Carbonate Alkalinity (adjusting TA)

[CarlD]

Evan,
Fascinating! Even Chem_Geek should be impressed!

Now a question from a non-chemist: When you add lye (Sodium Hydroxide) a big chunk of sodium ions are released. Don't those combine with the FC in the water to create NaCl, salt? Can't that effectively bind up chlorine ions in the water?

Just a thought!

[Ohm_Boy]

Cool. So, re-arranging components just a bit, my chart of how the carbonate alkalinity behaves/exists with regard to pH now looks like this:



We can shift back and forth in the elliptical area, but until we actually remove a component (CO2), nothing really changes.
Is that roughly what we're dealing with?

[waterbear]

Evan,
Fascinating! Even Chem_Geek should be impressed!
I was a chem major in college also! Switched after 3.5 years and got my degree in another area. I do understand the technical stuff.
Now a question from a non-chemist: When you add lye (Sodium Hydroxide) a big chunk of sodium ions are released. Don't those combine with the FC in the water to create NaCl, salt? Can't that effectively bind up chlorine ions in the water?
Well, chloride ions (Cl-) would form salt with the sodium. Chlorine in the water is in the form of hypoclorous acid (HOCl) and Hypochlorite ions (OCl-). (And clorinated isocynaurates if CYA is present, plus the various forms of combined chlorine.) If the sodium combined with the hypochlorite ions (which doesn't reall happen in solution) it would be sodium hypochlorite. I am sure you recognize that name!
If the level of salt formed from the sodium and chloride ions formed when the chlorine is used up by sanitizing (reduced to chloride ions after giving up their oxygen) was a problems then you are talking about TDS and a SWG system would be having major problems. We've talked about that before time and again in the forum.
Just a thought!

Hope this clears it up.

[chem geek]

Yes I'm impressed -- clear graphs beat equations for understanding. And Evan knows a lot of chemistry and probably remembers some things better than I and has interesting and relevant real-world experience. And sodium chloride (salt) when dissolved in water is really separate sodium and chloride ions so the sodium from sodium hydroxide doesn't really do anything significant at all. If you increase your ion levels A LOT as with saltwater pools, including SWG pools, then the higher concentration of ions affects the ionic strength and that affects all chemical equations that have ions in them (especially ions on one side and not the other, or higher charges on one side compared to the other). This is why the TDS is part of the saturation index -- it's for taking into account this ionic strength effect.

Richard