obsessed_chemist
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Suitable Membrane/Diaphram for DIY Electrochemical Cell?
I plan on building an electrochemical cell with a partition(s) to separate the products. I have never heard of anyone using a salt bridge in real life
with any success, and I imagine it would greatly impede current flow through the cell due to such a narrow passage. Also, I have my doubts as to how
well Agar would hold up at higher temperatures, or if it would resist very acid or alkaline conditions.
As a test, though, I am considering connecting two 3 gallon buckets with a salt bridge contrived from two 4" PVC/ABS elbows and a nipple connecting
them to form a U-tibe, or one made from heating and molding a length of PVC/ABS which would be filled with a solution of the salt to be electrolyzed
along with agar. The ends would be covered with fiber-glass fabric, then inverted with each end in one bucket.
The only other viable option I can conceive is to use a membrane/fabric-type partition that will allow ions to pass through without allowing the
solutions to mix. I am really hard-pressed for ideas regarding OTC available materials to use for the membrane. The only thing I can think of is
perhaps fiber-glass cloth, artifical sponges, or the material used in reverse-osmosis water purifiers. Most commercial cells use expensive fluorinated
polymers and such. I realize that the proposed membrane material really depends on what is being produced in the cell.
For my purposes, I am interested in the electrolysis of K2SO4, or potassium sulfate fertilizer, for the production of sulfuric acid, and potassium
hydroxide. Any thoughts or suggestions would be appreciated.
[Edited on 3/23/2007 by obsessed_chemist]
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not_important
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Do a little searching on the term electrodialysis which will lead you to such items as:
http://www.electrosynthesis.com/news/mwatts.html
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obsessed_chemist
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Thanks for the link. I'm actually a bit disappointed; If big research companies with tons of funding can't keep the two products seperated with
high-tech membranes, then I doubt I can with OTC available supplies. Then again, if ammonia is utilized in the anode side to neutralize the sulfuric
acid produced (as it is in one of the processes mentioned), I suppose it may be possible to produce fairly pure NaOH or KOH at the cathode side, but
not any decent yield of H2SO4.
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roamingnome
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I would propose that you come up with the half reactions for the electro-cell in question.
Then you can see the flow of ions and were they need to go.
Then I bet, in the end, you need a proton exchange membrane to allow H+ to some how make H2SO4…
Both AEM and CEM membranes are commercially available, and if your not wearing a slayer t-shirt you could order them over the net, probably from Japan
is best….
Improvised ion exchange membranes would be a nice write up that I would like to read.
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bio2
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Hi quality maintenance free auto batteries have the electrodes
encased in bags (membranes) made of a semi-permeable polymer.
These can be used as is or cut to size and mounted on a frame
as a cell divider.
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not_important
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I think you will need ion selective membranes to pull it off. What you are doing is sort of charging a battery, the original state is lower in energy.
Having a primary separation stage with a finishing stage for each of the two product streams might help. In the finishing stage one of the product
streams of the first stage would be used as the feed, again being split into two streams one of which is the desired product and the other recycled to
the first stage's input.
There's some old Soviet work done on this that I may be able to find refs for.
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chromium
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I have done some (minimal) experiments with salt spliting using only readily available materials.
What i can say is that some spliting surely occurs even if electrolyser is just two beakers connected with capsized U-tube which is completely filled
with electrolyte.
I, too, used K2SO4 soln as electrolyte and got at least 2% solutions of acid and alkali (determied by titration). This looks bad but do not be
disapointed because i just did two or three experiments and conditions (like current value, total amount of current and diameter of bridge) were
probably far from optimal and, after all, acid or alkali are rather easy to isolate from salt and water even if concentrations are very small.
Main problem i meet when doing these experiments was quite different from what you expect: logically one wants to use as thin tubing as possible for
bridge and resistance can be overcome by increasing cell voltage. No problem so far but as tubing is highest resistance part in the system allmost all
excess energy which is released as heat is released in the tubing and bridge becomes very hot. Electrolyte may start boiling in there and even if this
does not happen this heat degases water so well that released oxygen will soon block your bridge with nasty bubble (this determined end points of my
experiments).
Someone who has electronic pH meter at hand could try these experiments to see what concentrations one can achieve and make graphs to determine
optimal conditions.
[Edited on 24-3-2007 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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Levi
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Your half reactions should all have ions on the left. I'm not sure what the correct anode side reaction is, but I'm almost certain
SO<sub>3</sub> doesn't belong there.
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Aurum
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Has anyone tried using PET as a permeable membrane? PET is permeable and Mylar is PET with an aluminium layer. Could the aluminium be removed from
Mylar to produce a usable membrane?
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obsessed_chemist
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Still working on the half reactions...
[Edited on 3/29/2007 by obsessed_chemist]
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not_important
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| Quote: | Originally posted by Aurum
Has anyone tried using PET as a permeable membrane? PET is permeable and Mylar is PET with an aluminium layer. Could the aluminium be removed from
Mylar to produce a usable membrane? |
It's easy to get oPET films without coatings
http://www.grafixplastics.com/plastic_film.asp
But it's not permeable in the electrolysis sense, just a bit to gases.
http://www.dupontteijinfilms.com/FilmEnterprise/Datasheet.as...
http://www.roberts.fr/ballon/IMG/pdf/mylar_chemical_properti...
You can use microporous film, if the pores are large enough. But what's better are membranes that allow ion flow, best if they are selective to some
degree.
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obsessed_chemist
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| Quote: | | Originally posted by chromium... after all, acid or alkali are rather easy to isolate from salt and water even if concentrations are very
small. |
Care to explain how they can be easily extracted? I think the acid could be by boiling, then filter, but not the KOH, except maybe by drying,
then with hot alcohol.
But again, you said that the conditions were far from optimal. I was considering using a piece of PVC pipe of several inches in diameter, bent to
shape (with heat blanket), and then the tube would be filled with the electrolyte (same as rest of cell) and agar. Then the ends could be plugged with
glass wool, wetted and inverted, with each end dipping into the respective partition.
[Edited on 3/30/2007 by obsessed_chemist]
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chromium
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| Quote: | Originally posted by obsessed_chemist
| Quote: | | Originally posted by chromium... after all, acid or alkali are rather easy to isolate from salt and water even if concentrations are very
small. |
Care to explain how they can be easily extracted? I think the acid could be by boiling, then filter, but not the KOH, except maybe by drying,
then with hot alcohol.
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By fractional crystallisation. K2SO4 has low solubility (7.18g in 100ml water at 0C). In case of KOH this number is 87.6.
Sulfuric acid, as you know, has infinite solubility (although at very high concentrations it reacts with K2SO4 converting it to KHSO4).
Extracting of KOH with denaturated alcohol should also work (although i have not tried this).
[Edited on 30-3-2007 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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BeanyBoy
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Old methods?
Fenner's Complete Formulary (1888) describes using parchment paper for dialysis, "made by immersing firm, unsized paper in a mixture of two measures
of Sulfuric Acid with one measure of water, and afterward washing it throughly with pure water".
The dialyzer consists of a clay pot and a glass lamp-shade.
One web reference claims that ordinary brown wrapping paper doesn't have sizing added to it; sounds reasonable, and the writing parchment I've seen
looked like it could have started life as brown wrapping paper.
Writing parchment from the hobby shop may also, or may not, work.
The formulary is online at http://www.swsbm.com
I've not considered further whether this is suitable for use in conjunction with electrolysis.
hth,
-beanyboy
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bio2
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Pine wood diaphragm from Organic Synthesis.
.....Another convenient arrangement for the reduction cell is to divide the battery jar into two compartments by sealing in a diaphragm of thin
cork-pine wood by means of paraffin. This offers very little resistance to the passage of the current and allows the reduction to be carried on very
rapidly.........
http://www.orgsyn.org/orgsyn/prep.asp?prep=CV1P0485
So is "cork-pine" a species of pines?
Don't know if I have ever seen it.
Maybe they mean a soft wood type of pine.
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BeanyBoy
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Wood not Cork (SO's preference, too)
| Quote: | Originally posted by bio2
So is "cork-pine" a species of pines?
Don't know if I have ever seen it.
Maybe they mean a soft wood type of pine. |
Cork pines are white pine trees that have lived a relatively long time and have gotten rather tall, say 300 years & 200 feet. Michigan used to
have a lot of them. Now there is only a small stand of them left in a Michigan state park. They are called cork pines mainly because wheh the logs are
felled and sent downstream in the river to the sawmill, they float high in water.
Other names include northern white pine, northern pine, Weymouth pine, Quebec yellow, cork pine, and pumpkin pine.
Canada engaged in better stewardship of their land, and still have enough quantities of this pine tree to sell as lumber.
So as far as I can tell, you need soft pine wood, and not cork.
Cork generally comes from the bark of the cork oak, harvested from any given tree once per ten years.
hth,
-beanyboy
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froot
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I know this is an old topic but it's the 2nd hit when googling 'diy ion exchange membrane'.
Could one use an electrostatic charge to orientate ions in an electrolyte?
I was thinking along the lines of using a fine metal mesh that has been insulated by a layer of a non-conductive coating insulating it from the
electolyte, together with a porous membrane to restrict mixing between the half cells. The mesh is then charged with a positive voltage, just enough
to keep the positively charged metal ions (Cu+, Zn+, etc.) away from the separator, while attracting the anions (SO4-, O-, Cl-, etc.) and allowing
them to pass through.
If it does work I would imagine that this applied voltage would have little effect on the overall electrode potentials being insulated from the
system.
Any thoughts?
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