Hello and Alkalinity

[chemistrydropout]

Although I'm a new face to TFP, I have been in the BBB business of pool ownership since I had our pool built in June of 2006. I see a lot of familar faces/signatures here that helped me through the "UH-OH I have a new pool and no clue what to do" when the water went in and the pool builder said to throw away the test kit that came with the pool and just keep pucks in the skimmers. I would like to thank all you guys for the help.
Now for the alkalinity question I have. I was given advice last year to bring my alkalinity up to range. I was having such a trouble free time last year that I never tried to address a slightly low alkalinity. I decided this year to get everything in place at the beginning of the season and attempted to raise my alkalinity to 80. I have seen it explained how alkalinity and ph work together but I don't quite understand the chemistry behind it. My alk at 80 gives me a constant/persistent ph rise that requires acid to lower at least once a week. If I let alkalinity stay in the 60-70 range my ph will be consistent at 7.5. Why does this pool like to be in the 60-70 alkalinity range and will it hurt anything to run it that low?

20x40 vinyl liner, sand filter, No SWG, No heater, and approximately 25000 gallons

TC= 6
ph= 7.5
alk= 70
cya= 35
ch= approximately 70
temp = 80

 

[JasonLion]

Do you have a water feature, such as a waterfall, fountain, negative edge, or other source of aeration? A fairly high level of aeration could cause the effect you are seeing. When you aerate CO2 comes out of solution which raises the PH. This happens more quickly the higher the TA. At a low enough TA it will essentially stop and PH will be stable.

 

[duraleigh]

Chemdropout,

I recognize your username as well. Welcome to the forum. Your question is above my pay grade but I am very interested in the answer.

Chemgeek checks in frequently, as does Waterbear, and many others smarter than me. I look forward to gaining some knowledge from this post, too.

 

[The Mermaid Queen]

Welcome to TFP!

 

[medvampire]

chemistrydropout
If you have a stable pH with a lower ALK I would just leave it alone... ALK and pH are tied to each other with a rubber band effect... When you find the best ALK the pH will be more stable...I dont see running a lower ALK would cause any problems if your pH is stable.
Steve

 

[Guest]

If your pH is stable at a lower TA keep it there! I won't go into the reasons for that (chemgeek can do that if he likes) but running a low TA often improves pH stability when using unstabilized chlorine!

 

[chemistrydropout]

Alkalinity

I have no water features or such. I do have thunderstorms on occasion that bring in debris, here in Alabama this year it has been on very rare occasions lately with the drought, and I try to keep my returns turned to barely ripple the water so it will push any debris to the skimmers. I have turned them down as an experiment to see if it affects the ph/alk and I cannot tell any difference. I do know that if I want to fight it I can run my alkalinity up to 80 or 90 and pour acid every 3-5 days or so till the alk settles back in at around 70. I think Chemgeek said in some of his posts answering other questions on this topic that the CH has an effect on this. Something to do with Saturation Index or something to that effect. My Ch is somewhere between 50 and 100 so that's where I got the number of 70.

 

[JasonLion]

As waterbear said, there is nothing wrong with having ALK in the 60-70 range. And Ben always said that pools tend to have an ALK and PH that they try to return to and it is almost always better to just let them be there. But even though I am reasonably familiar with chemgeek's ALK and PH analysis I don't understand what is happening here.

 

[chem geek]

Theoretically, the difference in outgassing rates between the pH 7.5, CYA 35 with TA of 80 vs. 60 should be the difference between 6.8 and 4.5 which is noticeable, but is still surprising that it's as strong as being seen. It's also a bit surprising that without the water features you have lots of apparent aeration, but if your thunderstorms or any other rain is frequent enough, then that provides a LOT of aeration (think splashing drops). Anyway, low TA is no problem so long as calcium carbonate is balanced in pools that require such balance (i.e. plaster/gunite/grout), but for vinyl pools where the calcium isn't needed anyway, low TA is no problem at all.

 

[chemistrydropout]

Would an increase in water temp have anything to do with it? When I was trying to keep the alk up the water temp was in the lower 70's and now it is in the lower 80's. Also we have been having non-existent rainfall for the last 3 months. The other side to that is now with water temps in the low 80's we have been having a swimmer load that was not there in the 70's so that could be contributing to outgassing. Kids, diving board, cannon-balls and such!!
It doesn't really matter except that it would be nice to know what is going on for future reference.

 

[chem geek]

The rain probably contributes to more aeration than your kids splashing. Remember that aeration has more to do with surface area than anything else so lots of tiny drops splashing over your pool's entire surface over a continuous and extended period of time adds up to a whole lot more than some larger splashing that only hits a relatively small area of the pool. That's my best guess.