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Total Alkalinity and PH Question


Leonardo
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Hi,

I have been a hot tub owner for a few years now and try not to be too fussy about things.  I use the standard Taylor hot tub test kit and the standard Leisuretime chemicals.  I use Ahhh-some when changing the water once a quarter.  I'm running a bromine tub here.  Overall things have worked out well and I haven't experienced any problems.  It's a small tub, about 225 gallons.

I have been under the impression that total alkalinity should be in the 80-120 ppm range.  However, keeping total alkalinity in this range causes my PH to rise.  As a result, I am required to add PH down and Alkalinity Increaser on a daily basis in nearly equal amounts (but definitely more alkalinity increaser than PH down).  I have found that the PH does not creep up as much when the Alkalinity is in the range of 50-60 ppm.  I'm wondering if perhaps I am wrong about the 80-120 ppm range or perhaps the 80-120 ppm range isn't ideal for my hot smaller tub.

Two questions for the forum:

(1) Is it OK to keep total alkalinity in the 50-60 ppm range in a 220 gallon bromine tub?  

(2) Why is it that the PH increases more when the tub is maintained with a higher alkalinity in the 80-120 ppm range?

I'm assuming this is a normal phenomenon and was curious to understand it better.  I did a search and mainly found posts about aeration being a cause for this.  While aeration may be contributing to this situation (unfortunately my jet valve isn't functioning properly and a small amount of air is leaking out even on the lowest setting), I am most interested in understanding why the PH doesn't creep up as much when the total alkalinity is in the 50-60 ppm range (as opposed to the 80-120 ppm range).

Thanks!

Leo

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18 hours ago, Leonardo said:

(1) Is it OK to keep total alkalinity in the 50-60 ppm range in a 220 gallon bromine tub?  

Yes.

18 hours ago, Leonardo said:

(2) Why is it that the PH increases more when the tub is maintained with a higher alkalinity in the 80-120 ppm range?

Because alkalinity drags ph. If you need specifics you'll have to ask @waterbear.

 

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Thanks for your post. This answered my question in my own post.

Different spa stores specially the ones who offer water test suggest to keep the TA in 120-150 range. However, similar to your case, my tub's pH would drift higher in the 8.x range (even after bringing down with acid).

Since reading @waterbear's post about TA and pH, I've been intentionally keeping TA in the 60-70 range and my pH is ~7.6. Lowest I've seen it was 7.2 but after a while it goes up by itself to 7.6. I occasionally take a water sample to the store where I buy chemicals and they mention that the pH and TA is too low. Makes you uneasy...

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Many people in the industry really don't understand the actual chemistry that occurs in the water and are just repeating the same misinformation that they learned in a 'training' over and over again.

Acceptable pH range for a bromine tub is 7.0 to 8.0.

read this post for a better understanding of the actual chemistry in layman's terms:

https://www.poolspaforum.com/forum/index.php?/topic/28846-lowering-total-alkalinity-howto/

 

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On 12/18/2021 at 8:04 PM, Leonardo said:

I am most interested in understanding why the PH doesn't creep up as much when the total alkalinity is in the 50-60 ppm range (as opposed to the 80-120 ppm range).

Thanks!

The main cause of pH rise is outgassing of CO2. TA is basically a measure of bicarbonate in the water.  Bicarbonate is a buffer system (actually a bicarbonate/carbonic acid buffer. For our purposes think of carbonic acid as CO2 dissolved in water.). The higher the TA the more bicarbonate in the water. The more bicarbonate the more CO2 in the water. The more CO2 the more it will outgas and the faster the  pH will rise. Outgassing will occur whether the water is agitated by aeration or not. However the more the water is agitated (by water movement from the jets or people in the tub or by air injectors or salt water generators or ozone generators the faster the outgassing will occur.

Read both of these for a better understanding of the chemistry involved:

https://www.poolspaforum.com/forum/index.php?/topic/52522-some-truths-about-ph-and-ta/

https://www.poolspaforum.com/forum/index.php?/topic/28846-lowering-total-alkalinity-howto/

 

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