SWG Operation - Lessons Learned

[PatL34]

Tightly sealed bags should be sufficient. When re-opening in the summer, use a hair dryer to dry out the tri-sensor and cell connections on the cables and the cell manifold assembly.

Pat

[waste]

Pat and Waterbear, (this is only ~ 4 mos late) Re: posts 34 - 36 on this thread - why didn't you ask me???? - I have access to large quantities of vynal liner stock, salt and the best test kit. If you'll give me the specifications for the experiment (I think I know what is required, but would rather you tell me how to set it up to get your required results ie., volume of water, ppm salt, sealed or open, what would the control be?, etc), I'll gladly perform it.
If this is now a 'non-issue' that's fine, but if you want me to do this, I'll need to know in the next week, as I'll be layed off in the next 2 weeks (aaaahhhh, winter!)

[cschnurr]

I understand that when I take H20, and NaCl and pass them across the electrically charged plates, I can see how H2 and Cl2 can be formed.

Since the H2 comes from water, the remaining oxygen will dissipate.
However the Cl2 come from NaCl. What happens to the sodium, where does it go?

[chem geek]

I understand that when I take H20, and NaCl and pass them across the electrically charged plates, I can see how H2 and Cl2 can be formed.

Since the H2 comes from water, the remaining oxygen will dissipate.
However the Cl2 come from NaCl. What happens to the sodium, where does it go?

At the cathode (the negatively charged plate), hydrogen is produced from either of the following reactions:

2H+ + 2e- --> H2(g)
2H2O + 2e- --> H2(g) + 2OH-

Note that oxygen gas is NOT produced and does not dissipate. Instead hydroxyl ion (OH-) is created. Either of the above can be considered to be occurring since there is an equibrium that always exists in water between its split components, though the vast majority remains as the water molecule at normal pool pH:

H2O --> H+ + OH-

At the anode (the positively charged plate), chloride ion gets converted to chlorine gas that then dissolves in water to from hypochlorous acid and hydrochloric acid as follows:

2Cl- --> Cl2(g) + 2e-
Cl2(g) + H2O --> HOCl + H+ + Cl-
----------------------------------------
2Cl- + H2O --> HOCl + H+ + Cl- + 2e-

As you can see, the sodium doesn't participate at all in these reactions. When you add salt to water, it dissolves into its two components:

NaCl(s) + H2O --> Na+ + Cl- + H2O

Nothing happens to the sodium ion in a salt cell. Only the chloride ion participates in any reaction. The sodium ion does contribute to the general conductivity of the water. The area around the cathode accumulates negatively charged ions while the area around the anode accumulates positively charged ions so the sodium ion and hydrogen ion (which are positively charged) will tend to migrate from the anode to the cathode while chloride ion and hydroxyl ion (which are negatively charged) will tend to migrate in the opposite direction.

Now it is possible for another reaction to occur at the anode and competes with the production of chlorine as follows:

2H2O --> O2(g) + 4H+ + 4e-

So it is possible for oxygen gas to be produced in addition to chlorine, but due to the differential liklihoods (activation energies) of these two reactions, the production of chlorine is the primary reaction.

There are some websites that talk about "splitting" the sodium chloride as part of the electrolysis in the salt cell, but basically they have no idea what they are talking about. It is simply not true. Sodium Chloride (salt) "splits" all by itself when dissolved in water (in fact, that is how it is able to dissolve in water) and sodium has nothing to do with the electrochemistry of making chlorine.

Richard

[waste]

Carl, though I don't have all the reactions in front of me - the sodium becomes NaOH (I believe that it's lye). One of the 'real' chemists could tell you better, but it does combine an subsequently effects the reactions. (If I recall correctly, there are a few (2) loose electrons that help other chems 'match up')

[edit]Whoops, Richard alredy answered[edit]

[cschnurr]

Thanks for the info.

I'm theorizing that as NaCl is regularly added to water a number reactions occur that free up Chlorine.
As the chlorine is used up to kill stuff and evaporates, then eventually the Na concentration in the pool will increase.
Similiar to CYA levels using stabilized pucks.

If it is correct, should I be concerned.

[chem geek]

Carl,

Salt does not need to be regularly added to the water unless it is getting diluted through splash-out or backwashing. The consumption of chlorine in the water produces chloride ion so essentially the chloride ion is REGENERATED when the chlorine gets used up -- the chlorine/chloride does not (usually) evaporate. This happens regardless of whether the chlorine gets used up through oxidation (including disinfection) or through breakpoint with ammonia/urea or through breakdown from sunlight.

The only way that chloride will leave the system on its own (i.e. without sodium) is if there is outgassing of chlorine gas or hypochlorous acid gas. Both can outgas from a pool with hypochlorous acid being about 100 times faster to outgas (it's equilibrium quantity as a gas is much higher than for atomic chlorine gas). However, such chlorine outgassing is very slow and minimal in a pool. By far, most chlorine in a pool gets used up through the breakdown by sunlight and therefore gets regenerated back into chloride ion.

So the bottom line is that though it is true that theoretically you can build up sodium ion by having to add salt to make up for the lowering of chloride ion through the outgassing of chlorine (hypochlorous acid), the amount is exceptionally small. Anyone who has an SWG pool with a cartridge filter can probably tell you that they need to add very little salt to maintain the salt level (which is actually measuring chloride ion though the units are in ppm sodium chloride).

You can see the equations for chlorine consumption (and generation in a salt cell) at this link where you can see that the net reaction of chlorine generation AND consumption in an SWG system is:

4H2O + 2Cl- --> 2HOCl + 2OH- + 2H2(g)
2HOCl --> O2(g) + 2H+ + 2Cl-
2H+ + 2OH- --> 2H2O
------------------------------------------
2H2O --> 2H2(g) + O2(g)

6H2O + 3Cl- --> 3HOCl + 3OH- + 3H2(g)
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
3H+ + 3OH- --> 3H2O
------------------------------------------------------
3H2O + 2NH3 --> N2(g) + 3H2(g)

So the net reaction of chlorine generation AND consumption in an SWG system is that you simply produce hydrogen and oxygen gasses if the chlorine breaks down from sunlight (which is what most of it does) or you produce nitrogen and hydrogen gasses if the chlorine burns up (so to speak) ammonia. Other reactions are possible, but these are the common ones. Chlorine can combine with organics, but generally sunlight breaks these down and liberates the chloride ion (carbon dioxide may also be produced in this process).

Richard

[CarlD]

OOPS! Wrong "Carl"!!!!