Saturation Index - Ph 8.0 But Balanced

[Vince22]

Hi all,

My saturation index at rest (per the pool calculator) is at -0.07 so the water is nicely balanced as far as I understand it.

TA: 60

CH: 110

PH: 8.0

Bromine tub. Sodium bromide with bleach + floater.

450 gallon Viking Heritage II

However, I keep seeing in people's posts that the ideal PH is around 7.6. If I were to lower my Total Alkalinity to the point (around 40ppm)where PH settled around 7.6, my saturation index--given the Calcium level--would be quite unbalanced.

So...which is better: 1) Balanced water (SI near zero) with PH at 8.0, or 2) Unbalanced water with PH at 7.6?

This is all based on my Ch level of 110ppm. Would I be better off rasing CH to 150 in order to have a saturation index near zero with a lower PH?

[chem geek]

It's up to you because the higher pH in your situation is mostly only a problem for your eyes. So in a pool that would be more of a problem, but in your spa that depends on whether you dunk your head underwater or otherwise get spa water into your eyes. Also, a pH of 8.0 is near the high-end of the range of the pH kit so you might not know if it got higher. Finally, I get different results than you did for the saturation index (more on that below).

Normally, one would just have a higher CH and a somewhat lower pH closer to 7.7 or thereabouts. Again, that's up to you. Also, having a somewhat negative saturation index isn't a big problem in most spas since they don't have exposed plaster/gunite/grout and saturation with calcium carbonate isn't needed for vinyl (though for some fiberglass with a calcium carbonate based gelcoat it might be needed).

By the way, I get a different result than you do. I assume the CYA is zero since you are using bromine, is that correct? Or are you using Dichlor for shocking so some CYA has built up? With a pH of 8, TA of 60, CH of 110, CYA of 0, then I only get a saturation index of -0.07 if I set the temperature to 70F which doesn't seem right; at 104F the saturation index is +0.21 so at some risk for scaling (especially with the pH high). A reasonable balance would be a pH of 7.7, TA of 50, CH of 110, CYA of 0 and temp of 104F which results in -0.04.

[Vince22]

By the way, I get a different result than you do. I assume the CYA is zero since you are using bromine, is that correct? Or are you using Dichlor for shocking so some CYA has built up? With a pH of 8, TA of 60, CH of 110, CYA of 0, then I only get a saturation index of -0.07 if I set the temperature to 70F which doesn't seem right; at 104F the saturation index is +0.21 so at some risk for scaling (especially with the pH high). A reasonable balance would be a pH of 7.7, TA of 50, CH of 110, CYA of 0 and temp of 104F which results in -0.04.

Oops. Quite right. I forgot to change the temp. (102f). Using bleach with bromine floater. No CYA. I'll go home and goof with it tonight. The other question I have is this: Should the PH be optimal when the water is at rest (jets off) or when the jets are on? I ask because I've seen posts saying that you should balance the water so that the PH drifts up in to the target range with aeration. However, that's about 30min out of each day. What about the other 23h30m? Shouldn't that be when the PH is optimal?

Thanks,

Vince

[chem geek]

You are correct that if you have a choice, then it would be better to balance the pH so that it is where you want it to be after your soak rather than before. The pH will tend to rise during your soak if you've got the jets on.

[quantumchromodynamics]

Note: For bromine, the pH has less of an effect on the hypobromous acid/hypobromite ion ratio than it does for chlorine.

Hypochlorous acid or hypobromous acid is a much better sanitizer than the corresponding hypochlorite or hypobromite ion.

At a pH of 7.6, about 42 % of chlorine (that is not attached to cyanuric acid) is hypochlorous acid and about 58 % is in the form of the hypochlorite ion.

However, at a pH of 7.6, about 91 % of bromine is in the form of hypobromous acid and about 9 % is in the form of the hypobromite ion.

At a pH of 7.9, only 27 % of the chlorine is in the form of hypochlorous acid, while 83 % of the bromine is in the form of hypobromous acid. This is one reason that it is OK to operate a bromine tub at a higher pH than a chlorine tub.

Hypobromous acid pKa = 8.62 (About 8.6 to 8.8. Does not account for temperature or ionic strength.)

Hypochlorous acid pKa = 7.46 (About 7.4 to 7.6. Does not account for temperature or ionic strength.)