Understanding the Reaction of Copper Impregnated Anodes in Chlorate Cells

[Squall]

i just recently acquired a few pieces of copper impregnated graphite. this stuff is supposed to have high wear resistance in EDM applications So i thought i would give it a shot in a chlorate cell. After a few minutes of operation my electrolyte which is a solution of KCL, turned blueish green. Does anyone now what reaction is occurring. i have a few of my own speculations but i thought Id ask if anyone else has experimented with such anodes.

 

[mrjeffy321]

The Copper metal on the anode is being oxidized, forming Cu+2 ions in the solution, which go on to form a complex with the water thereby turning the solution blue.
You now have Copper [Chloride] contaminants in your electrolyte.
Are you also noticing some substance forming over the cathode of the cell? The Copper should be plating over from the anode to the cathode. If you remove the Copper from the anode, over time I think it should be possible to plate-out the Copper impurities in solution.

By the way, for a Chlorate cell, it is generally advisable to use Sodium Chloride as the electrolyte, first forming Sodium Chlorate and then (through a double replacement reaction with KCl) produce Potassium Chlorate from that.

 

[Squall]

Thats what I assumed, thanks for your reply, but i do have another question why is it better to use NaCl then do a double replacement does that result in higher yields. I've had decent results with using a solution of KCl.

 

[mrjeffy321]

Your right after running the cell for a few hours the blue color disappears. I am guessing it plated on the cathode . Why do you say I should use NaCl for electrolysis what are the benefits. I was able to achieve decent results using a solution of KCl.

When you use Sodium Chloride as the electrolyte you produce Sodium Chlorate.
Sodium Chlorate is very soluble and remains [ionized] in solution.
When you use Potassium Chloride as the electrolyte you produce Potassium Chlorate.
Potassium Chlorate has a decidedly lower solubility than NaClO3 (espeically at low temperatures) and if you are producing KClO3 in sufficient quantities, some of your KClO3 yeild will precipitate out as a solid. Not that this is necesarily bad, but it lowers the concentration of the electrolyte in the cell and thus makes it more dificult for an electric current to flow (dissipating more energy at heat) and also makes the KClO3 harder to clean in the end, especially if you are using Carbon electrodes which tend to erode and give off a graphite powder suspension...leaving you will a black/gray colored final product.

If you produce NaClO3 first, you can filter the final solution all you want and not loose very much of your Chlorate product. You can this (after you have removed the graphite / other impurities) precipiate out the KClO3 by adding KCl. Then you can filter the KClO3 out and wash with cool water to remove any of the soluble NaCl impurities.

 

[Squall]

Ok I see your right it does percipitate out while the cell is operating and i guess i do loose a good bit of product in the filtration process. You said it the precipitant causes the cell to dissipate more energy and heat could this be a cause for excessive anode wear. any way thanks for the advice I will try using a solution of NaCl next time.

 

[mrjeffy321]

The increase in the electrical resistance through the cell is not terribly noticeable as an effect of the KClO3 precipitating out of solution. It is due to the fact that the more ions you have in your solution, the easier it will be for the electric current to flow through. When K+ (aq) and ClO3- (aq) ions precipitate out as KClO3 (s), you are loosing ions and thus the resistance is going up.

But the graphite anode will erode away in time anyway. Ways to slow this erosion include lowering the current density (current per unit area) on the anode surface and keeping the temperature and pH adjusted to the proper level, as well as impregnating the graphite anodes with linseed oil which I have found to work quite well in slowing anode erosion.

 

[Squall]

I have not tried linseed oil yet but I'll probably give it a shot. I have a question about using NaCl solution the question is how can i know when most of the NaCl has been converted to NaClO3 if it doesn't precipitate out. Are there ways of checking the NaClO3 concentrations in solutions.

 

[mrjeffy321]

I do not directly measure the remaining concentration of the NaCl, since this would not be terribly easy to do.
What I do is start with some known amount of NaCl and calculate how much charge must be passed through the cell in order to convert 90% of it to NaClO3 (leaving 10% as NaCl, since past this point efficiency drops and anode wear increases dramatically). I then take whatever efficiency I expect to get (~50%) and compensate the amount of charge I need to pass through the cell.
I measure the charge which has passed through the cell using an Amp Hour meter which constantly measures the current going into the cell as well as how long the cell has been running.