A pH question

[red-beard]

I'm starting to see other pool sites start to promote the idea of FC based on CYA.

Chlorine, pH and Cyanuric Acid Relationships

pH controls chlorine strength (% HOCl), but NOT when Cyanuric Acid (CYA) is in the pool. CYA takes over control of chlorine strength. Did you know that? Chlorine and cyanuric acid.

blog.orendatech.com

Some of them are not new articles, but they are popping up in my news feed. And one article used all of Chemgeek's graphs and data.

But here is the issue I have coming from reading the article. pH is not important to FC activation between 7 and 8.5 pH. It is more used for balance. And at TFP, the suggested range is 7.6-7.9. My pH like be about 8.0. With lots of CH in the water, that tends in the direction of scaling, especially in light of my SWG, with a CSI above 0.

Could I reduce the CH level level and let my pH remain around 8? Playing with pool math, with 3000 Salt, TA 70, Borates 30, Water temp 85, pH 8, it seems if I let my CH slip to 250, I'd have a CSI of -0.11. At 200 CH, the CSI is -0.21.

At present, my CH is around 275 and I add Muriatic Acid when the pH hits 8.0 and drop it to 7.7. So my CSI range is -0.3 to -0.07.

Am I missing something else here? The referenced article indicates that pH vs eye irritation is not an issue. Osmotic pressure (salty eye vs pool water) and CC products are the real culprit. Would it be better to let my pool pH float to where it wants to live and then adjust everything else to keep a good CSI?

 

[JoyfulNoise]

The problem is your SWG cell. With only 30ppm borates you have very weak high pH buffering. Remember, inside the cell and near the cathode where hydroxide is being produced, the pH of the water locally is extremely high. This is why chlorine cells develop lots of scale in highly mineralized water. If you allow your pH to ride too high, you’re going to do damage to your cell. Increasing borates to the suggested 50ppm will help to keep a lid on pH, especially inside the cell. The ultimate pH your pool water wants to be at is driven by carbonate alkalinity, the higher the CarbA the higher the pH ceiling. Maintaining a CSI between -0.3 and 0 with a pH below 8.0 will keep all of your equipment happy.

 

[red-beard]

So, up the borates to 50, can do. Sounds like I could also reduce my TA from 70 to 60, which might help my pH from rising above 7.9.

With 50 Borates, TA 60, Calcium 350 and pH 7.9, I would be CSI -0.27. At pH 8.0 , the CSI would be -0.25.

How do the borates affect CA? The Orenda article says to divide CYA by 3 and subtract it from TA to get Carbonate Alkalinity. Do the borates have an affect on the TA when trying to calculate CA?

 

[JamesW]

How do the borates affect CA? The Orenda article says to divide CYA by 3 and subtract it from TA to get Carbonate Alkalinity. Do the borates have an affect on the TA when trying to calculate CA?

1÷(1+10^(pKa – pH)) = borate percentage.

1÷(1+10^(9.15 – pH)) X 100 = borate percentage.

pH.....Borate.....Boric Acid.
7.2.....1.1%..........98.9%
7.4.....1.7%..........98.3%
7.6.....2.7%..........97.3%
7.8.....4.3...........95.7%
8.0.....6.6...........93.4%.

Assuming 50 ppm borate.

You have 50 grams boron per 1,000 liters.

Boron = 10.81 grams per mole.

You have 4.6253469 moles of boron.

You have 4.6253469 moles of boric acid plus borate.

At a pH of 7.8, there will be 0.19888992 moles of borate and 4.42645698 moles of boric acid.

2 moles of borate = 1 mole of calcium carbonate.

0.19888992 moles of borate = 0.09944496 moles of calcium carbonate.

Calcium Carbonate = 100.01 grams per mole

So, you have 9.9455 grams of calcium carbonate equivalent per 1,000 liter, which is a TA of 9.9455.

So, 50 ppm borate at a pH of 7.8 contributes 9.9455 ppm to the TA.

pH.........TA contribution in ppm (Borate = 50 ppm).

7.2.........2.544
7.4.........3.93
7.6........6.24
7.8........9.9455

Adjusted TA = TA – (CYA X CYA C.F) – (Borate x Borate CF)

Borate C.F (correction) factor based on pH

pH.......CF
7.2.....0.051
7.4.......0.0786
7.6......0.1248
7.8......0.1989

Cyanuric Acid correction factor based on pH.

pH........CF
7.0.......0.22
7.1.......0.24
7.2.......0.26
7.3.......0.28
7.4.......0.30
7.5.......0.32
7.6.......0.33
7.7.......0.34
7.8.......0.35
7.9.......0.36

For example, if the pH = 7.6, TA = 90, Borate = 50 and CYA = 70, the adjusted alkalinity is 90 - (70 x 0.33) – (50 x 0.1248) = 60.66.

 

[JamesW]

pH.....Borate.....Boric Acid.
7.2.....1.1%..........98.9%
7.4.....1.7%..........98.3%
7.6.....2.7%..........97.3%
7.8.....4.3...........95.7%
8.0.....6.6...........93.4%.

As you can see from this chart, most of the borate is in the form of boric acid, which contributes to the total acidity.

Total acidity is the same concept as total alkalinity but total acidity buffers pH rise whereas total alkalinity buffers pH drop.

At a pH of 7.8, there will be 0.19888992 moles of borate and 4.42645698 moles of boric acid.

2 moles of boric acid = 1 mole of calcium carbonate.

4.42645698 moles of boric acid are equivalent to 2.21322849 moles of calcium carbonate

Calcium Carbonate = 100.01 grams per mole.

So, you have 221.34 grams of calcium carbonate equivalent per 1,000 liter, which is a Total Acidity of 221.34 ppm.

With a borate level of 50 ppm and a pH of 7.8, the total acidity will be equivalent to 221.34 ppm of total alkalinity as far as preventing pH rise.

Borates at 50 ppm provide as much protection from pH rise as 221 ppm of TA provides against pH drop.

Carbonic acid and cyanuric acid also contribute to the total acidity.

 

[JamesW]

Hydroxide is produced in the cell.

2H₂O --> H₂ + 2OH^-

2 water –> Hydrogen gas + 2 hydroxide.

The hydroxide converts bicarbonate to carbonate.

HCO₃^- + OH ^- --> H₂O + CO₃^2-

Bicarbonate + hydroxide --> water + carbonate.

Then, the carbonate connects to calcium and you get calcium carbonate.

Ca²⁺ + CO₃ ^2- --> CaCO₃

Calcium + carbonate --> calcium carbonate.

Boric acid protects from pH rise by accepting and binding to the hydroxides produced in the cell due to the production of hydrogen.

B(OH)₃ + OH^- --> B(OH)₄^-

Boric acid + hydroxide --> Borate.

So, it's really boric acid and total acidity that provides the protection from pH rise and cell scaling.

 

[mknauss]

@red-beard -- You asked.

 

[proavia]

@red-beard -- You asked.

I bet after his head stops spinning, he may be sorry he did ask.

 

[red-beard]

I like it! I am normally the one to do posts like this.

And the answer is...if I adjust my TA down, then add borates, to TA 60, I should lock my pH to 7.9. With a CH of 350, the CSI should be in the -0.25 to -0.28 which is perfect for a SWG.

 

[JoyfulNoise]

Excellent post @JamesW. This should be added to the TFP Wiki as it does an excellent job of demonstrating why borates help in an SWG cell.

 

[red-beard]

Here-here!

 

[Bperry]

I like it! I am normally the one to do posts like this.

And the answer is...if I adjust my TA down, then add borates, to TA 60, I should lock my pH to 7.9. With a CH of 350, the CSI should be in the -0.25 to -0.28 which is perfect for a SWG.

For us regular folk, how far down should I lower my TA before replacing with borates?

 

[JamesW]

For us regular folk, how far down should I lower my TA before replacing with borates?

You can add boric acid at any time you like.

 

[Bperry]

You can add boric acid at any time you like.

I’m hesitant to do anything I don’t understand. But looking at the above information leads me to believe I’ll never REALLY understand.

This is what I was interested in learning more about since I’m in a similar situation with the weather warming up.
—
And the answer is...if I adjust my TA down, then add borates, to TA 60, I should lock my pH to 7.9.
—

My reading of that is that adjusting the TA down, then raising it back up to TA 60 with borates will tend to hold the pH at 7.9. But boric acid is an...acid and not alkaline so kind confused.

 

[red-beard]

It's a weak acid