Calculating PH change from salt cells

[Diamond_Pool]

Hello Troublefree experts.

I was wondering if there has been an effort to calculate change in pH from salt cell using the calculator.
I believe this can be calculated using inverse of adding HCl to pool.
This is what I am thinking. For every Cl molecule create one H leaves the pool as hydrogen gas. Since weight of Hydrogen is 1 compared to Chlorine of 35.5, we can approximate for every ppm of chlorine created from salt cell, 1 ppm of HCl is removed. This may look odd but eventually chlorine create in pool leaves as chlorine gas.

The pH increase than be calculated using existing pool math equations by for effect if removing HCl

I did this math for my pool which is 16416 gallons. The HCL conc of 20baume is 500,000ppm. For every fl oz of 20baume net effect is 0.237 of HCl removed or added. At current alkalanity (100) every 7 oz of 20 baume it is 0.1 ph change. 7fl oz of HCl = 1.7 ppm of chlorine generated from salt cell. At current setting it takes my salt cell to generate this is 1.5 days and this comes very close to observed pH change for my pool.
Please let me know if I m thinking this wrong.

 

[JamesW]

It is a myth that SWGs cause pH rise.

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^-.

HOCl + OCl^- + uv light -> O₂ + H⁺ + 2Cl^-.

Following the process, we can see that there are 4H⁺ and 4OH^- created, which nets out to pH neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

 

[JoyfulNoise]

The pH rise you see isn’t from the cell generating chlorine (as @JamesW has shown). It’s from aeration of the water in the pool and the subsequent offgassing of CO2 from the pool water surface. At 100ppm TA, you have a lot of carbonate alkalinity in the water. It’s basically “stored CO2” like a bottle of pop. Open the lid and the acidic soda water eventually rises in pH as the CO2 bubbles off.

Throw a cover on your pool and watch what happens to the pH. The rise in pH will practically cease.

 

[JamesW]

This may look odd but eventually chlorine create in pool leaves as chlorine gas.

The chlorine gas dissolves into the water very quickly and you really do not lose many, if any, chlorine atoms at all.

 

[Diamond_Pool]

The pH rise you see isn’t from the cell generating chlorine (as @JamesW has shown). It’s from aeration of the water in the pool and the subsequent offgassing of CO2 from the pool water surface. At 100ppm TA, you have a lot of carbonate alkalinity in the water. It’s basically “stored CO2” like a bottle of pop. Open the lid and the acidic soda water eventually rises in pH as the CO2 bubbles off.

Throw a cover on your pool and watch what happens to the pH. The rise in pH will practically cease.

Thanks for explaining. So basically when alkalinity hits around 80 we should stop seeing pH increase?

 

[mknauss]

What is your fill water TA?

 

[JamesW]

So basically when alkalinity hits around 80 we should stop seeing pH increase?

The pH does not stop until the CO2 in the pool is in equilibrium with the CO2 in the air according to Henry's law.

This also depends on the pH.

At a pH of about 7.8 and a Carbonate Alkalinity of about 40, the pH increase is very low.

If the pH is about 8.3, the CO2 is usually low enough to stop the pH rise.

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 9 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[Diamond_Pool]

What is your fill water TA?

Fill water TA is 200+. It. Is taking me whole season to bring it down

 

[mknauss]

If you add any during the season, that will drive your pH higher. Like JamesW says, if you get the TA to about 60, the pH will be pretty much stable at a level of 8 or so. But add any of that fill water, and the pH will rise.

 

[JamesW]

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[Diamond_Pool]

The pH does not stop until the CO2 in the pool is in equilibrium with the CO2 in the air according to Henry's law.

This also depends on the pH.

At a pH of about 7.8 and a Carbonate Alkalinity of about 40, the pH increase is very low.

If the pH is about 8.3, the CO2 is usually low enough to stop the pH rise.

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 9 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

Thank you for insight. This makes me ask another question on TA range of 50-90. What is the rationale for this range ?

 

[JamesW]

Basically, the TA should be wherever it needs to be to make the pH stable.

If the pool is covered, the CO₂ will be kept in more than an uncovered pool and this will change how fast the pH rises.

Different types of chlorine have different effects on the pH.

For example, trichlor is acidic and it will lower the pH, so you need a higher TA.

For concrete pools, the CSI is also important to protect the plaster from dissolving.

 

[JamesW]

The CSI depends on the amount of carbonate, which depends on the TA and the pH.

So, there are three different forms that can be present depending on the pH.

CO₂ (Carbon Dioxide)/H₂CO₃ (Carbonic Acid), HCO₃^- (Bicarbonate ion) and CO₃^2- (Carbonate ion).

Below shows the amount of carbonate vs. pH.

In fact, if the pH is over 8.3 and you add bicarbonate, the bicarbonate will lose hydrogen ions and become carbonate, which means that the bicarbonate actually lowers the pH and acts like an acid.

HCO₃^- -->H⁺ + CO₃^2-.

y = (100/(1+10^(10.3– x))), x from 8 to 12.5 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[AUSpool]

It is a myth that SWGs cause pH rise.

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^-.

HOCl + OCl^- + uv light -> O₂ + H⁺ + 2Cl^-.

Following the process, we can see that there are 4H⁺ and 4OH^- created, which nets out to pH neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

I cant help but wonder though, why only one H⁺ due to UV. Seems a bit convenient. What happens in a slam situation with a high FC/CyA ratio? Are more hydrogen ions created? Or how about a shaded pool in winter with little to no UV effect?

I‘m guessing that there’s more to this story with the interaction of alkalinity that helps to stabilize the system?