VitaminX
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Setting up an electrolysis cell
Hello dear folks of SM,
I come to you asking for information regarding my first project in electrochemistry. I want to make a small amount of bromine starting from sodium
bromide by means of electrolysis.
I've stumbled across this link in my research:
http://woelen.homescience.net/science/chem/exps/OTC_bromine/...
and found it very intriguing, it also seems to me like a much cleaner method of making elemental bromine than oxidation with sulphuric acid and an
oxidant.
Since I am in no way an expert in electrodynamics (I barely passed the physics exams in college) I have a few questions:
1.) Will an 12V 95Ah ship gel-battery work here? In the link he only says that the power supply should have 12-15 volts. Afaik it should be DC so that
part is fine however I am not sure if my setup as a whole is going to work.
2.) I have 8x 1 Ohm 5 Watt resistors. When I put all of them in series I should have 8 Ohm resistance and 40 Watts. Keeping in mind U=RI that means
that I should have a current of 1.5 A. What do the amounts of watts change? What would happen if I used 1x 10Ohm 0.1Watt resistor instead of this
setup?
3.) Which wires should I use? Just regular copper wires? What kind of resistance do these have and do I have to include this in my calculations?
Thanks so much 
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MrHomeScientist
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I used woelen's procedure to make my own bromine, and it went flawlessly! His writeup was very detailed and easy to follow. I've got a video on my use
of the process: http://www.youtube.com/watch?v=NKjyM2AkZSY
As for your specific questions, I'm not really sure about the first two. I had a variable DC power supply where I could set the amperage to 1.5A, and
the voltage ended up around 4.5V for the whole duration of the experiment. Correct me if I'm wrong, but I think for just about every electrolysis
experiment 6V is about the maximum voltage you ever want to use. Anything higher than that just produces excess heat and can erode your electrodes. I
used plain old copper wire to connect my graphite electrodes to the power supply. Copper has very low resistance so I would imagine any calculation
you do could just omit it.
This was a really fun experiment. It's really nice because it keeps fumes to a minimum. I definitely recommend it if you have the apparatus to do it.
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Mildronate
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dont use resistors its waste of energy. Use some transistor and potentiometer.
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Metacelsus
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I have a setup with a variac (variable autotransformer) powered by mains with the output hooked up to a full wave rectifier with filter capacitor. I
adjust the voltage to control the current. I usually have it set at around 5 volts, and the supply can deliver up to 15 A.
I don't recommend using a battery, as the battery will run out. You could just use the charger for the battery instead (if the charger can safely run
the cell). If you had to run the cell off DC, you could use a switched-mode power supply to step down the voltage from 12 V to ~5 V. Building your own
may be too complex for you, however.
Example: https://en.wikipedia.org/wiki/Buck_converter
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Fantasma4500
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red and black cables from a computer power supply should do the job, althought its usually grey and green you need to connect to have fan running
aswell on startup
should give around 5V 18A, although 12V 18A seems to give off ALOT more chlorine and hydrogen, a violent burst infact, i guess this is because the
volts cuts properly through the electrolyte
12V ive heard many times to be very damaging to anodes, so dont use that high voltage..
also, make some dichromate, it should add a reasonable amount of performance to your cell, making sure the ClO3 or BrO3 doesnt reduce back into
chloride or hypochlorite
also, potentiometer -- the damn thing ive been searching for forever, now i just need a device that can put out a 60 amps and 12V, then reduce the
volts to 3.3 or 5, supposing a potentiometer doesnt overheat just like that
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Mildronate
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ad 3*tip35C transistors in pararell (or one powerful transistor) with heat sink and your potentiometer will be ok
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Varmint
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Mildronate: Your original objection that resistors are a waste of energy is in no way solved by moving to a potentiomenter and transistor. The
transistor will be dissipating the same energy the plain resistors were, but now you have an active device that needs supplemental heat sinking in
order to survive.
The only true energy efficient way to reduce the available voltage and amperage is to use a switching power supply design, in this case, a "buck"
regulator topology.
This works by essentially connecting/disconnecting power at such a frequency and duty cycle that it averages the power needed for the load, with very
little overhead required for the actual switching/filtering functions.
However, this is far more than just a reference voltage and a transistor, you need an oscillator, a temperature stable voltage reference, and a means
of feedback in order to maintain regulation. Having said that, "chip" level designs are available which require few external components to achieve a
given output, but the high current requirements for this application will typically exceed the capabilities of "pre-packaged" solutions and require
the addition of external heavy-duty switching components (Usually MOSFETs), along with suitable heavier-duty inductors and capacitors.
No references provided because I'm a EE with over 30 years real world experience, you can use THIS POST as reference if you need to.
DAS
[Edited on 11-2-2014 by Varmint]
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woelen
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I have to agree with Varmint. Keep in mind that I did this experiment in a home setup with the goal to make some bromine from very easy to obtain
(nearly OTC) chemicals and with a minimum of equipment and complexity. I know that my setup is horrible in terms of energy efficiency (of the supplied
12 volts only 4 to 5 volts are used for generating interesting chemical stuff, the rest is heat). But does that matter in a home setup? Suppose you
want to make a few tens of grams of bromine. Your power supply may need to run for 1 day or so while delivering 1 A of current and providing 12 to 15
watts of power. It may take 20 watts of power from the wall outlet. Even if you need to run for two days, you only take appr. 1 kWh of electrical
power, which is several tens of eurocents. What are we talking about? The cost of all those nifty semiconductor device is a multiple of this little
amount of energy.
I like the reply of MrHomeScientist. He shows that he understands things. The only goal is to keep the current reasonably constant and he apparently
has the luxury of a nice lab power supply with current control and I did not have, so I used a series of power resistors to keep the current more or
less constant (give or take 10% or so).
Using a 0.1 Watt resistor is not a good thing. It will dissipate several watts of power and will burn out in a fraction of a second.
What is important in this experiment is making the cell reasonably efficient in terms of current (which is not the same as power efficiency, see
above). For each mole of electrons transported through the cell you want nearly one mole of effective oxidation of bromide. For that reason some
chromate or dichromate is added. Since then, I found a very convenient way to make hexavalent chromium from easy to obtain chemicals:
- dissolve a pinch of chrome alum or chromium sulfate in water (easy to obtain, cheaply available from eBay without suspicion, 100 grams is enough for
hundreds of liters of electrolyte).
- add plain household bleach to this solution, until the solution becomes yellow.
Add this solution to your electrolysis cell. The small amount of chloride ions, introduced in this way, is of no concern at all.
[Edited on 17-2-14 by woelen]
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Metacelsus
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Another idea: Run two cells in series off 12 V. The cells would need to be very similar to ensure that both have the same voltage drop.
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