elementcollector1
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NaCl electrolysis
To date, all of my attempts have failed, so a few questions to pose to the group:
-For the salt bridge, do I have to use a salt-soaked strip or can I use metal? I feel like if I use metal, it'll just make for another electrode at
each and and both products will be hypochlorite. Without metal, or something as reactive, the electrolysis runs so slowly as to be useless.
-Is 3V, 300mA a good electrical output?
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bbartlog
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| Quote: | | For the salt bridge, do I have to use a salt-soaked strip or can I use metal? |
Metal won't work. You are trying to permit the passage of ions, not electrons. A salt soaked strip sounds like something that would work for a
theoretical demo of a salt bridge, but for any practical purpose would lead to very high internal resistance and very low rates of current /
electrolysis. A glass U-tube (possibly with cotton balls to slow diffusion) would be one way; the other is to construct a divided cell using some sort
of membrane.
| Quote: | | Is 3V, 300mA a good electrical output? |
As I recall 4V is required to electrolyze molten NaCl and I don't know that aqueous solution makes it any easier. I'd try to get something with higher
voltage. As to whether the current is sufficient, it depends on what you're trying to accomplish...
The less you bet, the more you lose when you win.
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BromicAcid
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You should ideally have more amps. More amps are going to compensate for any type of salt bridge and dictate the rate of the electrolysis. My
favorite divided cell is an unglazed flower pot hot glued upside down inside of the cell. This very effectively isolates one electrode from the other
although it certainly slows down the electrolysis. Despite this I just give the cell plenty of space and time and try to forget about it for the
weekend (or a week or so) before investigating. Chemistry in the background so to speak.
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elementcollector1
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Additionally, I heard that if you put some zinc in the cell, it reduces hypochlorite back to hydroxide or chloride. Is this true?
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ManBearSwine
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Yes, zinc will react with hypochlorite to make zinc hydroxide (which may or may not dissolve depending on pH) and chloride. It will also react with
the OH- from the cathode. Basically, if you just want a lot of Zn(OH)2, go for it. 3 volts should be plenty to carry out this reaction, considering
that the cell potential is -2.19 volts.
What exactly are you trying to make, anyway?
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barley81
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Zinc hydroxide dissolves in basic solution to form zincate ion. So if you want to make NaOH, using zinc won't work at all. To make useable amounts of
NaOH using electrolysis, you need a mercury cathode cell, diaphragm cell, or membrane cell. An easier method if you just want a bit of sodium
hydroxide is to react calcium hydroxide with sodium carbonate in aqueous solution.
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elementcollector1
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I'm going for the NaOH, because it's expensive and this is fun.
So, the product is sodium zincate? Then wouldn't electrolysis split this back up, or can I crystallize this out from solution?
It's fairly obvious I don't or can't use:
Mercury.
Diaphragms (point me to a cheap one and I might use it).
Membranes (see above).
If electrolysis does not split this, what can reduce hypochlorite without being annoyingly hard to separate in the process?
EDIT: Process has been upgraded to carbon electrodes and 6 amps of current. At this rate, it should be done in a few hours.
[Edited on 30-4-2012 by elementcollector1]
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elementcollector1
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Would filter paper work as a membrane? I don't think so, but...
In any case, what about magnesium? Will it reduce hypochlorite?
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DoctorOfPhilosophy
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Filter paper might be better than nothing, but to do it the right way you have to use a diaphragm (usually asbestos - as in US patent US4879009), or a
Nafion membrane (you can buy these online starting at 15 bucks - use a prepayed gift credit card from the gas station if you don't trust Internet
vendors). A lamb skin prophylactic might also work as a cell divider, that's one of my current projects.
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dann2
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You can get a pretty thin divider from a ceramic tile. Get one as a sample and get one as thin as possible. They will have one side glazed which has
to be removed. This is done using an 'angle grinder' also called friction cutter or disk cutter. Put a blade in it that is for stone. We are not
talking about a tungsten tipped or a carbide tipped or a diamond tipped steel blade. Its made from matrix of abrasive particles. Common as muck in
hardware stores.
You grind off the glaze. It is easy to do. You can grind some more to make the tile as thin as you dare. Seal into a cell using silicon rubber for
experimenting. You would probably need something more substantian that Si rubber for long term use.
Flower pots are inclined to be very dense and give very high voltage drops accross themselves.
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elementcollector1
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Yes, but will it conduct ions? That is the question. 
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plante1999
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clay flowerpot will make a very good diaphragm from my recent experimentation, it does not react, dissolve and it when imersed in a salt solution it
lets the electron travel trough it very well.
I never asked for this.
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dann2
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It will conduct OK when it gets soaked with solution.
Similar to an Asbestos which is (was?) used in industry.
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elementcollector1
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... (jawdrop) a ceramic pot. Someone explain *how* solid ceramic conducts ions?
Incidentally, would epoxy or clay work as a glue for the 'flowerpot cell'? Or hot glue, or something else?
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watson.fawkes
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Quote: Originally posted by elementcollector1  | ... (jawdrop) a ceramic pot. Someone explain *how* solid ceramic conducts ions?
Incidentally, would epoxy or clay work as a glue for the 'flowerpot cell'? Or hot glue, or something else? |
"Solid" ceramic is not the same thing as "volume-filling" ceramic. Ceramics are classified by porosity along the continuum of
increasing volume fill as non-vitreous -> semi-vitreous -> vitreous -> impervious. For a given ceramic body type (i.e. its ingredients), the
density goes up as the volume fill increases. What's in the volume that's not the ceramic body? As fired, it's air. You dunk it in water, the pores
fill with water. Terra cotta flower pots (the specific kind used in the present application) are in the non-vitreous category. They are quite porous,
which is what you want in a pot that holds plants, since it mimics gas exchange in the soil more faithfully than a vitreous pot would.
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elementcollector1
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Oh! It all makes sense now!
So, my guess of an ideal cell would be a large flowerpot with no hole in the bottom. Inside would be an overturned flowerpot with 1 hole (for the
electrode) on the top. The outer cell would be filled with salt solution, the inner with distilled water. The NaOH solution would be pipetted out.
Off to build! 
EDIT: Again, what of the 'glue' for the inner half of the cell? What is resistant to chlorine or carbon dioxide and NaOH?
[Edited on 9-7-2012 by elementcollector1]
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dann2
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Flower pots (IMO) seem to not be porous enough and give highish voltage drops.
I am not saying they will not work. The tile can be made much thinner (by grinding) and they are (if you pick a suitable one) more porous and will
give less voltage drop. I have seem flower pots where the large voltage drop accross the wall of the flower pot makes the cell unworkable. There are
many types and varieties of flower pots so it will vary from pot to pot of course. Don't forget to let the pot or tile soak for long enough.
One advantage of the pot over the tile is that once you get the hole pluged in the flower pot you have no need for 'glue' etc, unlike the tile where
you have to seal around the tile where it divides the cell.
Someone here used melted plastic to fill the hole in the pot.
What EXACTLY are you trying to do? Make Chlorine?
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elementcollector1
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Make NaOH, KOH, etc.
So the inverted pot is simply placed inside the larger one? Seems a bit risky if you ask me...
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