Author Topic: Practical electrolysis chamber construction  (Read 1449 times)

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Prince_Charles

  • Guest
All about glass frits
« Reply #20 on: May 20, 2003, 02:58:00 PM »
Is this overkill for reductive aminations? As I understand, the separator does not need to be ion-selective; something that is unreactive, ion-permeable and prevents the solutions from mixing is enough.

A u-tube might be conductive enough. You can fill the tube with a salt solution thickened with agar to reduce mixing.


roger2003

  • Guest
U - tube with glas frit (G4) You can buy
« Reply #21 on: May 20, 2003, 03:01:00 PM »

Prince_Charles

  • Guest
Re: U - tube with glas frit (G4) You can buy
« Reply #22 on: May 20, 2003, 07:26:00 PM »
That looks like it would have a cross-sectional area of several cm2. With several molar electrolyte concentration, the conductance would be low.

Reductive aminations seem to require low current densities at the electrode: a small setup might draw an Ampere or so. Hence, a u-tube looks doable! Some reductions I have seen used several hundred mA/cm2: I wonder whether the higher current will cause a problem? One way to find out, I suppose.


moo

  • Guest
You must remember, that for a reaction you...
« Reply #23 on: May 21, 2003, 02:48:00 AM »
You must remember, that for a certain reaction you want to use a certain electrode potential (selects the reaction that is being affected) with proper current density (minimizes other side reactions, like hydrogen evolution) and current (you want to do it within a reasonable time). If you have a cell separator with a relatively high resistance, you need to give the cell higher voltage between the electrodes to get the electrode potential (the potential between the electrode and the electrolyte in the vicinity of the working electrode) high enough. This is why you really want to have a membrane with high mass transport abilites, ie low resistance. Read the british patent already mentioned (

Post 231955 (missing)

(foxy2: "Nitrostyrene or Nitropropene electrochem reduction", Chemistry Discourse)
) and what they tell about the requirements for good yields. It is no bullshit either, I'm telling you.

Prince_Charles

  • Guest
Controlling the reaction
« Reply #24 on: May 21, 2003, 03:58:00 AM »
Moo,

Isn't that why a reference electrode is necessary? When you have a successful setup, it might be possible to dispense with the additional electrode and rely on current/voltage measurements across the remaining electrode pair.

Have you any experience of reference electrodes? Mercury chloride SCE looks nasty for obvious reasons, how about a silver chloride setup?

Imagining a laptop controlled current source for experimentation.


roger2003

  • Guest
setup
« Reply #25 on: May 21, 2003, 08:12:00 AM »
There is another setup:

Post 419410

(bottleneck: "Benzyl alcohols via electro. red. of benzoic acids", Novel Discourse)
:

with clay (maybe a flowerpot)

and another setup and reference for reductive amination

Post 343766

(Rhodium: "Electrochemical reductive amination of P2P's", Novel Discourse)
:

moo

  • Guest
That is exactly why you need a reference ...
« Reply #26 on: May 21, 2003, 03:38:00 PM »
That is exactly why you need a reference electrode... well, one could do without it, but it wouldn't be reliable anymore. It is possible to use other electrodes than the saturated calomel electrode, the greatest difference being that they have a different potential and thusly you get a different voltage (potential difference) in your measurements, a matter of simple addition/substraction. Of course, you must know that the electrode survives your reaction conditions.

Edit: I think the silver electrode would work well, I have the potential somewhere as well but if you have a proper physical chemistry textbook, the data you need is most likely listed there.

Prince_Charles

  • Guest
Construction of an AgCl reference electrode
« Reply #27 on: May 21, 2003, 10:51:00 PM »
In that case, here's something:

First, a Luggin capilliary - used to get the reference electrode to measure as close to the working electrode as possible.

http://www.voltaicpower.com/References/Luggin.htm



Secondly, instructions below have been shamelessly ripped and edited from a college lab. practical that is at a few places on the web in pdf.

**************************************
The instructions for this original experiment can be found in the Journal of Chemical Education (Volume 69, No. 1, Page 74, January 1992).

PROCEDURE:

Part One; Reagent Preparation:

   1. Obtain a piece of silver wire from the instructor.
   2. Prepare the following solutions:

      -25 mL of 0.05 M AgNO3
      -50 mL of saturated KCl solution

   3. A solution containing 5 mL deionized water, 5 mL of concentrated hydrochloric acid and 5 mL of concentrated nitric acid. Remember to always add acid to water!

Part Two; Electrode Construction:

   1. Gently clean a 5 mm portion of one end of a silver wire with emery cloth. Dip this end of the  silver wire 3 cm into the 1:1:1 water, nitric acid, hydrochloric acid solution for 15 seconds. Remove the wire, rinse it thoroughly with DI water and set it aside to dry. This forms a AgCl ppt coating on the wire, providing both species for the AgCl(s) + 1e-   Ag(s) + Cl
      -(sat)  reaction in the reference electrode.

   2. Obtain a Pasteur pipet (thin-wall glass pipet) and triangular file. Score the tip of the pipet 1 cm from the bottom of the pipet body with the file and snap off the tip. Cover the pipet with a paper towel to prevent injury and perform this operation over a paper towel to collect small pieces of glass.

   3. Light a bunsen burner and slowly melt the tip of the broken Pasteur pipet. The goal here is to melt the tip sufficiently so that only a small pinhole (~0.1 mm or less) remains. This pinhole functions as the salt bridge and provides electrical conductivity between the reference cell and the metal electrode. Allow the pipet to cool. SEVERAL ATTEMPTS AT STEPS 2 and 3 will probably be necessary to get a suitably small hole without sealing the tip of the pipet.

   4. While the electrode is cooling, obtain cotton and wooden shaft cotton swab. Break the cotton  into small pieces about 5-6 mm in diameter. Also obtain about a dozen filter paper circles (5 mm in dia.).  These are made by punching holes in filter paper using a single hole punch.

   5. Using the wooden shaft cotton swab, force cotton balls into the bottom of the electrode body and compress them. Add cotton balls and compress them until they are ~1 cm thick. Compress this as tightly as possible without shattering the electrode body. Add 6 layers of filter paper circles and compress these on top of the cotton balls.

   6. Add saturated KCl solution to the electrode to a depth of 3 cm from the top of the electrode. Then add ONE drop of AgNO3 solution to the electrode. Place the electrode in a 50 mL beaker or graduated cylinder filled with saturated KCl solution and soak it for 5 minutes. This saturates the cotton fibers/filter papers with KCl.

   7. Remove the electrode and dry it. Hold it at eye level and examine it to see if solution is leaking from the pinhole. It should leak at no more than 1 drop every 4-5 minutes, if it leaks at all. If it leaks at a greater rate, the electrode body must be reconstructed until it leaks at the indicated rate. Do not be discouraged if many attempts are needed to properly assemble the electrode, it is a difficult procedure!

   8. Place the silver wire into the electrode (tarnished end first) until the wire extends to 5 mm above the filter paper circles. Seal the top of the electrode with a plug of rolled up ParaFilm (wax film) secured by tape. Seal the top as best you can, but be careful not to crack the electrode body and cut yourself. Store the electrode in a beaker or graduated cylinder filled with saturated KCl up to the level of the fill solution in the electrode. The electrode is now ready for use.

Finally, here is another method:

http://www.consultrsr.com/resources/ref/agcl.htm%5B/blue

]

Oh yeh, AgCl reference electrodes are fine in acid electrolytes but slowly degrade in strongly basic ones. Short periods look OK (reductive aminations of ketones seem to work best at pH 11-12).


roger2003

  • Guest
A lot of years ago i saw software
« Reply #28 on: May 22, 2003, 01:15:00 PM »
A lot of years ago i saw software (and hardware) for controlling the electrochemical process. Anyone knows this software ?

roger2003

  • Guest
information to Nafion
« Reply #29 on: May 23, 2003, 05:18:00 PM »

Prince_Charles

  • Guest
Re: Control Software/Hardware
« Reply #30 on: May 24, 2003, 01:47:00 AM »
I've had a quick scout on the web and potentiostats are extremely expensive. I imagine software is specific to the model.

It is relatively easy to make a variable voltage, regulated supply, but a PC controlled system would be ideal. There are some nice (cheap) hobby kits to add ADC and DAC interfaces to the PC.


Prince_Charles

  • Guest
Re: information to nafion
« Reply #31 on: May 24, 2003, 02:03:00 AM »
Interesting stuff. Apparently fragile when wet. Expensive. Gore have developed a material consisting of goretex with nafion-filled holes: gives better strength when wet and excellent results with fuel cells.

Vanilla goretex I have found difficult to purchase by the square meter: resorted to cannibalising clothing. Dialysis membranes may also be good, but ones I have found are expensive. Seem to be 2 types of dialysis membrane: cellulose ester and regenerated cellulose. The latter has the best chemical resistance (e.g. acid/base/ketone).

A bastard elk just ate all my pot plants, so I have some terracotta pots to play with too.


Organikum

  • Guest
homemade membranes
« Reply #32 on: May 24, 2003, 07:22:00 PM »
see:

Post 325605

(Osmium: "> So what's the back-up plan?", Chemicals & Equipment)

for sulfonated membranes a la´ Osmium

and read the whole thread for more.

The information on the reference electrode is worth gold - thanks!

To the question at start: I don´t think there is a universal perfect electrolysiscell design. It depends very much on the reaction if it shall be more than a educational experiment.

For a cell which is easy and works in many perhaps most electrolytic reactions you can use an electrolytic cell. Sounds stupid, but thats exactly what a car battery is. You get the fixation frames for your membrane(s) and electrodes for free this way and lead electrodes are also included.
Be careful with the acid and the lead.

roger2003

  • Guest
car battery
« Reply #33 on: May 25, 2003, 11:15:00 AM »
anypne knows the kind of plastics (PP,PE etc.) from a car battery ?

roger2003

  • Guest
membranes
« Reply #34 on: May 30, 2003, 06:04:00 AM »
membranes from PP, PE and PTFE

http://www.esters.com/en/ptfe.php