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making hydrogen and oxygen from electrolysis

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IamSmooth

Thu Dec 08 2011, 01:59AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I know that when two graphite electrodes are put in salty water oxygen and hydrogen are produced. I also remember that one could get Chlorine gas and sodium hydroxide. What determines if one gets H2 and O2, or Cl2 gets produced? Is it the concentration of the salt?

Bjørn

Thu Dec 08 2011, 07:02AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The salt is there to make the water conductive and it is a bad choice for the reasons mentioned. A much better solution is to use NaOH to make the water conductive and stainless steel electrodes.

Only yesterday we had someone in the chatroom have their cell explode on them so some care is needed unless the volume is very small.

Hon1nbo

Thu Dec 08 2011, 01:41PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

BjÃ¸rn wrote ...

The salt is there to make the water conductive and it is a bad choice for the reasons mentioned. A much better solution is to use NaOH to make the water conductive and stainless steel electrodes.

Only yesterday we had someone in the chatroom have their cell explode on them so some care is needed unless the volume is very small.

I second this.

I was experimenting with MHD propulsion and used saltwater... I could smell chlorine, and I was just using a 9V battery and not even trying to really do any electrolysis. And when I did try for the purpose of making H2 and 02 I could really smell it!

You going to try to collect the gas?

-Jimmy

IamSmooth

Thu Dec 08 2011, 02:57PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I have no desire to do this now. I did this when I was in high school. My question is academic: what determines if one gets pure H2 and O2, or a combination mixed with Chlorine?

I take it that the explosion was from the mixture of H2 and O2, Bjorn?

GluD

Thu Dec 08 2011, 03:26PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

IamSmooth wrote ...
what determines if one gets pure H2 and O2, or a combination mixed with Chlorine?

The contents of the cell determines the gasses.

If you dont want chlorine, dont put it in your cell.

IamSmooth

Thu Dec 08 2011, 03:31PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Not quite the answer I was looking for. I understand the contents are important. Does this mean that one will *always* get H2, O2 and Cl2 if salt is used?

GluD

Thu Dec 08 2011, 04:31PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

I am by no means an expert on the subject but I would consider it reasonable to think that if there is chlorine in the cell it will be in the gas as well.

BjÃ¸rn mentioned NaOH, I think that is a good advice. Remeber to read the datasheet on this chemical if you are unfamilar with handling it.

You could also just limit the amount of NaCl used, so the amount of chlorine released is insignificant, or use a cold trap to cleanse the gas but that seems rather silly compared to just leaving the chlorine out of the equation altogether, by simply not adding it in the first place.

IamSmooth

Thu Dec 08 2011, 04:46PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I appreciate your responses, but I am not trying to make it. I've done this already. I am inquiring about what determines the products. I thought I read somewhere that the concentration of the salt had something to do with it.

Using NaOH would solve the Chlorine issue, but that was not my question.

AndrewM

Thu Dec 08 2011, 06:41PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I would expect that the proportion of liberated gasses is determined by a) the concentration and b) the applied voltage.

IIRC the standard potential to split the water is a bit over 1V, maybe 1.25V or so. I understand that the potential to split NaCl is quite high by chemical standards - wikipedia says 4V for molten NaCl; I don't know if it changes in solution though I would guess not.

Ergo, I believe if your applied voltage is less than 4V (or whatever the standard potential for the NaCl reaction is) then you should get little chlorine. I also suspect that people turn the voltage wayyy up on their electrolyser in order to make gas more quickly, and thats when they run into problems.

Also: I am not a chemist.

Steve Conner

Thu Dec 08 2011, 07:29PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

According to the Wikipedias (which is 100% accurate of course) no oxygen is produced by electrolysing brine. The chloride ion competes with the oxygen one, and you get hydrogen and chlorine. The reaction is:

2 NaCl + 2 H2O â†’ Cl2 + H2 + 2 NaOH

The explosions might well just be from hydrogen and air.

If you want oxygen, you have to use NaOH or KOH as your electrolyte. Imagine how much worse the explosions would be! Maybe once the reaction has run for a while with NaCl, the NaOH produced can give you oxygen.

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