TA-how low can you go?

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palmspriings

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Feb 18, 2013
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Living in Palm Springs, I continue to battle the ever-rising pH. I have started using tri-chloro pucks and keeping the TA at 60 or less. But it still climbs .2+ daily.

I recently brought the TA down under 40...today it tested at 30, yet the pH rose .3, the low TA isn't stopping the pH from increasing. When I imput the numbers onto the calculator, it shows the CSI to be in the safe zone, yet 30 seems very low to me. And the pH still climbs.

Any advice? I'm kinda at my wits end here......Thanks!
 
It sounds like there is something else causing the pH to rise other than carbon dioxide outgassing. If it is rising even using Trichlor pucks with the TA low, then something in or added to the pool is causing a pH rise. If for some reason the plaster were dissolving, then the calcium carbonate would cause the pH to rise. By any chance are you noticing a rise in Calcium Hardness (CH)? In Palm Springs you probably have a lot of evaporation and refill (unless you have a pool cover) so the CH will increase from whatever is in the fill water.

Also, with carbon dioxide outgassing, the pH rise will slow down as the pH gets higher, but your see it rise to 8.1 and beyond. This again points to something else causing the pH rise.

You did an acid wash in February so it's possible that leftover plaster dust could have the pH rise, but I would have expected all of that to dissolve a while ago. Or maybe you exposed lower levels of plaster with copious amounts of calcium hydroxide so that your plaster is "curing" though again I'd expect that rate to slow down over time. If this is the case, then you'd want your water to have a higher TA as with the bicarbonate start-up procedure and the pH not dropped as low as you have been doing.

I don't think you need to go below a TA of 50 ppm for any reason. You don't have water aeration features and it appears that your pH rise is from some other source.

If you have plaster dissolving adding more calcium carbonate to the water, then the lower TA would actually have the pH rise more quickly. With your current numbers with a TA of 40 ppm, then in 7300 gallons dissolving 4 ppm (1/4 pound) of calcium carbonate would cause a 0.3 rise in pH. If this were calcium hydroxide instead, then it would take half as much (1/8 pound) to cause a 0.3 rise in pH. However, I'd expect such dissolving to stop when the water is saturated.

Why don't you bring the TA up closer to 50 ppm or a little higher (not above 70 ppm) and see what happens with the pH when it's around 7.8 or so. See if the pH goes above 8 and how much and if it slows down at all at that point. It does seem like your acid wash has caused a situation similar to new pool plaster curing. That's my best guess at this point.
 
Thanks for the in-depth response chem-geek

The CH is always on the rise due to our hard water and evaporation. Out of the tap, the CH is 280, I am already sitting at close to 500. Before using the tri-chloro pucks, the pH was rising at .3-.4 per day, so it has slowed it somewhat, .2-.3, but not a huge change.

I’ve experimented several different ways. Held the TA at 70 for several weeks and the pH rose. Took it down to 50-60 for nearly a month and the pH rose. In both cases, I would allow the pH to rise until it hit the high scale of 8.2, to see if it would stop, but it didn’t appear to. It consistently rose .2- .4 daily.

I hope you are on to something by connecting it to the recent acid wash. The calcium scaling was so bad that they had to change the diamond blades several times and some of the scaling remains. Could it be possible that the pH rise is due to some of this left over scaling slowly dissolving? If so, I would expect it to rise faster with the lower TA. I don’t know, but I feel as if I am walking a tight rope with this pool. The ever increasing CH and pH is what caused the need for the extensive acid wash/polishing in the first place. I’m doing my best to avoid scaling but don’t know what else to do at this point.
 
What are "diamond blades"? Are you referring to a saltwater chlorine generator?

You are right that if scale is dissolving then that would have the pH rise along with CH. And yes, a lower saturation index would make that happen more quickly.

You could use 50 ppm Borates as an additional pH buffer. That would cut the rate of pH rise down but you'd still need to use about the same amount of acid over time -- just adding a larger dose less frequently.
 
The diamond blades were used to sand the scaling off the plaster. Was tough to get off.
I'm not seeing any "plaster dust" from the acid wash so maybe it's the residual scaling.

Maybe it's time to look into Borates, thanks again!
 
Turns out my TA was never 30, Dolt! I realized today that I have had a brain fart for the past few days and was using a water sample size of 10mL rather than 25mL. Sorry! So my TA has been roughly 50-60 instead of the reported 30. That makes me feel better. However the increasing pH issue remains the same.

Jason, here are todays numbers

CH 525
TA 60
pH 8.2
TC 4.5
CC 0.5
FC 4
CYA 35

Beginning 5 days prior, I took the pH down to 7.1, as the TA was testing at 70. Here is the daily rise:
7.3
7.5
7.7
8
8.2

Again, any insight would be greatly appreciated!
 
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