Carbonate Alkalinity (adjusting TA)

[Ohm_Boy]

In Ben's sticky on lowering TA (http://www.poolforum.com/pf2/showthread.php?t=191), he says that the 4 carbonate-ish (my term) players in the mix are :
bicarbonates (-HC03)
carbonates (=CO3)
carbonic acid (H2CO3)
carbon dioxide (CO2)

at high pH, the carbonate alkalinity is "in the form of carbonates: there is NO bicarbonate and NO carbonic acid"

and that at low pH, the carbonate alkalinity is "carbonic acid + carbon dioxide, with NO carbonates and NO bicarbonates"

Now, I will be the first to admit that I am not highly chemistry-oriented, so I don't always expect to grasp all of it. To visualize this in my own little empty head, I made a graph, of sorts...

http://www.graphicartssolutions.com/image_pool/carbonates_vs_ph.gif


It isn't graphed to any real numbers, it is just supposed to indicate that CO3 is high at high pH, H2CO3 is high at low pH, and I am guessing that the CO2 may increase as the pH drops, but I don't know that. Like I said, it is supposed to be an indicator/visual aid only.

What I am having difficulty with, is this: How do the bicarbonates fit in? They don't seem to be there in either high or low pH cases. Can someone tell me the relationship with it?

[waterbear]

A few things that might help. (This is the simplifed version so the chemistry is going to be, well, simple!;) I promise not to get into topics like dissociation constants, partial pressure, etc.:eek:)

Carbonic acid is just carbon dioxide dissolved in water(not exactly true but for our purposes we are going to assume it is, the distinction is not really important for what I am trying to explain), exactly the same thing (at least for our purposes here) as seltzer water. If there is carbon dioxide in the water it forms carbonic acid, H20 + CO2 ==> H2CO3.

Carbonate ions are (CO3--) and bicarbonates are (HCO3-). At very high pH there is an excess of (OH-) ions (hydroxide ions) in the water and they will pull the hydrogen off the bicarbonate ion to form water and a carbonte ion.
(OH-) + (HCO3-) ==> H20 + (CO3--)

Since carbon dixoide in the water is the same as carbonic acid (not really, but for our purposes lets say it is) it can't exist at very high pH because it is an acid and will be neutralized by the (OH-) ions in the water like this:
H2CO3 + 2(OH-) ==> 2(H2O) + (CO3--)

Now if the pH is not that high and there are both hydroxide ions (OH-) and hydronium ion (H+) in the water then bicarbonates can exist because some of the carbonate ions will take up a hydronium ion and form bicarbonate.
(H+) + (CO3--) ==> (HCO3-)

Bicarbonates are interesting because they can act like an acid or a base. If you add acid (H+) then they act as a base and combine with the acid to neutalize it (and form carbon dixoide and water, or carbonic acid)
(H+) + (HCO3-) ==> H2CO3 (or you can say H20 + CO2--carbon dioxde dissloved in water or selter!)
This is why baking soda fizzes when you pour an acid like vinegar on it. You are releasing carbon dioxide and are left with water (not exactly true but for our purposes it's close enough).

If you add base (OH-) then the reaction forms water and carbonate ions.
(OH-) + (HCO3-) ==> H20 + (CO3--)

This is why we use bicarbontes to buffer pool water, it can take up both excess hydroxide and hydronium ions and keep the pH from bouncing.

This also explains why lowering the pH and aerating will lower the TA while lowering the pH to lower the TA and then raising the pH chemically will raise the TA again.

At normal pool pH we will have both carbonate and bicarbonate ions in the water but mostly bicarbonate. We also have carbonic acid If we drop the pH the excess hydronium ions from the acid we added will convert some of the carbonate to bicarbonate and some of the bicarbonate to carbonic acid (or carbon dioxide and water). The pH stays low because we have creaated more carbonic acid. If we just add hydroxide ions (chemically raise the pH)the carbonic acid will convert back to bicarbonates and carbonates
H2CO3 + (OH-) ==> H20 + (HCO3-)
(HCO3-) + (OH-) ==> H20 +(CO3--)

If we aerate the water and drive off the carbon dioxide (remember that carbonic acid is seltzer!) then we lower the amount of carbonic acid in the water and the pH rises but we do not recreate more bicarbonates and carbonates. The TA is now lower.

Actually, TA is only a measure of the carbonates and bicarbonates (the alkaline part of the buffer) so the act of dropping the pH and converting the measurable carbonates and bicarbonates to carbonic acid, which is not measured when we test TA lowers the TA. The aeration only serves to raise the pH without raising the TA back again since we are removing from the water the carbonic acid that can reform bicarbonates and carbonates. By removing the carbon dioxide part of the carbonic acid we are just left with water.
H2CO3 ==> H20 + CO2 (removed by aeration--think shaking a bottle of seltzer to make it go flat.)


Also, the reason that soda ash raises BOTH pH and TA is becasue when it is dissolved in water it forms sodium hydroxide and sodium bicarbonate
Na2CO3 + H20 ==> NaOH + NaHCO3 or
Na2CO3 + H20 ==> 2(Na+) + (CO3-) + H20 ==> 2(NA+) + (OH-) + (HCO3-)
As you can see we have increased the pH by increasing the hydroxide ions in the water and also increased the TA by forming bicarbonate ions and carbonate ions. How much of each is dependant on how high we have raised the pH. For the normal pH range of pools we will form more bicarbonate ions than carbonate ions. The net effect is the same as adding baking soda and lye.

I hope I haven't totally confused you at this point!

[Ohm_Boy]

So, the 'general purpose' bicarbonate is happy in the neutral zone. (No Romulan jokes!)

I will still have to spend a few minutes drawing 'H's on a piece of paper, but I can change my magic graph to this:
http://www.graphicartssolutions.com/image_pool/carbonates_vs_ph_2.gif

[waterbear]

Pretty much. Carbonic acid and carbon dioxide will exist until the pH is exetremely high. Carbonates and bicarbonates will exist until the pH is extremely low. Everywhere else they all coexit in dofferent concentrations with the majority in the form of bicarbonate around pH of 8.2, if my memory serves me correctly.

[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:

http://www.graphicartssolutions.com/image_pool/carbonates_vs_ph_3.gif

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