I have 300 TA, 7.4 pH fillwater. Beginning of season I bring down the TA to 50 otherwise it is a huge battle with pH. I understand that aeration increases the pH as CO2 is released from the water. If I didn’t have any fresh water coming into the pool due the splashes and evaporation etc at some point CO2 should be depleted or at least chemical balance should be established so that net CO2 release should be zero. Am I right on this one? Secondly why do I have CO2 in my fillwater? We use underground water. Is it common for water to be so rich in CO2?
Where is CO2 coming from?
- Thread starter fanis.merk
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Bicarbonate converts to CO2 based on the pH.
So, as CO2 is lost to the air, more bicarbonate converts to CO2 to maintain the same percentage (On a molar basis).
For example, at a pH of 7.35, 90.91% is bicarbonate and 9.09% is carbon dioxide.
At a pH of 8.0, the percentage that is carbon dioxide drops to 2.19%
100-(100/(1+10^(6.35 – 8))) = 2.19%
As the bicarbonate converts to carbon dioxide, the pH rises because the bicarbonate picks up hydrogen ions.
HCO3- + H+ --> H2CO3 --> H2O + CO2
The Y axis is the percentage of baking soda that converts into carbon dioxide.
The X axis is the pH.
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So, as CO2 is lost to the air, more bicarbonate converts to CO2 to maintain the same percentage (On a molar basis).
For example, at a pH of 7.35, 90.91% is bicarbonate and 9.09% is carbon dioxide.
At a pH of 8.0, the percentage that is carbon dioxide drops to 2.19%
100-(100/(1+10^(6.35 – 8))) = 2.19%
As the bicarbonate converts to carbon dioxide, the pH rises because the bicarbonate picks up hydrogen ions.
HCO3- + H+ --> H2CO3 --> H2O + CO2
The Y axis is the percentage of baking soda that converts into carbon dioxide.
The X axis is the pH.
y = 100-(100/(1+10^(6.35– x))), x from 4 to 8 - Wolfram|Alpha
Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
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Yes but bicarbonate should deplete as well. No? Where is it coming from?Bicarbonate converts to CO2 based on the pH.
So, as CO2 is lost to the air, more bicarbonate converts to CO2 to maintain the same percentage (On a molar basis).
For example, at a pH of 7.35, 90.91% is bicarbonate and 9.09% is carbon dioxide.
At a pH of 8.0, the percentage that is carbon dioxide drops to 2.19%
100-(100/(1+10^(6.35 – 8))) = 2.19%
As the bicarbonate converts to carbon dioxide, the pH rises because the bicarbonate picks up hydrogen ions.
HCO3- + H+ --> H2CO3 --> H2O + CO2
The Y axis is the percentage of baking soda that converts into carbon dioxide.
The X axis is the pH.
y = 100-(100/(1+10^(6.35– x))), x from 4 to 8 - Wolfram|Alpha
Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
At a 300 TA, the amount of carbon dioxide is very high and this causes the pH to keep rising as the carbon dioxide offgasses.
If you lower the TA to about 60 ppm, the carbon dioxide in the water will be close to equilibrium with carbon dioxide in the atmosphere and the off gassing will be slowed way down.
If you are adding acid, then the TA should be going down.
If you have a lot of fill water due to replacing water lost to evaporation, then the fill water adds bicarbonate.
But do you normally expect a lot of bicarbonate in fillwater? My CH is 650. Is it related to bicarbonates? If I don’t top up how fast should it deplete so that CO2 emission would stop.
Every source of fill water has a different TA, so there is no "expectation" other than maybe what the Water Quality Report from your water source supplier indicates.
TA might range from about 50 ppm to about 400 ppm or more.
No, CH is related specifically to calcium ions.
The TA and pH determine the amount of CO2, so the lower you can get the TA, the lower the CO2 will be.
So, it depends on how much acid you can add and that depends on the pH and the TA.
All you can do is try to keep the pH as close as possible to 7.2 and maybe aerate to help drive off carbon dioxide.
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I understand this relationship. I am trying to understand the origins of CO2. Is it because I am using underground water I have so many bicarbonates? Are bicarbonates counting towards my TA? Can I filter bicarbonates from my source water with reverse osmosis device for example?
Is filtering the bicarbonates from the fillwater a feasable option for a residential pool as opposed to dumping acid into the swimming pool on a constant basis?
It depends on how much money you want to spend and how much water you need to add.
You can get an automatic acid dispenser if you want to make the acid additions easier.
I have an automatic acid dispenser. I am dumping 150kg acid in a summer season. It is around $120 per season. I am checking alternative solutions. If I can get rid of these bicarbonates from the source then it is a solution.
Sulphuric Acid. Yes I know HCl is better but in Europe this is the practice due to various reasons. We have discussed the topic here
Hydrochloric Acid vs Sulphuric Acid in automated dosing
Hello, I have been reading all the materials on this site and finding it to be immensely helpful. I also read the sections and forum posts related to Hydrochloric Acid (HCl) vs Sulphuric Acid (H2SO4). I understand that Hydrochloric Acid is quite a natural choice from a chemical perspective as...
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Is that about 125 liters (33 gallons) of 28% H2SO4?
That is a lot of sulfate to add to the pool (about 500 ppm).
HCl or Reverse Osmosis would be a better choice.
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It seems the bottles I am buying are 20 liters each and weigh 25kg (do we use the volume or mass for calculations?) Concentrations are usually not specified on the bottles but I am assuming around 30% based on some bottles I saw here and there by chance. My pool is 80000 liters. I spend around 6 bottles in a season. How do I calculate sulphate ppm? Some of it going out with backwash for sure but would be good to know how to calculate the worst case scenario.
- Mar 5, 2020
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- 66000
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- Astral Viron V35
Isn't it fair to say that a common way for calcium ions to get into the water supply is via calcium carbonate (CaCO3) entering the water and dissolving as Ca2+ and CO32- ions?
The calcium ions will register as CH and the carbonate ions (which will at pool pH actually mostly convert into bicarbonate ions) as TA.
Even though the CH value is independent from bicarbonate, the source for both being in the fill water can be common, as far as I understand. If the water had been flowing through limestone, then it will likely end up being high in both, CH and TA.