PH

[cledee]

I added some Trichlor last week to make up for the CYA loss the last couple of months to get it back to 40ppm.
My PH was 7.3 before the addition, my alk was 70. According to Pool Math I should expect PH to drop to about 6.8. I will confirm in testing, but should I add Borax to raise it up to 7.0-7.2 or just let it rise on its own, with alk still at 70 PH won't rise as fast?

I do have a pool heater and my current CH level is 100ppm.

 

[Bperry]

I added some Trichlor last week to make up for the CYA loss the last couple of months to get it back to 40ppm.
My PH was 7.3 before the addition, my alk was 70. According to Pool Math I should expect PH to drop to about 6.8. I will confirm in testing, but should I add Borax to raise it up to 7.0-7.2 or just let it rise on its own, with alk still at 70 PH won't rise as fast?

I do have a pool heater and my current CH level is 100ppm.

Note the message on that screen that says pH changes are estimates especially when changing more than 0.4. Don’t ever make changes based on estimates. That said, the pH will rise on its own.

 

[cledee]

Note the message on that screen that says pH changes are estimates especially when changing more than 0.4. Don’t ever make changes based on estimates. That said, the pH will rise on its own.

I'm going to still confirm with testing but I guess my question was if PH did fall to 6.8 should I just sit it out and let it rise on its own or get it to 7.0 right away.

 

[JamesW]

2C₃Cl₃N₃O₃ + 6H₂O --> 3HOCl + 3OCl^- + 2C₃H₃N₃O₃ + 3H⁺

3HOCl + 3OCl^- --> 3H⁺ + 3O₂ + 6Cl^-

2C₃Cl₃N₃O₃ + 6H₂O --> 2C₃H₃N₃O₃ + 6H⁺ + 3O₂ + 6Cl^-

Trichlor + water --> cyanuric acid + hydrogen ions + Oxygen + chloride.

1 mole of trichlor creates 3 moles of hydrogen ions.

232.40 g of trichlor creates 3 moles of hydrogen ions.

1 liter of hydrochloric acid (31.45% or 10 M) provides 10 moles of hydrogen ions.

1,160 grams per liter.

31.45% of 1,160 = 364.82 grams of HCL.

Molar mass: 36.46 g/mol.

This is 10 moles per liter.

300 ml of hydrochloric acid provides 3 moles of hydrogen ions.

232.40 g of trichlor is equivalent to adding 300 ml of hydrochloric acid.

8.2 oz (weight) of trichlor is equivalent to adding 10.1442 fluid ounces of hydrochloric acid.

0.5125 lb of trichlor is equivalent to adding 10.1442 fluid ounces of hydrochloric acid.

1.0 lb of trichlor is equivalent to adding 19.794 fluid ounces of hydrochloric acid.

1.0 lb of trichlor added to 10,000 gallons will lower the will lower the TA by 7.7 ppm.

50 lbs of trichlor (is equal to 7.73 gallons of 31.45% acid) in a 25,000 gallon pool will:

Raise fc by 219 ppm

Raise CYA by 133 ppm

Raise salt by 179 ppm

Lower TA by 154 ppm

To maintain the same TA, you would need to add 56.5 pounds of baking soda.

To maintain the same total alkalinity, you have to add 1.13 pounds of baking soda per pound of trichlor.

To maintain the same total alkalinity, you have to add 1.0 pounds of baking soda per (2) 3" tablets of trichlor (7 ounce weight per tab).

 

[Bperry]

I'm going to still confirm with testing but I guess my question was if PH did fall to 6.8 should I just sit it out and let it rise on its own or get it to 7.0 right away.

I’d sit it out, but I doubt it’ll get that low.

 

[JamesW]

I added some Trichlor last week to make up for the CYA loss the last couple of months to get it back to 40ppm.

How much trichlor?

Test the pH and TA before doing anything.

 

[cledee]

How much trichlor?

Test the pH and TA before doing anything.

1.5 lbs

Before I added the Trichlor, I tested PH at 7.3PPM and ALK at 70PPM.

 

[JamesW]

1.5 lbs

Before I added the Trichlor, I tested PH at 7.3PPM and ALK at 70PPM.

1.5 lb of trichlor is equivalent to adding 30 fluid ounces of hydrochloric acid.

 

[JamesW]

According to Pool Math I should expect PH to drop to about 6.8.

That is assuming the trichlor is added all at once.

If it is added over time, the pH recovers due to the TA creating carbon dioxide, which offgasses.

So, the TA should drop by 6.5, which might or might not be detected in the TA test.

The pH should mostly recover assuming the trichlor takes a few days to dissolve.

 

[JamesW]

The Y axis is the percentage of baking soda that converts into carbon dioxide.

The X axis is the pH.

If the pH is 6.8, about 26% of the carbonate alkalinity converts to carbon dioxide.

HCO₃^- + H⁺ --> H₂CO₃ --> H₂O + CO₂

This process raises the pH because the hydrogen ions are picked up.

The CO₂ offgasses and more carbonate alkalinity converts to CO₂ and the pH rises even more.

This is happening as the trichlor dissolves.

If the TA is correct, the pH should never be in danger of going too low.

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.

www.wolframalpha.com

 

[cledee]

The Y axis is the percentage of baking soda that converts into carbon dioxide.

The X axis is the pH.

If the pH is 6.8, about 26% of the carbonate alkalinity converts to carbon dioxide.

HCO₃^- + H⁺ --> H₂CO₃ --> H₂O + CO₂

This process raises the pH because the hydrogen ions are picked up.

The CO₂ offgasses and more carbonate alkalinity converts to CO₂ and the pH rises even more.

This is happening as the trichlor dissolves.

If the TA is correct, the pH should never be in danger of going too low.



View attachment 523352

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.

www.wolframalpha.com

That makes sense. So because my TA is still in the 60-70 range as the Trichlor is dissolving the PH is recovering back to it's original or close to original ppm prior to adding the Trichlor. Thank you @JamesW

 

[JamesW]

the PH is recovering back to it's original or close to original ppm

Note that pH is not reported in ppm.

 

[JamesW]

Cyanuric acid has a pK_a of 6.88.

At pool pH (7.6), there is about 16% that is cyanuric acid and the rest is cyanurate.

100C₃Cl₃N₃O₃ --> 16C₃H₃N₃O₃ + 84C₃H₂N₃O₃^- + 84H⁺

Adding CYA lowers the pH due to the released hydrogen ions, but the TA stays the same because there are the same number of cyanurate ions created as the number of hydrogen ions.

Each cyanurate ion counts towards the TA and each hydrogen ion counts against the TA for a net result of zero TA change.

When accounting for the TA change caused by trichlor, the effect of the CYA can be ignored by considering it as all cyanuric acid, which is what it will become as the sample is titrated down to a pH of 4.5.

 

[JamesW]

If you want to account for the pH, 1 mole of trichlor produces 3.84 moles of hydrogen ions (Based on a pH of 7.6).

Different pH starting points will produce different pH end points.

So, predicting the exact effect of the trichlor on the pH is very difficult and it depends on the amount of trichlor added, the TA and the starting pH.

The effect of trichlor on the pH is about 1.28 times as much as the effect on the TA.

For the TA, 1.0 lb (16 ounce weight) of trichlor is equivalent to adding 19.794 fluid ounces of hydrochloric acid.

For the TA, 1.5 lb (24 oz) of trichlor is equivalent to adding 29.69 fluid ounces of hydrochloric acid.

For the pH, 1.0 lb of trichlor is equivalent to adding 25.336 fluid ounces of hydrochloric acid.

For the pH, 1.5 lb of trichlor is equivalent to adding 38.004 fluid ounces of hydrochloric acid.

2C₃Cl₃N₃O₃ + 6H₂O --> 2C₃H₃N₃O₃ + 6H⁺ + 3O₂ + 6Cl^-

100C₃Cl₃N₃O₃ + 300H₂O --> 100C₃H₃N₃O₃ + 300H⁺ + 150O₂ + 300Cl^- (TA).

100C₃Cl₃N₃O₃ + 300H₂O --> 16C₃H₃N₃O₃ + 84C₃H₂N₃O₃^- + 384H⁺ + 150O₂ + 300Cl^- (pH).