Porous Membrane

Electrodialysis. It will draw all the cations to one side and the anions to the other.

You mean like the machine or are you talking about some sort of membrane? More info would help.

Thanks for the answer.

It's a process. It uses a machine called an electrodialysis stack. It uses high voltage DC power and 2 special types of membrane. One type is permeable to cations but not anions, and is mated to the negative terminal, while the other type is permeable to anions but not cations, and is mated to the positive terminal. In this way, the anions will be drawn to the anode and the cations to the cathode, resulting in the solution being separated into acid and base.

So what are the two materials?

Here's what I'm trying to do... I'm trying to create a battery using two half cells that act together to make a single cell. Here's a picture from Wikipedia:

What is that damn "Porous disk" made of - it's driving me nuts!!!

Honestly, I don't know what the 2 types of membrane are made of as I design water treatment systems, not membranes. Veolia Water or GE-Ionics, who do manufacture such stacks, will probably be better placed to advice you on this. Other membrane manufacturers such as Hyflux may also be able to tell you more. In this case, you membrane needs to be permeable to both cations and anions. It is the DC charge that separates them. In fact, if you show the membrane manufacturers or their agents the diagram, they'll probably be able to tell you what type of material it is.

The Gore corporation told me they make theirs out of a micro-teflon felt, but if I wanted a sample, I'd have to cough up $500.00!!!

By the way, they're the same people that make Gore-Tek.

http://en.wikipedia.org/wiki/Daniell_cell

It looks like a porous ceramic can be used (a flower pot would be the suggestion for the enthusiastic home experimenter or Myth Buster watcher, perhaps?), or failing that a salt bridge made of either sodium sulphate or potassium sulphate, both of which are highly soluble.

The original voltaic pile used cloth or cardboard soaked in brine (salt water), so it would appear that there is little need to keep the two solutions separate.

Thank you! That was a very good post. I'm glad you understand the processes involved in half-cells! I'm going to vote you a good answer!

^{PS: I really do like your avatar!}

Vermin,

you're welcome. This thread interests me very much since I actually experiment with home-made batteries as a hobby. I felt tempted to share some of my experimental results here, but they relate too closely to a potentially patentable idea of mine (I know that battery technology is already a very mature field, but I won't yet abandon my goal of contributing something original). Well, maybe I can describe one battery that is unrelated to my secret work. Using only materials available at a hardware store, I think that I can exceed the performance of the batteries proposed so far.

P.S. I still plan to explain the images in my avatar, and what they mean. But that will have to wait since I'm smack in the middle of finishing one teaching semester, and beginning a new one.

soaked in brine (salt water),

I'm so glad that was clarified. I'd add more, but all the smillies have gone !!

Anything that is porous, but does not allow gross mixing of the two liquids will work. So you could actually separate the two half cells with a thin tube, fritted glass, porous ceramic, porous polypropylene, a salt bridge, Goretex or other porous plastic etc.

If you are really getting into it, I am sure you can buy a device specifically designed for this at a scientific supply shop. This kit looks like it will do exactly what you have described, and more:

http://wardsci.com/product.asp?pn=IG0007475&sid=2008FS&eid=2008FS&mr:trackingCode=2039F3B7-5A16-DD11-AD5F-000423C27502&mr:referralID=NA&bhcd2=1209649578

For home experiments, you can use real cellophane (from a cigarette pack) or a lambskin condom (without lubricant). I'm serious. Tyvek should also work.

Gore does not want to deal with small orders. If you look up expanded or porous Teflon, I'm sure that you will find something suitable.

Are you trying to duplicate the cell you have shown, or something else?

Tad

I'm using that basic configuration, but my chemistry will be different.

You're right about Gore Corp. (GoreTex), they want big bucks for just a small sample.

I will check out the kit.

Thanks.

By the way, I actually have thought about using those condoms!!!

Hi Vermin,

something is disturbing me with your picture.

The cation flow is driving the Zn++ through the membrane and electrons neutralise the Cu++ so Copper will be deposited and the solution will be only zinc-sulfate on both sides.

So why to use a membrane?

RHABE

I think the copper ions go the opposite way, entering the zinc sulphate solution.

Hi vermin,

the picture says (and this is correct): influx of cations (to the right)!

This is driven by the electrons that travel in the wired connection between the poles, searching for and attracting positive ions to discharge.

Copper will be plated out first as being much nobler than zinc.

RHABE

PWSlack has the right idea with ceramics. Many fuel cells use a sintered ceramic as the ion exchange medium. It might be possible to use a sintered stainless steel, but the porosity with this material is not as uniform or as small as the sintered ceramic.

Hope that points you in the right direction.

Three questions...

1. How thick should the ceramic be?
2. How do I sinter it?
3. What type of ceramic?

Generally, Potters For Peace (PFP) kneads ground, screened sawdust (or rice hulls, etc.- really any granular, fine combustible material should work) into the mix with the clays for then form their pots before kiln firing. The issue they often face is how to guarantee the consistency of the clay-sawdust consistency before forming, as this obviously affects the porosity of the finished product (after firing). These pots are used for filtering contaminated water; pore sizes ~ 2 micron are supossedly achieved.

I am wondering about whether some sort of "salt bridge", such as was used in our chemistry classes years ago could work for you?

1. Thickness of the ceramic membrane is dependent on the time, temperature and pressure that your system will exhibit. So without knowing more about your system I can only recommend reviewing thermodynamic and rheological principles to determine that for yourself.

2. There are many sintering methods to choose from. Some people have mentioned wood burnout, which is a very good and relatively simple process, but uniform pore size is an issue. I would recommend a sol-gel process or a fine ground polymer burnout method to ensure uniform pore size.

3. The type of ceramic often depends on your system and without knowing more about it I can't say for sure because of reaction issues. But for cost effectiveness I would recommend alumina and for corrosive resistance I would recommend a sialon. Many people are recommending Earthenwares as a ready made product, but these are generally low grade, non-uniform porosity, and have poor corrosion resistance. If your process is more dependent on time and cost effectiveness over purity, this may be a good route. Otherwise I wouldn't recommend it if you are concerned about purity.

Have a look at molecular sieves. A membrane can be formed inside a porous pot dividing wall. This could be fired clay or plaster you just have to get the two reactants to meet in the middle - put one on one side and one on the other. Fill porous plant pot with one and submerge it in solution 2.

Also check out osmosis.

In protein chemistry purifications and those of other macromolecules where relatively small ionic materials (salts) are present, it is convenient to merely place an aqueous solution of the materials in question in a plastic bag of known porosity and suspend this in deionized water to effect removal of the salts by simple osmosis.

Hey, Vermin! There are a really wide number of "porous membranes" out there, capable of passing everything from ions to atoms to molecules and even really big chunks! The medical industry often uses porous ceramics for implants, especially for bone repair where the pores allow "biological fixation" to anchor the materials permanently. These ceramics are typically Al2O3, polycrystalline, or hydroxyapatite-coated porous metal sheets. For pore sizes > 100um, bone grows in the ceramic and acts as a "scaffold" for bone formation. There are polyurethane membranes that will pass only blood glucose, for example. There are techniques to construct various pore sizes. The older techniques called for adding very small crystals of salt, starch, sugar, etc to the membrane as it was formed, then dissolving away the soluble materials. Some of the membranes available include PTFE, polyolefins, silicone rubber, polyimides, polyurethanes, and so on. There are several hundred suppliers, from Asahi Glass (ion exchange membranes) to American Polyfilm, Air Products makes gas separation menbranes to concentrate nitrogen, etc. Millipore has been in the membrane industry for many years. Any of these companies can give you more physical chemistry on the process of separation, ion transport, etc. Good luck!

It would be a good idea to contact a company called PUR-FLOW in North Carolina. They are a water purification group, but they have a great EDI, (Electronic de-ionization). Its all in one compact unit. it can be used to make "heavy " water and also remove gasses. The more purified water is, the more aggressive it is and the more it wants to equilibrate to what it is in. So I am sure it can handle almost any liquid. Hope this can be a start.