pH balancers

[goose973]

I've had my new 340 gallon hot tub about a week. The dealer included leisure time pH balance in our startup chems. Our water was 7.2 pH after filling and coming up to temp. so we added the whole bottle (for spas up to 500 gal) without any adjustment as the bottle stated to do. The next day, the pH was around 8. Took a couple of doses of pH down over a few days to bring things back to around 7.4-7.6.  The day after my first week's oxy shock, the pH was down to 7. Two daily doses of pH up and we're at 7.4.

My question is, for something that is supposed to prevent pH swings, why do I still have to make so many adjustments?  Is there something I'm doing wrong? I wouldn't think it should take over a day for this stuff to come back to equilibrium, does it? BTW, my TA went way up after adding the balancer. My Ca hardness remained Ok, about 100ppm or so throughout. Didn't have cloudy water or any scale deposits form throughout all this.

[sorebikr]

I figure that this might turn into a PH discussion, and/or may lead to a link to a past post about it so I'd chime in with a similar question. 

How to raise TA while keeping your PH low enough.  Seems like a neverending back and forth between the Baking Soda and PH Down.

[goose973]

I have the opposite problem. My TA was originally right where it needed to be. Then, I added the pH balancer, TA shot up, and I have pH swings that the balancer is supposed to prevent.

[chem geek]

You need to remember that Total Alkalinity (TA) is not just a pH buffer, but also a SOURCE of rising pH in its own right.  It is not a pure pH buffer (such as a phosphate buffer).  This is because TA is mostly a measure of the bicarbonate in the water and is related to how much carbon dioxide there is in the water.  The spa is over-saturated with these carbonates so carbon dioxide outgasses from the spa, especially when there is aeration (so running spa jets is a lot of aeration) and the pH rises with no change in TA.

So the basic rule is if your pH tends to rise over time, then lower your TA level.  When you add acid, this lowers the pH and TA so over time if you do not add anything to raise the TA it will drop.  pH Up raises both pH and TA and is technically identical to adding lye (a pure base for raising mostly pH) and baking soda (used to raise mostly TA).  There should be a TA level where the acidity from your sanitizer/oxidizer additions balances the pH rise from carbon dioxide outgassing so that the pH is relatively stable.  At that point, only the TA will drop slowly over time and can be restored with baking soda.

If, on the other hand, your pH tends to fall over time, then you can raise your TA level (within limits -- if you water is high in CH, then you need to be careful to prevent scaling).

Note that Dichlor is net acidic because chlorine usage/consumption is acidic.  MPS is acidic.  Bromine tabs, when accounting for bromine usage/consumption, are net acidic.

[goose973]

Thanks, geek!  That was the best explanation I got, yet, on what is going on here. So, basically, I can expect a short-term pH rise after heavy use of the jets, but with the use of the MPS shock and dichlor sanitizer the pH should stabilize and TA will probably drop over time. I think I will try to not over correct slightly high pH especially after heavy use and realize that a slightly low pH after shocking is not unusual. Make sense?

[sorebikr]

Hi -

Not trying to thread-jack as it appears you're getting your answer too Goose.  Chemgeek - I hoped you'd chime in.  I've tried to learn a lot from your past posts, but I'm still easily confused when it comes to chemistry.

So I have a 500gal tub (+/-) and my TA was 40 with a PH of 7.8.  How would you suggest moving forward?  Raise the TA to acceptable ranges (100-120?) and then try to adjust the PH back down? 

[chem geek]

I think I will try to not over correct slightly high pH especially after heavy use and realize that a slightly low pH after shocking is not unusual. Make sense?

Yes, that makes sense.  You don't want to be too precise and frequent about your pH adjustment in a spa because it WILL go up and down.  You want to instead target parameters, such as TA, to have the longer-term pH be about where you want.

Also, note that you can have an additional pH buffer that is not carbonate based by using 50 ppm Borates in the spa.  This is in products such as Proteam Gentle Spa though some people have recently reported that it isn't as pH neutral as it used to be.  Technically, all you need is boric acid which you can buy directly if you want (it's only slightly acidic so doesn't lower the pH very much when adding it).  You can use The Pool Calculator to calculate dosages.  You can get boric acid at The Chemistry Store and at AAA Chemicals -- they vary on shipping costs depending on where you are in the U.S.

[chem geek]

So I have a 500gal tub (+/-) and my TA was 40 with a PH of 7.8.  How would you suggest moving forward?  Raise the TA to acceptable ranges (100-120?) and then try to adjust the PH back down?

You didn't say whether your pH tends to rise or fall.  If it tends to rise, which I would think with that low TA would only happen if you were using a hypochlorite source of chlorine such as bleach or lithium hypochlorite, then I wouldn't change the TA but would consider using the 50 ppm Borates (see my previous post).  If the pH tends to fall and what you reported was a temporary rise after adding pH Up, then you can raise your TA higher with baking soda to see if that makes your pH more stable.

If the pH tends to settle in at 7.8, then I wouldn't worry about it and just leave it there.  The rate of pH rise from outgassing slows down at higher pH and 7.8 is not going to be a problem (tap water is often 7.8 to 8.0 in pH).  This post shows how over-carbonated pool/spa water is compared to the carbon dioxide in the air at various pH and TA levels (all assuming a CYA of 30 ppm).  The color coding was more for pools -- with spas and their aeration jets and hotter water, you will get a lot more carbon dioxide even in the "green" areas of the chart with higher numbers.