Pretty sure chlorine gas will still raise TDS, cause you'd adding chlorine. The end result is chloride ions floating around. When you add chlorine gas, it reacts with water to create HOCl, the primary chlorine disinfectant:
Cl2(g) + H2O --> HOCl + H+ + Cl-
Some of the HOCl with dissociate (I think that's the right term) into OCl-, which does disinfect as well, but only to a very small amount:
HOCl --> H+ + OCl-
When used up:
2HOCl --> O2(g) + 2H+ + 2Cl-
2OCl- --> O2(g) + 2Cl-
In short, originally you had water, with nothing in it, now you have water, with Cl- ions floating around, which is TDS. Very similar for adding liquid chlorine (NaOCl) except you end up with Cl- and Na+ ions floating around after that chemistry all works out.
SWCG is the only form of chlorination that does
not raise TDS. You initially raise it by dumping in loads of salt (sodium chloride), which separates into sodium and chloride ions when it dissolves. But the magic of a SWCG is it takes those "dead" chloride ions and recycles. Using electricity, it strips electrons from the chloride ions, turning 2Cl- into Cl2, and, as a byproduct on the other plate, creates hydrogen gas and hydroxide ions:
2Cl- --> Cl2(g) + 2e-
2H2O + 2e- --> H2(g) + 2OH-
Combining those equations you get the following net:
2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
So, wait. A SWCG is making chlorine
gas? Yes, it is! And hydrogen gas too. The chlorine gas dissolves back into the water very rapidly and almost entirely, and works exactly the same as the first equations I posted for chlorine gas chlorination. The hydrogen gas does not dissolve much at all and bubbles out of the pool.
The end result though is you are not
adding anything during this chlorination process. You're taking chloride ions that are pre-added with salt, stripping electrons to make it chlorine gas, the chlorine gas dissolves back into the water and makes HOCl, which after it does the thing we want it to do ends up as Cl- ions floating around in the water again. They get picked back up by the SWCG, and the cycle repeats.
So, summary is chlorine gas still increases TDS (unless I'm not understanding something correctly), but a SWCG does
not increase TDS while running. You do a one time TDS increase when adding salt but it doesn't rise after that because you are recycling chloride ions, not adding new chlorine. Any other form of chlorine additions will increase TDS because you aren't doing this "recycling."
Having said all that, there's NO need to worry about TDS. It's a marketing gimmick, and used wrongly at that, by saying liquid chlorine will raise your TDS but pucks will not. BS! It
all will raise your TDS, except SWCG. Which is of course the LAST thing an average pool store will tell you to get cause it means no more money for them on overpriced chlorinating products!
Please note I didn't come up with these equations myself. I copied chem geek's equations in post #3 from his quite excellent stickied post on pool water chemistry over in The Deep End. And I did it poorly, cause somehow he had all the proper superscripts and subscripts that make the equations easier to read, but that doesn't copy over...
!!
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