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AC to DC Converter for Electrolysis hobby

by chemaddict
Tags: ac to dc, electrolysis, hofmann, rectifier
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chemaddict
#1
Jan4-11, 06:28 PM
P: 15
Hello All,

I am interested in constructing a small Hofmann apparatus for electrolysis (as a hobby), like this kind:
http://en.wikipedia.org/wiki/Hofmann_voltameter

I am confronted with the choice of either using a bunch of 9V batteries, or some kind of AC/DC rectifier.

As I am a chemical engineer and have taken 1 physics class on electricity, I consulted my old textbook and found that building one from pieces is probably beyond me.

My question, then, is this:
Is there some simple (cheap) AC to DC converter that can plug into a US residential wall socket and also has leads to hook up alligator clips or something similar for the apparatus?

It sounds kind of dangerous as I type it out. It would need ~9V out (somewhere in the 6-15V range is fine).

I've looked for one, but it seems you can either plug it into the wall or have nice leads, but not both. Any help and (especially) advice is appreciated.
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berkeman
#2
Jan4-11, 06:38 PM
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Quote Quote by chemaddict View Post
Hello All,

I am interested in constructing a small Hofmann apparatus for electrolysis (as a hobby), like this kind:
http://en.wikipedia.org/wiki/Hofmann_voltameter

I am confronted with the choice of either using a bunch of 9V batteries, or some kind of AC/DC rectifier.

As I am a chemical engineer and have taken 1 physics class on electricity, I consulted my old textbook and found that building one from pieces is probably beyond me.

My question, then, is this:
Is there some simple (cheap) AC to DC converter that can plug into a US residential wall socket and also has leads to hook up alligator clips or something similar for the apparatus?

It sounds kind of dangerous as I type it out. It would need ~9V out (somewhere in the 6-15V range is fine).

I've looked for one, but it seems you can either plug it into the wall or have nice leads, but not both. Any help and (especially) advice is appreciated.
Welcome to the PF.

What DC voltage level do you need? If it's only 9-15V or so, a simple wall wart transformer from Radio Shack should be fine.
vk6kro
#3
Jan4-11, 08:54 PM
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P: 4,027
You can get all sorts of power supplies. Something like this would be nice but possibly an overkill for what you need.


or this:


These show voltage and current and include current limiting so you could set a current and let the supply take care of the voltage. This is handy for electrolysis.

For a brief test, almost any small DC power supply would be OK.

Making a Hoffman Voltameter is a major project. The main problem would be getting platinum electrodes for it and then sealing these in glass and making the seal waterproof.
They need some very classy glassblowing skills too.

There are suppliers of these for schools and small ones are not expensive. Look up Laboratory Equipment in your Yellow pages.

If you want to produce Hydrogen and Oxygen from water, you will have to make the water conductive by adding something ionic to the water.
This can be Sulfuric Acid, but that is pretty dangerous and results in damaged clothing or skin if it gets spilt.
A better choice is 0.25 mole/litre Sodium Sulfate. This gives a good 2:1 ratio of Hydrogen to Oxygen produced.

chemaddict
#4
Jan4-11, 09:34 PM
P: 15
AC to DC Converter for Electrolysis hobby

Thanks for the pics and advice! That is very helpful info.

If you want to produce Hydrogen and Oxygen from water, you will have to make the water conductive by adding something ionic to the water.
This can be Sulfuric Acid, but that is pretty dangerous and results in damaged clothing or skin if it gets spilt.
A better choice is 0.25 mole/litre Sodium Sulfate. This gives a good 2:1 ratio of Hydrogen to Oxygen produced.
I was actually thinking epsom salt (a hydrate of magnesium sulfate) for the electrolyte. Also, I don't think the platinum electrodes are necessary, they just work best. I'm looking into graphite actually and I was thinking of using gouging rods for the anodes (with the graphite tip inside and the copper shaft outside) but I'm still looking into that. Electrochemistry was never my strong suit, hence, I am here.

Anyway, thanks for the help again, and I'll probably ask some more questions in the future.
vk6kro
#5
Jan5-11, 12:53 AM
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P: 4,027
The actual salt used has a big effect on the gas quantities produced and Sodium Sulfate was the best I could find. Luckily it is very cheap as well.
Magnesium Sulfate may be just as good or better.

I tried graphite electrodes, because they came with the Voltameter, but the Pt ones were superior. The Pt doesn't have to be very thick, because it is not consumed.
sophiecentaur
#6
Jan5-11, 05:39 AM
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This seems like a really straightforward home project as long as you don't want it to look pretty.
1. A possible source of Platinum could be a second hand jewellery shop or a pawnbroker's (or even Granny's jewellery box!). An old platinum wedding band could easily be cut in two and flattened for your electrodes.
2. You don't need glass for this. Plastic plumbing components would be fine for most of the equipment and there are plenty of small bore clear bits (taps / tubing) available from home-brewing shops.
3. You won't need a lot of current for this as you don't want things to overheat: you will get about 10^19 ions per second for a current of 1A (Plenty of cheap AC - DC adaptors will supply this). In an hour, you should be getting a vast volume of Hydrogen! A simple DC Ammeter would tell you the current that the cell is passing.
Phrak
#7
Jan5-11, 06:12 AM
P: 4,512
How much voltage potential is required to disassociate hydrogen and oxyen gases from a NaCl solution assuming carbon electrodes, or Pt, as long as the electrodes are the same and the electrode ions are don't complicate things? My guess is very low voltage, like less than 2 volts.

This is really a high school or first year Chem question (something about "half cell" reactions that I missed learning about somehow), but it's pertinent to the best choice of power supply.
sophiecentaur
#8
Jan5-11, 07:35 AM
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Wikkers seems to imply that.
See http://en.wikipedia.org/wiki/Electrolysis
chemaddict
#9
Jan5-11, 08:10 AM
P: 15
Quote Quote by Phrak View Post
How much voltage potential is required to disassociate hydrogen and oxyen gases from a NaCl solution assuming carbon electrodes, or Pt, as long as the electrodes are the same and the electrode ions are don't complicate things? My guess is very low voltage, like less than 2 volts.
An important note: While NaCl may seem like a good choice because it's ubiquitous, you have to realize that you will produce H2 gas and Cl2 gas, NOT O2 gas with it. This has to do with the electrode potential of the half reactions (and this is an EE forum so I'll stop there).

As far as the voltage required, this table is what you want:
http://en.wikipedia.org/wiki/Table_o...ode_potentials

The half reactions are as follows (for an acidic solution):
2H2O ----> O2 + 4H+ + 4e- (Anode) Eox = -1.23V
2H+ + 2e- ----> H2 (Cathode) Ered = 0.00V

Note: All of these are relative to that second reaction, the standard hydrogen electrode.

There are other considerations (google Nernst Equation), but suffice it to say that an absolute value would be approximately 6V (I think, correct me if I'm wrong) for the cell to drive the reaction. I know that some have used 9V batteries successfully.

So, in summary:
Thanks for everyone's suggestions! I don't intend to do glass blowing, I'm going with sophiecentaur's philosophy of "it doesn't have to be pretty." What I'm understanding is that I need a 9V DC power supply in the neighborhood of 1A and that while carbon electrodes will work, platinum may be worth it.
sophiecentaur
#10
Jan5-11, 08:39 AM
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Just a comment about power dissipation and excess curent. Whilst a lot of the energy supplied to the cell can go into electrolysis, I have a feeling that, if the current density is too high (limited by the number of ions available over the area of the electrodes) then there will be a limit to the rate of gas production and at which energy may be dissipated as heat.
I remember, once, setting up a half arsed electrolytic system for getting silver out of spent photographic fixer. Without using Cynaide compounds (even my garage was a health and safety aware area), the rate of silver deposition was very low and I found that increasing the current just caused things to get hot and produced clouds of nasty looking stuff. If I increased the electrode area, I could get away with higher current before this happened. It was a rubbish scheme, though, which would have taken years to produce any worthwhile amounts of silver, I'm sure.
vk6kro
#11
Jan5-11, 08:39 AM
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This reaction happens more quickly as you increase the current through the solution.

Invariably, you end up using as many volts as the power supply will deliver. I have placed two 12 volt supplies in series and used a 30 volt power supply to speed up the reaction.

The actual amount of gas being produced is quite small and getting 50 ml or so can take 10 minutes even with maximum voltages.

I don't recall actual currents, but it would be of the order of 100 mA or so, I think.

Depending on why you are doing this, it seems a shame to not be able to watch the process. It is fascinating.

I think simple Hoffman's were available for $50 or so and this included Pt and graphite electrodes. However that was 10 years ago, I guess.
I would try Google and Ebay.
Also, the glass tends to get broken and the old Pt electrodes are kept in a drawer, so these may be available if you ask nicely at schools and Colleges.
vk6kro
#12
Jan5-11, 08:44 AM
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I meant to mention, if you get two burettes full of solution and support them inverted vertically in a bowl of solution, you can put some suitable electrodes (of stainless steel if necessary) into the bowl and up into the inverted burettes.

This may give you a feel for what is required.
Borek
#13
Jan5-11, 09:04 AM
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Electrode reaction needs small single volts, everything else is lost to ohmic drops. Increasing concentration of dissolved salt will increase solution conductivity (up to some limits). If memory serves me well sodium sulfate will give higher conductivity than epsom salt, but both sulfuric acid and sodium hydroxide should be easily able to beat it. Volumes of gases produced should not depend on the dissolved salt. To make 24 L of hydrogen you need at least 2*96500 coulombs - almost 54 hours at 1A assuming 100% current efficiency.
vk6kro
#14
Jan5-11, 09:48 AM
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Greetings Borek.

I think the Voltameters I used were highly resistive due to the very small Platinum electrodes and relatively long distance between the electrodes. So, a high voltage was needed to get much current to flow.

Thanks for the comments.

Do you have any experience of Silver plating? The comment above described my attempts at doing this as well. Pretty much a disaster unless you want to play with Cyanide.
Phrak
#15
Jan5-11, 03:51 PM
P: 4,512
It seems to me, a variable current source would be preferable over a voltage supply.
sophiecentaur
#16
Jan5-11, 04:23 PM
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Yebbut you don't get a free one of those with every bit of DC kit that you buy. The cheap option is to use what you've got, surely.
Borek
#17
Jan5-11, 05:29 PM
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Quote Quote by vk6kro View Post
I think the Voltameters I used were highly resistive due to the very small Platinum electrodes and relatively long distance between the electrodes. So, a high voltage was needed to get much current to flow.
Not surprising In electrochemical measurements each cell - that translates mostly to the electrodes geometry - is described by so called cell constant, see http://en.wikipedia.org/wiki/Conduct...c)#Definitions. The closer the electrodes, the larges the surface, the lower the resistance. I guess that's intuitively obvious for anyone working with electric current.

Do you have any experience of Silver plating? The comment above described my attempts at doing this as well. Pretty much a disaster unless you want to play with Cyanide.
Never tried by myself. Cyanides complex silver, lowering concentration of free Ag+, that in turn means you need higher potential for the reduction, reaction is slower, but the quality of coating is much better. I have here an old Polish book with electrochemical recipes designed for home use - it proposes solution made of 100 mL of water, 35g of KI and 4.5 g of freshly prepared AgCl. But it honestly states that the surface quality - while acceptable - is lower than the one obtained with cyanides.
vk6kro
#18
Jan5-11, 10:38 PM
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Quote Quote by Phrak View Post
It seems to me, a variable current source would be preferable over a voltage supply.
Do you mean for silver plating? Yes, that is probably right.

For the Hoffman voltameter, you need the power supply at maximum everything or else you will be waiting around a lot.


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