantities/portions/weights etc.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Melgar
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| Quote: Originally posted by unionised | I rather doubt that will work. The terra has already been cotta
Chemical changes take place during the first firing, so a second firing won't work properly. |
It was my impression that firing clay bound the particles together via partial vitrification, which is more of a physical rather than chemical
process, and could be repeated. Oh shit, never mind. Forget I said anything. Last thing I want is to get into another argument with you about glass
chemistry. (In the real world, there's no such thing as vitrification, right?)
The reason so many experiments are done using porous unglazed pots, is because of how widely-available those were during the 19th century, when much
of the initial experimentation was done, regarding electrochemistry. They did not have the variety of inert materials that we have today, especially
plastics. PETE plastic is quite inert to decomposition, and conveniently, is also the polymer that's used to make polyester fabric. So if you had
some thick polyester fabric, and perhaps access to a sewing machine to make it the correct shape, this would allow you to make a superior divided cell
to those terra cotta pots.
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unionised
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| Quote: Originally posted by Melgar |
It was my impression that firing clay bound the particles together via partial vitrification, which is more of a physical rather than chemical
process, and could be repeated. Oh shit, never mind. Forget I said anything. Last thing I want is to get into another argument with you about glass
chemistry. (In the real world, there's no such thing as vitrification, right?)
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Your impression was mistaken.
It's a dehydration reaction, then a vitrification.
The dehydration isn't reversible on any sensible timescale.
You could get into fewer arguments with me and others by not posting stuff that's plainly wrong.
As you say, "The first step in the process of learning something is admitting that you don't know it already."
[Edited on 28-10-17 by unionised]
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Sedit
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I always just used a regular old flower pot with the hole plugged up. Sometimes you can find them without a hole which is even better. Standard use
calls to place a material that has the same Ion you wish to use in it as an electron transfer, you run the voltage backwards and it causes the Ions to
get trapped inside of the clay pot. You can then run take it out, wash it off and put in your working chemicals and it acts like a solid barrier to
the chemical you did not want to come through.
What Meglar said BTW has 0 chance of working. Fired Clay is an Aluminum Silicate, you cant reverse the process. Before hand Aluminum Oxides work with
Silica to form the fun sticky mess you know of as clay through hydrogen bonding but after firing its a totally different chemical.
| Quote: | | So if you had some thick polyester fabric, and perhaps access to a sewing machine to make it the correct shape, this would allow you to make a
superior divided cell to those terra cotta pots. |
Sorry this is just not true either, it would be very hard to get the fabric to produce pore sizes on the scale that you will get using fired clay,
Fired clay is great, its basically porous glass, what could really be better?
[Edited on 29-10-2017 by Sedit]
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Melgar
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For the record, I'm not actually that invested in whether crushing up a terracotta pot could make it so you could mold and fire it again. I still
think you could sort of do it, based on my understanding of ceramics, since there would be SOME partial melting where the vitrified parts touched each
other, but the results would probably be crap. I don't remember why I actually suggested doing that in the first place, but I meant it in the same
way that I might put "1) Smack yourself in the face for being that careless with nitric acid" as a step in a process. Trying to amuse myself while
giving advice, I suppose. The real point I was trying to make was that it's not that hard to fire clay yourself, even without a kiln.
| Quote: | | Sorry this is just not true either, it would be very hard to get the fabric to produce pore sizes on the scale that you will get using fired clay,
Fired clay is great, its basically porous glass, what could really be better? |
However I am somewhat invested in the idea that thick polyester cloth could make a suitable divider for a divided cell. Porous clay is really not
that great. Salt moves around in it via capillary action and clogs up those pores very easily if it dries out at all. Often, it will clog up those
pores even if it stays immersed, due to the higher resistance and greater heat generated within the ceramic. This heat will drive out the water, and
leave behind the salt. And once the pores are clogged up with salt, it can be next to impossible to unclog them. Additionally, how do you come to
the conclusion that the pore size in porous ceramic is ideal? Because so many people on these boards have had nothing but success with them? Of
course not; everyone is trying to find superior alternatives because of how much grief clay pots have caused them. The resistance is almost always
too high, and they run into the problem with heat driving out the water and leaving salt, if they turn up the voltage to compensate. Now, if you look
at every single divided cell that's in use in the modern world, they're either made of paper (cheap), fritted glass (long life), or a porous polymer
(compromise between the other two). Really, the divider just has to provide enough of a physical barrier to prevent too many ions from going back and
forth between the electrodes, getting subsequently oxidized and then reduced. (edit: or in the case organics and some inorganics, being irreversibly
oxidized or reduced) If a small fraction cross over though, and get on the wrong side, it's not typically a huge problem. The bigger problem is
likely to come from a resistance that's much too high. Also, clay is more like a zeolite full of miscellaneous ions, than it is like glass. Now, if
you don't mind random iron, aluminum, magnesium, calcium, sulfate, silicate, etc. ions getting into your solution and doing whatever they're prone to
do, then have at it. Maybe they were necessary for the reaction that OP is trying to replicate? However, for a general-purpose cell divider, a
terracotta pot is far from ideal.
[Edited on 10/29/17 by Melgar]
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unionised
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| Quote: Originally posted by Sedit |
| Quote: | | So if you had some thick polyester fabric, and perhaps access to a sewing machine to make it the correct shape, this would allow you to make a
superior divided cell to those terra cotta pots. |
Sorry this is just not true either, it would be very hard to get the fabric to produce pore sizes on the scale that you will get using fired clay,
Fired clay is great, its basically porous glass, what could really be better?
[Edited on 29-10-2017 by Sedit] |
OK, on this one, I agree with Melgar.
You can use cloth, and they do (albeit asbestos cloth was traditionally used in this case).
It seems you can get asbestos free versions; it's not clear if this stuff is asbestos or not, but it's a cloth diaphragm for electrolytic cells.
https://www.alibaba.com/product-detail/Non-Asbestos-Diaphrag...
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Sulaiman
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If you are experienced in electrochemistry (I am not) then choosing and using a membrane is a complex subject,
there are membranes far superior to a porous pot,
but I think that it is best to start with something which even though not perfect, it is predictable, and it produces results similar to those
published,
then go on to look for the ultimate membrane once you have an appreciation of the subject.
I just want to experience and understand the principles involved, for which porous pots work just fine,
if I wanted an efficient commercial cell - - I'd buy one made commercially
The pots that I used are I guess as good as were used by the pioneers, and all of the problems experienced in the early voltaic cells can be
investigated.
I have not yet tried these pots with electricity going in, only out, as primary voltaic cells,
so I do not know how they would perform in electrolysis cells for example,,
but the pots are inert, durable and cleanable (mostly), so they are very re-usable.
When starting a new topic of experimentation I like to minimise the number of ways that I can fail (I'm expert at failing !)
and as the membrane is a critical part of an electrochemical cell,
it is best for me to start with a known good performer.
For high current electrochemical cells I would expect the resistance of the electrolyte filled pores to be significant but not insurmountable.
In summary: I recommend porous pots for initial electrochemistry experimentation and learning,
(unless someone can point to a tried and tested, foolproof (especially in my case), reliable, available, cheap, general purpose membrane)
CAUTION : Hobby Chemist, not Professional or even Amateur
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bobdring
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Have you seen the porous pots on Haines educational?
https://www.haines.com.au/
They have two listed but the largest is
Porous pot, 150 x 50mm L x D
Item Code : GE364501
Price
$3.00 (excl. GST)
$3.30 (incl. GST)
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macckone
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polypropylene 5 micron hepa vacuum cleaner bags.
https://www.homedepot.com/p/Cen-Tec-Replacement-HEPA-Vacuum-...
You can get these things at just about any store that sells vacuum cleaners.
I would assume the same is true in other countries.
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Melgar
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Wow. I certainly wasn't expecting that.
Hug?
Err.. maybe later.
Reading more on the subject, I'm seeing that it's beneficial for the membrane to have ion exchange capabilities. This provides a sort of buffer layer
of neutral ions between the two sides, so that when an ion enters one side, it quickly bumps a stationary ion off of its site, which then takes its
place carrying the charge. With glass or fiberglass, these ions are typically sodium, calcium, silicate, and carbonate, possibly others. Clay has
plenty of its own ions, which vary depending on the clay. With polymers, you need the surface to be somewhat polar, because otherwise it'd repel
water and ions, and typically the treatment that makes the surface polar consists of partially oxidizing it. To provide ions for ion exchange, paper
and polymer membranes are often impregnated with a salt, chosen for having relatively inert ions. Potassium sulfate, for example. This step would
probably be unnecessary, though, if you only cared about what was going on in one of the two half-cells.
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Sulaiman
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Have you actually tried this material ?
Does water wet it (do the pores fil with electrolyte) ?
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unionised
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The usual cheat for that is to soak it in alcohol or acetone, then rinse out the solvent with water.
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Melgar
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BTW, apparently the ultimate membrane for divided cells is made of this stuff:
http://www.fuelcellsetc.com/store/N117
Nafion 117. Basically, ion-exchaging teflon with triflic acid exchange sites. If you were a positively charged ion trying to get through a membrane,
this stuff is like the SNES game "Ghosts and Goblins". 
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Corrosive Joeseph
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If this works it is absolute genius
/CJ
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macckone
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The polypropylene bags need to be rinsed with alcohol to get a
good wetting which may or may not interfere with a particular
reaction. Other surfactants can be used. For making sodium
hydroxide it is a fair material. It is definitely better than the
clay pot method. I have also used agar which breaks down if
the hydroxide gets too concentrated. Diaphragm cells are less
effective than membrane cells. Clay pots are actually a diaphragm
cell. Membrane cells are only permeable to certain ions like sodium,
hydrogen, sulfate or chloride depending on the material.
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Mesa
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You ever tried ordering anything from haines? They don't sell to the public.
EDIT: @OP You've gotten worried about a pretty meaningless detail tbh.
The specific physical dimensions of the pot are superfluous. The only details relevant to you is volume and surface area at the interface.
Get a pot that holds roughly the same amount of liquid, and has roughly the same wet surface area when submerged. The rest is irrelevant.
[Edited on 6-11-2017 by Mesa]
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bobdring
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I bought a few of the porous pots from Haines without any problems. They only supply chemicals to government/businesses however.
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JJay
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I remember doing lots of electrochemical reactions in chemistry classes, but I don't remember ever using a porous pot. I do remember using salt
bridges. It seems to me as though a salt bridge would provide much more reliable performance than a porous pot. Is there any reason a salt bridge
can't be substituted for a porous pot? Is there any advantage to using a porous pot instead of a salt bridge?
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Melgar
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| Quote: Originally posted by JJay | I remember doing lots of electrochemical reactions in chemistry classes, but I don't remember ever using a porous pot. I do remember using salt
bridges. It seems to me as though a salt bridge would provide much more reliable performance than a porous pot. Is there any reason a salt bridge
can't be substituted for a porous pot? Is there any advantage to using a porous pot instead of a salt bridge?
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Clay pots can transmit current through a much larger area. Salt bridges only make sense when you have no interest in scaling up, I guess. Also,
consider how alkaline batteries are shaped. The goal of the design is clearly to maximize the interface area between the two half-cells.
Anyway, clay pots are supposedly a decent off-the-shelf approximation of how cell dividers are shaped when the goal is to maximize current.
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JJay
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Hmm... if I'm reading this right, just looking at this chart, assuming that sodium sulfate is the electrolyte in the salt bridge, that it has a 1 cm^2
cross-section and is 10 cm long, the resistance would be an additional 3 ohms at high temperatures: https://pubs.acs.org/doi/abs/10.1021/i460002a019
My power supply only goes up to 3 amps but will do 15 volts, so I think I'm perfectly ok using something like a 1 inch PVC hose filled with agar and
sodium sulfate.
I have no idea what the resistance of a porous pot would be.
There are issues with scaling, but the same scaling issues that affect salt bridges affect porous pots (the volume of the pot follows a cubic law and
the surface area follows a square law); if there is a problem with scaling the size of the cell, you can scale up by simply using more cells.
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