Re: Carbonate Alkalinity (adjusting TA)
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.)
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!
Last edited by waterbear; 06-26-2010 at 01:15 AM.
Retired pool store and commercial pool maintenance guy.
Bookmarks