Frequency of MA Acid Addition

[Naplespool]

Saltwater, plaster pool, pentair IC40. TA 60 ppm, Boric 50 ppm. Using about 3, maybe 4, quarts of acid a week. About 1.5 quarts drops PH from 8 to 7.7. Adding all 3 qts at once drops PH to 7.1, with a CSI of -0.77.

i will be gone for several months. Can I have the pool guy add all the acid at once or should I have him come twice a week?

ch will be 350-450
ta will be 60-80
cya will be 60-70, so they don't overload it

 

[ping]

I would target the pH at 7.5 for acid additions and see if you can go a week between additions.

 

[Jimrahbe]

N,

I'm surprised why with a TA of 60 you have to add that much acid.

I suspect it is because you are trying to keep your pH at 7.2 or you are running some water feature all the time.

Most saltwater pools like the pH at about 7.8. My pool will stay at 7.8 for weeks at a time, but will only stay at 7.2 for about 30 seconds...

If you have a spillover spa, and it runs all the time, or you have other water features, they cause aeration which makes pH increase faster.

Thanks for posting

Jim R

 

[duraleigh]

How old is that pool? That is a TON of acid for an established pool.

Can I have the pool guy add all the acid at once or should I have him come twice a week?

Now, I have to be blunt, if you are using a gallon of acid a week and you will be gone "several months" , that would be an addition of maybe somewhere about 20 gallons of acid. That is not really a serious question, is it?

 

[Naplespool]

Very serious. And only trying to hold 7.8 PH.

The pool is 4 months old. I bleed a tiny amount of water into the spa to keep it full and it spills 1.5" into the pool. Current chemistry:

FC 7
CC 0
PH 7.9 (then I added 1.5 qts because I would be gone for a couple days)
TA 60
CH 400
CYA 70
Borate 50

Pool Math says 1.5 qts drops ph from 7.9 to ~7.6

naples water ph is 8, ch is 50, TA is 40. Evaporation is only 1/8" to 1/4" per day.

swg ic40 is running 60%

help!

 

[pooldv]

I would lower TA to 50 using this process, Pool School - Lower Total Alkalinity. Then wait for pH to rise to 7.9-8.0 and lower it to 7.6 and see how long it takes to rise back above 7.8. With TA at 50 and borates at 50 and not much aeration it should last more than a week.

 

[Naplespool]

I'll give 50 a try, what do you think of the CSI at ta =50? It will run at -0.6.

There is one other source of aeration, my wife swims laps for 30 minutes every other day.

 

[Patrick_B]

You can't put multiple gallons of Acid in a pool at one time like that. If you are worried about -0.6 CSI with the small reduction in pH you are talking about, just imagine what 20 gallons of acid would do at once. That is why Dave asked if you were asking a serious question, because it is far more important to understand that than CSI.

You'll be fine around those levels of pH and TA, just don't keep your pH too low too often.

 

[duraleigh]

Can I have the pool guy add all the acid at once

Doing a rough calculation, that would yield a pH down around 4, and your csi not meaningful...........that's why I asked if it was a serious question. So, the answer is "No". Please stretch out your acid additions to keep the pH in the 7's range.

As Patrick says above, understanding the consequences of a monumental pH drop like that is far more important than the tweaks (almost always unnecessary) that csi will provide you.

 

[pooldv]

CSI at -0.6 and rising is ok, especially if it works with your pool maintenance schedule. If acid additions can be be more frequent then TA at 60 might be better. Ideal CSI is -0.3 to 0. Definitely don't want to stop the aeration from swimming!

 

[Naplespool]

Doing a rough calculation, that would yield a pH down around 4, and your csi not meaningful...........that's why I asked if it was a serious question. So, the answer is "No". Please stretch out your acid additions to keep the pH in the 7's range.

As Patrick says above, understanding the consequences of a monumental pH drop like that is far more important than the tweaks (almost always unnecessary) that csi will provide you.

I see the misunderstanding. I wasn't asking about putting 6 months of acid in at once. The "all at once" I was asking about was the weekly dose. I was asking can I put 3 qts in at once versus two applications of 1.5 qts. Issue is that my pool guy will only come once a week while I was gone. If I were here it would go in in two doses, days apart.

- - - Updated - - -

CSI at -0.6 and rising is ok, especially if it works with your pool maintenance schedule. If acid additions can be be more frequent then TA at 60 might be better. Ideal CSI is -0.3 to 0. Definitely don't want to stop the aeration from swimming!

i get a great CSI at 80 ppm TA, with PH varying from 7.6 to 8.0 but I might have to dose 3 times per week. I've got till the end of April to figure it out.

 

[JoyfulNoise]

There are two problems I see here that are working against you, one I'm familiar with and one would require the advice of other experts.

First off, why on earth did you borate your pool water so soon after build? By adding 50ppm to your pool water, you have effectively made it A LOT more difficult to adjust pH. If you go to PoolMath and redo your calculations with zero borates, you'll find that it would take only a little over a 1-1/4 cup acid to change the pH by the same amount you are trying to now. I'm not sure if you were advised to add borates so soon after plastering or if you just decided to do it on your own, but that was not something I would have ever advised doing on a new pool.

In conjunction with my first comment, your pool's plaster is only 4 months old. I am not a plaster expert but, if the plaster startup process was not properly performed (and many plaster applicators nowadays do a poor job starting up plaster), then your surface is likely the cause of your pH rise because it is emitting calcium hydroxide (lime) into the pool water. Lime is incredibly caustic and causes large increases in pH. Normally, one would start off plaster using a bicarbonate or traditional startup process where the water is maintained at high alkalinity and pH in order to speed up the formation of calcium carbonate in the plaster matrix. Calcium carbonate acts as a barrier against the emission of plaster components into the water. Usually one spends the first 30 days or so doing the startup process and brushing & vacuuming the plaster dust until it subsides. Then, once the plaster surface is cured and a good carbonate structure is formed on the surface, the plaster is much more inert and has a less dramatic effect on water chemistry. As I said, many plaster applicators do a poor job in this area and they leave it to the pool owner to fight with the pH in the early months of the pool's life. If the plaster was not started correctly, then keeping the TA low is also slowing down the curing of the plaster surface because the carbonate alkalinity in the water is where the plaster gets it's carbonate from in order to cure. As I said, I'm no plaster expert and the startup details matter a lot, but fresh plaster surfaces can take a long time to settle down.

These two conditions (high borates and new plaster) are working against you and you'll be adding a lot of acid for the foreseeable future until the plaster surface cures. If it's not too costly, I would almost advise you to drain the pool and refill it with water that has no borates in it.

 

[elwood58]

Our 14,100 gallon salt water pool, with a four year old stonescapes finish uses 1.5 cups every other day to maintain a pH of 7.5. This tends to slow in winter, and increase a little with a heavy swim load. So in total we use about 16 gallons per year. We are currently building a new pool with the IntellipH System, which has a four gallon tank. Once cured, I am assuming a once per quarter fill up.

 

[pooldv]

There is no reason to maintain PH at 7.5, 7.8 is much easier to maintain in a pool with rising pH.

 

[Naplespool]

There are two problems I see here that are working against you, one I'm familiar with and one would require the advice of other experts.

First off, why on earth did you borate your pool water so soon after build? By adding 50ppm to your pool water, you have effectively made it A LOT more difficult to adjust pH. If you go to PoolMath and redo your calculations with zero borates, you'll find that it would take only a little over a 1-1/4 cup acid to change the pH by the same amount you are trying to now. I'm not sure if you were advised to add borates so soon after plastering or if you just decided to do it on your own, but that was not something I would have ever advised doing on a new pool.

In conjunction with my first comment, your pool's plaster is only 4 months old. I am not a plaster expert but, if the plaster startup process was not properly performed (and many plaster applicators nowadays do a poor job starting up plaster), then your surface is likely the cause of your pH rise because it is emitting calcium hydroxide (lime) into the pool water. Lime is incredibly caustic and causes large increases in pH. Normally, one would start off plaster using a bicarbonate or traditional startup process where the water is maintained at high alkalinity and pH in order to speed up the formation of calcium carbonate in the plaster matrix. Calcium carbonate acts as a barrier against the emission of plaster components into the water. Usually one spends the first 30 days or so doing the startup process and brushing & vacuuming the plaster dust until it subsides. Then, once the plaster surface is cured and a good carbonate structure is formed on the surface, the plaster is much more inert and has a less dramatic effect on water chemistry. As I said, many plaster applicators do a poor job in this area and they leave it to the pool owner to fight with the pH in the early months of the pool's life. If the plaster was not started correctly, then keeping the TA low is also slowing down the curing of the plaster surface because the carbonate alkalinity in the water is where the plaster gets it's carbonate from in order to cure. As I said, I'm no plaster expert and the startup details matter a lot, but fresh plaster surfaces can take a long time to settle down.

These two conditions (high borates and new plaster) are working against you and you'll be adding a lot of acid for the foreseeable future until the plaster surface cures. If it's not too costly, I would almost advise you to drain the pool and refill it with water that has no borates in it.

Thanks for for your comments. I'm really not having a hard time adjusting PH. I added the Boric Acid after reading in detail here at the throublefreepool that Boric Acid serves as a ph buffer against the rise in PH. My pool was fairly stable but to keep PH in the 8-7.5 range I was adding a small amount of acid almost daily. The explanation (below) is that Boric slows the rate of rise with the result that you use the same amount of MA but have to add it less frequently.

Thanks chem geek, that table is another useful resource.

It may be worth noting here a point that you have explained elsewhere: Buffering works in both directions. Over the same pH range the capacity of the same buffer is identical against pH rise and pH fall. Those who need regularly to reduce pH will find that with borate present they need to add higher acid doses, albeit less frequently, to restore their target pH. The buffer reduces the pH rise from addition of alkali (for example NaOH in commercial bleach) but equally it reduces the pH drop from addition of acid (for example HCl). Total acid usage will be identical, with or without the buffer (borate in this scenario).

Digging a bit deeper about borate, which may help to explain different experiences from different pools: The apparent pKa of boric acid is about 9.24 in fresh water at 25 C, and it declines with increased ionic strength or temperature (Owen and King 1943; Manov 1946). pKa is lower in the presence of small amounts of Ca or Mg than in pure NaCl solutions (Hershey et al. 1986). It is likely to be around 9.0 at the salt levels in many SWG pools. A practical consequence of all this is that in warm SWG pools, borate may show greater pH buffer strength than expected ‘at first glance’, especially as pH rises to around 8.

Owen BB, King EJ (1943) The effect of sodium chloride upon the ionization of boric acid at various temperatures. J Am Chem Soc 65:1612-1620
Manov GG, Delollis NJ, Lindvall PW, Acree SF (1946) Effect of sodium chloride on the apparent ionization constant of boric acid and the pH values of borate solutions. J Res Natl Bureau Standards 36:543-558
Hershey JP, Fernandez M, Milne PJ, Millero FJ (1986) The ionization of boric acid in NaCl, Na-Ca-Cl and Na-Mg-Cl solutions at 25 degrees C. Geochimica et Cosmochimica Acta 50:143-148

 

[JoyfulNoise]

Thanks for for your comments. I'm really not having a hard time adjusting PH. I added the Boric Acid after reading in detail here at the throublefreepool that Boric Acid serves as a ph buffer against the rise in PH. My pool was fairly stable but to keep PH in the 8-7.5 range I was adding a small amount of acid almost daily. The explanation (below) is that Boric slows the rate of rise with the result that you use the same amount of MA but have to add it less frequently.

Yes, that is all true, but are you using less acid, the same or more than you did before adding borates? I guess I'm confused as to what you are asking help for?

Borates are good buffers against pH rise, that is where they have the largest buffer capacity (the amount of pH change per mole of hydrogen ion added or removed); borates do very little for pH fall. Carbonates are the active species against falling pH but also contribute to pH rise themselves when CO2 outgasses from the pool water and the chemical equilibrium of bicarbonates/CO2 shifts to compensate. Because carbonates are the dominant chemical species in your overall TA and most responsible for pH changes, you want to be able to adjust the TA easily. Borates complicate that process by slowing down the ability to adjust TA (since it takes longer for the pH to be restored after lowering it with acid). This is one of the reason why, in a new pool, TFP counsels against adding borates initially because it just makes the chemical management more complicated.

 

[Naplespool]

Yes, with Boric Acid in the water, it is harder to get the TA down but that has gone okay for me. Let me reiterate my original question, which is how to manage my pool for the months I'm not in Florida. When I'm here, I can check and adjust the chemistry daily if needed. For the months I'm gone, I'll have to rely on someone who visits the pool weekly. So, my question ...

Saltwater, plaster pool, pentair IC40. TA 60 ppm, Boric 50 ppm. Using about 3 quarts of acid a week. When I add 1.5 quarts the PH drops from 8 to 7.7--very acceptable. If I were to add all 3 qts at once, the PH drops from 8 to 7.1, with a CSI of -0.77.

I will be gone for several months. Can I have the pool guy add three quarts of acid all at once during the weekly visit, or should I have him come twice a week and add 1.5 quarts at a time?

​

Another question, which I don't understand is why at 4 months old am I using so much acid?

Today, Thursday
FC 7.5
CYA 70 ppm
PH 7.7 I added 1.5 quarts MA on Tuesday which dropped the PH to about 7.5 at that time
TA 70 ppm (up a bit from 60 on Tuesday) ?
CH 400 ppm
Borate 50 ppm
Salt 3600 ppm
VS Pump at 2000 rpm
SWG IC40 at 60%

N.B. Pool water looks perfect and has for a couple months

 

[ping]

I would only add half the acid and dose twice a week. The pH will not raise as quick when it is higher.

How did the CH rise to 400 with fill water of 50? Was calcium added to the fill water? If calcium was not added to the fill water then I'm guessing the original water was very aggressive and leached the calcium from the plaster at start up. If you go back to JoyfulNoise's post I believe you will find that the rising pH issue is from the original aggressive fill water that was not balanced that led to the plaster not curing properly.

 

[JoyfulNoise]

I will be gone for several months. Can I have the pool guy add three quarts of acid all at once during the weekly visit, or should I have him come twice a week and add 1.5 quarts at a time?

If you can ask the person taking care of your pool to add the acid in two 1.5Qt increments, that would be optimal. The short explanation is this - Outgassing of CO2 drives pH rise. When you lower the pH of the water all the way to 7.1, you create A LOT more dissolved CO2 in the water than when you lower to 7.5 (it's an exponential change). Since the concentration of the CO2 determines the rate at which the CO2 outgasses (Fick's Law of Diffusion), the higher the concentration then the higher the outgassing rate. So what that all means is that the pH rise is not linear in time - the time it takes the pH to rise from 7.1 to 7.5 is much, much faster than the time it takes for the pH to rise from 7.5 to 8.0. So, if you hit your pool with acid all at once and drive it down to 7.0, you're only wasting acid and not gaining much. Or, to put it more succinctly - two doses of acid with "last" a lot longer than one big slug of acid for the same total amount of acid added.

Another question, which I don't understand is why at 4 months old am I using so much acid?

Because fresh plaster causes the pH to rise. Plaster pools can take anywhere form 12-18months before the pH rise settles down. It's mostly a consequence of how the plaster was started up when water was added. The was the origin of my second point and question - how was your plaster started? If a hot start (acid start) process was used, your plaster will emit calcium hydroxide for quite some time and be a large source of pH rise in your pool water. If a bicarbonate startup process was used (rare, because most plasterers are too lazy to do it), you'll get a better finish and one that emits less caustic plaster components.

 

[Naplespool]

The TFP target for a saltwater pool is CH 350 - 450. I added calcium to the pool. It has been very stable.