Raising Alkalinity w/o raising pH

[Walter White]

In my spa I can balance the TA, or pH, but not both.  Some pros have told me to start by bringing up my TA before balancing the pH.  Others have said the opposite.  In my experience, it doesn't matter: With TA=30 and pH=7.65, i started adding Leslie's Alkalinity Up, about an ounce at a time, until I got a TA=100 reading (a total of about two ounces).  But my pH started spiking upwards, and I ended up having to add almost 4 oz of dry acid to bring my pH back down to where I started!  At this point, my TA had dropped back down to -- you guessed it -- 30.  My experience was that Alk Up and Dry Acid ended up offsetting each other precisely.

I saw some old discussions on this in which Chem Geek said -- hopefully I'm summarizing your words right -- a byproduct of Alk Up is carbon dioxide, which can indirectly raise pH.  But, I can't find any practical advice for how to raise my TA w/o the end result of raising my pH.  I don't wanna keep dumping more and more offsetting chemicals into my spa, so for now I'm just ignoring the TA and making sure my pH is within specs.

Can anyone suggest a better plan?  Does it matter that I have an ozonator?  Should I set the spa to minimal filtration (twice a day for two hours each)?

[sorebikr]

I've had success adding Baking Soda to get TA up to 100+ and then a few hours later adding SpaDown (Sodium Bisulfate) to lower the PH. 

[Walter White]

I've had success adding Baking Soda to get TA up to 100+ and then a few hours later adding SpaDown (Sodium Bisulfate) to lower the PH. 

That's what I did too...  Alkalinity Up is Baking Soda, and Dry Acid is Sodium Bisulfate.  Question for Chem Geek: I saw on the Leslie's website that Alkalinity Up is pH 8.3.  If I just keep raising and raising the TA w/ A-Up, eventually my pH should stabilize at 8.3.  If I stop raising TA as soon as I get to 100, I find that as soon as I bring the pH back down to specs, my TA is down to 30.  But what if I just keep on adding A-Up, and I way overshoot the TA, bringing it up to 250 or something.  Then, when I start adding dry acid, I only need to bring my pH down from 8.3.  If I have good aim, the side effect of lowering TA should bring both my TA and PH into balance, right?

Will that work?

[TwinCitiesHotSpring]

just use baking soda to bump TA to the 50-70ppm range and be done with it...you'll drive yourself nuts trying to achieve the "perfect number" especially since your water is in a way "odd"...instead of adding whatever quantity to get too 100 use the numbers above and pH should not drift on you

[chem geek]

The pH of the Alkalinity Up is not relevant.  It is true that if you don't add any other acidic or basic chemicals to the water that at a TA near 100 ppm the pH won't go up above 8.5 from carbon dioxide outgassing, but that's a pretty high pH and if you have Calcium Hardness (CH) then you are at the risk of getting scaling, especially in the heater.  The equilibrium pH at which carbon dioxide outgassing stops for each TA level is shown in the table in this post where the TA is net of any CYA contribution.  So even with a net TA of 40 ppm, the pH could rise to 8.1 before stopping, but in practice it significantly slows down before then.  How fast it rises is largely a function of how much aeration you have -- running aeration jets for a long time will make the pH rise faster.

Instead of worrying about this, just focus on the very basic and simple rule.  If your pH tends to rise over time, target a lower TA, even 50 ppm if need be.  If you want additional pH buffering when the TA is low, then add 50 ppm Borates.

For a spa, the only reason to have a TA higher, such as 80 ppm or 100 ppm, is if you are using net acidic chemicals where the pH tends to drop unless you keep the TA higher.  Note that even Dichlor is net acidic when accounting for chlorine usage/consumption.  Non-chlorine shock (MPS) is also acidic.  Bromine tabs are net acidic.  But here again, if the pH is tending to rise, target a lower TA.

[Walter White]

Very interesting stuff.  Thanks Chem Geek.

Looking at your adjusted alkalinity table makes me wonder...  would you go so far as to say that in a spa with steadily rising pH (maybe because we run the jets enough to outpace the pH drop from shock and bromine), it is not just tolerable but preferable to have low TA?  My ignorant reading of your table leads me to believe if I keep the TA at 10, my otherwise endlessly rising pH will stabilize at 7.55.

Currently I have pH 7.5 and TA 30.

[chem geek]

Yes, if your pH tends to rise, then lower your TA target.  However, I wouldn't go down to 10 ppm and that's not necessary anyway since the rate of pH rise slows down significantly even if it doesn't stop completely.  You can see the degree of over-carbonation of the water which leads to carbon dioxide outgassing in this chart where the takeaway is that lower TA and higher pH have less outgassing so slower pH rise.  Technically, the outgassing varies as the square of the TA which is why a lower TA becomes more stable in spite of less pH buffering.

So target a TA of 50 ppm if that's where the pH is more stable AND target a pH of around 7.7 or 7.8 instead of trying to keep it at 7.5.  If you want additional pH buffering, use 50 ppm Borates as well.

[Walter White]

thanks for all the help, chem geek.  I got it to 7.8/50 and I'll try to keep it there.