Chemistry of TA Reduction

[Smykowski]

I've learned and incredible amount from this forum, and in searches I've gotten the exact answers I've wanted regarding chlorine, how it enters the pool, and how it works.

However, in my searching, I haven't seen anything about TA. If this has been covered already, please give me the link. If not, here's my question for the chemistry experts (cough cough, chemgeek, cough)....

I understand the process for reducing TA (aerate, add acid, repeat). Can someone spell out the chemical reaction, and explain why this works? I had an AHA! moment a few months ago perusing the forums here. Last summer, I was constantly adding acid but also adding "Alkalinity Up" on the direction of the pool store. Turns out, when I opened the pool, the eyeball return was pointed up, and I was aerating the pool without even knowing it. Now I know for next year, but I was curious about the nitty gritty of the process.

Thanks in advance.

 

[JamesW]

CO2(air) <> CO2(aqueous) + H2O <> H⁺ + HCO3 <> 2 H⁺ + CO3

The carbon dioxide in the air wants to be in equilibrium with the dissolved (aqueous) carbon dioxide in the water. The dissolved (aqueous) carbon dioxide in the water wants to be in equilibrium with the bicarbonate, which wants to be in equilibrium with the carbonate. For the species in the water, the pH determines the equilibrium ratios. In water, the equilibrium is reached fairly quickly.

In most cases, the dissolved (aqueous) carbon dioxide in the water is at a higher concentration than what would be in equilibrium with the air. Therefore, carbon dioxide is continually off gassing to try to achieve equilibrium. This process usually happens much more slowly than the equilibrium processes in the water depending on the aeration, water temperature, pH and TA.

As the carbon dioxide off gasses, some of the bicarbonate ions connect with hydrogen ions to become dissolved (aqueous) carbon dioxide, and some of the carbonate ions connect with hydrogen ions to become bicarbonate ions to try to regain equilibrium. As some of the free, active hydrogen ions are removed from the water, the pH rises. The TA remains the same because as you lose a free hydrogen, you also lose a free carbonate or bicarbonate.

When you add acid (HCL), the acid provides hydrogen ions (H⁺) and chloride ions (Cl^-). The hydrogen ions lower the pH and TA. In the process, the hydrogen ions create more carbon dioxide, which off gasses and raises the pH without raising the TA.

The equation for adding acid and then sodium bicarbonate is the following:

HCl + NaHCO3 --> NaCl + H2O + CO2

Acid + Sodium Bicarbonate --> Salt + water + carbon dioxide.

Therefore, you just waste money adding acid and then bicarbonate. If your pH is constantly rising, then your TA is too high, and your pH might be too low. If your pH is constantly falling, then your TA is too low. A higher TA and a lower pH create more carbon dioxide and therefore more off gassing of carbon dioxide and more pH rise.

It may seem counterintuitive, but maintaining a lower TA and a higher pH can significantly reduce pH rise. However, using a pH higher than 7.8 can cause other problems, so 7.8 should be the upper limit to the pH.

When using an acid source of chlorine, such as trichlor, using a higher TA will create more pH rise to offset the pH drop due to the acidic trichlor.

Note: TA, in this example, means carbonate alkalinity. There are other chemical species in the water that also contribute to TA (total alkalinity), such as borate and cyanurate (and phosphate in some hot tubs that use a phosphate buffer).

 

[chem geek]

There are also three earlier posts that go into detail about TA in various ways. This one shows the reactions as James did and does the accounting for TA with each reaction. This one similarly shows the reaction and the accounting for TA from various species. This one shows the pH buffering effect of the carbonates, borates and CYA. This post shows the concentration of various carbonate species at different pH and the effects of adding acid or base vs. carbon dioxide injection or outgassing.