Question about TA and pH

[AndrewFH]

The most recent tests with my swim spa shows the TA at 70 and pH at 7.2
I would like the pH a little higher (around 7.5-6) and have used borax to do so. It does raise the pH up to the level I am looking for, but then it drops back down (within a day or so) to 7.2
I understand that TA is a buffer and helps keep the pH steady, but I guess I am not totally understanding of the relationship between the two.
I had kind of assumed that the buffering effect of TA would work to keep the pH at whatever level I adjusted it to, but that does not seem to be how it works.
Is there a kind of "sweet spot" where with a certain TA, the pH will always move towards that pH? In other words, in my spa with a TA of 70, the pH is always going to try to level out at 7.2? Even if I raise the pH to 7.5, it will move back to 7.2?
If that is true, then I assume that if I want the pH to level out at something higher (7.5), then I need to raise the TA until I find the level of TA that has a "sweet spot" of 7.5?

 

[Bama Rambler]

How are you chlorinating the spa?

Raising the TA may help, but if you're using an acidic form of chlorine it will be a struggle.

 

[AndrewFH]

I am using liquid bleach, which as I understand it is essentially pH neutral.

 

[Smykowski]

I am using liquid bleach, which as I understand it is essentially pH neutral.

This is correct.

As for your statements above you're basically correct. TA acts as a buffer in that it will prevent wild pH swings and keep it more stable. If TA is too high, pH will tend to rise. If TA is too low, pH will swing around. If TA is nonexistent, pH can crash. I'm sure ChemGeek will be along shortly to expand on this.

The general consensus is that every pool seems to have a sweet spot where pH will settle, given small gradual changes in pH and TA. If your pH is drifting downward, you could try to raise the TA a little bit. How much aeration do you have?

 

[AndrewFH]

It is a swim spa so there is a good bit of aeration. Whenever we are in it, the jets are on. However I have tested both before and after running the jets with plenty of aeration and see little if any rise in pH.
I added some borax yesterday and got the pH up to 7.6. It was still at 7.6 today. I will continue to monitor and see what it does.

 

[chem geek]

If your pH is fairly stable and at a level that you want, then your TA is in a good place. It is strange that your pH keeps dropping when using bleach. It should rise a little when you add the bleach and then drop back roughly to where you started. Something is adding acid to your spa, but I don't know what it is if you aren't adding any other chemicals that would be acidic, such as MPS or Dichlor. If your pH continues to behave this way you can try a higher TA level to see if the outgassing of carbon dioxide helps to counteract whatever is causing the pH drop.