How does pH Increaser work?

[TreeFiter]

I was working on a hot tub yesterday, and as I added pH increaser to the tub, I realized that I had never really given much thought to how pH increaser actually works. This tub had been neglected, and the pH was EXTREMELY low, and difficult to raise. It took about 3lb of Sodium Carbonate to bring the pH into the normal range. So when I added pH increaser, I saw the water fizz. I remember fizzing being mentioned in other threads with regard to adding Sodium Carbonate to pools with very low pH.

I assume that the fizzing is dissolved CO2 coming out of solution. If this is correct, the rise in pH is the result of removing CO2. But what about the Alkalinity? We can raise pH by raising Alkalinity and aerating the water to drive out CO2. When we add Sodium Carbonate, the Alkalinity increases along with the pH. Is the rise in pH really a response to the increase in Alkalinity? What causes Sodium Carbonate to raise pH almost instantly, while Sodium Bicarbonate takes time and aeration to do the same?

 

[pooldv]

Aerating will raise PH and acid will lower TA. Here is an article from Pool School - Lower Total Alkalinity

But, rereading your question I'm not sure I fully answered it.

 

[chem geek]

Yes, the fizzing is from acid in the water reacting with the carbonate to form carbon dioxide that outgases. However, the rise in pH comes not only from that outgassing, but also from the acid converting the carbonate to bicarbonate. Each stage uses up hydrogen ions so raises the pH:

H⁺ + CO₃^2- ---> HCO₃^-
Hydrogen Ion (i.e. acid) + Carbonate Ion (i.e. from pH Up) ---> Bicarbonate Ion

H⁺ + HCO₃^- ---> H₂CO₃ <---> CO₂ + H₂O
Hydrogen Ion (i.e. acid) + Bicarbonate Ion (from above) ---> Carbonic Acid <---> Carbon Dioxide + Water

So pH Up will raise the pH even if you don't see carbon dioxide bubbles or even forming much carbonic acid or aqueous carbon dioxide because in water where the pH isn't terribly low the first reaction above mostly dominates. Note that the second reaction may happen when you add sodium bicarbonate (Alkalinity Up), but mostly what happens is that the bicarbonate ion stays as is so the pH doesn't change very much.

Adding sodium carbonate (pH Up) always raises the alkalinity. The carbon dioxide outgassing itself does not raise the alkalinity -- the alkalinity raised when the pH Up was added to the water.

So sodium carbonate (pH Up) raises the pH and the TA, but the pH always rises quite a lot because you are adding carbonate ion that very much wants to be bicarbonate ion. The only time carbonate ion will be happy without changing the pH is if the pH were extraordinarily high where adding pH Up will not change the pH at around 9.6. With sodium bicarbonate (Alkalinity Up) it raises the TA and usually doesn't change pH much and in fact it usually lowers the pH slightly upon addition. In practice you often see the pH rise not because of this addition, but because of the carbon dioxide outgassing that occurs as a result of that addition. If you were to add it and prevent the outgassing, you would find the pH slightly dropping upon addition.