What really goes on in a Salt Water Generator.

[RickstersPool]

I'm a computer programmer, not a chemist, so forgive me in advance...

I get the disassociation NaCl's sodium and chloride to make chlorine.
So, why don't I need to eventually add more salt to keep me from running out of Cl?

Well several pool "experts" have told me that the "used up cl", recombines with the sodium and the cycle repeats...
Bah humbug!

I'm guessing, the water coming out of the SWG has chlorine and "sodium something" that is a base and is why the ph constantly goes up.
I add muriatic (hydrochloric) acid to bring PH down.
I'm thinking the chloric part of the acid recombines with the sodium as it balances the ph and that's how things recycle back to NaCl.

If I'm right, in the end you're really getting your endless supply of chlorine really from the muriatic, not the salt?

 

[bk406]

There is no recombination of the sodium and chloride. Once salt hit the water, the two dissacociate. The sodium has no function at all in generating chlorine. The chloride ion passes over the electrified plates in the SWCG. The chloride gets converted to Cl2, which is chlorine gas. When the gas hits the water, basically instantaniously, its converted to hypochlorous acid, or plain old bleach.
Chem geek can give you the reaction particulars.
The pH rise is caused by a number of things. I'm sure chem geek will be along and give you a more precise run down of things.

 

[Pb2Au]

Here it is.

Electricity passed through a solution of salt and water (called brine) produces three things:

1. Hydrogen gas (H2) at the negative electrode
2. Sodium hydroxide (also called caustic soda—NaOH) at the negative electrode
3. Chlorine gas (Cl2) at the positive electrode

In the presence of electricity, here’s what happens to salt and water:

2NaCl + 2H2O Cl2 +2NaOH + H2
salt + water chlorine (gas) + sodium hydroxide + hydrogen (gas)

Ideally, we produce pure hydrogen gas, pure chlorine gas and pure sodium hydroxide in this process.

Once the chlorine generated from the salt is used, it reverts back to chloride (Cl-) to be used over and over again. The HOCl (hypochlorous acid) reduces to chloride (Cl–). Then the chloride (Cl–) can be converted by electricity back into chlorine gas (Cl2). Then the chlorine gas dissolves in the water to form hypochlorous acid (HOCl) again. This recycling process keeps on going, so it is only necessary to add more salt to replace loss due to bather drag-off, splash-out, overflow, leaks and filter backwash.

 

[Pb2Au]

Sorry, the yield arrow didn't show. Here is the equation.

2NaCl + 2H2O --> Cl2 + 2NaOH + H2

 

[chem geek]

The various reactions for adding chlorine from bleach or chlorinating liquid or from a saltwater chlorine generator and then having that chlorine used up by breakdown from sunlight or oxidation of an organic is described in this post. The easiest way to look at it is that the chloride from salt gets its energy boosted (to a higher oxidation state) to become chlorine and when that chlorine either breaks down in sunlight or oxidizes some organic (or kills a pathogen, etc.), then it gives up its energy and becomes chloride salt again. The only chloride/chlorine that is removed is that which is diluted from water replacement or that which combines with some organics without oxidizing them or that which outgasses. There is very little that is lost with these latter mechanisms -- most just gets recycled and most in residential pools is cycled via breakdown from sunlight (i.e. bather load contributes relatively little to chlorine demand in most residential pools).

As for pH rising, much of this is due to carbon dioxide outgassing from the water and you can reduce this by lowering the Total Alkalinity (TA) level to 70 ppm or so. Some of the rise may be due to some undissolved chlorine gas outgassing which you can reduce by lowering the SWG on-time. You can accomplish that by lowering chlorine demand via raising the CYA level to 80 ppm. Using 50 ppm Borates also usually helps. This is all described in the Pool School article Water Balance for SWGs.

 

[susa]

>> I add muriatic (hydrochloric) acid to bring PH down.

Keep in mind that the muriatic acid also breaks down and will contribute to the overall salt level. In my system, with everything fully automated, the MA injections maintain the salt at 4000-4100 ppm for one year now since build.