PoolGuyNJ said:
When salt, aka sodium chloride, hits the water, the crystals dissolve in solution with the water. There is no chemical reaction other than the loss of the crystalline structure. When the salt molecules pass an energized salt cell, electrons in the valence shells of each element are received from the cell power to each atom, temporarily splitting the molecule into two seemingly stable atoms and forming ions. These ions may drift alone or combine with other atoms or molecules.
As the energized electrons lose energy and drift away from the valence shell, the atoms or molecules form different ions or chemically reactive materials. Often, sodium atoms and chlorine atom meet and recombine. Chlorine atoms also meet their fate by drifting off in the air or combine with organics. Because of this, a non leaking and non evaporating pool would develop a salt demand.
Scott
I know that this is what many SWG manufacturers say, but it's really not true. Salt at the level found in saltwater chlorine generator pools and indeed even in levels found in the ocean is DISSOLVED. It does not form any significant amount of molecules of salt in the water. In the water, the sodium and the chloride in the salt are separated and each atom is an ion (i.e. electrically charged) surrounded by water molecules. That is, there are sodium ions and chloride ions. The sodium is irrelevant as far as the saltwater chlorine generator is concerned (except for providing more electrical conductivity to migrate to balance charges).
There are two types of plates in a saltwater chlorine generator cell. At the positively charged anode, chloride ions (which are negatively charged) have an electron stripped from them producing chlorine atoms that quickly combine to form chlorine molecules (essentially chlorine gas). This then dissolves in the water, combining with water to from hypochlorous acid which is the form of chlorine that kills pathogens and algae and oxidizes bather waste.
At the negatively charged cathode, hydrogen ions (which are positively charged) have an electron added to them producing hydrogen atoms that quickly combine to form hydrogen molecules which is hydrogen gas that you can actually see coming out of the returns if you look carefully, especially at night.
Notice that sodium from the salt wasn't involved in what I described above, at least not in terms of any sort of transformation or splitting or combining or anything like that. There are no electrons transferred to or from the positively charged sodium ions.
This electrochemical process is described more technically in
this post in the section "Salt (SWG) Pool".
As for the fate of most of the chloride from salt that has become chlorine, most of the chlorine is broken down in sunlight to become chloride salt again. Some of the chlorine oxidizes ammonia and organics from sweat and urine and also becomes chloride salt again. Some of the chlorine oxidizes the stabilizer (Cyanuric Acid, CYA) in the pool and also becomes chloride salt again. Only a small portion combines with organics and stays that way to possibly get caught in the filter (if the organics got consolidated with clarifiers or were insoluble themselves) and some gets splashed out with water or backwashed (both sodium and chloride get removed in this way). There is also some chlorine that escapes by getting outgassed from the pool, though it's a fairly small amount unless the temperatures are hot as in a hot tub.
I think that the easiest way to think about this entire process is to consider the saltwater chlorine generator system as a chloride-to-chlorine booster, taking an atom of chlorine that is relatively benign as salt (it's called chloride in that state) and boosting it to a very active form which is essentially a potent chlorine atom attached to water (it's called chlorine or hypochlorous acid in this state). When this active form reacts with something, it becomes deactivated returning to its benign form as chloride salt. It essentially uses its extra energy to kill a pathogen or algae or to oxidize some other chemical. Sunlight is a form of reaction in this regard and essentially forces the active chlorine state to revert back to inactive chloride salt by essentially transferring this extra energy into the formation of oxygen gas (the oxygen coming from the water).
[EDIT]
Some more info on how salt dissolves and is NOT "split" by the saltwater chlorine generator since this misinformation is quite widespread:
When salt is a solid, it is a crystal of sodium and chloride atoms in a lattice, but when it dissolves in water it separates into individual positive sodium ions and negative chloride ions. It is NOT a sodium chloride molecule in water. Substances that dissolve in water do so because they either 1) break apart into charged ions such as most salts, acids and bases including sodium chloride salt, hydrochloric acid, sodium hydroxide, etc. or 2) they are polar molecules (usually organic) that have separated charges that form hydrogen bonds in the water such as ethanol.
See this link for an animated demo of sodium chloride salt dissolving in water as well as
this link,
this link and many others. Note that when the water evaporates, the salt becomes more concentrated until it eventually saturates the water at which point it begins to precipitate reforming salt crystals. This does NOT prove that there were molecules of salt -- the ions simply reform into crystals. One can add hydrochloric acid (Muriatic Acid) together with sodium hydroxide (lye / caustic soda) and this will form sodium chloride salt dissolved in water, meaning sodium ions and chloride ions in water, NOT sodium chloride molecules. This is similar to adding calcium carbonate to the water by adding calcium chloride and sodium bicarbonate -- these are all IONIC SUBSTANCES that separate into charged ions in the water. There are not calcium carbonate molecules in the water -- there are calcium ions and carbonate ions (at pool pH, most are bicarbonate ions). It is only when there are so many that it becomes saturated that they begin to reform (combine) into solid crystals. The saturation point varies for different substances, sodium chloride salt being much more soluble than calcium carbonate, for example.
Some say that the fact that one can taste salt proves that the molecules are still there, but the salty taste comes from sodium ions ALONE in sodium ion channels and not from chloride, though potassium and lithium ions also trigger some of this taste as well as they are similarly sized to sodium ions. See
this link, for example. The anion (negative charge) does have an effect on the saltiness as described
here, but it is a secondary effect as it is sodium ion that is being measured/sensed. It is NOT sodium chloride as a molecule.
Electrolysis in the cell has nothing to do with "splitting" sodium chloride molecules because there are none to be split. It is true that at the negatively charged cathode one can consider water to be "split" to form hydrogen gas and hydroxyl ions rather than looking just at hydrogen ions alone forming hydrogen gas. If there were little chloride in the water, then oxygen gas would form at the positively charged anode, again by "splitting" water into oxygen gas and hydrogen ions. Perhaps this "splitting" of water by electrolysis into hydrogen and oxygen gasses is where the myth began that something similar was happening with sodium chloride salt.
When chloride is converted to chlorine, either as chlorine gas or hypochlorous acid (or hypochlorite ion), it does NOT combine with sodium. Only neutral molecules gas off -- sodium chloride is not volatile and does not gas off because they are separately charged sodium ions and chloride ions. Hypochlorous acid is neutral and can gas off as is molecular chlorine though the latter is in much smaller quantity in water and the chlorine that mostly gasses off is hypochlorous acid itself. So when this happens, chlorine atoms are lost and since an acid is removed from the water, the pH rises. This requires acid to be added so if one uses hydrochloric acid (Muriatic Acid), then one not only restores the pH, but adds more chloride ions to partially replenish what is lost. However, most pH rise is not from hypochlorous acid getting outgassed (there is more outgassed in spas, however), but rather is from carbon dioxide outgassing. Most salt, however, is lost through water dilution (splash-out, backwashing).
[END-EDIT]
Richard
