What happens to the Na+ ions

[Suley]

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Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reactions:
At the anode (positive plate):
2Cl^- --> Cl₂(g) + 2e^-

At the cathode (negative plate):
2H₂O + 2e^- --> H₂(g) + 2OH^-

which nets out to the following where the chlorine gas dissolves in water:

2H₂O + 2Cl^- --> Cl₂(g) + H₂(g) + 2OH^-
Cl₂(g) + H₂O--> HOCl + H⁺ + Cl^-
H⁺ + OH^- --> H₂O
----------------------------------------------
2H₂O + Cl^- --> HOCl + OH^- + H₂(g)

or equivalently

H₂O + Cl^- --> OCl^- + H₂(g)

Note that the products of HOCl and OH^- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

[EDIT]
The net reactions in an SWG pool for chlorine addition from the SWG and then breakdown from sunlight and oxidation of ammonia are as follows:

2H₂O --> O₂(g) + 2H₂(g)
2NH₃ --> N₂(g) + 3H₂(g)

The chlorine is not "seen" in the above net reactions because the chloride that became chlorine goes back to being chloride again. The oxygen gas comes from water when chlorine gas dissolved in it (i.e. from hypochlorite ion or hypochlorous acid) while the nitrogen gas comes from the ammonia (the oxygen or hydroxyl in the chlorine reverts back into water in this case, using the hydrogen from the ammonia to do so).
[END-EDIT]

Richard

I was wondering what happens to the Na+ ions in the salt water in a SWG pool.

Thanks!

 

[SunnyOptimism]

Re: Pool Water Chemistry

I was wondering what happens to the Na+ ions in the salt water in a SWG pool.

It's the counter ion in the redox reaction and can be effectively ignored. It obviously provides the charge balance to the negatively charged species. In solution it will balance the -OH or -Cl ion to form either salt or lye (sodium hydroxide).

So you can think if an SWG as increasing your waters pH by adding sodium hydroxide. However it really doesn't matter what the Na is doing.


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[JasonLion]

Re: Pool Water Chemistry

The Na+ sits in the water doing nothing.

 

[SunnyOptimism]

To edit chemgeek's post, it looks like this :

2H2O + Na+ + Cl- --> HOCl + Na+ + OH- + H2(g)

Since sodium is on both sides of the equation with no net change, it can be ignored.


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[SwimAustin]

The Na+ does nothing, but there is an increase in OH- which keeps the ions balanced. That increase in OH- is why people with salt water generators see a raising Ph and need to add acid. You could think of it as the salt turning into sodium hydroxide, but that's usually not the best way to think about dissolved ions.

 

[JasonLion]

there is an increase in OH- which keeps the ions balanced. That increase in OH- is why people with salt water generators see a raising Ph

Once you take into account the chlorine getting used up the reaction as a whole is PH neutral. So the OH- ends up having no impact on PH at all. The reason many people see PH increases are because of the hydrogen bubble producing aeration, which encourages CO2 outgassing, which raises the PH. You can counter this mostly or completely by keeping TA fairly low. There can also be some PH increase due to Cl2 outgassing, but this appears to be either rare or extremely minimal.

SunnyOptimism, writing the equation that way is true in once sense, but misleading. There doesn't need to be any Na+ in the water for the reaction to work, so it is normally not notated. It could just as well be potassium, or not present at all and the SWG would still work.

 

[SunnyOptimism]

SunnyOptimism, writing the equation that way is true in once sense, but misleading. There doesn't need to be any Na+ in the water for the reaction to work, so it is normally not notated. It could just as well be potassium, or not present at all and the SWG would still work.

JasonLion,

Agreed. Normally counter ions that stay in solution are not written out in redox equations unless they contribute to the overall reaction and/or change phase and leave the solution. I only put it back in the equation because the OP asked about the sodium specifically.

Not sure I totally agree with your last sentence though. You are quite right that the counter ion could be any chloride salt (HCl, NaCl, KCl, LiCl, etc) and it wouldn't matter one bit for the electrochemical reaction in the cell .... but "not present at all" ?? That's not physically possible as there has to be a counter ion for overall charge neutrality.

For sure a positively charged counter ion (H+, Na+, K+, etc) and the negatively charged Cl ion can easily be surrounded by water molecules and have their charge effectively screened due to H2O's electrostatic dipole configuration (Mickey Mouse ears structure of water molecule) but overall the aqueous solution must be charge neutral. There is no such thing as a free Cl atom in an aqueous solution under normal conditions.

I realize the above is deep in the weeds of electrochemistry and that we're probably far beyond the OP's original question so I'll just drop off the thread at this point. I hope what's been posted so far, especially ChemGeek's excellent summary, was sufficient to clarify the concept of SWG chlorine production.


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[chem geek]

He meant that the specific nature of the charged counter ion doesn't matter. Obviously the total charge balance in the pool needs to be zero.

The short answer to the OP's question is that the sodium ion stays in the pool and doesn't participate in any of the reactions in the pool including what goes on in an SWG cell. Now there is a movement of ions between the cells since where chlorine gas is generated negative charge from chloride ions are taken away while at the other plate where hydrogen gas is generated negative charge is being introduced as hydroxyl ions. So migration of charge happens and any of the charged ions in the water can move because of that and of course the water is physically mixing as well.

As for the pH, you cut out part of the post where I explain that -- the full post shows what happens when chlorine gets used/consumed which is an acidic process and this exactly counteracts the increase in pH from chlorine addition either from hypochlorite or from an SWCG. The net reactions are at the end of what you quoted and are pH neutral.

 

[Suley]

Thanks to all the explanation! Now I remember my childhood experiment of electrolysis of brine and had the same result... Lots of chlorine!