pH creep

[msachen]

Aloha,
Crystal clear pool!!! Thanks experts. CYA 40, FC 4-6. Easy to maintain. Haven't added anything since the water replacement. No nothing, but calhypo tabs. pH has been in the 7.4-7.5 range. CH 250. TA 50. Seems low, so I add 40# baking soda (over 4 days). Now TA 80, but pH has risen steadily to this am's 7.8.

What could be causing this? I don't want it to go higher and it's been 3 days since the baking soda treatment.

Thanks,
Mike

 

[Richard320]

Sorry to tell you the baking soda is causing it. TA combined with aeration (includes swimmers splashing) makes pH rise. When you add acid to lower pH, you'll also lower the TA you just raised. The way to stop the vicious cycle is to let the TA find the level where pH holds steady for a week or so at a time, and it's different for every pool. That number, whatever it is -- and it sounds like yours was in the 50's -- is what you should target for TA.

 

[msachen]

A self inflicted wound? How can that be? I went to pool school!

My inclination is to let time heal the wound. Is there a downside to this?

Where is the, 'pH is now x.x, I need to add acid to bring it down,' line in the sand?

Thanks.

 

[Richard320]

A self inflicted wound? How can that be? I went to pool school!

My inclination is to let time heal the wound. Is there a downside to this?

Where is the, 'pH is now x.x, I need to add acid to bring it down,' line in the sand?

Thanks.

You have a public pool, so you're bound by regulations. If they specify the limits, you're stuck. Otherwise, when pH hits 7.8, lower it to 7.2. If the pH stays in range for a week or more, you've found the TA target.

You'll get the hang of what affects what and how far you want to nudge things soon enough. You chose 80 TA as the target because it was square in the middle of recommended. I would have shot for 70 because it would take less Baking Soda. But if it left some odd amount of baking soda in the bag, I'd probably just dump it in, too.

 

[chem geek]

You might be interested in seeing the short TED-talk type of presentation I gave at last year's World Aquatic Health Conference (WAHC) entitled Reduce Facilities Chemical Costs By Proper Management of Total Alkalinity (TA) and pH. As noted in the swimming pool rules for Hawaii, the pH must be from 7.2 to 7.8 and the FC must be at least 0.6 ppm. There are no rules regarding Cyanuric Acid (CYA), Calcium Hardness (CH), nor Total Alkalinity (TA).

However, with a plaster pool if you decide to go lower in TA and you target a pH near 7.8, then you might need to raise your CH to protect your plaster.

 

[msachen]

Thanks guys. The pH of the water coming out of the tap is 7.5. I want that to be my goal for logistical reasons. It's not plaster, just concrete. So the question is, add acid or let it settle back on its own over time?

Chem question: Everything I've read here says that baking soda will raise pH a little, unless the pH is really low. I don't think 7.4 is really low. Is a little raise .4? If this is an exception, what made my pool special?

Thanks again.

 

[Richard320]

Thanks guys. The pH of the water coming out of the tap is 7.5. I want that to be my goal for logistical reasons. It's not plaster, just concrete. So the question is, add acid or let it settle back on its own over time?

Chem question: Everything I've read here says that baking soda will raise pH a little, unless the pH is really low. I don't think 7.4 is really low. Is a little raise .4? If this is an exception, what made my pool special?

Thanks again.

If you plug your numbers into poolmath, you'll see that the CSI - Calcium Saturation Index - shows it to be corrosive to plaster at the low pH. If you change each parameter individually, you'll see for yourself how each affects the CSI. So... if you take pH all the way down to 7.2, something else needs to be higher, which could be CH. Or TA. Or pH. You have a limited range for pH and if you find the sweet spot for TA where the pH holds stable, that leaves you with raising CH. Play with the numbers a bit, including water temp. It's enlightening.