Why doesn't Cl build up in a saltwater pool?

[Jeff S.]

If you have to add chlorine to a saltwater pool to maintain pH, why doesn't the chlorine level build-up over time. When I look at the equations for saltwater production with chlorine added to maintain pH (taken from Wikipedia):

4NaCl → 4Na+ + 4Cl− Salt dissolves in water.

4Na+ + 4Cl− → 4Na+ + 2Cl2 By electrolysis.

4Na+ + 4H2O → 4Na+ + 4OH− + 2H2 By electrolysis.

2Cl2 + 2H2O → 2HClO + 2H+ + 2Cl− Hydrolysis of aqueous Chlorine gas.

2HClO → HClO + ClO− + H+ Dissociation of hypochlorous acid at pH 7.5 and 25C.

4NaCl + 3H2O → 4Na+ + HClO + ClO− + OH− + 2Cl− + 2H2 Net of all the above.

Addition of Hydrochloric Acid to restore the pH to 7.5

HCl + 4Na+ + HClO + ClO− + OH− + 2Cl− +2H2 → HClO + OCl− + H2O + 4Na+ + 3Cl− + 2H2.

4NaCl + HCl + 2H2O → HClO + OCl− + 4Na+ + 3Cl− + 2H2 Net of the last two.

The end result is a total of 5 chlorine atoms and 4 sodium atoms. The start was four of each. The added HCl is the source. Recombining into NaCl would seem to leave one excess Cl.

I know I'm not understanding this correctly, but don't know where I've gone wrong. Could someone please explain?

 

[wireform]

If properly sized and calculated it'll produce what the daily loss is so it won't just keep climbing.

 

[Newdude]

I know I'm not understanding this correctly, but don't know where I've gone wrong. Could someone please explain?

I believe you are too focused and stuck inside the lab. Step outside into the sunshine and the UV burns off a couple ppm a day, requiring various runtimes of the above equations according to the time of season.

 

[Bperry]

If you have to add chlorine to a saltwater pool to maintain pH…

that’s where the confusion is right there. Chlorine is not added to maintain the pH but acid is.

 

[JamesW]

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^-.

HOCl + OCl^-+ uv light -> O₂ + H⁺ + 2Cl^-.

Following the process, we can see that there are 4H⁺ and 4OH^- created, which nets out to pH neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

 

[Jeff S.]

Isn't the acid HCl?

 

[JamesW]

6H₂O + 4Cl^-→ 2H₂+ O₂ + 4OH^-+ 4H⁺+ 4Cl^-

This is the net reaction.

There is no pH rise and no need for acid.

4OH^-+ 4H⁺ --> 4H₂O

Basically, the only thing you lose is water.

The process uses 1 mole of water for every mole of chlorine gas produced.

 

[JamesW]

Chemical formula H2O
Molar mass 18.01528(33) g/mol
Chlorine Cl₂ molar mass = 70.9 grams per mole.

You lose 18 grams of pure water for every 70.9 grams of chlorine you produce.

The chloride ions are just recycled.

You lose 0.25 lbs of water for every lb of chlorine gas produced.

 

[Jeff S.]

OK, I follow the chemistry of the saltwater generation being pH neutral, yet people are adding HCl to lower the pH in their saltwater pools (sometimes they are adding quite a bit, other times very little, judging by the threads at TFP). Whether they're having to do this because their overall water chemistry needs adjustment or not, they are doing it. When they do it, what happens to the extra Cl atom based on the equations I pasted into my original post? Alternatively, where am I confused? Maybe it is a "laboratory" type of question, but can't it be answered anyway? Thank you.

 

[Donldson]

people are adding HCl to lower the pH in their saltwater pools

People use HCl to lower the pH of all pools. Why is your question specific to saltwater pools?

 

[JamesW]

SWGs are pH neutral.

Some people get pH rise from the TA being too high and they add acid.

The TA should come down and then you do not need more acid.

However, if they keep raising their TA, then it is an endless cycle and the salt will continue to build up.

NaHCO₃ + HCl --> Na⁺ + Cl^- + CO₂ + H₂O

 

[Jeff S.]

People use HCl to lower the pH of all pools. Why is your question specific to saltwater pools?

Good point. My question is better put this way: if you dump a gallon of HCl in a pool, do the chlorine atoms ever leave the pool (ignore splashing, rain, etc.)?

 

[JamesW]

When they do it, what happens to the extra Cl atom based on the equations I pasted into my original post?

Acid adds chloride.

However, you should not need endless amounts of acid.

If you keep adding acid, the salinity will go up.

 

[JamesW]

My question is better put this way: if you dump a gallon of HCl in a pool, do the chlorine atoms ever leave the pool (ignore splashing, rain, etc.)?

Basically, no, they do not leave.

Dissolved solids mostly do not go anywhere.

 

[Jeff S.]

Thanks, maybe I see. The Cl minus ions are combining with (whatever) to form chlorides? At some point wouldn't there be a lot of dissolved solids (chlorides/salts) just from adding HCl periodically to control pH?

 

[JoyfulNoise]

Good point. My question is better put this way: if you dump a gallon of HCl in a pool, do the chlorine atoms ever leave the pool (ignore splashing, rain, etc.)?

No.

pH rise in a pool is dominated almost entirely by carbonate equilibrium. All water in planet Earth has carbonates in it unless it is specifically manufactured to not have any dissolved inorganic carbon. The transfer of carbon dioxide into and out of water is what causes most pH changes. People have to add water to their pools, when they do they are adding carbonate alkalinity which raises pH. Correcting for that requires acid. Muriatic acid is the most common and safest acid to use. All water will build up chloride as a result of this.

 

[JoyfulNoise]

Thanks, maybe I see. The Cl minus ions are combining with (whatever) to form chlorides? At some point wouldn't there be a lot of dissolved solids (chlorides/salts) just from adding HCl periodically to control pH?

There is so much water exchange in an outdoor pool, even in dry climates like mine, that the amount of chloride added is barely detectable.

 

[Jeff S.]

There is so much water exchange in an outdoor pool, even in dry climates like mine, that the amount of chloride added is barely detectable.

Thanks for your patience in answering this question.

 

[JamesW]

The Cl minus ions are combining with (whatever) to form chlorides?

The Cl minus ions are chloride ions.

They really do not combine with anything.