Lowering TA... does it do any good to aerate at 7.5 pH?

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Bubba17

Bubba17

0
Jun 30, 2008
14
Pinehurst, NC
Hi, noob here. Been reading and gathering lots of good information here. Just started using my TF100 test kit and am trying to get off the dichlor/trichlor/CYA train, or as I call it, pool crack. Drained and refilled HALF the pool over the weekend, yes, ~13,000 gal. out of a total 27,500. (The governor will be coming to see me after my water meter is read.) Here are my current readings:

FC 6.0
CC .5
TC 6.5
pH 7.5
TA 240
CH 70
CYA 50

Yes, these are after replacing half the water. Pool store had me add 34 lbs. of baking soda two weeks ago, that's why I'm here. Pool is still green and cloudy, but the algae seems to be getting under control. Any suggestions as to where I go from here? We'd like to be able to swim this weekend (July 4th) so any serious shocking might have to wait until Monday.

I know I need to get TA down. I just added 3# of dry acid yesterday, dissolved in a bucket to get pH down from 7.8. Wife was complaining of burning skin but pH was 7.5 when I checked it. Not sure if she swam too soon (like an hour later).

Can I add more dry acid so soon or will it do any good to start aerating at 7.5? Any guidelines on how much to add per day/week, and how long to wait before swimming? Thanks for all the great advice.
 
Chemgeek has a chart somewhere that I can't find that compares the out-gassing rates at different PH levels. Basically lower is better. That's why the usual recommendation of dropping PH to 7, aerating to 7.2, drop to 7, repeat until TA is where you want it. Out-gassing potential is the highest at the lower end of the PH spectrum, but we don't want to go too low to avoid pool and equipment damage.
 
The chart that shows relative carbon dioxide outgas rates is shown here. You can see the very large effect of lowering the pH. This chart is not correct in terms of the rate relative to the TA level. Experiments by Wojtowicz have shown that the outgas rate varies as the square of the TA level. That would explain why lowering the TA helps reduce the rate of pH rise by much more than implied by the chart since the lower TA doesn't buffer the pH as much.

Basically, the outgassing is more than twice as fast at a pH of 7.2 than 7.5. It's about 3-1/2 times faster at a pH of 7.0 than 7.5. Lowering the pH definitely makes the process go much faster. Note that it takes a little more acid to lower than pH from 7.2 to 7.0 as it does to lower the pH from 7.5 to 7.2 -- lower pH is more resistant to change (the pH buffer is stronger).

Richard
 
Bubba17 said:
FC 6.0
CC .5
TC 6.5
pH 7.5
TA 240
CH 70
CYA 50

I know I need to get TA down. I just added 3# of dry acid yesterday, dissolved in a bucket to get pH down from 7.8. Wife was complaining of burning skin but pH was 7.5 when I checked it. Not sure if she swam too soon (like an hour later).

Can I add more dry acid so soon or will it do any good to start aerating at 7.5? Any guidelines on how much to add per day/week, and how long to wait before swimming? Thanks for all the great advice.
Click to expand...


I started with a TA in the high 300's and was able to drop it to 80 in 3 days. But! I dropped Ph to 7.0 about 5 times and aerated for approx. 5 hrs. after each drop with a 10' waterfall. It made the process go much faster. I started with dry acid and quickly realized it was not cost effective. Also, dry acid will put other by-products in your water. I got myself comfortable with liquid MA and it's really not bad at all.

Hard to say if your wife was sensitive to the acid, or maybe the Ph was lower when she swam? I know we had trouble with our skin when the Ph and TA was high and now that it's in check, no more problems.
 
Thanks for all the replies. It's hard to tell how much acid to add since the Pool Calculator amount is so sensitive to starting pH and TA. I was getting anywhere from 5 to 14 lbs. just by tweaking the pH and TA numbers within my range of certainty. So I added all the dry acid I had, about 7 lbs. That should get the pH down around 7.0 - 7.2. I also set up the cover/sump pump to shoot water back into the pool, and the jets to do as much splashing as possible. We'll see what happens. Thanks again.
 
With the numbers you posted, I calculate that 7 pounds of dry acid would lower the pH to 7.23.

Starting with that pH, it will take cumulatively 7.3 gallons of Muriatic Acid (31.45% Hydrochloric Acid) to lower the TA to 80. Obviously not added all at once, but rather over time in aeration and acid addition cycles.

Richard
 
You calculations appear to be correct. I added the ~7 lbs. of dry acid at 11:30 last night and started aerating. This morning (7:30 am) the pH is around 7.2. Thanks for the info, now I know what to go shopping for. I don't know how many times I've been to the grocery store, Lowe's, and Wal-Mart in the past week, trying to make the changeover to BBB. Hopefully one trip to the pool store for MA and I'll be done with them! Thanks!
 
You are correct about MA at Lowes. I looked near the pool stuff and it's nowhere to be found, and the Lowes website shows that it's not carried at ANY store in central NC, but your message reminded me that I'd seen it somewhere. It's actually in the aisle with the paint strippers, furniture refinishers, brush cleaners, solvents, etc. Gallon jugs say they're used for changing pH in swimming pools so that's da stuff!

I picked one up to get started, but tonight I checked my TA and it's all the way down to 80-90 already, after only 7 lbs. of dry acid and 24 hours of aerating. pH is at 7.3 and slowly climbing. Is this possible??? Please tell me I fixed it!!! This is after vacuuming and brushing, and multiple trials with the TF100 test kit.
 

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That's certainly quicker than most folks are able to do but it sure looks to me like you're good to go.

(I live on Falls Lake, Raleigh's reservoir, and it's been overflowing for 6 weeks...they're releasing over .5 million daily back into the river. Why the blowhard politiciams here in Raleigh are still telling us we have to "cut back" is beyond my comprehension. I doubt they'll miss rhe water you used to get your pool in order :lol: )
 
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