Question about TA

[Katana]

OK...since switching to liquid chlorihe and getting rid of pucks, my PH tends to drift up and I add acid 2-4 times a week to lower PH (7.8 to 7.4). I have been letting TA come down to rec levels for using chlorine (70-90+) and it is about 80 or so (down from 110).

My issue is the two below statments from pool school:

1) 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.

2) There are two reasons to lower your total alkalinity (TA) right away, because you want to slow down the rate that the PH rises,

They seem to contradict themselves so I thought I'd ask. Statement 1 says a lower TA will make my PH swings worse/wild. Statement 2 says lower TA makes PH rate/swings slower?

I'm looking for my PH "drift up" from 7.4 to 7.8 to take longer. So do I need a higher TA (rasie with baking soda) or a lower TA...let it drift down to find happy spot?

Thanks

 

[Bama Rambler]

Statement 1 could be a little more clear. You only run into the pH swing issue if the TA is very low, usually about 40 ppm or less.

People tend to put way too much emphasis on TA. Unless your TA is extremely high or low you shouldn't worry about specifically adjusting it. Just adjust the pH as needed and the TA will find a happy place over time. The exception to that is people that use pucks need to look at it from time to time because pucks are acidic and keep driving the TA (and consequently the pH) down constantly.

Basically, just keep adjusting the pH down whenever it needs it and the TA will reach a point where the pH gets pretty stable. You should notice the pH adjustments getting less and less over time.

 

[JasonLion]

The trick here is that there are two completely unrelated effects from the same chemical. TA helps buffer PH, reducing PH changes. High TA can also, completely separately, can cause the PH to increase. The goal is to find a TA level that balances out those two effects, so the PH neither fluctuates wildly nor is constantly going up.

If your PH is consistently going up, you want to lower TA. If PH wanders around, both up and down, then you want to raise TA.

 

[Katana]

Thanks for the quick responses...

I will let it drift down and see where it settles...

 

[elik]

Talking about the TA buffering abilty, in what pH+- would the TA buffer the water?

 

[chem geek]

TA buffers the water over a reasonably wide range of pH typically found in pools, but it buffers more strongly at lower pH peaking in buffer strength at a pH of 6.3 which is why it takes more acid to lower the pH as the pH gets lower. For example, with a TA of 80 ppm and no CYA or borates or chlorine in the water (just to keep things simple by removing any other buffers), the amount of full-strength Muriatic Acid (31.45% Hydrochloric Acid) it takes to lower the pH by 0.2 units in 10,000 gallons is shown below:

8.0 to 7.8 takes 3.7 fluid ounces
7.8 to 7.6 takes 4.4 fluid ounces
7.6 to 7.4 takes 5.9 fluid ounces
7.4 to 7.2 takes 8.4 fluid ounces
7.2 to 7.0 takes 12.2 fluid ounces
7.0 to 6.8 takes 17.5 fluid ounces

So TA is good at helping to prevent the pH from crashing downwards when using acidic sources of chlorine such as Trichlor tabs/pucks. When using hypochlorite sources of chlorine, the TA does not need to be nearly as high, though one still needs some for other reasons such as to protect plaster surfaces by saturating the water with calcium carbonate (the carbonate comes from the TA).

As Jason noted, there are TWO distinct effects from TA, specifically from the carbonate alkalinity since TA also includes alkalinity from other sources such as from CYA and borates. TA is mostly measuring the amount of bicarbonate in the water and there is also dissolved (aqueous) carbon dioxide as a result. Pool water is intentionally over-carbonated so as a result the carbon dioxide outgasses into the air. When this happens, the pH rises. As shown in this chart, the water is more over-carbonated (in terms of having more carbon dioxide in the water) when the TA is higher and the pH is lower. For technical reasons, the rate of carbon dioxide outgassing actually varies as the square of the TA (this is not reflected in the chart) so lowering the TA level can help lower the rate of pH rise substantially.

As I mentioned, there are other pH buffers in the water. CYA also buffers the water and its buffering is strongest at a pH of 6.8. Borates (boric acid) also buffers the water and is strongest at a pH of 9.1 so buffers more strongly as the pH rises. If I were to take the same water example I had above but add 50 ppm Borates to the water, then the amount of acid would be the following:

8.0 to 7.8 takes 17.4 fluid ounces
7.8 to 7.6 takes 13.3 fluid ounces
7.6 to 7.4 takes 11.5 fluid ounces
7.4 to 7.2 takes 11.9 fluid ounces
7.2 to 7.0 takes 14.2 fluid ounces
7.0 to 6.8 takes 18.6 fluid ounces

This post shows the buffer strength of the different pH buffers in the pool at varying pH. Note that except for the carbonate buffer, the other buffers don't change the amount of acid needed over time if the pool is rising in pH -- they just reduce the frequency of which you need to add that acid. In other words, it takes more acid to move the pH back down, but you do this less frequently. For the carbonate part of TA, it is a SOURCE of rising pH so lowering it by removing carbon dioxide from the water helps to reduce the rate of pH rise from such outgassing AND reduces the total amount of acid needed over time. One can accelerate removal of the carbon dioxide by lowering the pH, aerating the water, and adding acid to keep the pH low. This is described in the Pool School article on how to Lower Total Alkalinity.