Lowering Total Alkalinity in a Spa

[BobN54]

Here are the numbers.

TA 300 to 60: half of 21.5 is 10.7 ounces; pH drops to 6.41; if one cup is used, pH drops to 6.60
TA 200 to 60: half of 12.5 is 6.3 ounces; pH drops to 6.50
TA 100 to 60: half of 3.6 is 1.8 ounces; pH drops to 6.83

As you pointed out, this pH probably doesn't last very long given the aeration.

Quoted from: http://www.troublefreepool.com/threads/11598-Lowering-Total-Alkalinity-(A-Modified-Approach)

Topic split. Please ask questions in your own topic, rather than hijacking someone else's existing topic. JasonLion

I'm having trouble understanding how to calculate the amount of acid required. In my 300 gal. spa the starting TA is 200 and ph is at least 8.0. Your numbers from this thread suggest I need to add ~ 11 oz. acid (you don't specify what type, concentration, or whether by volume or weight). However, I can't duplicate that result using the pool calculator or the guidance that came with my acid demand test kit. Both of those suggest far less acid; plugging 350 gal, 200 starting TA, 60 goal TA, 8.0 starting ph, goal 6.8 ph into the calculator I see 3.9 oz 31.45% muriatic or 5.3 oz. by weight dry acid. I've tried the pool calculator recommended amount and sure enough, the ph barely moves even when not aerating, so I presume the numbers in this thread are more like it. Where am I going wrong in my understanding or operation of the calculator? Why don't the pool calculator numbers agree with your recommendations in this thread?

 

[chem geek]

The Pool Calculator (now we use PoolMath) does not calculate the amount of acid needed to lower the TA because you don't add it all at once. If you put in a lower Goal than Now, it doesn't say to add any baking soda and there's nothing but the text that says "To lower TA you reduce pH to 7.0-7.2 with acid and then aerate to increase pH.". This is the same as the procedure in the Pool School to Lower Total Alkalinity. In that procedure, you lower the pH to around 7.0, aerate, then keep adding acid whenever the pH rises until you reach your TA target.

Now you CAN indirectly find out how much total amount of acid will be needed to lower TA in PoolMath by putting in your spa water volume and then in the "Effects for adding chemicals" section you can select the type of acid you are going to use and see the effect on TA. So for every 1 ounce weight of dry acid you add to 300 gallons, the TA drops by 9.7 ppm. The numbers you are quoting from the thread assume 350 gallons (you would see that if you looked earlier in the thread at this post and the acid being used is full-strength Muriatic Acid (31.45% Hydrochloric Acid).

So if you want to follow the alternate method that doesn't worry about going below 7.0 in pH because in a spa with aeration it won't stay that way for very long, you can figure out the total amount of acid needed so to get from 200 to 60 it would take (200-60)/9.7 = 14.4 ounces weight of dry acid. You could then add half of that (i.e. 7.2 ounces), aerate to get the pH back up, add half of the remaining amount (i.e. 3.6 ounces), aerate, etc. though when you get to the point where adding the rest doesn't go very low in pH, just add it so after you add 1.8 ounces and aerate to get the pH back up you can probably just add the last 1.8 ounces. Obviously you can't keep adding half forever.

 

[BobN54]

That's helpful; thanks. Yes, I am using Poolmath. I followed the iterative procedure and overshot slightly. My TA=20 and ph=7.2 in ~300 gallon spa. What do you recommend I do from here? PoolMath suggests 2 oz of baking soda and 0.5 oz of borax to go to TA=50 and ph=7.4. My wife suggests leaving it alone

 

[chem geek]

I'd bump up the TA some -- don't worry about the pH as it should rise on its own, but have you added 50 ppm Borates (say, with boric acid) yet, assuming you're using the Dichlor-then-bleach method?

 

[BobN54]

I'd bump up the TA some -- don't worry about the pH as it should rise on its own, but have you added 50 ppm Borates (say, with boric acid) yet, assuming you're using the Dichlor-then-bleach method?

I've added PoolMath's calculated amount of baking soda to bring the TA up. I'm planning on using the 3-step bromine process because it requires less testing and attention. Boric acid still recommended in that case? I'll need to go to Ace an pick up some insect dust.

 

[chem geek]

With bromine tabs that are acidic you shouldn't need the borates. You can certainly see if your pH is stable. And if it is tending to drop, you can raise your TA target higher. So just see how things go and you can do what is needed (if anything) later.

 

[BobN54]

Okay, got it; no boric acid. We'll see how the bromine 3-step process works for us. If that doesn't work out, I think we'll do a water change and try a DO SWG with chlorine. Sodium bromide is too expensive to get the concentrations recommended for a SWG.

 

[chem geek]

Yes, for a chlorine SWG there is ControlOMatic TechniChlor unless you wanted to go with an inline system. You should make sure your spa components are corrosion-resistant since 2000 ppm sodium chloride salt is used with this system.