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Topic: Electrolysis Of Brine  (Read 17029 times)

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zootreeves

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Electrolysis Of Brine
« on: August 10, 2005, 04:43:25 PM »
Hi,

I want to manufacture a small quantity of sodium by the electrolysis of sodium chloride. I know as soon as the sodium is produced it reacts with the water to produce sodium hydroxide, but once all the water has reacted (so it is sodium chloride dissolved in sodium hydroxide) will the sodium collect on the anode in a pure state (if it is made of graphite)?

Thanks,
Ben

Offline jdurg

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Re:Electrolysis Of Brine
« Reply #1 on: August 10, 2005, 07:09:47 PM »
You will never, and I mean NEVER, produce sodium metal from electrolysis of an aqueous NaCl solution.  There is nothing you can do, no products you can use, no containment you can think of.  You will NEVER do it.  In the history of man, nobody has ever done it and nobody will ever do it.

The ONLY way to make sodium metal from table salt is to electrolyze molten NaCl.  Also, the theory that as soon as the sodium metal is produced it will react with the water is false.  Sodium metal is actually NEVER produced.  At the anode, you'll get the production of chlorine gas as the oxidation potential for the Cl- ion is about the same as that for the oxidation potential for water.  As a result, chlorine gas forms in a moderately concentrated solution of sodium chloride.  At the cathode, hydrogen gas is produced because the reduction potential of the conversion of water to hydrogen gas and hydroxide ions is -0.83 while that for converting Na+ to Na is -2.71.  The more positive the reduction potential is, the more favorable it is for the reaction to occur.

So at the cathode, the reaction of H2O + e- => 0.5H2 + OH- is MUCH more favorable than the reaction of Na+ + e- => Na (metal).  So what happens when you electrolyze a sodium chloride solution is you make chlorine gas at the anode and hydroxide ions at the cathode.  The sodium ions go unchanged, so the sodium hydroxide is the result of the formed OH- ions and the remaining Na+ ions.
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zootreeves

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Re:Electrolysis Of Brine
« Reply #2 on: August 11, 2005, 09:13:58 AM »
Thanks for the reply...

Is there any way to lower the melting point of sodium chloride or is there another sodium compound (easily available) with a lower melting pint that can be used to produce sodium?

Also if I keep electrolysing the same sodium chloride solution will I eventually be left with concentrated sodium hydroxide?

Offline jdurg

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Re:Electrolysis Of Brine
« Reply #3 on: August 11, 2005, 10:23:55 AM »
Yes.  In fact, that's how sodium hydroxide is produced on an industrial scale.  They electrolyze a very concentrated sodium chloride solution producing chlorine gas and sodium hydroxide.  

The melting point of NaCl can be lowered a few hundred degrees centigrade by the proper addition of some calcium chloride.  Still, you need a fairly elevated temperature to make the mixture molten.  You will also need electrodes which are completely inert to the high temperatures used and the corrosive chlorine gas that is produced.  Another very important thing that's needed is an inert atmosphere over your electrolysis chamber.  Because of the high temperatures, the sodium that's produced will be in a liquid form.  Sodium is reactive enough as a solid, but at a high temperature in the form of a liquid it can actually be explosive.

Another compound that can be electrolyzed which has a lower melting point is sodium hydroxide.  The problem there is that anhydrous NaOH quickly absorbs water from the air so it's very difficult to keep the whole thing dry.  Also, molten NaOH is an INCREDIBLY caustic mess that can dissolve many supposedly 'inert' materials.  In addition, if some sodium forms in that molten mess and reacts with the tiniest bit of water, you'll get a little explosion which will fling molten sodium hydroxide all over the place.  That's a VERY dangerous thing.

If you truly want to learn more about the formation of sodium metal, go onto google and do a search for the 'Down's Cell'.  That should give you plenty of information about the electrolysis of NaCl.   ;D
« Last Edit: August 11, 2005, 10:25:26 AM by jdurg »
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zootreeves

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Re:Electrolysis Of Brine
« Reply #4 on: August 11, 2005, 04:08:07 PM »
Thanks a lot for your help i think i'll give up on trying to make sodium that way, I don't want to risk burning myself that much...

Will the sodium hydroxide keep absorbing moisture from the air? so you could never end up with it highly concentrate?

Thanks

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Re:Electrolysis Of Brine
« Reply #5 on: August 12, 2005, 10:07:36 AM »
Yeah, NaOH is a real pain in the arse in terms of keeping it pure.  You can buy it in an anhydrous form of powder/flakes, but as soon as you open the bottle it will start to absorb water from the air.  KOH is really good at doing this, and when you open a bottle of KOH on a humid day, you'll immediately see it start absorbing the water.  Also, NaOH will readily absorb carbon dioxide from the air and form NaHCO3 on you.  So whenever you have powdered NaOH around, you need to keep the bottle closed.  For most purposes (Such as making soap or unclogging a drain), the sodium hydroxide remains pure enough for the intended purposes.
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Offline billnotgatez

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Re:Electrolysis Of Brine
« Reply #6 on: August 14, 2005, 05:41:16 AM »
Is the result of electrolysis on brine at room temperature as follows?
First you get the hydrogen and chlorine released
When there is no more chlorine available
Then hydrogen and oxygen is released until all you have left is sodium hydroxide
You would need a higher temperature to continue the electrolysis so that there would only be sodium left
Also that continued electrolysis should be done in an inert atmosphere


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Re:Electrolysis Of Brine
« Reply #7 on: August 14, 2005, 04:11:35 PM »
You would need a higher temperature to continue the electrolysis so that there would only be sodium left. Also that continued electrolysis should be done in an inert atmosphere

At this stage, you will be left with solid sodium hydroxide because all the water has either been electrolysed or evaporated due to heating effect. How do you intend to electrolyse a solid compound? The only way to obtain sodium is via electrolysis of a molten sodium compound.

I don't recommend it at all, if you carrying out the electrolysis in a home make-shift lab. you need protective equipment. let me quote some information from sodium MSDS:
Quote
Emergency Overview
--------------------------
DANGER! FLAMMABLE SOLID. CORROSIVE. WATER REACTIVE. CATCHES FIRE IF EXPOSED TO AIR. HARMFUL OR FATAL IF SWALLOWED. HARMFUL IF INHALED OR ABSORBED THROUGH SKIN. CONTACT MAY CAUSE BURNS TO ALL BODY TISSUE.

Health Rating: 3 - Severe (Life)
Flammability Rating: 3 - Severe (Flammable)
Reactivity Rating: 3 - Severe (Water Reactive)
Contact Rating: 4 - Extreme (Corrosive)
Lab Protective Equip: GOGGLES & SHIELD; LAB COAT & APRON; VENT HOOD; PROPER GLOVES; CLASS D EXTINGUISHER
Storage Color Code: Red Stripe (Store Separately)


Potential Health Effects
----------------------------------

Inhalation:
Inhalation produces damaging effects on the mucous membranes and upper respiratory tract. Symptoms may include irritation of the nose and throat, and labored breathing. May cause lung edema, a medical emergency.

Ingestion:
Extremely dangerous, corrosive material. Will react immediately with saliva to cause serious burns and possible local combustion and even explosion of hydrogen in the mouth or esophagus. The metal's low melting point can cause further complications.

Skin Contact:
Corrosive, can cause serious burns due to almost immediate reaction with water, especially on moist skin. If metal ignites, very deep burns and tissue destruction can occur.

Eye Contact:
Corrosive. May cause redness, pain, blurred vision, and damage from severe alkali burns.

Chronic Exposure:
Continued or repeated skin contact may cause dermatitis, mucous membrane irritation, and lung damage..
« Last Edit: August 14, 2005, 10:19:47 PM by geodome »
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Offline billnotgatez

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Re:Electrolysis Of Brine
« Reply #8 on: August 14, 2005, 10:10:59 PM »
From a web site 318 C not 800 C
=========================
 General

      Synonyms: caustic soda, soda lye, lye, white caustic, aetznatron, ascarite, Collo-Grillrein, Collo-Tapetta, sodium hydrate, fotofoil etchant, NAOH, STCC 4935235, sodium hydroxide pellets, Lewis red devil lye
      Molecular formula: NaOH
      CAS No: 1310-73-2
      EC No: 215-185-5
      Annex I Index No: 011-002-00-6

Physical data
Appearance: odourless white solid (often sold as pellets)
Melting point: 318 C
Boiling point: 1390 C
Vapour density:
Vapour pressure: 1 mm Hg at 739 C
Specific gravity: 2.12
Flash point: n/a
Explosion limits: n/a
Autoignition temperature:
Water solubility: High (Note: dissolution in water is highly exothermic)



Offline billnotgatez

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Re:Electrolysis Of Brine
« Reply #9 on: August 15, 2005, 03:59:27 AM »
I had no intention to suggest sodium hydroxide heating should be done.
It appeared to me that many of the answers to this thread were incomplete or piecemeal.
So I put all the answers in one post and asked if I got it correct.
Obviously I was mistaken.

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Re:Electrolysis Of Brine
« Reply #10 on: August 15, 2005, 11:35:29 AM »
Is the result of electrolysis on brine at room temperature as follows?
First you get the hydrogen and chlorine released
When there is no more chlorine available
Then hydrogen and oxygen is released until all you have left is sodium hydroxide
You would need a higher temperature to continue the electrolysis so that there would only be sodium left
Also that continued electrolysis should be done in an inert atmosphere



Just to get to all of your questions.

1):  Hydrogen and chlorine are initially produced.  In the meantime, hydroxide ion is also produced so sodium hydroxide is made the entire time.  (The hydrogen is produced when an H+ ion is ripped off of a water molecule and the OH- ion is left).

2):  Once all of the chloride ions have been used up, then it's just the electrolysis of a sodium hydroxide solution.  If you heat the solution to dryness, you are left with sodium hydroxide.

3):  Sodium metal is only produced from the electrolysis of a molten sodium salt.  (I.E. molten NaCl).  Molten NaOH can be used to generate sodium, but NaOH is so hygroscopic that it will absorb water from the air and that can cause severe problems with the electrolysis of the mixture.  You do NOT want molten sodium metal, molten sodium hydroxide, and water coming into contact with each other.
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Offline billnotgatez

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Re:Electrolysis Of Brine
« Reply #11 on: August 15, 2005, 09:06:17 PM »
From your explanation ===
In item 2 if you stay at room temperature will you not get hydrogen and oxygen?
Also eventually will you not get solid sodium hydroxide because you electrolysis the remaining water leaving just the solid.
Item 3 then ensues which we are not recommending.

My point is that we can get rid of the water without heating – just continue the electrolysis.

Is this correct?

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