pH keeps rising 'off the clock'..! Any ideas?

[Daf-Tekno]

Hi TFPers,

My pH keeps slowly rising, at one point it was the equivalent of 8.2 (off the top of my Taylor tester scale).
My TA is pretty stable at 50, CYA is around 32, CH is 120, Borates of around 50 and Temp of either 99 or 103°F (depending on whether we bathe with our little daughter or not).

I seem to have to keep adding Muriatic acid about once a week to keep bringing it back down, with similarly regular addition of Baking Soda to address the TA alteration this causes.

I can slow the pH rise by reducing my waterfall to a mere trickle, but it easily stops itself altogether when set so low, and the pH still rises just not as quickly.

I assume that I cannot leave a pH of 8.2+ unchecked..?
So, is it just one of the normal 'weekly' routines that a Spa owner must endure, in adding c.50 mL of 20°B Muriatic acid along with 45g of Baking Soda?

And when I add Bleach, I assume that increases pH, as Bleach has a pH of c.11..?

My SWG often creates slightly too much chlorine, so often I come to check the water, and the Chlorine level is 4.5 or higher.
(It is almost impossible to get it to sit at the right level. "2" makes the Ch level decrease, slowly at first then very quickly; yet "3" makes it climb towards 4.5).

Would a slowly increasing Chlorine level from a SWG be part of such a 'pH rise' issue?

Any thoughts or ideas would be very much appreciated.
Thank you - D

 

[Swampwoman]

Its quite normal to adjust the ph weekly or more frequently, especially with a swg, I'd expect.
Also, no need for more cya if its a covered jacuzzi,mat least in terms of preservation from sunlight.

Though I use MA for my pool, Daf, for the hot tub I prefer to use the spa ph down liquid products simply because the quantities are small and handling simpler. Eg. Wearing safety goggles near my open tub at 104 steam up, particularly in sub zero temps

 

[JasonLion]

In a spa you should lower the TA until PH is stable. A TA of 50 results in stable PH for some people, but others need to lower TA further before PH will be stable.

 

[elwood58]

With such a small body of water, try knocking pH down to 7.2. It will aerate back up between water features and SWG. TA will drop, and not go back up with aeration.

 

[chem geek]

The usage/consumption of chlorine is an acidic process so the only net rise in pH from the bleach would be from the "excess lye" in it. Some bleach has more excess lye than others.

A rise in chlorine level does increase the pH, but not that much in your situation. Increasing FC by 5 ppm would raise the pH from 7.5 to 7.64. When the FC dropped back down the pH would go down with it (if there were no other sources of changing pH).

Some of the chlorine gas produced from the SWG may not be getting fully dissolved and outgassing of chlorine gas will raise the pH.

However, since you are having to add baking soda since your acid addition is lowering the TA, this points to carbon dioxide outgassing. That is also consistent with the waterfall increasing the rate of such outgassing.

Did you really mean "washing soda" or did you mean "baking soda". If you are adding washing soda (sodium carbonate), then that not only increases TA but it increases pH even more instead. That would have you add a lot more acid. Baking soda is sodium bicarbonate and is what is added to raise TA.

50 ml of 20º Baumé (31.45%) hydrochloric acid and 45 grams of baking soda in 1700 liters would lower pH from 8.0 to 7.45 with only 1.2 ppm TA increase so this implies that nearly all of your pH rise is from carbon dioxide outgassing. If the pH rise was due to excess lye in bleach or from chlorine outgassing from the SWG then both pH and TA would rise and the acid addition would bring both back down with no need for baking soda.

What is strange, though, is that the pH should have stopped rising before a pH of 7.9 in your situation since with your numbers that is when carbon dioxide in the water would be in equilibrium with that in air.

With your low TA and CH, you don't need to worry about calcium carbonate scaling at the high pH. Even with a pH of 8.2, the saturation index is still -0.4 (the saturation index doesn't change much with rising pH in your situation because the carbonate alkalinity drops as the pH rises and yours is low to begin with). So the main risk with the higher pH is metal staining if you have metals in the water. Otherwise it shouldn't be a problem unless your skin is sensitive to the higher pH. Tap water is often higher in pH in the 7.8 to 8.0 range to reduce the risk of metal corrosion (there is no EPA standard, but a secondary range of 6.5 to 8.5).

 

[mas985]

What is strange, though, is that the pH should have stopped rising before a pH of 7.9 in your situation since with your numbers that is when carbon dioxide in the water would be in equilibrium with that in air.

Even though this a spa with perhaps venturi jets?

 

[chem geek]

Regardless of the amount of aeration, when the pH gets to the equilibrium pH where the amount of carbon dioxide in the water is at equilibrium with the amount in air, then the outgassing should in theory stop. The pH of equilibrium with his numbers is 7.9. It's this low because of the borates. If there were no borates then at TA 50 ppm with CYA 32 ppm the equilibrium pH would be 8.05. The pH would only keep going up above that if there were alkaline/base chemicals being added or if undissolved chlorine gas or hypochlorous acid were being outgassed.

 

[Daf-Tekno]

Thanks y'all for your insights.

Yes, Chem Geek, as always, you're right! My mistake. I meant to say Baking Soda instead of Washing Soda.

One thing that may influence things is this:
I have a floating plastic (bubble-wrap like) thermal cover on top of the water, to help reduce heat loss in the depths of our Canadian Winter.

Might this be causing the creation of a 'sub-climate' underneath it, with different CO2 levels / equilibrium point in comparison to the air above yet underneath the main Spa weather cover..?

I could indeed try lowering the TA to 40, and see if that 'ballasts' the pH.
As long as low TA is equally safe to venture into as high pH.

The only metal of consequence in the Spa that come to mind are the elements of both the heater and Chlorinator.

I don't mind adding stuff to the tub on a weekly basis, but I kinda thought that if I'd eradicated all problems, then this would be reflected in happily stable chemical levels.
I continue to learn, thankfully faster due to your combined assistance!

Many thanks - D

 

[chem geek]

If you have a spa cover and don't have aeration (water bubbling, spillover outside the cover, etc.) then the amount of carbon dioxide outgassing should be reduced.

A lower TA is not a problem, most especially if you have other pH buffers in the water such as borates or CYA itself. As for protecting metal, it's low pH that is most detrimental. A low TA does not harm it. You aren't going to be saturating the water with calcium carbonate in an attempt to form a thin layer to protect metal -- that's very iffy to do and controversial with whether it works reliably (see this link).

It could just be that the continued aeration from the ozonator and other sources forces more carbon dioxide outgassing, but as for what causes the pH to continue to rise above that I don't know. Maybe it's chlorine gas outgassing though I would have thought that a cover would have minimized that. The SWG may have undissolved chlorine gas outgassing and the hot water may outgas some hypochlorous acid though with CYA in the water that should be reduced.

 

[Daf-Tekno]

Hi y'all,

I've reduced my TA to 40, yet my pH is still rising.

I no longer open my cover every day or so, as I used to to allow the Chlorine powder to 'gas off'.
In fact, unless being used for bathing, I don't have the cover open atall really. (Canadian Winter temperatures mean keeping the heat in seems logical).
Even during Shocking, I tend to close it up straight away to avoid heat loss.

Maybe this is part of the cause of my rising pH..?

Also, I remember reading that, as pH rises, then Chlorine becomes less effective, requiring higher amounts to sanitise effectively.
So, surely this means that any 'Acceptable Chlorine Level' table should be directly linked to pH level as well..?
So, if pH is say 7.6, then Chlorine level is good at say between 2 to 4 ppm. But if pH is say 8.0, then the Chlorine range will have to be much higher, yes..?

If my pH level stabilises at over 8.0, then what should be my acceptable Chlorine range? How / to what magnitude are the 2 interlinked?

And just how low can I take my TA level until it becomes problematic?
I could just keep lowering it until my pH stabilises, if there's nothing to lose

Thanks - D

 

[jblizzle]

The pH vs chlorine impact is only true when there is no CYA in the water. Once there is CYA, then the influence is negligible.

 

[chem geek]

I won't go lower in TA without having 50 ppm Borates in the water. As for chlorine and pH, the active chlorine level only drops by 15% when going from a pH of 7.5 to 8.0 when CYA is in the water.

 

[Daf-Tekno]

I indeed have c.50 ppm Borates in my water, and c.32 ppm CYA.

I shall reduce my level of concern accordingly (to c.13 ppm..!)

Thanks guys - D

 

[chem geek]

You could try turning off your Balboa ozonator to see if that makes a difference. Its aeration might be the main cause of pH rise though a lower TA should slow that down if that were the case.