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Concentrating bleach (evaporation just leaves impure table salt)

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LAN_403

Backyard Skunkworks

Tue Jan 29 2008, 02:07AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

EDIT:
Opps, I was first learning chemistry and I got over-zealous describing what I thought was a method of concentrating sodium hypochlorite by evaporation. Turns out it leaves NaCl and NaClO3, not solid NaOCl as I originally posted about.

Of course I later produced my own high concentration NaOCl through the chlor-alkali process...

Hazmatt_(The Underdog)

Tue Jan 29 2008, 03:42AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

1. You can't boil NaOCl because if you do, you get a mix of 5 NaOCl to 1 NaClO3 as your end product.

2. Go to a Pool Supply store! Couple bucks gets you 1LB bag! 95% Hypochlorite

Shaun

Tue Jan 29 2008, 03:56AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Hazmatt, I think you meant a mix of NaCl and NaClO3, right?

3NaClO ---> 2NaCl + NaClO3

Sodium Hyplochlorite does not exist in solid form; its too unstable.
The pool stuff is calcium hypochlorite.

Hazmatt_(The Underdog)

Tue Jan 29 2008, 08:20AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Oh I'm sorry, you're right, NaCl.

The pool stuff is Calcium, but it works just as well, and sometimes you want a Calcium salt because of its insolubility.

Bored Chemist

Tue Jan 29 2008, 04:48PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Backyard skunkworks,
It's not going to work.
The most likely outcome of your procedure is some very expensive salt, contaminated by NaClO3.

2 NaClO -->2 NaCl +O2

Backyard Skunkworks

Tue Jan 29 2008, 10:29PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Yeah, I tried evaporating the water away from household bleach and got white crystals, I (erroneously) figured these must be pure sodium hypochlorite. Sorry about all thisâ€¦ Iâ€™ll be sure to think then act, not act then think.

So I guess letting any water evaporate out of bleach would also let it decompose, rendering the entire idea of concentraion by evaporiton useless? I just read up on the properties of bleach and it is pretty unstable.

Hazmatt_(The Underdog)

Wed Jan 30 2008, 12:07AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

One of the experiments we did in our Chem 111A class was to determine the concentration and the prof's were getting agitated because the stuff was decomposing pretty fast and our answers varied from 2%-6% CLO ion.

I think the Bleach is pretty well saturated as it is, so concentrating it further isn't going to work. If you were to try to concentrate it you would need to bubble CLO2 into a cold solution of Bleach or NaOH.
I probably wouldn't bother with that because you can get a bag for a couple of bucks, like I mentioned earlier. Also, generating CLO2 is nothing to toy with, I know, I did it, its very dangerous. Very nasty gas. Even with a respirator its very nasty stuff.
So if you need concentrate, just buy the Calcium salt.

Backyard Skunkworks

Wed Jan 30 2008, 12:14AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Yeah its easier to buy it, but wheres the fun in that?

Since I did some reading around the net I did see that there is 40-50% NaOCl, also it appears sodium hypochlorite might form a solid at under 18C so I just stuck 60ml of bleach in the fridge to test this.

Shaun

Wed Jan 30 2008, 04:35AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

What can you do with solid NaOCl? Or even concentrated NaOCl solution?
Anything fun?

I mean, the chlorate ion has its uses...but won't get into that here. And thats usually the potassium compound, anyway...

Backyard Skunkworks

Wed Jan 30 2008, 05:55AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

It should be a pretty powerful oxidizer, 40% solution is listed as a hazardous oxidizer (I think 50% is about the max). Bleaching stuff would be a pretty good use for concentrate bleach! In addition it would be handy as a general purpose powerful oxidizer.

Despite a couple guides and figures I've seen, bleach does not seprate into crystals below 18C (I just tested this). Next I'll try partial freezing to see if this will allow excess water to be trimmed off (as ice), leaving a more concentrated bleach solution in the center of the ice. I think this might actually work! I'll be testing tommorow and get the results posted on thursday, seeing as ice is very pure solid water I'd think the "impurities" (hypochlorite) would be forced to the center of the ice block.

BTW by partial freezing I mean leaving the bleach below freezing long enough for ice to form on all the surfaces of the liquid, while the center is not allowed to freeze.

EDIT:
Well my freezer goes down to -20C and the bleach will not freeze, if anyone has a better freezer/dry ice sitting around they are welcome to try this out and see if bleach can be concentrated by freezing water away from it.

I've seen a couple MSDSes list the freezing point at -6F, just better then my freezer. On the other hand sodium hypochlorite does dissolve very well in water and as a result freezing it may just result in "bleach ice" instead of the hypochlorite "impurities" being forced to the center.

Looks like a concentrated solution of bleach might form from electrolyzing brine, on the other hand the chlorine gas may not dissolve and may bubble out of the water, causing uh, problems (of the poison gas kind).

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