Chemistry behind chlorine salt water generators

[dschlic1]

Hello;
I understand how electrosis can seperate chlorine from the sodium in salt, but what I do not understand is why doesn't the chlorine recombine with the sodium immediately after leaving the cell?

 

[chem geek]

Electrolysis does not separate the sodium from the chloride in salt. When salt (which is solid sodium chloride) dissolves in water, it dissociates into sodium ions and chloride ions. The sodium has nothing to do with the electrolysis or chlorine production in spite of what some websites say. You could use potassium chloride or calcium chloride salt and have the same effect (in fact, when you add calcium chloride to a pool to initially raise the Calcium Hardness level, the chloride from this is some of the chloride that is used by the SWG to make chlorine -- it doesn't all come from the added sodium chloride).

This post describes in more detail what goes on in a saltwater chlorine generator (and with manual dosing of chlorine). Essentially, at the anode (positive plate) the two chloride ions are converted to a single chlorine gas molecule. Chlorine gas dissolves readily in water producing hypochlorous acid, which is the disinfecting form of chlorine, and hydrochloric acid. At the cathode (negative plate), two hydrogen ions are converted to a single hydrogen gas molecule. The hydrogen gas bubbles out of the returns and aerates the water and is the main cause of pH rise in SWG pools. The acidity from the anode's hypochlorous acid and hydrochloric acid and the alkalinity from the cathode's removal of hydrogen ions net out to what was shown in the link where the SWG production of chlorine is identical to adding hypochlorite to the pool (plus hydrogen gas).

When the hypochlorous acid gets used up by oxidizing an organic or ammonia or killing a pathogen (bacteria, virus, algae), it gets converted back to chloride ion. The oxidation produces carbon dioxide and/or nitrogen gasses (and water) depending on what the chlorine oxidizes. This oxidation process produces hydrochloric acid and is acidic and exactly compensates for the alkalinity of hypochlorite sources of chlorine (bleach, chlorinating liquid, Cal-Hypo, Lithium hypochlorite) so the addition of hypochlorite or the production of chlorine in an SWG plus the later usage of that chlorine is a pH neutral process.

One way of looking at this is that the electrolysis raises the potential energy of chloride to a more powerful form in hypochlorous acid. This higher potential energy is then used as a disinfectant and oxidizer and when this energy gets used up, the hypochlorous acid reverts back down to become chloride ion again.

Richard

 

[dschlic1]

Ok then why doesn't the chlorine gas or later the hydrocloric acid combine with the sodium ions. Does the sodium ions form sodium hydroxide? What I am getting at is the chlorine is there, the sodium is there, what keeps them from recombining or re-forming immediately after the cell?

 

[Guest]

In layman's terms, because of the engery added by the electricity. The sodium ions just stay in the water. Depening of the pH of the water there may or may not be an excess of hydroxide ions in the water (this depends on the amount of outgassing of carbon dioxdie and the level of carbonic acid in the the water. You have to realize that when an ionic solid like sodium chloride is dissoved in water it dissociates into it's ions and stays that way until the water is evaporated. The ions can then act somehwat independantly, depening on what else is going on--such as what other ionic species are present or whether there is any enery being applied in the form of light, heat or electricity.
I am trying to keep this as simple as possible but it is really a complex topic.

 

[chem geek]

waterbear answered this well. I'm going to try and answer it from a slightly different angle.

The basic question that is being asked is "if something is separated, then why doesn't it combine later?" The answer is that chemistry (and physics) doesn't work that way. Chemical and physical systems move towards their lowest energy state. They don't always do so quickly -- otherwise our bodies would simply decompose (i.e. "burn") into carbon dioxide, water and possibly soot -- but the direction is towards lower energy. Salt that is solid when not in water is in a lower energy state than salt in a liquid or gaseous form (at room temperature) so is why you see it as solid salt when not in water. When salt is exposed to water, the sodium and chloride in the salt that are attracted to each other when there is no water present become separated when in water because the sodium and chloride are separately more attracted to water than they are to each other (in some sense) and are in a lower energy state when separate as charged ions. This occurs because water is polar so has a positive charge on one side of the molecule and a negative charge on the other so the net result is that the water and the sodium and chloride all bounce around in a way that is at the lowest energy.

Sodium in water, as sodium ions (that is, charged atoms interacting with the polar water molecules), is at a very low energy state so it will pretty much not react with or combine with anything. It is very hard to precipitate anything with sodium in water without actually removing the water itself (i.e. without evaporating the water). So the hypochlorous acid that is in the water and the hypochlorite ions that are in the water do not react with nor combine with the sodium. When you add bleach or chlorinating liquid, this contains sodium hypochlorite and in water the sodium ion and the hypochlorite ion remain separated, just like sodium and chloride from salt dissolved in water. The principle is very similar -- pretty much anything initially combined with sodium (and potassium, to a slightly lesser extent) remain separated when in water.

Even with extra hydroxide in water, sodium hydroxide is still separate sodium ions and hydroxide ions whenever water is present. The salt cell does produce an excess of hydroxide ions, but they just remain as separate hydroxide ions and don't combine with the sodium. Also, when the chlorine (hypochlorous acid) gets used up either from breakdown from sunlight of from oxidation of organics, this produces an excess of hydrogen ions and these combine with the hydroxyl ions to form water so the net result of chlorine addition from a hypochlorite source of chlorine (such as bleach or chlorinating liquid or Cal-Hypo) or from creation in an SWG cell PLUS the later breakdown of chlorine is a net result that is pH neutral -- no excess of hydroxyl ions or hydrogen ions. (Technically, there is a small amount of extra hydroxyl ion in the hypochlorite sources of chlorine, but it is negligible except after months of usage).

Hope that helps in understanding.

Richard