Co2 for PH control

[RSpool]

Adding CO2 converts hydroxide alkalinity into bicarbonate alkalinity.

CO2 + OH --> HCO3

So, carbon dioxide is not responsible for increasing the total alkalinity.

For the total alkalinity to increase, something alkaline needs to be added.

If a sodium hypochlorite solution has excess lye (hydroxide), it will raise pH and TA. It's the hydroxide that raises the total alkalinity.

Adding CO2 to lower the pH converts hydroxide alkalinity into bicarbonate alkalinity.

To lower pH rise caused by something other than CO2 loss, hydrochloric acid should be used.

H + OH --> H2O

When CO2 is lost, pH rises with no change in TA.

HCO3 + H --> H2O + CO2

So, the injection of CO2 is the reverse of aeration. You're just forcing CO2 into the water instead of forcing it out.

CO2 + H2O --> HCO3 + H

So, bicarbonate is created, which increases the alkalinity but a hydrogen ion is also created, which lowers the pH and alkalinity for a net change in the total alkalinity of zero.

If pH rise is only due to carbon dioxide loss, reinjecting the carbon dioxide brings you back to where you were before the carbon dioxide was lost.

If the carbon dioxide is injected at the same rate as it's lost, the pH and TA remain stable.

The problem is that CO2 is expensive and you have to keep reinjecting it to replace what offgasses into the air.

Adding CO2 converts hydroxide alkalinity into bicarbonate alkalinity.

CO2 + OH --> HCO3

So, carbon dioxide is not responsible for increasing the total alkalinity.

For the total alkalinity to increase, something alkaline needs to be added.

If a sodium hypochlorite solution has excess lye (hydroxide), it will raise pH and TA. It's the hydroxide that raises the total alkalinity.

Adding CO2 to lower the pH converts hydroxide alkalinity into bicarbonate alkalinity.

To lower pH rise caused by something other than CO2 loss, hydrochloric acid should be used.

H + OH --> H2O

When CO2 is lost, pH rises with no change in TA.

HCO3 + H --> H2O + CO2

So, the injection of CO2 is the reverse of aeration. You're just forcing CO2 into the water instead of forcing it out.

CO2 + H2O --> HCO3 + H

So, bicarbonate is created, which increases the alkalinity but a hydrogen ion is also created, which lowers the pH and alkalinity for a net change in the total alkalinity of zero.

If pH rise is only due to carbon dioxide loss, reinjecting the carbon dioxide brings you back to where you were before the carbon dioxide was lost.

If the carbon dioxide is injected at the same rate as it's lost, the pH and TA remain stable.

The problem is that CO2 is expensive and you have to keep reinjecting it to replace what offgasses into the air.

James W,
I noticed this discussion about CO2 vs. acid, perhaps you can help on an issue with my 15,000 gallon gunite/plaster pool I had built in 1981 in Atlanta, no automation, DE filter, using tri-chlor tabs. I had it replastered Oct 31, 2016, followed NPC start up procedure, and by end of December 2016 chemistry was balanced (FC4, CH 260, CYA 30, TA 120, pH 7.4, TDS 500) and appeared stable so it was closed . Reopened pool April 2017, chemistry way out of balance (FC 0, CH 380, CYA 30, TA 160, pH 8+, TDS 1000), crystals had formed on walls around pool perimeter. Plastering company sent representative of water chemical supplier who identified the crystals as calcium, added Blue Stuff, instructed me to run pump 24/7 and use stainless steel brush daily. Crystals were gone 3-4 weeks and have not reappeared.

Since then, now 2 years, the pH is still unstable and rises out of range 2-4 days, requiring frequent addition of acid to control, CH climbs to top end of range, as does CYA, water has been drained some and diluted several times to lower those levels. Even though the pH rises rapidly, the plaster is etching since the calcium crystals formed while closed Dec. 2016 - April 2017.

For a better understanding of the total water chemistry, I began using an LSI calculator when the new plaster was one year old in Oct. 2017. Through Dec 2018, 14 months, I have done 57 LSI calculations and 48 times the water was in the acceptable range of -0.3 to +0.3, 8 times it was out of range on the scale size >.3, only once on the etch side <.3. In some water tests the pH was over 8, but the test kit measures only to 8, so LSI was actually more on the + side than calculations show. In Oct 2018 I switched from tri-chlor to liquid chlorine to bring down the CYA, now at 50. I also added borates to prevent pH from rising which raised the TA from 120 to 160 but pH is stable at 8.1 to 8.2. I also started using a digital pH meter for accurate pH reading . During this cold weather period I have not tried to lower pH believing that scaling is a preferred condition vs. etching.

I have been told that even though the LSI is almost never on the etch side of the LSI scale, the etching is caused by the frequent addition of acid and that I should consider switching to CO2 injection to control pH without the corrosive effects of acid. No one has been able to explain why, over the 2 year and 4 month time period, the pH continues to rise or why the plaster is etching when it should be scaling. What do you think in going on with my pool and do you think CO2 injection is appropriate?

Thanks Richard
PS - I have a spread sheet of all water tests since replastering including chemical additions and water dilution.

 

[kimkats]

Mod note-this post was moved to it's own thread.

@JamesW can you help?

 

[ajw22]

Welcome to TFP.

Please put details of your pool in your signature. See read-before-you-post

Does your pool have water features? Does it have a spa spillover?

Trichlor pucks contain acid. When you stop using them it will require more additions of MA to manage pH.

TA of over 100 will contribute to rising pH. What is the pH & TA of your fill water? Folks have found that lowering TA to 60-70 can help stabilize pH. Needing to lower pH every few days is not unusual for some pools.

I doubt etching is caused by the frequent addition of acid.

Do you have pics of what you call etching in your pool?

 

[JamesW]

The rising pH is mostly due to the TA being too high.

The industry standard of 80 to 120 is nonsense. It creates too much carbon dioxide, which offgasses and raises the pH.

A TA of about 60 typically works best.

A higher pH also helps because it reduces the amount of TA that is converted into carbon dioxide.

A pH of about 7.8 to 7.9 works well for most people.

Use the CSI as determined by PoolMath for the best CSI calculation.

When doing the CSI calculation, it's important to subtract the cyanurate alkalinity and borate alkalinity from the total alkalinity to get the carbonate alkalinity.

PoolMath does the calculation automatically. If you're using a formula, you have to do the math.

The initial scaling during the closed period was due to the new plaster. It probably damaged the plaster somewhat.

If you're still getting plaster dissolution, you might have bad plaster, but that's hard to determine.

Right now, the best that you can do is to keep the CSI as close to 0.0 as possible and brush the pool regularly.

Make sure that you are entering the correct salinity and water temperature in the PoolMath calculator.

 

[RSpool]

Welcome to TFP.

Please put details of your pool in your signature. See read-before-you-post

Does your pool have water features? Does it have a spa spillover?

Trichlor pucks contain acid. When you stop using them it will require more additions of MA to manage pH.

TA of over 100 will contribute to rising pH. What is the pH & TA of your fill water? Folks have found that lowering TA to 60-70 can help stabilize pH. Needing to lower pH every few days is not unusual for some pools.

I doubt etching is caused by the frequent addition of acid.

Do you have pics of what you call etching in your pool?

ajw22,

I added pool details to signature, thanks for the reminder. There are no water features or spa spill over. Stopped trichlor Oct 2018, replaced with 10% liquid cholorine, CYA dropped from 80 to 50. TA was 90 when I added first 15# dose of borates on Dec 13, 2018, jumped to 130, added 2nd 15# dose Dec 29, jumped to 160. Borates are 40 ppm and holding. TA is back down to 130. I've hoped pH would find equilibrium in low 8's over the winter, not wanting to ad acid but looks like I will have to bring down TA and pH.

Well water is the source, tested before refilling after replaster: pH 8.0, TA 100, CH 80, TDS 1000. Heavy rains over last year in Atlanta have required draining, no filling.

I have thought the calcium crystals formed the first winter when closed followed by unstable pH and etching was the result of poor plaster job (soft plaster) but there is no consensus from knowledgeable pool people as to the cause. Attached are photos taken one year after replastering (Oct 2017) when the water was drained 15" to dilute. Image 1883 shows etching below the water level that occurred the three months pool was closed after replastering........note there is no etching above the water line. Image 1890 shows the top of a wedding cake step. Image 1893 show the etching around a return.

Obviously my biggest question is why the pH won't stabilize, why the plaster etched when pH was high, and is CO2 a better choice than acid to control pH? CO2 is an expensive alternative but I will do it if it will solve the problem.

Thanks for your help, Richard.

 

[RSpool]

The rising pH is mostly due to the TA being too high.

The industry standard of 80 to 120 is nonsense. It creates too much carbon dioxide, which offgasses and raises the pH.

A TA of about 60 typically works best.

A higher pH also helps because it reduces the amount of TA that is converted into carbon dioxide.

A pH of about 7.8 to 7.9 works well for most people.

Use the CSI as determined by PoolMath for the best CSI calculation.

When doing the CSI calculation, it's important to subtract the cyanurate alkalinity and borate alkalinity from the total alkalinity to get the carbonate alkalinity.

PoolMath does the calculation automatically. If you're using a formula, you have to do the math.

The initial scaling during the closed period was due to the new plaster. It probably damaged the plaster somewhat.

If you're still getting plaster dissolution, you might have bad plaster, but that's hard to determine.

Right now, the best that you can do is to keep the CSI as close to 0.0 as possible and brush the pool regularly.

Make sure that you are entering the correct salinity and water temperature in the PoolMath calculator.

James W,
Thanks for your reply. I've had no luck following industry standards for TA so will try your recommendation for TA and pH. I have been using LSI calculators by Orenda Tech and NPC (it includes borates), both automatically adjust for carbonate alkalinity. I'm happy to try the TFP calculator. You suggest entering salinity in the calculator, do you mean the salinity in a salt pool? If not please explain, mine is a chlorine pool.

I believe I got bad plaster or bad plaster finishing, "soft plaster" as described by the trio of experts at onBalance, and it is the cause of my problems but haven't yet figured out how to confirm it because bad water chemistry can do the same thing, any suggestions on how to tell the difference?

I attached some photos of etching 12 months after replastering in my response to ajw22, please take a look. Thanks Richard

 

[mknauss]

Chlorine pools are saltwater pools and saltwater pools are chlorine pools. Pools chlorinated with liquid or some solid chlorine still have salt added to them. The majority of non-saltwater pools have 600-1000 ppm of salinity.

 

[Donldson]

Just want to add on to Marty's comment: All chlorinated pools contain salt. This is because when chlorine is consumed it produces chloride, which remains in the water. On top of that people swimming add small amounts of salt from their sweat (and other bodily fluids that people totally aren't doing in the pool ) so it would be nearly impossible to find a pool without some measurable amount of salt in it.

 

[mknauss]

Realize that CO2 used for pH control does nothing to reduce TA. So it is not a viable alternative with your fill water. Also with borates in the water, you typically add large amounts of acid less often to reduce the pH. The pH moves slowly so when you need to drop it it takes just as much acid as it would if done every day or two but you add the acid all at once.

 

[RSpool]

Realize that CO2 used for pH control does nothing to reduce TA. So it is not a viable alternative with your fill water. Also with borates in the water, you typically add large amounts of acid less often to reduce the pH. The pH moves slowly so when you need to drop it it takes just as much acid as it would if done every day or two but you add the acid all at once.

mknauss,
Thanks for your reply. My interest in CO2 was to lower pH without acid but you say it won't lower TA. My interest in borates was to slow the rise of pH, but the negative is it raises TA. So regardless of whether or not I use CO2, if I continue with borates, I will also have to use acid. Bummer. The frustration of this chemistry challenge is the first 36 years of managing our pool, we did not have these issues. It only started with the replastering 2 years and 4 months ago. So if we didn't have this rising and unstable pH issue, accompanied by etching, with old the plaster that lasted 36 years, does this mean this problem after replastering is the result of the new plaster mix or its application? Thanks Richard

 

[JamesW]

I suspect that the old plaster probably contained asbestos, which can improve the longevity of the plaster but it is no longer allowed.

Borates don't really affect pH rise due to their effect on TA. Borate alkalinity only makes a small difference in the total alkalinity and only carbonate alkalinity causes pH rise.

Borates might slow pH rise some, but not the total amount of acid needed.

Just lower the TA and keep the pH at 7.8.

 

[RSpool]

Just want to add on to Marty's comment: All chlorinated pools contain salt. This is because when chlorine is consumed it produces chloride, which remains in the water. On top of that people swimming add small amounts of salt from their sweat (and other bodily fluids that people totally aren't doing in the pool ) so it would be nearly impossible to find a pool without some measurable amount of salt in it.

I downloaded the PoolMath app per James W's suggestion. I thought it is was an LSI calculator but it appears to be a dosing calculator, it this correct?

Chlorine pools are saltwater pools and saltwater pools are chlorine pools. Pools chlorinated with liquid or some solid chlorine still have salt added to them. The majority of non-saltwater pools have 600-1000 ppm of salinity.

The Taylor kit I have has salt test instructions on the label but no reagent, I thought it was used for salt pools. Learned something new!

 

[JamesW]

Poolmath can calculate the CSI.

PoolMath

 

[AUSpool]

Total alkalinity (TA) in pool water is made up of three parts, carbonate alkalinity, cyanurate alkalinity and borate alkalinity. If you add or increase cyanuric or borate alkalinity you can still maintain a TA in the range of 60-70ppm by reducing the carbonate alkalinity portion via the acid aeration method.

Managing pH via CO2 will be a constant up hill battle. Your pushing CO2 into a pool which is already over carbonated and venting CO2 to the air above. Using acid to manage pH is direct and makes more sense.

 

[RSpool]

I suspect that the old plaster probably contained asbestos, which can improve the longevity of the plaster but it is no longer allowed.

Borates don't really affect pH rise due to their effect on TA. Borate alkalinity only makes a small difference in the total alkalinity and only carbonate alkalinity causes pH rise.

Borates might slow pH rise some, but not the total amount of acid needed.

Just lower the TA and keep the pH at 7.8.

James W,
I added a quart of acid today, hopefully pH and TA will be lower tomorrow. I suspect it may take more but this will be my target. Thanks Richard

 

[ajw22]

James W,
I added a quart of acid today, hopefully pH and TA will be lower tomorrow. I suspect it may take more but this will be my target. Thanks Richard

Check your pH and TA 30 minutes to an hour after you added acid to see the effect of the acid. By tomorrow your pH and TA will have risen again.

 

[RSpool]

Check your pH and TA 30 minutes to an hour after you added acid to see the effect of the acid. By tomorrow your pH and TA will have risen again.

Allen,
I did not read your reply until this morning so my test was 24 hours after adding 32 oz. acid. pH dropped from 8.1 to 7.9 but TA remained at 130, LSI is now +0.29, finally back into to acceptable range. FC was at 0, added 26 oz of 10% liquid chlorine, pH will be up again tomorrow, bummer! Thanks Richard

 

[RSpool]

Poolmath can calculate the CSI.

PoolMath

James,
I have been using the LSI calculator for 16 months, trying to better understand how each chemistry factor effects the other. I was not aware of CSI until joining this forum and am very willing to use it as well. My Taylor kit has instructions on how to do the salt test but it doesn't have the reagents/test tube to conduct test so I have to rely upon LSI till I can obtain what I need to test salt. From what I've read the LSI and CSI calculators are similar, both developed by the same man, but I don't yet understand why the CSI is the better calculator for my plaster pool. Can you explain? Thanks

 

[JamesW]

The LSI and CSI should give similar results. The CSI as calculated by PoolMath does it correctly for pool applications. Use 1,000 for salinity until you can test the salinity. When using only liquid chlorine, the salinity can increase faster than most people realize.

 

[RSpool]

James W,
I added a quart of acid today, hopefully pH and TA will be lower tomorrow. I suspect it may take more but this will be my target. Thanks Richard

James,
My plastering company insured me the current ingredients of the plaster mix are the same as in 1981, you are the second person recently to mention that asbestos was in the old mix which improved it's performance. Too bad there is not a good replacement. I'm working on lowering TA and getting pH to 7.8. Thanks