Salt water generators are closed systems... what?

Here is my basic understanding:

Salt is NaCl. The salt water chlorine generator splits the NaCl into Sodium (Na) and Chlorine (Cl) molecules. The chlorine molecule cleans your pool, then joins back up with an orphaned sodium molecule and becomes NaCl again. Then the process repeats over and over until you backwash or splash out that NaCl molecule.

JJ_Tex said:

Here is my basic understanding:

Salt is NaCl. The salt water chlorine generator splits the NaCl into Sodium and Chlorine. The chlorine molecule cleans your pool, then joins back up with an orphaned sodium molecule and becomes NaCl again. Then the process repeats over and over until you backwash or splash out that NaCl molecule.

Click to expand...

These are the simple answers I crave. No gobbly -gook. I can’t spell kimistree either.

Abnaxis said:

But by "cleans" that means "find some critter and chemically bonds with it" right?

The more I think about it, the less I realize I understand what stuff is doing. The picture I had in my head was of a bunch of Cl molecules floating around as FC meet a bunch of biomatter in a backalley and bond with it to make CC.

From there it's kind of a mystery? I guess if there's enough chlorine the CC breaks down into gas and chlorine, and if there's sodium in the pool the chlorine stays in solution but if not it floats off as chlorine gas?

Click to expand...

I like your questioning - it is a bit mysterious!

Abnaxis said:

But by "cleans" that means "find some critter and chemically bonds with it" right?

The more I think about it, the less I realize I understand what stuff is doing. The picture I had in my head was of a bunch of Cl molecules floating around as FC meet a bunch of biomatter in a backalley and bond with it to make CC.

From there it's kind of a mystery? I guess if there's enough chlorine the CC breaks down into gas and chlorine, and if there's sodium in the pool the chlorine stays in solution but if not it floats off as chlorine gas?

Click to expand...

If you wade through this thread and can follow the chemistry it will answer many of your questions...

The SWG creates chlorine gas (Cl2) at the anode from the chloride ion (Cl-) in the water. The two chlorine atoms that make up chlorine gas then reacts with water (dissolves and undergoes hydrolysis) to form one molecule of hypochlorous acid (HOCl), the stuff that kills things and oxidized bather waste, and one molecule of chloride ion again (Cl-). When the hypochlorous acid kills something (algae) or oxidizes something (waste), the chlorine atom doesn't typically attach to it. What it does is technically complex but, simplistically, you can describe it as the reactive chlorine ion (Cl+) stealing electrons (e-) from other atoms that make up the molecules that it is attacking (organic matter, algae cells, etc). When the chlorine steals those electrons it converts from Cl+ (reactive chlorine) to Cl- (unreactive chloride salt). The thing it stole the electrons from gets all screwed up and either breaks apart forming new chemical compounds OR, in the case of biological things, dies. You can think of the reactive chlorine (Cl+) like a taser ... it zaps whatever it comes into contact with and then becomes a useless piece of plastic with wires sticking out of it.

Simple enough?

This is the chlorine atom as it is in chlorine gas, which is produced in the cell.

At the plates, the chloride ion loses one electron to become chlorine.

It has seven valence electrons in the outermost electron shell.

So, two chlorine atoms combine to make chlorine gas.

As the chlorine gas dissolves in water, one atom ends up with eight valence electrons in the outermost electron shell and one ends up with six valence electrons in the outermost electron shell.

The one with eight valence electrons in the outermost electron shell is called chloride like in sodium chloride.

The one with six valence electrons in the outermost electron shell connects to the oxygen from water to form a hypochlorite ion or it connects to a hydroxide to form hypochlorous acid.

As the chlorine oxidizes something, it takes two electrons, which fill the outer valence shell to provide a full 8, which makes the atom chloride again.

So, in the process, the chlorine atom is merely losing and gaining electrons, but the atom is not lost.

Abnaxis said:

But by "cleans" that means "find some critter and chemically bonds with it" right?

The more I think about it, the less I realize I understand what stuff is doing. The picture I had in my head was of a bunch of Cl molecules floating around as FC meet a bunch of biomatter in a backalley and bond with it to make CC.

From there it's kind of a mystery? I guess if there's enough chlorine the CC breaks down into gas and chlorine, and if there's sodium in the pool the chlorine stays in solution but if not it floats off as chlorine gas?

Click to expand...

Good questions and I'm using all 10 of my brain cells to answer the best I can. I'm sure many of the more sciency members with more than 10 brain cells will chime in, but until then here goes:

- NaCl passes through the SWCG and forms HOCL (Hydrogen and Oxygen from the water, Chlorine from the salt)
- The Oxygen is what truly cleans (oxidizes) the bacteria and nasty stuff, and it comes from the HOCL.
- Without the oxygen the Hydrogen offgasses and the CL stays in the water to either re-form into salt, or go back through the SWCG and make more HOCL.

Edit - The > 10 brain cell guys type faster than I do.

Basically, this is the chlorine atom.

There is a single electron that is removed by the cell to turn chloride into chlorine.

When chlorine works, the electron goes back on and the atom becomes chloride again.

So, the process is just a single electron being remove and reinstalled on the chlorine atom that remains floating in the water.


Not actual size. Actual atom is slightly smaller than this.

My whole point in bringing up the "closed system" idea was to debunk a myth that floats around out there, and is often used by pool builders to dissuade customers from getting SWG's, that because an SWG produces chlorine, it's going to constantly need salt added to it to replenish all the chlorine it creates. Then they scare people by saying things like,

"You'll be hauling 40 lbs salt bags to your pool every week..."
"Putting all that salt in your pool will corrode everything you own and kill your dog. ..."
"Your landscaping will die from all the splash out ..."

And my favorite one I heard once ... a builder told a friend of mine who wanted a pool -

"The county routinely monitors salt levels in the sewer lines and if you drain to your household waste line they will charge you a penalty for adding salt to the wastewater stream ..."

It took me several minutes to recover from some extremely painful side aches after having laughed so hard that I nearly passed out ...

JoyfulNoise said:

Then they scare people by saying things like,

Click to expand...

They actually measured fear chemistry (adrenaline and cortisol) in people visiting a haunted house and a pool store and the amount of fear was roughly the same.

True story.

72% of facts are made up.

Abnaxis said:

where does it go in a liquid chlorine system that it needs replaced?

Click to expand...

Liquid chlorine comes with the chlorine atoms missing electrons.

Once the atoms gain electrons, they become chloride.

The the cell pulls electrons from the chloride to make it back into chlorine.

Abnaxis said:

Right, but the chloride ions go somewhere, or else the pool would continuously increase in chloride saturation.

Click to expand...

The chloride does continuously build up.

One can think of an SWG in very simplistic terms as it is nothing more than an oxidation/reduction reactions. Ignore everything but the chlorine atoms -

Inside the SWG, chloride ions are OXIDIZED to chlorine -

Cl- ----> 2 e- + Cl+

(Chloride ion) ---> (electrons sent to the power supply) + (chlorine ion)

Chlorine ions in a +1 oxidation state are powerful oxidizers and at a very high chemical potential energy. They want nothing more than to get those 2 electrons back from anything they can find. Once they do find something the chlorine ions are REDUCED back to chloride -

Cl+ + 2 e- ----> Cl-

(Oxidizing chlorine) + (electrons taken from some other molecule) -----> (chloride ion)

This cycle repeats itself constantly. At no point does the chlorine atom ever leave the water.

Now there are some technical details that were left out of that oversimplification. Namely, the SWG generates chlorine gas which eventually becomes the powerful oxidizing chlorine. It is possible with extremely high flow rates and a very short pipe between the SWG and the pool return to have that chlorine gas not fully dissolve in water and then off gases along with the hydrogen gas that is created inside the cell. That is called the "short pipe effect" and it will also cause the pH of the pool water to rapidly increase as chlorine gas is lost. You would, in that case, also lose chloride ion from solution. However, the amount of chlorine gas created and lost to a short pipe effect is so vanishingly small that it is almost never seen in practical pool layouts.

Manual chlorination, in simplistic terms, is nothing more than just that second chemical reaction being done without the first. In other words, you are adding a powerful oxidizing chlorine ion into the pool (in the form of sodium hypochlorite), it reacts with something and is reduced to chloride and then you add more chlorine. As @JamesW stated, the chloride constantly builds up because you are constantly adding it to the water. It is not at all unusual to find that manually chlorinated pools (in areas without a lot of rain dilution and overflow) have a salt content up around 1000ppm. Many people who convert to SWGs make the mistake of thinking their water has zero chloride ion in it and calculate their salt need based on that when, in fact, they are almost 1/3rd of the way to a salt water pool. Every gallon of 10% LC added to 10,000 gallons of pool water adds 16ppm salt which comes from both the excess salt water in liquid chlorine from the manufacturing process as well as the breakdown of hypochlorite to chloride ion.