[ChrisG_UK]

Hi, I haven't done chemistry since school (long time ago) but am desperate to understand the full cycle of reactions going on in my swimming pool salt water chlorinator. I have found a huge amount of conflicting information about this and I am now pretty well convinced that no-one in the swimming pool industry really understands the full story of what happens in a salt chlorinator pool. I'm really hoping that a real chemist will be kind enough to put me out of my misery and explain it properly.

This is what I have pieced together so far from a lot of different sources. I hope someone can tell me if I have this right, and then hopefully answer my question at the end. So:...

Salt is added to the pool water to a level of 2000 to 6000 ppm depending on the manufacturer. Low voltage DC electricity is applied to the cell plates and the following reaction occurs (the Chloralkali process):

   2 NaCl + 2 H₂O => Cl₂ + H₂ + 2 NaOH

This is an "in-line" cell with no membrane or diaphragm, so the products are free to mix, and further reactions take place very quickly:

Chlorine reacts with a cold solution of sodium hydroxide to give a mixture of sodium chloride (NaCl) and Sodium Hypochlorite (NaOCl):

   Cl₂ + 2 NaOH => NaCl + NaOCl + H₂O

Sodium hypochlorite in water makes hypochlorous acid (the sanitiser we want) and sodium hydroxide (raising the pH):

   NaOCl + H₂O =>  HOCl + NaOH

Some chlorine also dissolves in the water to form "chlorine water". When chlorine water is exposed to bright sunlight, it breaks down into hypochlorous acid and hydrochloric acid (lowering the pH). The amount of chlorine dissolved in the water depends on the water temperature.

   H₂O + Cl₂ => HCl + HOCl

In the presence of sunlight, hypochlorous acid decomposes into hydrochloric acid and oxygen:

   2 HClO => 2 HCl + O₂

Hypochlorous acid oxidises dirt and bacteria:

   HOCL + XXXX => HCl + NaOH + XXXXO

Hydrochloric acid combines with sodium hydroxide to form water and salt:

   HCl + NaOH => H₂O + NaCl

Note that in certain conditions, chlorine and hydrogen could combine explosively to give hydrochloric acid, although in the dark, this does not occur.

Inside the cell, the chlorine level can reach 20ppm, but as the water reaches the pool it typically dilutes down to 2-3ppm. Obviously, many chemical reactions are involved here, and their relative extent presumably depends on temperature, pH, types of contamination, and other factors. However, following the various oxidation reactions in the pool, the sodium and chlorine eventually recombine to form salt, so the salt is never actually used up.

Most people report that in a salt chlorinator pool, the pH steadily rises and must be brought down, usually by adding some hydrochloric acid. However, in my pool, the pH steadily falls! Why?
 

[nj_bartel]

CO2 + H2O  H2CO3

CO2 dissolves in water and undergoes reaction to form small quantities of carbonic acid.  The carbonic acid neutralizes the basic water, and more CO2 dissolves, forming more carbonic acid.

That'd be my guess anyway.

[mnakhla]

Is your pool above ground or below grounds, what surrounds it and what are its walls made off, just the regular pool tile?

Also, I'm not too familiar but, I am guessing that it pumps the water directly from a water line or something of the sort, is the water in your area hard or soft?

[mnakhla]

Also the equilibrium reaction you have described is PH dependent so definitely how soft or hard your water is, maybe a significant factor.

[RichardFalk]

The chemistry in the first post is not quite correct.  Sodium is an ion in water so not participating in the reactions shown.  There is no "cold solution of sodium hydroxide" though hydroxyl ions are produced at the cathode and half of these combine with the hydrogen ions produced from the chlorine gas that dissolves in the water.  Most of the chlorine gas dissolves into the water to form hypochorous acid.  A more complete description of what goes on with both manual dosing of chlorine and from the saltwater chlorine generator (SWG) is given in this post.

Most SWG pools have rising pH due to the increased aeration of the water which increases carbon dioxide outgassing and from undissolved chlorine gas that outgasses.  The former effect can be reduced by lowering the Total Alkalinity (TA) of the water since the outgassing rate varies roughly as the square of the TA while the pH buffering from the TA varies more linearly.  As for why this specific pool tends to drop in pH, that must be coming from other factors of acidity and is indeed very unusual.  Normally, a drop in pH in pools comes from use of Trichlor, Dichlor or non-chlorine shock on a regular basis.  Hypochlorite sources of chlorine are net pH neutral (except for "excess lye") as described in the above post due to the acidity of chlorine consumption/usage.

Richard

[RichardFalk]

I suggest you read this World Health Organization document so that you understand the increased risks in natural waters.  "Natural" pools that are not sanitized by an EPA-approved sanitizer (chlorine, bromine, Baqua/biguanide/PHMB, Nature2 with MPS in hot spa water) would not be used in commercial/public pools due to the vastly greater risk of transmission of disease from one person who can infect dozens or more.  What you do in your own private pool is your own business and the risk is lower, but if you keep the water warm then bacterial growth can still be substantial and you'll also need to deal with preventing algae.

[Jmae]

We just believe that having saltwater in a pool is really just an alternative way to chlorinate the water. Because from the salt itself that it was sodium, it makes a cleaning action. It only provides the raw material to make chlorine. Don't forget that a salt chlorinator is only doing its job when the pump is running.

[RichardFalk]

The sodium is irrelevant.  Any source of chloride (that dissolves in water) would work, such as potassium chloride.  It is the chloride in the pool that the saltwater chlorine generator turns into chlorine via electrolysis.  Again, the sodium is irrelevant and performs no "cleaning action".

[rickythomas]

There is an huge difference between saltwater pool and chlorine based water, swimming pool should be free from harmful bacteria and pollutants, you should have the basic chemistry of these chemicals in mind so as to confirm the best water conditions for divers.