What is TA, anyway?

[Maestro]

I would love to know the difference between total alkalinity and high pH. I have a very basic (sorry about the pun!) knowledge of chemistry, but I always thought that adding baking soda to water would make it more basic and raise its pH. Does it have the same effect on the pH of the water as borax or soda ash, or is there a different process at work?

Also, what am I really controlling by keeping the TA in my pool between 70 and 90 (is that ppm?)? What are the consequences if it's too low or too high? Since PoolMath has "90+" for the upper range, does that mean there's no upper limit, and is more always better?

 

[pabeader]

I can't give you the level of detail that Chem Geek, JoyfulNoise, and others can, but I can tell you how I understand it. TA is a pH buffer. It acts to stop pH from wild swings that can sometimes happen. If TA is too low, pH is hard to control. If TA is too high, pH will usually rise. TA is the one thing in pool school that can be left to find it's own happy place. What I usually do is watch pH, if it starts to rise, use muriatic acid to lower it. The TA will go down with it. If the pH starts to rise again, do the muriatic acid to lower again. At some point the p?H should start to hold for longer periods.

Things that cause pH to rise include SWGs, fountains, plus stuff related to plaster.

 

[pooldv]

The simple answer is this,

TA - Total Alkalinity
Total alkalinity indicates the water's ability to buffer PH changes. Buffering means you need to use a larger quantity of a chemical to change the PH. At low TA levels, the PH tends to swing around wildly. At high TA levels, the PH tends to drift up. You can raise TA with baking soda. It is often best to make large TA adjustments in a couple of steps, testing the water after each one, as adding large quantities of baking soda can raise the PH a little and you don't want the PH going out of range. If you need to lower your TA level, see Pool School - Lower Total Alkalinity.

The practical answer is this,
Generally, you want to control PH to 7.2 to 7.8 and let TA find its own equilibrium. Usually somewhere between 50 as a minimum and maybe 120. When PH becomes balanced and doesn't require weekly or more adjustments then that is probably a good place to try to keep your TA in the future. Some people have high or low TA fill water and have to manage it more. Also, people with very high CH water have to manage their PH and TA closer to offset their high CH.

 

[chem geek]

Total Alkalinity (TA) is a measure of the concentration of chemicals in the water that can accept a proton (hydrogen ion) down until the pH reaches 4.5 at least in pool water test kits. It is a measure of the resistance of the water to having the pH get lowered from the addition of acid either directly (e.g. adding Muriatic Acid to the pool) or indirectly (e.g. chlorine usage/consumption). The units are parts-per-million (ppm which in pool water is essentially milligrams per liter) calcium carbonate.

pH is a measure of the concentration of hydrogen ions in the water. It is a logarithmic scale so a difference of 1.0 is a factor of 10 difference in concentration. It defines how acidic or basic (alkaline) the water is.

Baking soda increases TA but at normal pH it does not increase pH very much. This is because at normal pH the bicarbonate added by baking soda mostly remains as bicarbonate. TA in pools is mostly a measure of bicarbonate.

 

[Maestro]

Thank you, chem geek! That is much more in layman's terms than the article on Wikipedia.


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[Ale_Brewer]

Great explanation! I'm still learning the TFP method and I am curious...if my CH level is pretty well within range in the pool and my fill water, is there any suggestion on which side of the "70-90+" range I should be at? It sounds like my current TA of 140 will just require more acid for pH lowering so I would think lower is better, plus if my TA goes down to 70-80 then my CSI will be closer to 0. But if I go from 140 to 70 on the TA, will I be sacrificing a significant amount of pH buffering ability?

 

[chem geek]

Every pool is different because the amount of aeration and carbon dioxide outgassing is different. So we cannot tell you where to exactly set your TA. If your pH tends to rise over time, then lowering the TA level may help reduce that rate of rise and the amount of acid you need to add. If your pH is fairly stable, then your TA may be fine where it is assuming the overall water balance is OK (i.e. that your CSI isn't so high as to risk scaling).

As for pH buffering capability, there is still plenty of that with 70 ppm TA. The only reason one would need higher TA levels is if one is using net acidic sources of chemicals such as Trichlor, Dichlor, or non-chlorine shock, where their acidity will lower the TA over time so you need more over time and the higher TA will help keep the pH from dropping as quickly since the higher TA outgases more carbon dioxide.