#01 Electrolysis Of Sodium Chloride (Cl2 is produced!)

When electrolysing a solution of NaCl, you'd normally expect water to be oxidized to oxygen and oxonium because of the electrode potentials (given values are E0):

Cl2 + 2 e− ⇌ 2 Cl− +1,36 V
O2 + 4 H3O+ + 4 e− ⇌ 6 H2O +1,23 V

Normally, the oxidation reaction with the lower potential happens, so one would expect that water oxidation would be preferred over chloride oxidation.
But in reality, the potentials depend on many factors such as concentration, type of the electrodes used, temperature, etc.
That's why you can "trick out" the potentials by using a graphite anode and a saturated NaCl solution.
The smaller chloride ion can attach to the graphite more easily than the bigger H2O molecule, so the chloride ion will be oxidized to chlorine ("over voltage" effect). This favours the chloride oxidation.
Using a higher concentration of chloride ions will also shift the preffered oxidation reaction towards chloride oxidation (This is understandable if you look at the Nernst Equation).
However, some of the chlorine produced that way will react with the OH- ions produced by the cathode to form ClO- and Cl- (in succession ClO3- and ClO4- as well):
Cl2 + 2 OH(-) ⇌ Cl(-) + OCl(-) + H2O
3 OCl(-) ⇌ 2 Cl(-) + ClO3(-)

To reduce such disproportionation effects, HCl can be added. On the one hand, this increases Cl(-) concentration, on the other hand it reduces OH(-) concentration. Both effects hinder the chlorine from reacting to hypochlorite , because the reactions are pushed towards the educt side.
To prove that chlorine has really been produced, I let it react with iron to form yellow iron(III) chloride (FeCl3).
So you don't need to melt NaCl if you want to obtain Cl2 (but you have to if you want to get Na).
BTW the gas bubbling up from the metal cathode is hydrogen (H2).

Here's a very informative site about (per)chlorate cells:
http://www.geocities.com/CapeCanaveral/Campus/5361/chlorate/chlorate.html

Category:

Science & Technology

Tags:

• Electrolysis
• Sodium
• Natrium
• Chloride
• NaCl
• Na
• Cl2
• Chlorine
• Chlor
• Hydrogen
• Iron
• Eisen
• FeCl3

[AccountTredecim]

Yeah, you don't want to put your hands in the sodium hydroxide solution. It's a highly corrosive base and can cause blindness on contact your eyes.

[bla287]

@AccountTredecim LOL, after such a short time of electrolysis the pH won't go that high. Assuming that 1ml of pure chlorine gas was made, I calculated ~0.083 mmol or ~3.3 milligrams of NaOH (that equals about some grains of salt). That in about 200ml makes 0.22 mmol/l or a pH of ~10.6. Furthermore I added a bit HCl in the beginning (for other reasons) so the actual pH was maybe even slightly acidic

[bla287]

@AccountTredecim (2/2) If you have a chlorate cell which electrolyses for several days at high currents, then of course it's an entirely different story (I wouldn't worry about the hydroxide in that case but rather about the hypochlorite and chlorate ions)

[123456789106]

what is produced in the cathode?

[bla287]

@123456789106 hydrogen and OH(-)

[nickli251]

@bla287 OH? Are you sure? negative charge go to cathode... Doesn't they repull? Cl is halogen so it will be on Anode, OH must leave in solution...

[bla287]

I think you got it mixed up: cathode is (-). There, the water (or to be more exact: H3O(+) which is in every aqueous solution at low concentration via autoprotolysis) picks up an electron from the metal:

2 H2O + 2 e(-) --> H2 + 2 OH(-) or

2 H3O(+) + 2 e(-) --> H2 + 2 H2O

The OH(-) then stays in the solution. And to avoid that the OH(-) then goes to the anode (+) and becomes discharged to oxygen and water, I added a bit HCl to keep OH(-) concentration low.

[hanamichi885]

keep it up my friend, and thanks for sharing this video

[rafaelmachineski]

very nice

[Berowra290997]

Dude, it's not the chlorine we're after - it's the sodium!

[AccountTredecim]

@bla287 Oh, okay.  Got it. I hadn't done my stoichiometry, I suppose :)