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?