Sodium Bicarbonate / TA - pH Hydrochloric Acid /TDS

[Retep]

I need some opinion / advice from the experts here. . . (chem geek - I hope you are reading this)

OK - I went to see a public indoor pool today ( 250.000 gal) in order to take a look at their filtration system.
Filters are old, rusty and they are thinking of replacing them.
Crystal clear water ( 0.12 NTU ) ph=7.55 FC = 1.81 TC=2 TDS= 1180 TA= 100
Pool is well kept - Staff follows recommendations of the CPO Handbook. . .
Sodium Hypochlorite as a Chlorine donor , Hydrochloric Acid ( 22 Baume) for lowering the ph is used.

Now - I asked the guy how much acid he is using since I saw quite a few drums (50 gal) in the filtration room.
He is averaging about 6 gal / day , which I find quite a lot. . .
In order to keep his TA at 100 ppm he naturally uses quite a lot of Sodium Bicarbonate as well (average 20 lbs / day)

So - here I saw a typical "yo yo game" scenario. Sodium Bicarbonate -> pH up , compensate with Acid -> ph and TA down.
Basically he is "fighting" one thing with another one.
He puts about every 2nd day the Sodium Bicarbonate straight into the Pool.

I suggested to pre-mix the Sodium Bicarbonate in a container ( proportion of 10 parts of water to 1 part of Sodium Bicarbonate)in the filtration room and inject this solution with a small metering pump slowly and continuously after the filters so it goes evenly into the swimming pool until he reaches his desired TA. While doing so he should turn off his Acid pump till all the water is well distributed.

Theory behind this approach is to avoid those spikes he's got now with wild fluctuating swings in pH and TA.
I also anticipate quite a reduction in both Acid and Sodium Bicarbonate consumption.

Naturally his supplier sells him Sodium Hypochlorite , Acid and Sodium Bicarbonate and more or less tells them that it's going to be the end of the world
and catastrophic consequences will happen if his TA drops below 90 ppm and the pH drops below 7.5. . . lol

According to my experience I've never seen any problems even if the TA drops down to 50 ppm.
Ideally I want them to run at a pH of 7.3 max - if they want to keep the TA at 100 it's fine with me too.
FC levels should be max. 1 ppm (as long as we are able to keep the ORP's at around 750mV. . . (chem geek - you knew this would come. . lol

Question : I know it is simply not possible (unfortunately we got this law of nature / chemistry) to decrease the pH by not decreasing the TA,
but which would be the smartest way to avoid this "yo yo game" as much as possible ?

Thanks in advance

 

[JamesW]

Question : I know it is simply not possible (unfortunately we got this law of nature / chemistry) to decrease the pH by not decreasing the TA,
but which would be the smartest way to avoid this "yo yo game" as much as possible ?

Stop adding sodium bicarbonate when the pH is high or rising. They are just wasting chemicals.

NaHCO₃ + HCl --> NaCl + H₂O + CO₂

Sodium bicarbonate + hydrochloric acid --> salt + water + carbon dioxide

It really does not matter if the TA is "low" as long as the carbonate alkalinity is definitely not zero and the CSI is [edit]close to[end edit] zero (I am assuming that this is a concrete pool).

 

[Retep]

Question : I know it is simply not possible (unfortunately we got this law of nature / chemistry) to decrease the pH by not decreasing the TA,
but which would be the smartest way to avoid this "yo yo game" as much as possible ?

Stop adding sodium bicarbonate when the pH is high or rising. They are just wasting chemicals.

NaHCO₃ + HCl --> NaCl + H₂O + CO₂

Sodium bicarbonate + hydrochloric acid --> salt + water + carbon dioxide

It really does not matter if the TA is "low" as long as the carbonate alkalinity is definitely not zero and the CSI is not zero (I am assuming that this is a concrete pool).

They are also adding sodium bicarbonate even if their pH is higher - as long as they bring up the TA ( remember. . . their chemical supplier told them that's the way to do it. . .lol)
Now - when you refer to carbonate alkalinity shouldn't there be a minimum of about 30 ppm (in CaCo3)? Doesn't carbonate alkalinity make up the majority of TA ? I am confused. . .
It is a concrete pool (with tiles) and don't know the CSI ( even though I think that perhaps in the industry there is too much emphasis on the CSI)

 

[JasonLion]

In this situation carbonate alkalinity is no doubt the great majority of TA, but in many of the SWG pools we work with carbonate alkalinity is around half of TA, with most of the remainder coming from cyanurate alkalinity and just a little from the borate alkalinity.

The only way to end the cycle is to allow TA to come down.

 

[Retep]

In this situation carbonate alkalinity is no doubt the great majority of TA, but in many of the SWG pools we work with carbonate alkalinity is around half of TA, with most of the remainder coming from cyanurate alkalinity and just a little from the borate alkalinity.

The only way to end the cycle is to allow TA to come down.

I should have mentioned that this pool is not a SWG pool.
So - if I want to have a pH of 7.2 and a TA of 100 - how does one proceed in getting those numbers ?

 

[JamesW]

So - if I want to have a pH of 7.2 and a TA of 100 - how does one proceed in getting those numbers ?

You don't. You are just setting yourself up for problems.

Your pH will be much more stable if you keep it at the higher end of the range, rather than the lower end.

Allow the TA to go down until the pH stabilizes.
__________________________________________________________

Actually, if you really wanted to do it, you could use carbon dioxide to lower the pH without lowering the TA.

CO₂ + H₂O --> H⁺ + HCO₃^-

Carbon dioxide + water --> acid + bicarbonate

However, the carbon dioxide will just keep off gassing and you will have to keep adding more carbon dioxide.

This method also does not address the TA rise due to introduced bases combining with carbon dioxide, which will require muriatic acid to reduce anyway.

 

[chem geek]

TA, and specifically the carbonate portion of TA, is a SOURCE of pH rise through carbon dioxide outgassing since pools/spas are intentionally OVER-CARBONATED. So the solution to getting out of the yo-yo effect is to simply target a lower TA level and not try to raise the TA as much. As you add acid to lower the pH, the TA will drop, so if you just stop adding baking soda the TA will drop further and likely get to a point where the rate of pH rise and amount of acid that is added gets down to an acceptable level. You will still need to add some acid since there is "excess lye" in sodium hypochorite (and other hypochlorite sources of chlorine) that needs to be compensated, but it is relatively low. The pH of chlorinating liquid varies, but high-quality 12.5% chlorinating liquid with a pH of 12.5 has 0.25% excess lye (sodium hydroxide) in it so for every gallon you would need 0.88 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) to compensate. Lower-quality 12.5% chlorinating liquid has a pH of 13.0 with 0.8% excess lye which would need 2.7 fluid ounces of acid while poor quality 12.5% chlorinating liquid with a pH of 13.5 has 2.5% excess lye which would need 8.4 fluid ounces of acid. You cannot go lower than these amounts even if carbon dioxide outgassing were completely eliminated.

With the TA then lower, even at 50 ppm, you can then compensate for the saturation index in two ways: one is to have a higher Calcium Hardness (CH) level and another is to target a higher pH of 7.7-7.8 instead of 7.5. The rate of carbon dioxide outgassing, and therefore pH rise, is a function of TA and pH (and aeration) as shown in this chart.

By the way, this effect was described (briefly) in the Certified Pool Operator (CPO) training -- What is not taught post under "TA and its Effect on Rising pH" as well as more info under "Procedures for Lowering TA".

Also, what you described with respect to pre-mixing the bicarbonate isn't going to help. There is a differing effect on adding bicarbonate with respect to the amount of pH rise in that distributing it slowly with careful mixing (but not aeration) into a large body of water can minimize the pH rise from immediate carbon dioxide outgassing, but does not affect that outgassing over time. If you dump the baking soda in quickly, then you can create very high TA locally that causes carbon dioxide to outgas so the pH could rise more. Note that no matter how you add the bicarbonate, the TA will rise by the same amount -- the method of addition only somewhat affects how much the pH will rise immediately. Now as for acid addition, it doesn't matter how you add it, the TA will drop by the same amount and pretty much the pH will drop as well, but the "slug" or "acid column" method is unsafe as was described in the paper linked to in the "Procedures for Lowering TA" section in the CPO post. At any rate, as mentioned above, the way out of the yo-yo effect is to stop adding bicarbonate and let the TA get lower to a point of greater pH stability and less required acid addition.

 

[cya_sux]

Is it too far out of the budget to get a CO2 tank to address the pH problem? I've found that when using bleach as your Cl, CO2 works well for lowering the pH and raising alkalinity. You would then use HCl to lower the alkalinity (instead of using it to lower the pH).

 

[chem geek]

Injecting carbon dioxide lowers the pH with no change in TA. The TA rises over time from the excess lye in bleach or chlorinating liquid (or Cal-Hypo, etc.). It doesn't make sense to use carbon dioxide for pH control. It only directly counteracts carbon dioxide outgassing and that can be minimized by having a lower TA level. To counter the pH and TA rise from the excess lye, a strong acid such as Muriatic Acid is best to use.

Only if you had to have a higher TA level or had carbon dioxide outgassing due to a lot of aeration would you want to consider using some carbon dioxide for pH control.

 

[CaOCl2]

Injecting carbon dioxide lowers the pH with no change in TA.

CO₂ + H₂O --> H⁺ + HCO₃^-

The reaction produces H+ ions (drops the pH) but it also produces bicarbonate ions. How can it produce bicarbonate ions without increasing TA

 

[chem geek]

The reaction produces H+ ions (drops the pH) but it also produces bicarbonate ions. How can it produce bicarbonate ions without increasing TA

H⁺ lowers TA just as OH^- raises it. TA counts all chemical species that can accept a hydrogen ion (which for the TA test is down to a pH of 4.5 which is when the indicator dye switches from green to red), but hydrogen ion itself counts negatively towards TA. Another way of looking at it is that [OH^-]-[H⁺] represents the net TA from the dissociated components of water (and yes, TA can be negative, though obviously the TA test doesn't measure below 0).

Adding carbon dioxide to the water creates carbonic acid but has no change in TA for the same reason that outgassing of carbon dioxide removes carbonic acid. The two H⁺ exactly balances the CO₃^2- in terms of TA for carbonic acid, or in terms of bicarbonate the one H⁺ exactly balances the HCO₃^-.

 

[cya_sux]

I guess I learned something new today :lol:. I was always told when I worked at a water park that the CO2 was to lower pH and raise alk. I believed them too because we would have low pH in many pools along with high alk.

So the excess lye, which I'm assuming is OH-, counteracts the HCO3-, thus lowering pH and having no affect on alk?

Maybe I'm making this more difficult than it really is

 

[JamesW]

I guess I learned something new today :lol:. I was always told when I worked at a water park that the CO2 was to lower pH and raise alk. I believed them too because we would have low pH in many pools along with high alk.

So the excess lye, which I'm assuming is OH-, counteracts the HCO3-, thus lowering pH and having no affect on alk?

Maybe I'm making this more difficult than it really is

The hydroxide introduced by the lye increases the pH and the TA. Carbon dioxide creates a hydrogen ion and a bicarbonate ion. The hydrogen ion lowers the pH and TA, and the bicarbonate raises the TA. The hydrogen ion and the hydroxide combine to form water, which leaves the bicarbonate. Therefore, the overall effect of using carbon dioxide to lower the pH, when the pH rise is due to introduced bases, ends up with an increased TA.

If the pH rise is strictly due to the offgassing of carbon dioxide, then the net effect of using carbon dioxide to lower the pH is no effect on the TA.

Total Alkalinity is a measure of all types of alkalinity (not just bicarbonate). Hydroxides, carbonates, bicarbonates, cyanurates, borates, phosphates etc. are all types of alkalinity. The Total alkalinity is the sum total of everything that can accept a hydrogen ion down to a pH of 4.5.

 

[chem geek]

I guess I learned something new today :lol:. I was always told when I worked at a water park that the CO2 was to lower pH and raise alk. I believed them too because we would have low pH in many pools along with high alk.

The CO₂ lowered the pH with no change in TA, but the large amount of hypochlorite chlorine that was used raised the TA due to the excess lye. The net result, assuming that one was adding enough CO₂ to keep pH fairly constant is the following:

................................................. pH .... TA
Carbon Dioxide Outgassing ...... + ...... 0
Excess Lye in Chlorine .............. + ...... +
Carbon Dioxide Added ............ -- ....... 0
-----------------------------------
Net Result ................................ 0 ....... +

If one lowers the TA significantly, then one can minimize the outgassing. That would then just leave the excess lye to deal with where using Muriatic Acid would be better than using carbon dioxide since only the acid would lower the TA to keep it in check.