Every year I add another bag of salt to my pool to keep it at 3100 ppm, more or less. Everyone knows that salt is just sodium chloride (NaCl). We all are aware (at least peripherally) of what happens to the chlorine part of the equation, ultimately gassing off in one form or another. The sodium part, however, is a highly reactive metal, and can't gas off. Where does it go?
SWG: Where does the sodium go?
- Thread starter outbacktommy
- Start date
Chlorine, for the most part, does not get lost any more than sodium.
Very minor amounts of chlorine can get lost to offgassing compounds, like nitrogen trichloride, but it's not significant.
Sodium ions are inert. They are not the same as sodium metal. They mostly just move around randomly not doing anything.
If you're losing salt, you're losing water some way other than evaporation.
Very minor amounts of chlorine can get lost to offgassing compounds, like nitrogen trichloride, but it's not significant.
Sodium ions are inert. They are not the same as sodium metal. They mostly just move around randomly not doing anything.
If you're losing salt, you're losing water some way other than evaporation.
- Jul 3, 2013
- 9,028
- Pool Size
- 15000
- Surface
- Vinyl
- Chlorine
- Salt Water Generator
- SWG Type
- CircuPool SJ-40
usually splash out or overflow and every time you get out of the pool 10 ounces of water or so are in bathing suits 
Splashout and swimsuits would have to be pretty extreme to account for 2000 gallons of water/salt loss per season. On average, our pool gets used by 3 people for about an hour, once per week, 20 weeks per year. Note in the signature that we have an autocover which is closed 100% of the time the pool is not in use, so evaporation is minimal, but wouldn't contribute to salt loss if it did. I started another post regarding water loss, and the comments that came back was that adding 5-6" of water per year was not uncommon with no contribution from rain or loss from uncovered evaporation. I don't fully believe it yet, though.
I'd like to know more about where the chlorine goes (either SWG or more common chlorine pools). It goes somewhere, and if it doesn't offgas, it has to go somewhere, otherwise there would never be a need for anyone to add more.
I'd like to know more about where the chlorine goes (either SWG or more common chlorine pools). It goes somewhere, and if it doesn't offgas, it has to go somewhere, otherwise there would never be a need for anyone to add more.
Elemental chlorine is a gas, so I always assumed that the chlorine gassed off, hence the need to continually add more. Are you saying that it doesn't gas off?
The chlorine atoms take 2 electrons and that turns chlorine into chloride. The chloride ions just move around randomly and don't do anything.
cfherrman
TFP Guide
I will probably get flamed for this, but chlorine does combine with solid organic material in the pool. This material is then removed by filtering etc. Also if you have a SWG, chlorine gas can escape as bubbles. Chlorine will also out-gas from the surface of the pool water (smell chlorine on a still morning).
The sodium hangs around, if it can't find a non-metal to associate with it will associate with water forming sodium hydroxide. Which is one reason pH raises in a pool. (flame away!).
The sodium hangs around, if it can't find a non-metal to associate with it will associate with water forming sodium hydroxide. Which is one reason pH raises in a pool. (flame away!).
What organic material does chlorine combine with that gets filtered out?
Chlorine gas generated in a cell dissolves pretty fast in the water. As long as the cell is far enough from the return, no significant amount of chlorine should be lost.
Minor amounts of chlorine can be lost to cloramines being outgassed, but it should not be much.
Sodium doesn't do anything but randomly move around. It doesn't combine with hydroxide to form sodium hydroxide.
SWGs are pH neutral. Any pH rise is from bicarbonate converting to carbon dioxide and the carbon dioxide offgassing.
Chlorine gas generated in a cell dissolves pretty fast in the water. As long as the cell is far enough from the return, no significant amount of chlorine should be lost.
Minor amounts of chlorine can be lost to cloramines being outgassed, but it should not be much.
Sodium doesn't do anything but randomly move around. It doesn't combine with hydroxide to form sodium hydroxide.
SWGs are pH neutral. Any pH rise is from bicarbonate converting to carbon dioxide and the carbon dioxide offgassing.
You can get some chlorine disinfection byproducts, like chloroform CHCl3.
Carbon based disinfection byproducts should not be a significant factor in residential pools.
I think that carbon based organic molecules where the carbon is in the -4 oxidation state is more likely to have chlorine combine than carbon based organic molecules where the carbon is in a higher oxidation state like -1, 0 or +1.
Algae is primarily made of carbon based molecules based on glucose or glucose chains. In glucose, carbon has 4 carbon atoms in the 0 oxidation state, 1 in the -1 oxidation state and 1 in the +1 oxidation state. Chlorine is more likely to oxidize the carbon than to combine with it.
Molecules with methyl groups tend to form more disinfection byproducts where chlorine in the +1 oxidation state displaces a hydrogen ion that has a +1 oxidation state.
A methyl group is a molecule that has 1 carbon atom in the -4 oxidation state and 3 hydrogen atoms in a +1 oxidation state.
Carbon based disinfection byproducts should not be a significant factor in residential pools.
I think that carbon based organic molecules where the carbon is in the -4 oxidation state is more likely to have chlorine combine than carbon based organic molecules where the carbon is in a higher oxidation state like -1, 0 or +1.
Algae is primarily made of carbon based molecules based on glucose or glucose chains. In glucose, carbon has 4 carbon atoms in the 0 oxidation state, 1 in the -1 oxidation state and 1 in the +1 oxidation state. Chlorine is more likely to oxidize the carbon than to combine with it.
Molecules with methyl groups tend to form more disinfection byproducts where chlorine in the +1 oxidation state displaces a hydrogen ion that has a +1 oxidation state.
A methyl group is a molecule that has 1 carbon atom in the -4 oxidation state and 3 hydrogen atoms in a +1 oxidation state.
This is a real eye opening discussion. Clearly there are a lot of reactions happening in the swimming pool, many of them chaining together, apparently in an endless loop.
If I'm reading this correctly, most of the carbon based molecules (organics, oils, plant matter, algea) are ultimately gassed off as carbon dioxide. Did I get this part right?
I'm still confused though, if the chlorine doesn't go anywhere as JamesW suggests, then why must additional chlorine be continually added? If it all stays in the pool, and more is routinely added (conventional, not SWG) eventually you would have quite a build up of various chlorinated compounds. The only exit I see is the creamy white sludge that gets hosed off of the (cartridge) filters, and that doesn't amount to much, but since it is concentrated as a solid and not a gas or liquid, maybe its enough?
Thank you to all in this conversation for keeping it clean and positive. Sorry I'm not a chemist (but I paid attention in my high school and college chem classes). I'm trying to see it from a "black box" viewpoint: stuff keeps going in, so eventually when the box is full (or saturated), stuff has to start coming out somewhere. In this case, my 'stuff' of interest is chlorine and/or sodium.
If I'm reading this correctly, most of the carbon based molecules (organics, oils, plant matter, algea) are ultimately gassed off as carbon dioxide. Did I get this part right?
I'm still confused though, if the chlorine doesn't go anywhere as JamesW suggests, then why must additional chlorine be continually added? If it all stays in the pool, and more is routinely added (conventional, not SWG) eventually you would have quite a build up of various chlorinated compounds. The only exit I see is the creamy white sludge that gets hosed off of the (cartridge) filters, and that doesn't amount to much, but since it is concentrated as a solid and not a gas or liquid, maybe its enough?
Thank you to all in this conversation for keeping it clean and positive. Sorry I'm not a chemist (but I paid attention in my high school and college chem classes). I'm trying to see it from a "black box" viewpoint: stuff keeps going in, so eventually when the box is full (or saturated), stuff has to start coming out somewhere. In this case, my 'stuff' of interest is chlorine and/or sodium.
The difference is in how many electrons are in the outer valence shell of the chlorine atom.
In active chlorine, the outer shell has 6 electrons but it wants a total of 8 to fill the shell.
Since chlorine has a strong ability to take electrons from other atoms, it takes 2 to get a full shell.
Once the shell is full, the chlorine atom is satisfied and won't try to take any more electrons.
A chlorine atom with 8 electrons is called chloride.
Chloride is what is added to the water when sodium chloride is dissolved in water.
So, chlorine just becomes salt and builds up. The chloride ion is stable and just moves around randomly and does not do anything.
3 ppm per day of chlorine added will add 1,800 ppm salt in one year.
If the pool uses a SWG, the SWG can remove electrons from chloride ions to reactivate the chlorine atom back to active chlorine with 6 electrons in the outer shell.
Most of chemistry is based on how many electrons atoms have in their outer shell. The number of electrons in the outer shell determine what other atoms an atom will bond to and what compounds can form.
Chemical reactions can include a transfer of electrons from one atom to another.
Carbon in algae has 4 electrons in the outer shell and are combined with hydrogen and oxygen to form glucose.
2 active chlorine atoms take the 4 electrons.
So, chlorine goes from 6 to 8 in the outer shell to become chloride.
Carbon goes from 4 to 0 electrons in the outer shell. Carbon with no electrons can combine with oxygen with 8 electrons in the outer shell to form carbon dioxide.
When an atom takes electrons, it's called oxidation.
So, when chlorine oxidizes carbon in algae, the chlorine becomes chloride and the algae becomes carbon dioxide and water.
In active chlorine, the outer shell has 6 electrons but it wants a total of 8 to fill the shell.
Since chlorine has a strong ability to take electrons from other atoms, it takes 2 to get a full shell.
Once the shell is full, the chlorine atom is satisfied and won't try to take any more electrons.
A chlorine atom with 8 electrons is called chloride.
Chloride is what is added to the water when sodium chloride is dissolved in water.
So, chlorine just becomes salt and builds up. The chloride ion is stable and just moves around randomly and does not do anything.
3 ppm per day of chlorine added will add 1,800 ppm salt in one year.
If the pool uses a SWG, the SWG can remove electrons from chloride ions to reactivate the chlorine atom back to active chlorine with 6 electrons in the outer shell.
Most of chemistry is based on how many electrons atoms have in their outer shell. The number of electrons in the outer shell determine what other atoms an atom will bond to and what compounds can form.
Chemical reactions can include a transfer of electrons from one atom to another.
Carbon in algae has 4 electrons in the outer shell and are combined with hydrogen and oxygen to form glucose.
2 active chlorine atoms take the 4 electrons.
So, chlorine goes from 6 to 8 in the outer shell to become chloride.
Carbon goes from 4 to 0 electrons in the outer shell. Carbon with no electrons can combine with oxygen with 8 electrons in the outer shell to form carbon dioxide.
When an atom takes electrons, it's called oxidation.
So, when chlorine oxidizes carbon in algae, the chlorine becomes chloride and the algae becomes carbon dioxide and water.
Let me see if I got this right. Since I have a SWG, I shouldn't need to add any salt because the chlorine/chloride is continuously recycled by the SWG. But since I'm adding about 120 lbs of salt each season, this means that I'm losing about 4600 gallons of water from splash out or leaks each season. It takes about 14 lbs of salt to bring 100 gallons of water from 0 to 3100 ppm. My actual water loss is about 2000 gallons per season from all causes including evaporation, so there is an inequality here of about 2600 gallons (more because evaporation is not present on one side of the theory). The leading question is only partly answered.
It is a different discussion for conventional pools, because the chloride does not get regenerated back to chlorine. My guess is that because chlorine is being continually added, a buildup of some sort of chloride occurs in such installations.
Am I on the right track here?
It is a different discussion for conventional pools, because the chloride does not get regenerated back to chlorine. My guess is that because chlorine is being continually added, a buildup of some sort of chloride occurs in such installations.
Am I on the right track here?
Every gallon of 12.5% sodium hypochlorite adds about 1.72 lb of salt to a pool. So, salt will build up. 3 ppm fc added per day would add 1,800 ppm salt to the pool.
For a SWG, the salt is added at the beginning and the Swg turns the chloride into chlorine.
The salt level should stay the same because the same chlorine atoms get used over and over.
You will lose salt if you lose water other than evaporation and refill.
You can lose some chlorine to offgassing chlorinated gasses, but I don't think that it would be a significant amount.
How are you calculating water loss?
For a SWG, the salt is added at the beginning and the Swg turns the chloride into chlorine.
The salt level should stay the same because the same chlorine atoms get used over and over.
You will lose salt if you lose water other than evaporation and refill.
You can lose some chlorine to offgassing chlorinated gasses, but I don't think that it would be a significant amount.
How are you calculating water loss?
outbacktommy,
Yes you probably shouldnt be including evaporation in your 4600 value, because the salt doesn’t evaporate.
I too am interested in the answer to your question. I know the salt levels should stay the same, but there seems to be a deficit somewhere!
Yes you probably shouldnt be including evaporation in your 4600 value, because the salt doesn’t evaporate.
I too am interested in the answer to your question. I know the salt levels should stay the same, but there seems to be a deficit somewhere!
Each season I add about 5-6 inches of water in 2" increments. Rectangular pool. 18' x 36' x .5' x 7.48gal/cu.ft. = 2400 gal +/-. I keep track because I'm paranoid about frozen pipes and underground plumbing leaks with an outdoor, in-ground pool in Minneapolis. I've had very similar results for the 3 years that we've owned this house.
Something else must be a play. What am I missing?
Something else must be a play. What am I missing?
How many inches of rainfall do you get per year?
How many inches of snow do you get per year?
How many inches of snow do you get per year?
Most of that would be evaporation I’m guessing?
so yes, the question is a good one - where are the chemicals going?
so yes, the question is a good one - where are the chemicals going?