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DJF90
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Would a nickel electrode suffice for electrolysis in these conditions. I know it isn't as available as graphite, but it is much more available than
platinum and wouldn't leave particles in the product. What conditions should be avoided with a nickel electrode (either as anode or cathode)? Sorry if
this is a bit OT.
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woelen
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I don't expect it would work as anode. A cathode, made of nickel definately will do the job, but an anode, made of nickel will be corroded and I think
that not a single gram of bromate can be made with an anode made of nickel.
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DJF90
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electrochemistry isnt exactly my strong point but I'm guessing it has something to do with standard potentials of nickel cells? Would you be able to
provide some rough guidelines for choosing an appropriate material for the electrodes of an electrolysis cell, or perhaps suggest a text I should read
to help me with this task?
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woelen
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The cathode is not critical at all. Nickel, stainless steel, titanium, graphite and of course platinum all do the job, especially if the cathode is
protected as long as it is in the liquid. As long as the power supply is connected, the cathode, especially the compact metal ones, are protected
against corrosion by oxidation. As this is the normal mode of operation, the cathode usually hardly is attacked during electrolysis.
The anode is the critical part. The positive voltage (potential) on the anode gives very strong oxidative strain. If there are several volts between
anode and cathode, then the oxidizing potential can be very high, even as high as if fluorine or some other extreme oxidizer is present. In fact,
electrolysis does nothing else than providing very strong oxidizing conditions at the anode and very strong reducing conditions at the cathode.
Only few materials can withstand the strongly oxidizing conditions near the anode and for a cell, in which halogens are formed at the anode, or
oxygen, only platinum, platinum/iridium alloys, platinum/tungsten alloys, some conductive oxides of metals in high oxidation states (e.g. PbO2) and
carbon are suitable. All other materials are oxidized (e.g. even gold is quickly eroded).
The problem with carbon rods is that they are not made of a single piece of carbon, but they consist of many little particles sintered to each other.
These particles can detach from the rod and under the stressful conditions near the anode, they usually do. I see no simple cheap option, the cheapest
option is graphite, and after that, I think PbO2.
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DJF90
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Thanks, that clears it up alot.
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woelen
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Do not use anodes with titanium strap (e.g. MMO/DSA) for bromate production
I now have a few MMO anodes on a titanium substrate. I tried one of these in a small bromate cell.
Experimental conditions:
50 grams of KBr dissolved in lukewarm water, topped up to appr. 100 ml.
200 mg or so of K2Cr2O7 dissolved in hot water and added to the solution of KBr.
Total volume is around 100 ml.
I transferred this to a sealed bottle (Schott Duran, with the cap modified such that the anode, cathode and a litte PVC tube for gas release can be
pressed through the cap). No mists and sprays were coming out of the bottle during electrolysis.
I ran the cell for 2 hours at a power supply of 5 volts, which delivered 3.5 amperes.
Cell became quite hot, I could just stand the touching of the bottle. It must have been somewhere between 60 and 70 C in the cell.
Anode size: 80 mm x 20 mm DeNora mesh welded on a 1 cm wide titanium strap (free anode area is appr. 60 mm x 20 mm).
Cathode: 80 mm x 20 mm titanium strip.
After 2 hours, I already had a nice layer of white KBrO3 in the cell, but to my shock, there also were thick green flocculent pieces of material
floating around in the liquid, and also dark spots in the layer of KBrO3 
The titanium strap of the MMO anode is severely corroded, especially at the place where the MMO mesh is welded to the titanium strap. I immediately
switched off the cell and inspected the anode. The titanium strap was covered by some greyish/green goop, which simply could be wiped off with a
finger. The MMO mesh itself still is in perfect condition.
So, for making KBrO3, stick to graphite, or if you can afford it, use platinum. The MMO coating is capable of handling bromide/bromine/bromate, but
the titanium substrate isn't. There might be other MMO anodes around with other substrate metal.
---------------------------------------------------------------------
I used this same MMO anode already for making small quantities of chlorate and it worked perfectly in the same cell with 30 grams of KCl dissolved in
luke warm water and topped up to 100 ml and some K2Cr2O7 added. Current was between 3 and 3.5 ampere, just as in the bromate cell.
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plante1999
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Quote: Originally posted by woelen  | I now have a few MMO anodes on a titanium substrate. I tried one of these in a small bromate cell.
Experimental conditions:
50 grams of KBr dissolved in lukewarm water, topped up to appr. 100 ml.
200 mg or so of K2Cr2O7 dissolved in hot water and added to the solution of KBr.
Total volume is around 100 ml.
I transferred this to a sealed bottle (Schott Duran, with the cap modified such that the anode, cathode and a litte PVC tube for gas release can be
pressed through the cap). No mists and sprays were coming out of the bottle during electrolysis.
I ran the cell for 2 hours at a power supply of 5 volts, which delivered 3.5 amperes.
Cell became quite hot, I could just stand the touching of the bottle. It must have been somewhere between 60 and 70 C in the cell.
Anode size: 80 mm x 20 mm DeNora mesh welded on a 1 cm wide titanium strap (free anode area is appr. 60 mm x 20 mm).
Cathode: 80 mm x 20 mm titanium strip.
After 2 hours, I already had a nice layer of white KBrO3 in the cell, but to my shock, there also were thick green flocculent pieces of material
floating around in the liquid, and also dark spots in the layer of KBrO3
The titanium strap of the MMO anode is severely corroded, especially at the place where the MMO mesh is welded to the titanium strap. I immediately
switched off the cell and inspected the anode. The titanium strap was covered by some greyish/green goop, which simply could be wiped off with a
finger. The MMO mesh itself still is in perfect condition.
So, for making KBrO3, stick to graphite, or if you can afford it, use platinum. The MMO coating is capable of handling bromide/bromine/bromate, but
the titanium substrate isn't. There might be other MMO anodes around with other substrate metal.
---------------------------------------------------------------------
I used this same MMO anode already for making small quantities of chlorate and it worked perfectly in the same cell with 30 grams of KCl dissolved in
luke warm water and topped up to 100 ml and some K2Cr2O7 added. Current was between 3 and 3.5 ampere, just as in the bromate cell.
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I just made a bromate cell using 35%NaBr sol. and a mmo anode mesh without any titanium exposed. There is no aditive and the anode have about 3 time
more surface area than the titanium cathode and the cell is made from PP. voltage is 3.35V and temperature of the electrolyte is 7 degree C (the cell
is outside and the temperature outside is -18 degree C). It as been working for 5 hour and the cell look healty and there is no corrosion on the
anode, the cell contain 750-800ml of sol.. At the start the anode make bromine which fall on the bottom of the cell but the cathode gas mixed the
bromine with the hydroxide at the cathode and the sol. turned yellow (like NaOCl Sol., but more yellow).
Photo will be aded later.
[Edited on 16-1-2012 by plante1999]
[Edited on 16-1-2012 by plante1999]
I never asked for this.
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plante1999
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After 27hour the anode is still looking healty. After 16hour after the starting of the electrolisis I changed the 3.35v to 4.5v (I need to check the
temperature). The cell is somewath hot and there is no precipitate.
Edit : the temperature is now 57degree C.
[Edited on 16-1-2012 by plante1999]
[Edited on 17-1-2012 by plante1999]
I never asked for this.
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woelen
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If you use NaBr, then you won't get much precipitate. NaBrO3 is much more soluble than KBrO3. You can add more solid NaBr to the hot cell and then at
a certain point in time you'll get precipitate of NaBrO3. The lack of an additive also is not good. Much of the formed bromate will be reduced back to
bromide. Look at the production of hydrogen. When hardly any hydrogen is produced at the cathode while you have a hot cell and a good electrical
current, then the bromate is reduced back to bromide.
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plante1999
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Large ammount of hydrogen is generated at the cathode, so bromate should not be reduced. NaBrO3 solubility is supposed to be at 0degree C, about
24.2g/100ml and my solution is 35% so there should be some precipitate , but when the sol. level go down I ad 35% NaBr sol.
Edit: After 51 hour the MMO coating is still looking healty but the exposed titanium on the sides erroded somewath , I will need to ad epoxy to seal
of the titanium from the electrolyte.
[Edited on 17-1-2012 by plante1999]
I never asked for this.
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Muzz1969
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Has anyone tried using Thoriated tungstan rods for their electrolysy unit,they're cheap and readily available and are chemically resistant to all but
a few acids and is an excellent conductor.
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