Sciencemadness Discussion Board

Electro' reduction questions

RobT - 14-10-2012 at 02:38

Hi,

I'm planning on reducing some nitro alkenes to amines. I've got absolutely no experience with electro chemistry and I have a few questions:

Does the type of cell divider matter? I've read all sorts of posts on here mentioning things like flowerpots, ceramic tiles, asbestos, lamb skin condoms and the membranes from car batteries. Does it matter which I use? Wouldn't something like a Nafion membrane be better suited? Does the material used for the electrodes or the electrolyte dictate the type of cell divider needed? (Obviously certain acids are going to melt certain materials - apart from that.)

Do the catholyte and anolyte have to be equal in volume?

When scaling up electrochemical reactions does every variable scale up in a linear fashion? E.g. If I need 100ml of anolye, 100ml of catholyte and 1000000 columbs to reduce 1g of nitrobutene then would it take 10 times as much to reduce 10g?

When increasing the surface area of the electrode does the ampage required also increase in a linear fashion? E.g. If an electrode is 60cm² and the current density is 0.074A/cm² and 5A of current is passed over an hour (3600 coulombs.) Then with a 600cm² electrode would I require 50A in order pass 36000 coulombs?

To avoid paying the high taxes associated with buying absolute ethanol could I just use over the counter methylated spirits? (I believe that these contain: ethanol, methanol and methyl violet.) Would these contaminate the reaction?

Can anyone recommend a good website/book on elctrochemical reductions?

Sorry for all of the questions!

Thanks,

Rob

hissingnoise - 14-10-2012 at 04:40

An earlier discussion on the topic . . .

http://sciencemadness.org/talk/viewthread.php?tid=4145

RobT - 14-10-2012 at 16:10

Thanks for the link. It does touch upon a few of my questions but what I'm mainly asking about is just general electrolysis, not specifically reducing nitro alkenes.

Rob

elementcollector1 - 14-10-2012 at 18:16

-Cell dividers: A wide range of items, as you saw, can be used, but for small-scale, I usually use a paper towel or some such (unless the solutions get too acidic or basic).
-Coulombs: I would think so, yes.
-Surface area: I'd actually like to know a little more about this as well. More surface area = better results, correct?
-Ethanol/Methanol: No idea.
-Books on electrochem: Use teh Internetz, specifically Google Books tends to be helpful.

RobT - 15-10-2012 at 13:10

Thanks elementcollector1,
Quote:

Coulombs: I would think so, yes.


Do you think volume of electrolyte would also need to increase?
Quote:

Surface area: I'd actually like to know a little more about this as well. More surface area = better results, correct?


I think that the way it works is the more surface are you have the more amps you pass through the electrolyte per hour, this equates to more coulombs and a faster reaction.

Also does anyone know if electro reductions scale well? (with sufficient cooling) What I am invisioning is something on the 500g level. With my (probably wrong) calculations this works out to about 10L of electrolyte and would require about 13710720 coulombs. Is this kind of thing feasable? Or are electrochemical reductions generally done on a small scale?

Thanks,

Rob

ElectroWin - 16-10-2012 at 09:35

Quote: Originally posted by RobT  

When increasing the surface area of the electrode does the ampage required also increase in a linear fashion? E.g. If an electrode is 60cm² and the current density is 0.074A/cm² and 5A of current is passed over an hour (3600 coulombs.) Then with a 600cm² electrode would I require 50A in order pass 36000 coulombs?


yes. electrode effective surface area and current should scale linearly in order to fix the current density.

because of this, note also that current density at anode and cathode will differ if their effective area differs.

RobT - 16-10-2012 at 10:07

Thanks ElectroWin, and do electro reductions scale well? I've only seen references to small scale experiment, do you think that I will really need 10L of electrolyte or does this sound like overkill?

Thanks,

Rob

ElectroWin - 17-10-2012 at 14:39

if the anode and cathode are large surface area, then you can still use small amounts of electrolyte if you pack and space things nicely.

they do this in batteries. i opened up a non-rechargeable AA lithium battery, once, and it unrolled like multi-ply toilet paper

elementcollector1 - 17-10-2012 at 15:49

Not sure if the lithium battery is a good analogy (though they really do remind one of toilet paper). I would think concentration would be better than sheer volume...

ElectroWin - 20-10-2012 at 09:20

Quote: Originally posted by elementcollector1  
Not sure if the lithium battery is a good analogy (though they really do remind one of toilet paper). I would think concentration would be better than sheer volume...


the example i was thinking of was about electrowinning of copper, where you would use a crude copper anode and deposit pure copper on the cathode; electrolyte would then be recycled, unless anions were themselves decomposed

fixing membrane between cathode and anode

lullu - 5-11-2012 at 13:40

If you don't mind I would like to ask another general question regarding divided electrochemical cells.

Has anyone a method for fixing a membrane (PE, Nafion, etc pp.)
between a two compartment cell?

It has to be acid resistant and tight.
Like a flansch or adhesive but so far I did not came up with a good solution.

so if anyone has done this before I would be happy to get a heads up.

best regards
lullu