Why the need for acid?

[Isaac-1]

Ok, this is one I have been wondering about for a while, what is it about swimming pool chemistry that tends to require the addition of acid to maintain pH. And more importantly why is there such a wide variation in the amount needed. I ask because my pool needs very little acid to maintain pH (around 1 quart per year of muriatic acid, although since it is so low I do tend to use dry acid instead for convenience) vs some pools that seem to need that much acid each day. My standard method of chlorinating is use of 6% bleach, plus very occasional use of dichlor when I also need to boost CYA (something under 5-10 pounds per year if I don't have a water leak). Any thoughts on why there is such variation, and is there anyone out there that uses no acid at all?

Ike

 

[jblizzle]

Well the pH and TA of the fill water certainly has an impact. If the TA is high, pH will rise more. If there is a lot of aeration (spa, waterfall, SWG), pH will rise more.

Posted from my Droid with Tapatalk ... sorry if my response is short

 

[JasonLion]

I didn't use any acid at all last year, and about 1 pint the year before.

PH drift varies dramatically with several factors, notably TA level, fill water TA level, and the amount of aeration the pool gets (which varies dramatically).

 

[bk406]

I have very low TA fill water, and keep my TA around 70 and have a chlorine generator. I havent had to add a drop of acid in 2 years. OTOH, I usually add 4-10 pounds of borax during the swim season to keep the pH from getting too low.

 

[linen]

I too use very little acid, just a little at the beginning of last season and I was good for the year.

I would guess as the others have said, aerating water features (waterfalls, jets, etc.), swgs, and high TA, are factors that make the the acid demand seem higher then if you only looked at pools with no swgs, low TA, no aerating features.

I sometimes wonder if "active swimmers" (I have a lot of splashy kids constantly using my precious pool ) also can cause a significant increase? Have not noticed it here though.

Do you have borates in your pool?

 

[JohnN]

I have never had to use acid. For the first 6 or so years, I used trichlor, so that would explain those years. However, last year, for the first few weeks, I used trichlor, and then switched to bleach. Even with the bleach, I had to use some Borax. I don't have any aerating features, and the pool is not very active, so maybe that explains it. In fact, I am going to need some baking soda and borax when I open.

 

[Isaac-1]

I have never used Borates, but my pool is indoors which changes a number of factors, lack of exposure to rain being one.

Ike

p.s. to add one more factor my water is from a private well

 

[linen]

Yeah, being indoors may also decrease your evaporation rate and therefore reduce the need for make up water. Many pool owners fill water (mine included) have high TA, which of course would require more acid to counteract.

I also fill from a private well and my TA is typically in the upper 200s . This encourages me to keep my pool covered when practical.

 

[HouTex]

I add about a quart every 10 days or so and having been doing so since the pool was about 3 months old and it's now over 2 years old. It seems to be getting a little better. My TA is around 70 and I have a SWG but not much aeration. My fill water is high in TA so it's hard to get my TA low enough to stop the pH from rising.

 

[anonapersona]

My fill water TA has varied from 340 to 220 in two years. So whenever I add water, I will drive the TA up. With a spillover spa and a return on the swimout that breaks the surface constantly and worse when the water level drops just a bit, and evaporation losses from the big waterfall, my pH is always climbing. I normally consume something like 3 cups of acid at a time, twice a week I guess.

 

[ComputerGuyInNOLA]

My TA is not high but I use a LOT of acid, to the tune of about 3 gallons per month. New plaster, SWCG, and waterfalls make it happen.

 

[chem geek]

Just to note that there is another option to dealing with high TA and that is to operate a "high pH" pool instead of trying to lower the pH via acid. Ben Powell wrote about this in this link at PoolSolutions. You can't do this approach if you've got metals in the water and a pool surface that can stain and you can't use this approach if your water is high in Calcium Hardness (CH). Since oftentimes high TA and high CH go together in fill water, the high pH option isn't a good one for that situation. That's why we normally don't talk about it, but for those with a high TA but low CH and no metals, it's something to consider.

Now, that said, one can at least minimize the amount of acid by targeting a somewhat higher pH and not going down as low. As shown in this chart, there's more carbon dioxide outgassing not only at higher TA, but also at lower pH.