Sciencemadness Discussion Board

Collection of Chlorine by Electrolysis

Abromination - 9-8-2018 at 08:23

Everyone knows how to make chlorine by the electrolysis of sodium chloride in solution, but it disolves into the water as HClO and reacts with the sodium hydroxide also formed to make sodium hypochlorite. How many grams of NaCl per liter of water are required to make excess chlorine that doesn't react? Better yet, how many g/L are required to make a visible amount? I know I could just add sodium hypochlorite to HCl, but I dont have any HCl and need something controllable.

DavidJR - 9-8-2018 at 09:25

Electrolysis of sodium (and potassium) chloride is done industrially on a very large scale to produce the hydroxide, hydrogen gas, chlorine, hypochlorite, and chlorate. This is called the chloralkali process.

Now, if you just put some salt water in a beaker and electrolyse it, the sodium hydroxide which is produced will make the solution strongly basic. Chlorine reacts with the hydroxyl ions and is disproportionated to chloride and hypochlorite:
Cl2 + 2OH- -> Cl- + ClO- + H2O

Since the hydroxide is formed in the exactly the quantity which would be needed to react with the chlorine, there is no amount salt that will be enough. What you need to do is to keep the hydroxyl ions away from the chlorine. Industrially, ion exchange membranes are used to do this.

Here's what I'd try instead:

Get 2 beakers, and fill one with tap water and the other with a strong solution of sodium chloride. Wet a strip of filter paper with tap water and use it to bridge between the two beakers. Place one electrode in each beaker and connect the brine side to the positive terminal of your DC power supply and the water side to the negative.

At the anode, chloride ions are oxidized to chlorine:
2Cl- -> Cl2 + 2e-

And at the cathode, water is reduced to hydrogen and hydroxide:
2H2O + 2e- -> H2 + 2OH-

The sodium ions from the salt pass through the paper bridge towards the cathode, resulting in sodium hydroxide in solution.



[Edited on 9-8-2018 by DavidJR]

Abromination - 9-8-2018 at 09:46

Quote: Originally posted by DavidJR  
Electrolysis of sodium (and potassium) chloride is done industrially on a very large scale to produce the hydroxide, hydrogen gas, chlorine, hypochlorite, and chlorate. This is called the chloralkali process.

Now, if you just put some salt water in a beaker and electrolyse it, the sodium hydroxide which is produced will make the solution strongly basic. Chlorine reacts with the hydroxyl ions and is disproportionated to chloride and hypochlorite:
Cl2 + 2OH- -> Cl- + ClO- + H2O

Since the hydroxide is formed in the exactly the quantity which would be needed to react with the chlorine, there is no amount salt that will be enough. What you need to do is to keep the hydroxyl ions away from the chlorine. Industrially, ion exchange membranes are used to do this.

Here's what I'd try instead:

Get 2 beakers, and fill one with tap water and the other with a strong solution of sodium chloride. Wet a strip of filter paper with tap water and use it to bridge between the two beakers. Place one electrode in each beaker and connect the brine side to the positive terminal of your DC power supply and the water side to the negative.

At the anode, chloride ions are oxidized to chlorine:
2Cl- -> Cl2 + 2e-

And at the cathode, water is reduced to hydrogen and hydroxide:
2H2O + 2e- -> H2 + 2OH-

The sodium ions from the salt pass through the paper bridge towards the cathode, resulting in sodium hydroxide in solution.



[Edited on 9-8-2018 by DavidJR]


Haven't tried with a paper bridge yet, have always used a mebrane. And the chlorine will be visible and will not remain in solution? I'm terrible at electrochemistry.

Ubya - 9-8-2018 at 10:07

well if you are using an ion exchange membrane use that!
if it's just a mesh membrane it won't work, hydroxyl ions still come in contact with chlorine gas

MrHomeScientist - 9-8-2018 at 10:08

I've done this with a common clay flower pot as my membrane. My goal was the NaOH, though, so I just let the chlorine escape. Just place a small clay flower pot inside a larger container, and fill both up with salt water to below the rim of the flower pot. You may need to let it soak for an hour or so before it becomes conductive. One electrode goes in the outer container, and one in the inner container. In my setup, NaOH was made in the flower pot and chlorine made at the outer electrode. It's been quite a while so I don't recall all the details, but I believe I used steel rods as electrodes. You can also use salt water in just one of the chambers and distilled (with a tiny bit of electrolyte) in the other one, but again I don't recall which is which. Maybe with your recent experience you'll already know what to do!

To collect the chlorine, you could use an outer container with a lid. Make a hole in it and lead the gas through a drying tube into your collection flask. Chlorine isn't very soluble so you should be fine, but you could always boil the water afterward to drive it out. Whether it's visible or not would depend on the rate it's made and the final volume, I guess. If you seal it in a long glass tube, it's easier to see (look down the length of the tube).

Abromination - 9-8-2018 at 12:08

Nah, just a PP mesh membrane. Flower pot is a cool idea, but for this I'll just use a salt bridge. Also, while im at this I might as well do something the the HClO that I make. Could I just leave it out in the sun to decompose to HCl?

GrayGhost- - 9-8-2018 at 13:52

I used time ago flower pot ( terracotta) like membrane and work, Chlorine dissolve in ClNa solution and when is satured evolve like Cl2 gas.

I used PET bottle cut half like bell for pick up the gas.

My wife use the pot for you flower and that dry:D .

She say , what make you with this pot man?

nothing nothing .

Possibly was impregnated with Na0H and kill the plant.

Abromination - 9-8-2018 at 18:09

Quote: Originally posted by GrayGhost-  
I used time ago flower pot ( terracotta) like membrane and work, Chlorine dissolve in ClNa solution and when is satured evolve like Cl2 gas.

I used PET bottle cut half like bell for pick up the gas.

My wife use the pot for you flower and that dry:D .

She say , what make you with this pot man?

nothing nothing .


Possibly was impregnated with Na0H and kill the plant.



Same with thermite ;)

Also, if I add copper oxide to the hypochlorous acid it supposedly will decompose into HCl in solution. I think.

GrayGhost- - 10-8-2018 at 15:32

Quote: Originally posted by Abromination  
Quote: Originally posted by GrayGhost-  
I used time ago flower pot ( terracotta) like membrane and work, Chlorine dissolve in ClNa solution and when is satured evolve like Cl2 gas.

I used PET bottle cut half like bell for pick up the gas.

My wife use the pot for you flower and that dry:D .

She say , what make you with this pot man?

nothing nothing .


Possibly was impregnated with Na0H and kill the plant.



Same with thermite ;)

Also, if I add copper oxide to the hypochlorous acid it supposedly will decompose into HCl in solution. I think.



meanwhile electrolysis run you have ClOH +CLH between Ph 4,5 to 7 approx, when Ph decreases You have Cl2 in solution,, Whren PH is minor to 1 Cl2 escape like gas.

http://www.sbcontrol.com/hocl.jpg



I dont professional or expert but I experimented with this.