Sodium Hypochlorite

Kes

Well-known member
Apr 24, 2010
46
Worcester UK
It depends what you mean by the same. As far as I can tell the Sodium Hypochlorite constituent of both is the same. However, I'm no expert, so I'll cut and paste what Chem Geek posted.

'6% Bleach is, by weight, 6.0% sodium hypochlorite (i.e. chlorine), 4.7% salt, and 89.3% water. All sources of chlorine will add salt to the pool as chlorine turns into chloride (salt) when it gets used up, but bleach adds salt upon addition as well.'

and...

'For every 10 ppm FC added by ANY source of chlorine, it will result in 8.2 ppm salt from the consumed chlorine that turns into chloride salt. With bleach, chlorinating liquid, and lithium hypochlorite, there is an additional 8.2 ppm salt upon addition so the net total result is 16.5 ppm.'

So I interpret that as you would get additional salt from using bleach instead of sodium hypochlorite. Why additional salt is added to bleach I don't know, and CG didn't say. (Perhaps he will here, and correct my chemistry.)
 

JasonLion

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May 7, 2007
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Household bleach and sodium hypochlorite sold for pool use are both produced by the same companies in the same plants in the same way. Bleach/sodium hypochlorite is produced by running electricity through salt water in a process very similar to what happens inside a SWG, but on a much larger scale. The "extra" salt is required for the production process to work correctly.
 

chem geek

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Mar 28, 2007
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Kes said:
Why additional salt is added to bleach I don't know, and CG didn't say. (Perhaps he will here, and correct my chemistry.)
As Jason noted, the manufacturing process of making bleach or chlorinating liquid, all of which are "sodium hypochlorite", has salt created in the product by the way it is made. It isn't added intentionally. Essentially, the electrolysis of brine water produces chlorine gas and this then gets added to a solution of lye (sodium hydroxide). The chemical reaction that occurs produces salt:

Cl2(g) + 2NaOH ---> NaOCl + NaCl + H2O
Chlorine Gas + Lye ---> Sodium Hypochlorite + Salt + Water

Essentially, half of the chlorine gas goes to making hypochlorite and the other half goes to making salt. Unfortunately, there is no easy or inexpensive way to make hypochlorite without the salt.
 

ttomp

Member
Jul 22, 2010
20
Gonzales, LA
In case anyone is interested the chlorine, sodium hydroxide (caustic) and hydrogen are all produced at the same time in the same cell. Caustic comes out the bottom as a liquid, chlorine and hydrogen come out the top as a gas but they are seperate. The hydrogen and sodium hydroxide are produced on the same side of the cell and the chlorine is on the oppisite. We do it by adding salt water to a cell and running 150 thousand amps through it that break it into the three different chemicals.

We also make bleach (sodium hypochlorite) by circulating sodium hydroxide (caustic) in a tower and running chlorine through it. It is really strong but unfortunantely I can't use it because it is not very clean.

Pretty much what ChemGeek said but just a little extra useless info LOL !!!

CHEM GEEK
One thing I have always wondered is does a SWG add pH to the pool. In other words to produce the chlorine you release the Cl's, so what happens to the Na's. Do they join with OH's and form a little sodium hydroxide?
 

chem geek

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Mar 28, 2007
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I was only giving the chlorine gas and bleach production side of the chlor-alkali process that you describe (a very simplified description of different chlor-alkali methods is here).

As for what happens in an SWG, this post describes how the production of chlorine in an SWG is essentially identical to adding a hypochlorite source of chlorine so is pH neutral when accounting for chlorine consumption/usage. However, in practice we find that the pH rises in most SWG pools. There are probably two reasons for this. First is that the increased aeration from the hydrogen gas bubbles increases the outgassing of carbon dioxide. Lowering the Total Alkalinity (TA) helps to reduce this effect, but doesn't usually completely eliminate the pH rise. I'm not sure about the other source of pH rise, but it may be undissolved chlorine gas outgassing as that would definitely have the pH rise. Shorter pipe runs from the SWG to the pool would have more of this effect. Lowering the SWG on-time by having a higher CYA level to lower chlorine demand and using 50 ppm Borates to inhibit algae growth and possibly lower chlorine demand can also help (the borates are also an additional pH buffer that help reduce scaling in SWG cells).

The sodium ions are irrelevant to the SWG chlorine production, just as they are in the chlor-alkali process. They are only there for charge balance and as ions for conductivity, but they do not directly participate in any of the electrochemistry.
 

ttomp

Member
Jul 22, 2010
20
Gonzales, LA
I know you were giving the simplified version, but I just had to act like I know something. LOL !!! I just wrote that post cause most people I explain that to find it quite interesting how you get those three products from salt water.

Thanks for the post. I think I understand about the SWG. I am not real good with chemistry just the little I know from work. But one thing you said caught my attention, and please don't think I am arguing,just learning, but you mentioned that the sodium Ion is irrelevent in the chlor-alkali process. But without it how would you get the sodium hydroxide. Is it that the -OH ion is all you need and the Na is just a by product. If the answer is way to deep for me then just tell me. Dont want to waste your time :-D Thanks again!!!
 

chem geek

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What I meant was that the sodium ion had little to do with the electrolysis itself. It wasn't "created" or transformed by the electrolysis. What is produced is actually hydroxide ions in water. It's only when the water is removed that you can get solid sodium hydroxide. The sodium was there all along from the salt (though more migrates from the chlorine gas generation side to the hydrogen gas generation side to keep charge balance). The hydroxide ions, on the other hand, come from electrolysis of water that also produced hydrogen gas.
 
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