Sciencemadness Discussion Board

Chlorate cell first attempt

Mixe - 2-6-2017 at 23:20

So I aquired an MMO anode and a titanium cathode, bought some pure, laboratory grade KCl and got hold of an old 6V/6A car battery charger. Hooked it all up in a 2 L clear plastic container.

I made a saturated 1,7 L solution of KCl and topped it with 600 mg of potassium dichromate to prevent back-reduction at the cathode.

Powering on the cell, it draws 4,9-5.0 A, and the cathode bubbles nicely. First off, there was a yellowish-green discolouring of the solution around the anode, and a chlorine smell was felt around the cell (there was some leakage from the lid that I have since fixed). From the exhaust-tube that's submerged in a solution of NaOH there is a constant stream of bubbles. All seems well.

It's now been 24 + hours, and not a single chlorate crystal is visible on the bottom of the jar. I corrected the pH, which had run up to 12 yesterday. Still nothing.

I seem to be producing only hypochlorite. Why is this?

I've read dozen of tutorials and discussions on this forum and elsewhere, and the amount of current people use are extremely varied. Everything from 2-50A. I choosed 5A since it was said to spare the electrodes more. But is it too low? Or am I just too impatient?

Neme - 3-6-2017 at 00:14

If I remember correctly, temperature above 70°C is required for hypochlorite to disproporcionate back to chloride and chlorate.

Mixe - 3-6-2017 at 00:38

Really? In none of the tutorials or videos I have viewed has anyone heated the solution during electrolysis.

Neme - 3-6-2017 at 02:01

Well the process is basicly:

2KCl + 2H2O ----EL----> Cl2 + 2KOH + H2

Then the chlorine reacts with hydroxide.

CL2 + 2KOH -----> KCl + KClO + H2O

When heat is applied, the chlorine in hypochlorite disproporcionates.

3KClO -----T-----> 2KCl + KClO3

Chloride is then recycled in the reaction.

By quick search on wikipedia, I found this as possibly helpfull for you.

"The heating of the reactants to 50–70 °C is performed by the electrical power used for electrolysis"

Above about 60 °C, chlorate can be formed.

Mixe - 3-6-2017 at 02:38

Ah, ok. That would do it. I measure 40 C in my solution runtime. But why isn't it heating up like it should? Current level, right?

Metacelsus - 3-6-2017 at 04:40

Yes, usually the heat is provided by the waste heat from the electrolysis.

If you want to increase the temperature without increasing the current, you can try putting insulation on the cell.

Mixe - 3-6-2017 at 05:17

Well, I tried putting the cell in a waterbath on a hot plate, keeping it at 70 C. This will test the heat theory, I suppose.

Mixe - 3-6-2017 at 11:50

And then again, I find this:

@mnick12: I do not think that making hypochlorite is possible by simply keeping the cell cold. As I wrote in my webpage, when the hypochlorite cannot be converted to chlorate in the liquid (either by low temperature, or by too high pH), then hypochlorite ions are oxidized to ClO radicals and these recombine and react with water to give chlorate, chloride and oxygen. So, when you keep the cell cold you also get chlorate, albeit at a lower current efficiency.

in this thread:

phlogiston - 3-6-2017 at 13:51

Both are correct.

Hypochlorite can auto-oxidise to form chlorate. This only happens at a significant rate at higher temperatures.
It can also be oxidised at the anode to chlorate (at any temperature).

The first mechanism is preferred because it results in much better current efficiency.

If you don't care about current efficiency, just be patient. Your setup sounds about right, and you will most likely see chlorate begin to precipitate at some point. Should you decide to try to raise the temperature, adding insulation is the best method. Note that when the cell is hot, a lot more potassium chlorate can dissolve before it is saturated and precipitation starts.

[Edited on 4-6-2017 by phlogiston]

yobbo II - 3-6-2017 at 14:44

There is a large chlorate page here

Mixe - 3-6-2017 at 23:40

Thank you all for the kind and wise replies! I don't know why people say to stay away from this forum since people are too nasty here. You guys are great!

I shut off the hotplate during the night for safety reasons, and this morning, lo and behold, beautiful white crystals have covered the bottom of the cell. What can I say? Patience is not a virtue I'm plagued with.

Anyhoo, I corrected the pH this morning again, and it was at 12. That's +6 units in less than 12 hours. Made me start thinking of constructing an automatic pH adjuster, and I found this:
Arduino campatible pH meter

I'm thinking: connect it to an arduino nano, connect that to a solenoid valve and add dilute HCl dropwise to the solution according to measurements.
The only thing I wonder about is if the electrode can be submerged in the solution on runtime without beeing completely ruined? It's a nasty brew, after all.

[Edited on 2017-6-4 by Mixe]

Mixe - 19-6-2017 at 05:20

So I ended up with a simpler solution for pH adjustment. In this next batch I just hooked up an old saline drip filled with 10% HCl. After some adjustments of the drip speed (it really needs to drip very slowly), it now keeps an even pH 6,4 in the electrolyte. Next, I need to find a way to raise the temperature, which now only reaches aprox 40 C. But crystals are forming already, about 12 hours after initiation running only 5 amps.

Unfortunately I couldn't rotate the image, but this is the setup.

The sickly orange colour is due to the added potassium dichromate and doesn't affect the final product, which is snow white. Some impurities are seen lining the bubbling electrolyte. These are probably due to corrosion from the hypodermic needle i use for adding the HCl. For my next batch I'm adding a few things things:

1) An all plastic alternative for adding the drip
2) An oil-filled temperature probe extending from the lid into the electrolyte, so I can use a digital thermometer inside it
3) Probably a pH meter with display
4) Insulation

[Edited on 2017-6-19 by Mixe]

[Edited on 2017-6-19 by Mixe]

hyfalcon - 20-6-2017 at 04:38

Open the distance between your electrodes or try a dedicated 5v 10amp power supply. To get the temp up you need to flow more current.

Lots of good reading.

[Edited on 20-6-2017 by hyfalcon]

phlogiston - 20-6-2017 at 07:11

He is using a constant-voltage car charger, so he should decrease the distance between the electrodes to obtain more heat.
However, it would cause a higher current to flow, so it would increase the current density as well.

Can you find a constant current supply?
That would allow you to freely vary the distance between the electrodes to optimise the amount of heat being generated without affecting the current density.

[Edited on 20-6-2017 by phlogiston]

Mixe - 21-6-2017 at 10:29

Sure I could find a constant current supply, but I dont know if I'm really that interested in applying more current. It will apparently increase the wear and tear of my electrodes, and I'd rather wait a few more hours or apply heat from outside . But I'd might try and bend the electrodes closer together.

Also, I have a newly scavenged computer power supply I'll try next time. The car battery charger has a peak amp of 6.

phlogiston - 21-6-2017 at 12:17

You could keep the current what it is right now.
When you increase the distance between the electrodes using a constant current supply, what will happen is that the ohmic resistance of the cell increases and the power supply will output a higher voltage to maintain the constant current.
As a result, you'll generate more heat in the cell while maintaining the same current.

Mixe - 21-6-2017 at 12:27

That's true. But a higher voltage will also put more strain on the electrodes, which are expensive and pretty hard to find. Do you have any suggestions on where to get a constant current PS?

woelen - 22-6-2017 at 02:28

You can have a fairly good approximation of constant current if you use a higher supply voltage, combined with a series resistor.

Assuming your cell runs well at 5V, and you want a current of 5A, and you have a power supply of 12V (a PC power supply works well), you can take a resistor of 1.4 Ohm in series with your cell. You cannot buy a 1.4 Ohm power resistor, but you can make that yourself. Buy 6 power resistors of 1 Ohm, make three sets of 2 parallel resistors and put these 3 sets in series. That makes 1.5 Ohm, which is fine. You are putting 35 W of power in these 6 resistors, so you need to use big power resistors (10 Watt, or even better 25 W). They will become HOT. You can use this heat for heating your cell.

A more energy efficient setup is to take two of your cells in a series connection and use 4 resistors of 1.8 Ohm in parallel and connect this in series with your two cells. The advantage is that with the same current you make two times as much chlorate, but of course you have less heat and your cells may remain too cold.

It is a matter of tweaking and experimenting, but with resistors and a power supply of 12 V you can have endless variations and you have a sufficiently good approximation of a fixed current supply. The voltage over the resistors acts as a form of negative feedback, making the cell MUCH less sensitive to variations in temperature and concentration. The higher the voltage drop across the resistors, the better the regulation.

This page may help you:

It is intended for somewhat lower currents, but the general idea of using resistors can be applied in your case as well.

[Edited on 22-6-17 by woelen]

yobbo II - 22-6-2017 at 06:58

You can make one if you wish

Also LED drivers are constant current like

You can get smaller ones.


AJKOER - 24-6-2017 at 11:50

As mentioned above and my prior writings on SM, it is radical formation and especially, the .ClO radical.

Interestingly, bicarbonate is cited as a catalyst, at least in non-electrolysis settings. The explanation, in my assessment, is that it converts hydroxyl radicals to the more selective carbonate radical. The latter further reaction with hypochlorite as outlined below:

.OH + HCO3- = .CO3- + H2O

.CO3- + OCl- = CO3(2-) + .ClO

The carbonate ion help neutralize pH while forming more bicarbonate.

Some of my prior work, citing reaction paths, but not in an electrolysis setting:

Note, as a point of disclosure, my understanding is that the electrolysis pathway to chlorate is expected to deviate from the above cited theory.

[Edited on 24-6-2017 by AJKOER]

DocX - 26-6-2017 at 13:51

Great info. I opted for the electolysis method after a completely failed attempt at converting calcium hypochlorate through boiling. But since the formation of chlorate from hypochlorite is mainly catalysed by heat and not the electrolysis itself (as I have understood it, but I am a bear of very little brain that loves to be schooled), sodium bicarbonate is maybe worth trying as an additive? But how will it react to the electrolysis in the presence of K+, ClO-, Cl+ and potassium dichromate?

Now, I just started a third batch ( good 60% yield on the second ), and moved the electrodes a little closer to each other. Strangely, when I started to run the cell, no hydrogen gas was bubbling through the vent tube, and the electrolyte went from a clear yellow to an almost opaque, brown liquid within the minute. I added some HCl, and after quite some unhealthy fizzing it cleared up. Soon after there was hydrogen gas bubbling from the vent tube into my NaOH filter solution, but just a few hours later the solution is opaque and brownish again. The pH is at 6 though.
Why this strange discolouring? It hasn't been this bad in any of the prior batches.

Edit: LOL just noticed that my phone has an older login to these forums than my pc :D. One that I thought was lost. So "Mixe" and "DocX" is the same user, me. Honest mistake, I will return under my new name and scrap this one:).
[Edited on 20171717/6/26 by DocX]

[Edited on 20171717/6/26 by DocX]