Lowering Total Alkalinity (A Modified Approach)

[Nitro]

The problem with lowering Total Alkalinity (TA) in general is that when you add acid to lower TA, pH also decreases. This limits the amount you can lower TA without pH becoming too low. However, there is something you can do to raise pH that leaves TA alone, Aeration. Aeration is the process of turning on all the jets and air features in your tub, in order to pump as much air into the water as possible. By adding Acid and Aerating you can lower TA, while keeping pH the same. I won't get into detail why you would want to do this here, but I will say this. The key to a balanced tub is, you should NOT try to adjust pH directly. Instead, you should focus on Tuning TA to an ideal level, in order to keep pH in an ideal range.

This method is a slight variation to the Acid/Aeration method. The main problem with the old method is that it's difficult to know how much acid to add each time. The amount of acid it takes to lower pH to 7.0 will vary a lot depending on the TA. Also, if you have a very high TA (>300 ppm), it can take a long time to lower it if only small amounts of acid are added. So, I decided to refine the method a little. If done correctly, you can lower TA quickly, without pH ever dropping below 7.0.

The first thing to remember is, the amount that TA decreases is directly proportional to the amount of acid you add. That is, in a 350 gal tub, 8 oz of Dry Acid will lower TA by 100ppm. It is important to know (by calculating) how much Total Acid you need to add to your tub in order to get to your Target TA. The second (key) thing to remember is, the higher your TA, the less effect acid will have on your pH. If your TA is really high (>300), adding acid won't effect pH much at all. This confuses beginners (and some experts alike), which causes them to take much longer to lower TA, or give up altogether. Because of this key point, you need to add more acid in the beginning, and less as you get closer to your Target TA.

So, here is the process.

1. Turn on all your jets and air features and keep them on the entire process. (i.e. Aerate)
2. Check starting TA/pH.
3. If pH is < 7.6, goto step 6.
4. Calculate how much you want to lower your TA, and how much acid you need to add to lower it.
5. Add to the tub HALF (not > 1 cup nor < 1 TBS) of the amount of the remaining acid you need to add.
6. Aerate for 30min, and check TA/pH.
7 If TA > Target and pH is > 7.8, goto step 4
8 If TA > Target and pH is < 7.6, goto step 5 one time.
9 If TA > Target and you can't get pH > 7.8 via aeration, Target too low. Stop here.
10 If TA = Target and pH is > 7.8, Target too high. Lower Target TA and goto step 4.

Example:
Your TA is 300ppm, you want to lower it to 60ppm, in a 350 gal tub.
To lower TA by 240 ppm you need to add a total of 2.4 cups (~19oz) Dry Acid.

Turn jets/air on.
Add 1 cup acid and Aerate 30mins.
Check TA/pH. TA=200 and pH>8.0
Add 6 oz acid and Aerate 30mins.
Check TA/pH. TA=125 and pH>8.0
Add 3 oz acid and Aerate 30mins.
Check TA/pH. TA=90 and pH>8.0
Add 2 oz acid and Aerate 30mins
Check TA/pH. TA=65 and pH=7.8
Add 1 TBS acid and Aerate 30mins
Check TA/pH. TA=60 and pH=7.6

Again, the main thing to remember is, you add more acid in the beginning of the process, and taper off toward the end. This method will allow you to lower your TA in just a few hours, regardless of how high your TA is, without pH ever dropping too low.

Lastly, safety is priority one. Be careful with any type of acid, especially Muriatic. If you're using Muriatic acid, wear rubber gloves and eye protection. Muriatic Acid is nasty stuff. Dilute all acid in a bucket of water and slowly pour mixure over a return flow, or middle of tub. Be careful not to splash. Turn the Air off when adding if necassary. When mixing Acid and Water, always add Acid to water! NEVER ADD WATER TO ACID!!!

Happy Tubbing!

 

[Guest]

IF you have an acid demand test there is an even faster way.
Determine the total amount of acid needed to lower TA
Determine how much acid is needed to drop your pH to 6.8 or 7.0 (depending on your testkit) and not any lower and add it, turn on aeration and leave it on.
in thirty minutes repeat acid demand and add full amount of acid needed to drop ph to 6.8 or 7.0
Do this until the entire amount of acid needed to lower TA by desired amount has been added and then just continue aeration until pH is where you want it.

The amount of acid needed to lower TA by a certain amount is not going to change for different TA or pH readings (i.e. it will take the same amount of acid to lower your TA by 10 ppm no matter if your starting TA is 100 or 300 ppm. It also does not matter if your starting pH is 8.2 or 7.2 as far as the TA lowering effect but remember that adding acid will also lower pH and you don't want that dropping too low!)

 

[Nitro]

IF you have an acid demand test there is an even faster way.
Determine the total amount of acid needed to lower TA
Determine how much acid is needed to drop your pH to 6.8 or 7.0 (depending on your testkit) and not any lower and add it, turn on aeration and leave it on.
in thirty minutes repeat acid demand and add full amount of acid needed to drop ph to 6.8 or 7.0
Do this until the entire amount of acid needed to lower TA by desired amount has been added and then just continue aeration until pH is where you want it.

That will work fine if the jets/air aren't running. However, I found it takes more acid than the acid demand test says to get pH down to 7.0, because of the constant aeration.

The amount of acid needed to lower TA by a certain amount is not going to change for different TA or pH readings (i.e. it will take the same amount of acid to lower your TA by 10 ppm no matter if your starting TA is 100 or 300 ppm.

Yes, that's what I said above. "The first thing to remember is, the amount that TA decreases is directly proportional to the amount of acid you add. That is, in a 350 gal tub, 8 oz of Dry Acid will lower TA by 100ppm." However, the amount of acid it takes to lower pH will change depending on TA.

It also does not matter if your starting pH is 8.2 or 7.2 as far as the TA lowering effect but remember that adding acid will also lower pH and you don't want that dropping too low!)

I just find it easier to first calculate the Total Acid needed, then add HALF the Remaining Amount every 30 mins with jets/air running. I lowered the TA in a tub from ~450 to 60 ppm in just a few hours using this method. Also, if TA is high, aerating will raise pH within mins. However, I could change it to first wait for pH to rise >7.8 before adding the first acid dose.

 

[chem geek]

This is an interesting variation on the basic method. I was a little concerned about how low the pH might get with the method, so I ran some calculations using the pH 7.6 starting point and various starting TA levels and the "half the remaining acid" (ignoring the > 1 cup and < 1 tabelspoons, for now) and assuming 350 gallons.

TA 300 to 50: half of 22.4 is 11.2 ounces; pH drops to 6.24; if one cup is used, pH drops to 6.46
TA 200 to 50: half of 13.4 is 6.7 ounces; pH drops to 6.32
TA 100 to 50: half of 4.4 is 2.2 ounces; pH drops to 6.59

The amount of pH drop in subsequent additions isn't as extreme, but this "half the remaining acid" even throttled with one cup does have the pH drop below 7.0 or 6.8. Of course, with aeration, the pH doesn't stay that low for very long.

Richard

 

[Nitro]

Richard,

Aeration must have an extreme effect on raising pH when it's <= 7.0 and TA is high. Every time I've used this method the pH never dropped below 7.0. I measured pH immediately after adding acid, and every 10 mins, to be sure. As a matter of fact, when TA is above 300, I couldn't get a pH reading under 8.0.

I put an 8 oz cap on it to just to be safe. However, I believe even without the cap the pH won't stay below 7.0 for more than a few minutes.

The main point is, the higher TA, the more acid it takes to lower pH. Using this method you can lower TA within hours, regardless of how high you start with. That comes in handy when TA is really high.

 

[JasonLion]

This is a complex situation to model, the acid won't mix instantaneously, the PH will be rising from all the aeration, how quickly you pour the acid could well make a difference, etc. I have no doubt that the PH does get very low in some parts of the spa for at least a short time. But it won't stay that way for very long.

The question becomes, does low PH for a short period, most likely under one minute, pose any risk?

 

[chem geek]

I suspect that the pH is only low for a relatively short time and that this is OK. Recall how pools abused with Trichlor and with pH below 4.5 (i.e. TA test turns red immediately) take time to show corrosion problems. So this modified procedure is probably fine so long as it isn't done frequently. Lowering the TA once every fill, so every 3 months or longer, is probably not going to be a problem.

 

[Guest]

We are also talking about a special situation here, fill water with extremely high TA which is the exception, rather than the rule so we need to keep this in mind also!

 

[Nitro]

If you think about it, if you tell someone to add enough acid to lower pH to 6.8-7.0, considering aeration, that might turn out to be more than this method. The only reason I'm purposing this method, is so you know exactly how much acid to add each time. I see a lot of users try to lower pH, but have know idea how much to add.

Also, Richard, I see you're using a target of 50 ppm TA. I wouldn't recommend anyone lower their TA that low to begin with. That might be too low for some tubs, especially if using Dichlor. If you use 60 ppm TA, what do the pH values come to?

 

[chem geek]

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.