Cannot Seem to drop pH

[Duk]

I have just replaced approx. 50% of my pool water. I made the mistake of taking my water to be tested at a local pool store where the the numbers are very differnt from my test kit (PS 2333). I thought my CYA was in excess of 100 and CH was running a little high @ 500. The pool has started showing a white scale at the waterline. The pool is a 23,000 gal, in-ground gunite pool w/ spa, DE filter and water falls. I have been trying to lower the pH by addition of 4 cups MA twice a day, but the pH just hangs in at 7.6 for the last 2 days after the addition of a little over a half gallon of acid. Maybe I am being a little too impatient but I was expecting some down movement. My test numbers were as follows:
FC-9
CC-0
CH-230
TA-80
CYA-40-45
pH-7.6
T- 75 deg. F
Water has good clarity
Bleach used for Chlorination

Is there a reason why my pH would be buffered to where it would not be going down w/ the addition of acid. I do run the water falls for some time to get aeration, but is my pH low enough to be gassing off much CO2?? I am trying to lower my pH down to 7.0- 7.1 to see if it will help soften up some of the scale line for easier removal. Any ideas?
Thanks,
Duk

 

[duraleigh]

Duk,

I'm not a guru in this area by any stretch, but my guess is your aeration is causing a little pH rise offsetting the acid addition. The somewhat lower Alk would tend to go along with that thinking. One of the smart folks will be along soon to give you better input but that makes sense based on what I have gleaned over the years.

BTW, 75 degrees??? Are you sure you don't really live in Alaska and simply wishing you lived in Texas?? :lol: :lol: :lol:

 

[Duk]

Duk,
BTW, 75 degrees??? Are you sure you don't really live in Alaska and simply wishing you lived in Texas?? :lol: :lol: :lol:

It won't stay at 75 deg long. That is the result of all the rain we have been receiving for the last 3 weeks plus swapping out to new water from the tap. :-D I wish that water temp would stay around for the summer.
Duk

 

[chem geek]

Duk,

Virtually every pool (unless it's covered) is outgassing carbon dioxide to the air. The question is how much. This chart shows how far out of equilibrium a pool's carbonates are with carbon dioxide in the air at various pH and TA levels. It shows that your pool has about 5.3+1 = 6.3 times as much carbon dioxide dissolved in the water as would normally be found if in equilibrium with the air (the chart is for a CYA of 30, but that's just to adjust the TA so your 40-45 CYA isn't that far off). Anyway, aeration from waterfalls can be a lot and rain provides even more aeration so it is quite possible for that to cause the pH to rise.

Though lowering the TA helps significantly reduce the amount of acid that needs to be added over time since that's proportional the the above chart (the actual amount depends on aeration), this chart shows that the relative change in pH with lower TA doesn't change nearly as dramatically because though lower TA has less outgassing, it also less pH buffering.

If you used 30-50 ppm Borates in the pool, then that would provide an additional pH buffer that would not increase carbon dioxide outgassing. This would reduce the rate of rise in pH, but would not change the amount of acid you needed to add over time (i.e. you would just need to add it less frequently).

Richard

 

[Duk]

Thanks Richard! I will try the Borax mix once I get my pH down around 7.1. I calculate that this would require about 23.5#'s of Borax to attain 50 ppm in a 23,000 gal. pool. Am I close?
Thanks for the advice,
Larry

 

[Guest]

To raise a 23k pool to 50 ppm borates would require 86.25 lbs of sodium tetraborate decahydrate (20 mule team borax) or just slightly more than 18 boxes. It will also require approx 5 and 1/3 gallons of muriatic acid to neutalize the pH rise from the borax and create the boric acid/borate buffer system in the water.
If you used BleachCalc to calculate the amount of borax needed (and from you numbers it looks like you did) then be aware that there is an error in the Borax calc that has been documented.

 

[Duk]

If you used BleachCalc to calculate the amount of borax needed (and from you numbers it looks like you did) then be aware that there is an error in the Borax calc that has been documented.

Thanks Waterbear,
You are correct, I did use Bleachcalc to calculate. I was not aware of any problems w/ the program. I'm still learning about the use of borates. Thanks for the catch and the correct numbers. I've got to do some searches on this subject so I know what I'm getting into with the use of Borax.
Duk

 

[Guest]

With a TA of 90 ppm and a pH of 7.6 you should have a very stable pool in terms of pH and minimal acid usage. why are you trying to lower the pH? I would leave it where it is and when the pH climbs to 7.8 drop it back down to 7.6. IF you add the borates your pH will become even more stable. If you want to know what borates wil do just read the advertising materials for Proteam Supreme, Bioguard Optimzer, or any of the other sodium tetraborate pool water 'enhancers' on the market. This is one product that actually lives up to the claims! If you want to read a detailed experiment on using 20 mule team to achieve the same results check out this thread..