We have heard it a thousand times, “I don’t use chlorine, I have a salt water pool.” Homeowners do not always understand that they are indeed using chlorine and that the only difference between this and a tablet is the chlorine delivery system. So why do they insist that it is not a chlorine pool? Had they been lied to?
Written out in its formula, salt is sodium chloride (NaCl). It is simply a combination or compound of elements taken directly from the periodic table, Na and Cl are the chemical symbols. The sixth most ample of all elements on the planet, sodium is a soft metal with the symbol of Na and an atomic number 11. The Cl in NaCl is chlorine. It is in a group on the periodic table known as halogens and has an atomic number of 17.
I hate to poop in your corn flakes, but a salt generator does not actually produce chlorine; the salt cell serves a very different purpose. It actually separates the bonded elements (compounds) that are already present in the swimming pool water through a process called electrolysis. By passing an electric current through an aqueous NaCl solution, we actually separate the Na from the Cl. Therefore, it is not producing chlorine; instead, it is freeing the element so that it can be chlorine. Once separated, the Cl (Chlorine) becomes Chlorine gas (Cl2) and the Na becomes Sodium Hydroxide (NaOH).
So, where does the hydroxide (OH-) come from? Do not forget that water (H2O) is also subject to electrolysis; hydroxide is a single hydrogen and a single oxygen atom with a molecular bond formed in the electrolyzed water after the 2 hydrogen (H2) atoms are separated from the Oxygen (O) atom. Some Hydrogen does become Hydrogen gas in the process and the Oxygen, as you can see from all of the “O”s above and below (ie: NaOH, HClO, ClO-, H2O, OH-), does not leave the water.
To simplify, we add 3,000 to 4,000 ppm of salt to a swimming pool (about 250 to 333 lbs per 10,000 gallons). Yes, this does also increase the Total Dissolved Solids level (1 pound of salt will increase the TDS of 10,000 gallons of water by 12 ppm). The saltwater solution we created passes through the electrolytic cell that contains electrically charged plates (a cathode and an anode). The sodium chloride separates into both Na (sodium) and Cl (Chlorine). The chlorine gas becomes hypochlorous acid (HClO) and hypochlorite ion (ClO-), the combination of the two known as FAC (Free Available Chlorine). The H2O separates and becomes hydrogen gas, and combining with the now separated Na, sodium hydroxide (NaOH). Sodium hydroxide, also known lye (or caustic soda), with a pH of 14 is the strongest base known to exist. This is why saltwater pools always have such a high pH.
Wait a minute; I thought the pH was always high in a saltwater pool because the pH of salt is high? Not quite, NaCl actually has a pH of 7.0. The only way sodium chloride (table salt) will change the pH of water is if the starting pH is actually very low, or very high, drawing the pH toward neutral upon addition. Do not forget, with a pH of 7.0, the closer to 7.0 the pH gets, the more salt it will take to make a change. Still skeptical? Try this: Take a sample of water with a pH of 7.0, add a teaspoon of salt and wait until dissolved. Do not shake the sample as turbulence can cause the pH of your sample to increase (that is a completely different conversation), then retest your pH. Your pH will not have changed. It is the sodium hydroxide, the byproduct of the electrolysis of saltwater, which drives the pH upward.
In theory, SWGs will not change the pH of the water. In theory, this is true. You start with a neutral salt and split it into Na and Cl and put it all back into the water. In practice, the hydrogen produced is a gas and because it is a gas, it leaves the water. The hydrogen would normally neutralize the OH produced but the hydrogen leaves the water. So the OH raises the pH.
So why then does your saltwater pool owner think that they are not using chlorine?
When these chlorine generators first came about, the industry was extremely hungry for a magic pill and so were the pool owners. A one-stop end all, used to treat all pool related problems in one shot. Unfortunately, there were, among the many honest professionals, a few overzealous sales people in the industry at the time. In a snake oil salesperson kind of way, the capabilities of these systems being extremely exaggerated. “Get away from the harmful effects of chlorine”, “Never add chlorine again”, were just a couple of the phrases used to close the deal. “You don’t want your kids to suffer from chlorine allergies, do you?” kicking it into a used car lot, borderline life insurance sales type pitch. “You’ll never have to adjust the pH, buy chlorine tablets, brush the walls, skim the surface, vacuum the pool, mow your lawn, empty the dishwasher, or make your bed – the saltwater system for your pool will do it all!” Okay I strayed, but you get the point. Sadly, the Pool Operator is usually the one who has to explain how these things really work to the often-doubting homeowner, who not only uses chlrine, but technically is a chlorine manufacturer. ?
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