How Much Dry Acid At One Time?

[seligman]

I have a plaster spa that holds about 600-700 gallons. How much dry acid can I safely add at one time (dissolved in a gallon bucket)? What is the maximum amount per day?

Reason I ask is that our tap water has a pH well over 8 with total alkalinity over 200ppm.

Yesterday I added a total of 3 ounces dry acid.

Today I added another 3 ounces dry acid.

According to my Taylor test kit, the pH is still around 8.0 and the total alkalinity is 140-150ppm. Calcium Hardness is around 260ppm.

[chem geek]

3 ounces weight of dry acid into 650 gallons with your pH of 8 and TA of 145 would result in a pH of 7.25 with the TA dropping to 132 and then if you run your jets the pH will rise again. If you want to lower the TA faster, you can safely add 5 ounces of acid to go from a pH of 8.0 to 7.0 and then immediately run your jets to make the pH rise and repeat this process. I wouldn't go below 7.0 in pH. So the amount of acid you can add depends on your starting and ending pH as well as the TA. Fortunately, it takes more acid to move the pH when it is lower so it's hard to overdose -- adding 5 ounces of acid starting at a pH of 7.5 would result in a pH of 6.9.

So if you are at a pH of 8.0 or above, you can add up to 5 ounces of acid when your TA is at 145. If your TA drops closer to 100, then you wouldn't want to add any more than 3.5 ounces since at that TA this would take the pH from 8.0 to 7.0. Aeration can then be used to raise the pH without changing the TA.

If you lower your TA, then you should find that the pH has less of a tendency to rise over time and you'll need to add less acid to restore the pH to where you want it. This is counterintuitive, but is how it works -- TA does buffer against pH changes from external sources, but it also contributes directly to a rise in pH via carbon dioxide outgassing (the spa is essentially over-carbonated, much like a carbonated beverage -- and you know how blowing bubbles in a carbonated drink will make it go flat faster).

Richard

[Whoneedsavacation]

I have the opposite problem... my pH and TA are constantly drifting down. I usually wind up adding quite a bit of baking soda to get the TA where it wants to be, then adding some dry acid to adjust the PH. What is the chemical process here? Also, how do pH lockers work chemically? We are getting near to refill time and I was considering using one.

[seligman]

OK, it took approximately 6 more ounces of dry acid, but my pH is now 7.5 with TA 100-110.

Is this a relatively common occurrence for a spa to require 12-14 ounces of dry acid whenever it is drained and refilled? Without much experience in this area, I keep worrying that this is too much.

[navsav]

OK, it took approximately 6 more ounces of dry acid, but my pH is now 7.5 with TA 100-110.

Is this a relatively common occurrence for a spa to require 12-14 ounces of dry acid whenever it is drained and refilled? Without much experience in this area, I keep worrying that this is too much.

Hey I have your same problem. I seem to always be adding acid since my area had very hard water witha high PH. The other day I added around 5 OZ of acid to get it down to 7.4 - but then checking my TA it was only at 60ppm. But of course when I add baking soda the PH goes up - this is a never ending battle!!

I am going to check the spa tonight and see where I am at and post it on here.

[chem geek]

I have the opposite problem... my pH and TA are constantly drifting down. I usually wind up adding quite a bit of baking soda to get the TA where it wants to be, then adding some dry acid to adjust the PH. What is the chemical process here? Also, how do pH lockers work chemically? We are getting near to refill time and I was considering using one.

If your pH and TA are drifting down, then I suspect you are using an acidic source of chlorine or shock or both. Dichlor is actually acidic when one accounts for both the addition AND consumption of chlorine (unlike bleach or chlorinating liquid or Cal-Hypo or any other hypochlorite source of chlorine all of which are actually pH neutral when one accounts for addition and consumption). The consumption (using up) of chlorine is acidic and most people don't account for that. So using Dichlor is acidic.

1 Tablespoon of Dichlor (dihydrate) in 500 gallons will raise the FC by 4.15 ppm and after that FC gets used up, the pH will go from 7.5 to 7.35 (assuming a TA of 100 with a CYA of 30). So continued use of Dichlor is acidic and if nothing else was going on, the pH (and TA) would drop over time.

If you use a non-chlorine shock (e.g. potassium monopersulfate), then that is acidic as well, but not nearly as much as Dichlor (for equivalent amounts of weight).

Whether pH goes up or down depends on which effect is stronger -- the acidity from using Dichlor vs. the outgassing of carbon dioxide from the aeration. Even if the pH were stable in such a situation, the TA would slowly drop over time.

[chem geek]

OK, it took approximately 6 more ounces of dry acid, but my pH is now 7.5 with TA 100-110.

Is this a relatively common occurrence for a spa to require 12-14 ounces of dry acid whenever it is drained and refilled? Without much experience in this area, I keep worrying that this is too much.

Don't worry. If you have initial fill water that is high in pH and TA, then it can take quite a bit of acid. Theoretically, going from the TA of 200 to 105 and a pH of 8.0 to 7.5 would take 21 ounces weight of dry acid plus a lot of aeration. If you didn't have the aeration, then going from a TA of 200 to 190 and a pH of 8.0 to 7.5 would only take 2.3 ounces weight of dry acid. The large amount of acid in the first case is for the reduction in TA and occurs along with aeration. Since aeration is common in spas, due to the jets, getting the TA lower is really the price you have to pay with high TA fill water. It's not really the pH that is as much of the issue -- the high TA causes the pH to rise so it seems to get "stuck" high or always be rising, at least until you get the TA to a lower level.

I hope that makes some sense -- it's one of those counterintuitive things in water chemistry. If you think of a spa or pool as a carbonated beverage, then you know that blowing bubbles or stirring it up causes it to go flat faster. If you were to measure the pH, you would find that it rises. The same thing goes on with your spa except that adding acid brings the pH down and at lower pH this outgassing process is accelerated.

Richard

[seligman]

Don't worry. If you have initial fill water that is high in pH and TA, then it can take quite a bit of acid. Theoretically, going from the TA of 200 to 105 and a pH of 8.0 to 7.5 would take 21 ounces weight of dry acid plus a lot of aeration. If you didn't have the aeration, then going from a TA of 200 to 190 and a pH of 8.0 to 7.5 would only take 2.3 ounces weight of dry acid. The large amount of acid in the first case is for the reduction in TA and occurs along with aeration. Since aeration is common in spas, due to the jets, getting the TA lower is really the price you have to pay with high TA fill water. It's not really the pH that is as much of the issue -- the high TA causes the pH to rise so it seems to get "stuck" high or always be rising, at least until you get the TA to a lower level.

I hope that makes some sense -- it's one of those counterintuitive things in water chemistry. If you think of a spa or pool as a carbonated beverage, then you know that blowing bubbles or stirring it up causes it to go flat faster. If you were to measure the pH, you would find that it rises. The same thing goes on with your spa except that adding acid brings the pH down and at lower pH this outgassing process is accelerated.

This is really helpful, thanks. I just needed some reassurance that I wasn't doing something wrong or overlooking something.

[Whoneedsavacation]

How well do pH lockers work? HOW do they work? Why is it recommended to use them only at fiill time?

[chem geek]

How well do pH lockers work? HOW do they work? Why is it recommended to use them only at fiill time?

Do you have an example product I can look up? I'm not sure what you are talking about unless it's something like Proteam Supreme which is Borates. Borates are an additional pH buffer, but they are also an algaecide so in SWG pools they can help reduce the problem of rising pH by killing algae so that chlorine demand is lower thereby letting you reduce SWG on time thereby reducing aeration thereby reducing carbon dioxide outgassing thereby reducing pH rise. A long and complicated chain of events, but it has worked well for some SWG users.

[Whoneedsavacation]

Well Leisure Time has a product called "pH Balance Plus" that is supposed to "lock in proper pH from refill to refill". Their website doesn't say what it actually is, though.

[chem geek]

Well Leisure Time has a product called "pH Balance Plus" that is supposed to "lock in proper pH from refill to refill". Their website doesn't say what it actually is, though.

The MSDS for that product is here and indicates that it is a proprietary mixture, but they list Boric Acid and Sodium Carbonate as its ingredients. So essentially it's like combining pH Up with Borax and acid. Depending on the relative amounts of Boric Acid and Sodium Carbonate, the pH will probably remain relatively neutral. The MSDS indicates that the mixture has a pH of 7-8 so apparently is balanced to be around 7.5. By the way, pH Up (sodium carbonate) is identical to Arm & Hammer Washing Soda (caution: not the detergent). I believe that additional acid, such as sodium bisulfate, is needed for this product to be truly pH neutral (or near 7.5), but they don't have that listed and I could be mistaken (it's just that Boric Acid is very weak compared to sodium carbonate at the pH around 7.5 so the tendency should be for the pH to rise when combining these two -- perhaps there is quite a bit more Boric Acid than Sodium Carbonate in this product).

So the net effect of using this product would be an increase in TA just as if you had added Alkalinity Up plus an increase in Boric Acid (i.e. borates) which would be equivalent to adding a combination of 20 Mule Team Borax plus Muriatic Acid (but without the extra salt produced from this combination). It also increases the Borates level in the pool and as I indicated earlier, this is an additional pH Buffer.

So it's a combo product, but you can accomplish the same thing using the following combination of grocery/hardware store products:

50 ounces weight 20 Mule Team Borax

50 ounces weight Arm & Hammer Baking Soda

26 fluid ounces Muriatic Acid (31.45% Hydrochloric Acid)

The above in 10,000 gallons will increase the TA by about 22 ppm and the Borates level (measured as ppm Boron) by about 4 ppm and there is no change in the pH. The above also increases the salt level by 10 ppm which would not occur if using the pH Balance Plus product directly, but that's no big deal. The pH Balance Plus may be balanced more heavily towards Boric Acid in which case there would be less Baking Soda and more Borax and Muriatic Acid added than what I indicated above. For example, the following:

50 ounces weight 20 Mule Team Borax

10 ounces weight Arm & Hammer Baking Soda

25 fluid ounces Muriatic Acid (31.45% Hydrochloric Acid)

The above in 10,000 gallons will increase the TA by about 4.5 ppm and the Borates by about 4.25 ppm so is more balanced for each and there is still no change in pH.

Generally speaking, it is best to add chemicals individually so you can control the parameters separately. With Borates, you want to add the amount you want as a buffer or as an alagecide (30-50 ppm range for algaecidal properties) and then stop. TA will reduce over time if acid is added to lower the pH after it rises from carbon dioxide outgassing.

Richard

[seligman]

It's me again.

Yesterday the pH was around 7.5, but tonight it's over 8.0 like before. The TA is unchanged at 100ppm. Since testing the water yesterday, I have only run the jets for 2, maybe 3 hours total.

How should I handle this? Do I keep adding dry acid to lower the pH, combined with some baking soda to keep the TA from getting too low?

[chem geek]

It's me again.

Yesterday the pH was around 7.5, but tonight it's over 8.0 like before. The TA is unchanged at 100ppm. Since testing the water yesterday, I have only run the jets for 2, maybe 3 hours total.

How should I handle this? Do I keep adding dry acid to lower the pH, combined with some baking soda to keep the TA from getting too low?

No, if you add acid, then the pH and TA get lower, but if you add baking soda you'll have the TA go back up and be back to where you are now -- still experiencing rising pH. The good news is that at least now the rising pH takes less acid to lower again and it may be rising more slowly (a little), but it's hard to see since you went over 8.0 before. So long as you don't plan on using an acidic source of chlorine (Trichlor, Dichlor), you can let your TA go lower. That is, add the acid to adjust the pH, but do not add any baking soda. You can lower the TA all the way down to 50-60 and still have enough buffering in your pool water (assuming your CYA is less than 30). If you find that at the lower TA you have less pH rise, then great, you've got a solution. If not, then you can always easily increase the TA back up. So it's worth trying a lower TA -- at least get down to 70-80 and see how that is.

And keep us posted. This usually works for those with pools, but spas have a lot more aeration so this would be good info to have (i.e. to see if this is as effective in spas).