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Author Topic: Electrolysis of NaCl solution  (Read 25621 times)

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vmelkon

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Electrolysis of NaCl solution
« on: October 31, 2010, 09:07:00 AM »

I'm sure everyone has done this. I'm using 12 V. I would like to collect the Cl2 so I'm using a graphite anode. I also tried with silicon and it works but since it is not the right shape, I am using a graphite rod instead.

My question is, do you really get Cl2 as a gas or does it dissolve in water? Does oxygen get produced?
I am getting bubbles but I don't know if it is Cl2 gas. I would have to wait a long time to collect enough to see the green color.

I know that there are faster methods to make Cl2. Not interested in them.
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Grundalizer

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Re: Electrolysis of NaCl solution
« Reply #1 on: October 31, 2010, 11:02:55 AM »

No matter how hard you try, you cannot get chlorine gas from the electrolysis of aqueous sodium chloride (NaCl).  You will get Hydrogen and Oxygen gas. 

If you want O2 gas it's much easier to just take apart an alkaline battery and extract the black manganese dioxide, then add store bought hydrogen peroxide.  Oxygen gas will bubble out which you can then use to make things burn more brightly, which is pretty cool.

Why do you want chlorine gas anyway?  Breathing that stuff in will instantly destroy your lungs
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vmelkon

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Re: Electrolysis of NaCl solution
« Reply #2 on: October 31, 2010, 01:01:46 PM »

But it is clear that chlorine is getting formed because the solution smells but perhaps it stays dissolved in the solution. There is also 1 mL of gas collected.
It's for another experiment. I won't be next to it.
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Borek

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Re: Electrolysis of NaCl solution
« Reply #3 on: October 31, 2010, 01:39:08 PM »

You are getting mixture of chlorine and oxygen, but it is mostly oxygen.
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Grundalizer

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Re: Electrolysis of NaCl solution
« Reply #4 on: November 01, 2010, 02:49:01 AM »

If you want to make chlorine gas a lot easier go to the local hardware store and buy some Muriatic acid (Hydrochloric acid) which I think is ~30% concentration.  It should only be $8 or so.

Then take apart an alkaline battery and take out the black manganese dioxide like I had mentioned above.

Manganese dioxide catalyzes the decomposition of HCl into Chlorine gas and water.  You will need to add a little heat, but it produces a lot more than the electrolysis of salt water.

I'm sure you know chlorine can kill you if you inhale too much as it will eat your lungs and you'll drown in bodily fluids.  Good luck, may the science be with you.
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horsebox

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Re: Electrolysis of NaCl solution
« Reply #5 on: November 13, 2010, 09:35:39 AM »

No matter how hard you try, you cannot get chlorine gas from the electrolysis of aqueous sodium chloride (NaCl).  You will get Hydrogen and Oxygen gas. 

If you want O2 gas it's much easier to just take apart an alkaline battery and extract the black manganese dioxide, then add store bought hydrogen peroxide.  Oxygen gas will bubble out which you can then use to make things burn more brightly, which is pretty cool.


When I did this I could clearly smell Cl2. It was a decent sized lab and the whole place stank of chlorine after a few minutes of people experimenting with electrolytic cells so I assume a significant quantity of the stuff can be formed this way. No idea how it could be collected though.

As for H2O2 and MnO2, I'm gonna try this out since I have a fair bit of MnO2 lying around.
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Catsceo

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Re: Electrolysis of NaCl solution
« Reply #6 on: November 13, 2010, 10:23:05 AM »

Would't this form hypochlorites, chlorites, chlorates, and perchlorates?  Wikipedia says unless you use a membrane cell, you won't get, or at least won't get much, chlorine.  I intended to use this method to get ahold of chlorates and perchlorates.  Would this not work for what I was planning?
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Grundalizer

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Re: Electrolysis of NaCl solution
« Reply #7 on: November 13, 2010, 04:11:04 PM »

It would still be 100x times cheaper and easier to just buy perchlorates in their salt form rather than even attempting to make them from NaCl electrolysis.  I've done that electrolysis 1000 times and never once smelted chlorine.  Perhaps it's like Hydrogen Sulfide in that we can detect parts per billion, because it is made in such small quantities it's not even worth trying to collect by that method.
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bromidewind

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Re: Electrolysis of NaCl solution
« Reply #8 on: November 13, 2010, 09:25:51 PM »

You won't get (much) Cl2 out of a simple salt-water solution because it's at a constant concentration, i.e. everything's jumbled together. The membrane cell Catsceo mentioned separates the Na+ and Cl- ions via a membrane, allowing Cl2 to be collected at the anode and oxygen at the cathode, with NaOH as a byproduct.

You can see a nice Flash animation of the membrane cell at work over here.
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Catsceo

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Re: Electrolysis of NaCl solution
« Reply #9 on: November 14, 2010, 03:23:32 AM »

It would still be 100x times cheaper and easier to just buy perchlorates in their salt form rather than even attempting to make them from NaCl electrolysis.  I've done that electrolysis 1000 times and never once smelted chlorine.  Perhaps it's like Hydrogen Sulfide in that we can detect parts per billion, because it is made in such small quantities it's not even worth trying to collect by that method.

I tried a carbon anode and got a big wiff of chlorine.  Perhaps I need a better one that won't release too much of it.  Also, I'm trying to do a demonstration by making a solution of sodium per/chlorate and adding KCl to precipitate out potassium per/chlorate.
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Jango

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Re: Electrolysis of NaCl solution
« Reply #10 on: November 23, 2010, 06:21:18 AM »

In electrolysis of concentrated NaCl solution (using graphite electrodes and, for example: 12V), H2 is formed at the cathode, while Cl2 is formed at the anode. I believe no oxygen is formed at the anode.

The water (H+ + OH-), has its H+ taken away, so the OH- ion is left in solution. The Na+ ion left in solution goes together with the OH- ion to form NaOH.

So the half equations are:

(at cathode) 2H+ + 2e- -----> H2(g)

(at anode) 2Cl- -----> Cl2(g) + 2e-

(in solution) Na+ + OH- -----> NaOH(aq)

The full equation goes like this:

2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)

Therefore, no oxygen is made in this way, as it is all used in the process of making the NaOH. Using a different voltage and electrodes, you may do.

If you used iron electrodes, hydrogen would form at the cathode as usual, but most of the chlorine at the anode would react with the iron to from a yellow/green solution of iron(III) chloride because chlorine is so reactive.
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billnotgatez

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Re: Electrolysis of NaCl solution
« Reply #11 on: November 23, 2010, 01:43:03 PM »

Jango -

There are indication that many think that oxygen is formed as well when in an aqueous solution of sodium chloride even though many of us learned otherwise
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Grundalizer

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Re: Electrolysis of NaCl solution
« Reply #12 on: November 23, 2010, 04:23:27 PM »

Quote
There are indication that many think that oxygen is formed as well when in an aqueous solution of sodium chloride even though many of us learned otherwise

What does that mean?
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Zerm

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Re: Electrolysis of NaCl solution
« Reply #13 on: November 23, 2010, 04:52:01 PM »

I know for a fact that chlorine can be produced by electrolysis of a brine solution and I would think evolution of chlorine would be more thermodynamically favorable than oxygen but there are a lot of variables and I certainly can't say that I know its not produced because it likely is to an extent.  I therefore propose an experiment! 

Setup three inverted test tubes in three beakers of water.  In one tube, use the afore mentioned catalytic decomposition of HCl with MnO to trap pure Cl2.  In the second tube, use catalytic decomposition of H2O2 with MnO to trap pure O2.  In the third tube, trap the gasses given off at the anode of a NaCl solution under electrolysis.  Compare the gasses by color first.  If color isn't a clear indicator, carefully insert an iron nail or wire into each tube and observe the reactions.  Yellow ferrous chloride should form rapidly in a pure Cl2 environment. 

I just might do this over the weekend.  If I do, I'll let you know how it turns out.
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billnotgatez

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Re: Electrolysis of NaCl solution
« Reply #14 on: November 23, 2010, 06:45:59 PM »

Grundalizer -

http://electrochem.cwru.edu/encycl/art-b01-brine.htm
http://en.wikipedia.org/wiki/Chloralkali_process
http://scienceaid.co.uk/chemistry/applied/electrolysis.html

I have seen somewhere that oxygen is also produced in some quantity, but I can not find it now. So here are some discussions I found while Googling "electrolysis of brine". Maybe I just remembered incorrectly.

Thanks for getting me thinking.

In any case if any oxygen were created in combination with Chlorine, how would we easily determine the purity.


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