Normal acid usage

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stev32k

stev32k

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Mar 29, 2009
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Mobile, Alabama
My pool has been requiring 1 liter of HCL every 5 days to maintain the pH between 7.4 - 7.8. Since I don't have anything to compare to I'm wondering if this addition rate is high, low, or normal - or maybe it varies with each individual pool. I keep the TA at 100, FC at 4 - 6 and the CYA is 40. My chlorine consumption averages about 1.0 - 2.0 ppm/day depending on the hours of sunlight. There is a small leak in the pool around the overflow and I have to add water about once every two weeks. The pool volume is approx. 32,500 gal.
 
TA does two different, somewhat contradictory things. The well know thing is that TA helps resists PH changes. The higher the TA, the more of a chemical needs to be added to change the PH by a given amount. But TA also causes a second effect. TA puts upward pressure on the PH. The higher the TA and the more aeration there is the faster the PH will go up.
 
JasonLion said:
TA does two different, somewhat contradictory things. The well know thing is that TA helps resists PH changes. The higher the TA, the more of a chemical needs to be added to change the PH by a given amount. But TA also causes a second effect. TA puts upward pressure on the PH. The higher the TA and the more aeration there is the faster the PH will go up.
Click to expand...

I have always thought that the higher TA level would result in lower acid (and caustic) usage because of the buffering effect. I remember the titration curves from chemistry and once they hit equilibrium it requires a lot to change the pH.
 

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The bicarbonate portion of TA (which is usually the largest portion) is a SOURCE of rising pH. Pools are intentionally over-carbonated both to provide pH buffering and to provide carbonate to protect plaster surfaces. So think about a carbonated beverage and how it goes flat faster when there is more aeration and how it outgasses faster at first when there is more carbonation and then slows down. That is essentially like a pool except it's not so carbonated that you see the bubbles. When the carbon dioxide leaves the water, it makes the pH rise. When a carbonated beverage is created by injecting carbon dioxide under pressure, it makes the pH drop which is why carbonated beverages are so acidic, at least until they go flat.

There are other pH buffers that can be used that don't have this carbon dioxide outgassing effect. The use of 50 ppm Borates in the pool is an alternative pH buffer and has some other benefits as well, but it's optional.
 
bobodaclown said:
Also isn't it easier for PH to rise from 7.2 to 7.4 then 7.4 to 7.6 and so on. I thought it's logarithmic. So it might be easier to run at the higher end. Less, but more often adding of MA to stay in numbers.
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The pH buffering from the bicarbonate is stronger at lower pH. So it takes more acid to go from 7.4 to 7.2 than from 7.6 to 7.4. On the other hand, when it comes to pH rise from carbon dioxide outgassing, it occurs faster at lower pH and higher TA as shown in this post that shows how over-carbonated a pool is at various pH and TA levels. The actual outgassing rate is apparently proportional to the square of the TA and that effect is not shown in the table which is closer to linear with TA. So you are right that you add less acid if the pH target is higher, but I wouldn't say that it is more often and in fact is usually less frequent. The outgassing effect outweighs the pH buffering the effect unless the TA is very low.
 
Just an update on this older thread. On May 25 I added 20 lb of sodium bicarbonate to raise the TA from 70 to 100 and have not added any since then. The TA drifted down to 80 in 16 days and during that time I added 3 liters of 31% MA to maintain a pH of 7.4 -7.5. In three more days the TA drifted down to 70 ppm with no acid additions. It then took 15 days to decrease to 60 and I added one liter of acid. On July 18 (15 days later) the TA was 50 and 1/2 qt of acid was added. I ran tests this morning and the results are:

FC 5.5
CC 0
TC 5.5
pH 7.4
TA 50
CYA 40

The TA seems to have stabilized at 50 ppm, and the pH at 7.4. The last time I added acid was on July 14 and that was 1/2 qt. The pH has been very stable since then at 7.4. I spent 6 days days shocking the pool with 64 gallons of chlorine and when it was over the pH was still 7.5 and the TA 50. That is a lot different than adding 1 liter of acid every 5 days to keep the pH down.

Does anyone think there will be a problem leaving the TA at 50 ppm?
 
First, maybe this is better addressed in The Deep End, for it is surely over my head.

I've got a question here (very bad grades in chemistry, so bear with me please).

I read that at lower TA, pH is less stable. Does that mean that if something occurs to disturb the pH, higher OR lower, it will move rapidly? Just wondering because in aquariums, there are processes going on that acidify the water and with more RO water in a fish tank you were more likely to have pH to crash. Now my question is, are there similar processes going on in a pool? In the fish tank that was the processing of waste by bacteria. Now in the pool, you hope to have all waste processing going on via chlorine, with no bacteria in the pool. But what is going on in a pool? Are there processes going on that would push the pH either lower OR higher?

I'm just wondering, with low TA, would you be just as likely to one day find it super low as you are another day to find it super high? Is it possible to be managing with a low TA, and a normal pH of whatever, then one day it is 8.2 for no apparent reason? I mean, the first thing one might do would be to grab some muriatic acid to deal with the pH but if that happened due to low and destabilizing TA, that might be the wrong answer. If the pH was super low, the first response would be correct, but could it go the other way?
 
If you read some of the earlier posts in this thread you will see that outgassing of CO2 is one cause (among others) of pH rise. As the TA rises the amount of CO3 rises and that can lead to more C02 outgassing. So there appears to be some optimal TA that reduces the tendency for outgassing which results in a more stable pH. There are other factors - splashing, return jets breaking the surface, and intentional aeration seem to be important as well.
 
anonapersona said:
I read that at lower TA, pH is less stable. Does that mean that if something occurs to disturb the pH, higher OR lower, it will move rapidly?
Click to expand...
That was my understanding too.

You can raise TA without raising pH using baking soda (that's the middle B of BBB) - although I'm told if you add it too quickly, then it converts from bicarbonte to carbonate and raises pH as well.
 
TA has TWO effects. One is as a pH buffer which will make pH more stable from OUTSIDE sources that would otherwise change the pH, such as acids or bases or net acidic sources of chlorine (including Dichlor and Trichlor). However, TA is also a SOURCE of rising pH in its own right due to increasing the amount of carbon dioxide in the water that then outgasses and raises the pH. So "pH stability" has two meanings here. If the TA is higher, then the pH can rise over time due to carbon dioxide outgassing so that makes the pH seem less stable, but the higher TA means that it takes more acid or base to move the pH.

The bottom line is that even 50 ppm TA is plenty of pH buffering if one is using hypochlorite sources of chlorine. The reason for the higher TA recommendations is primarily because of the acidic sources of chlorine, especially from Trichlor, where it is disastrous for the pH to "crash" which it will if you don't maintain the TA since the acidity from the chlorine lowers the TA over time (as well as the pH).

Note also that TA provides carbonate which along with calcium protects plaster surfaces. So if you have a plaster pool and operate with a lower TA for a more stable pH when using hypochorite sources of chlorine, then you need to raise your Calcium Hardness (CH) level to compensate and probably your pH target to around 7.7 as well -- basically, get the saturation index up closer to 0.
 

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