TA- Should it always trend downward with acid addition?

[Kidneydoc]

Deep chemistry question here:

Lets say your pH is 7.5 and your TA is 100. If you have a SWG, the pH is always rising up due to Hydrogen gas production (and then gassing off) with SWG. So, you always add HCL to the pool 1 or 2 times a week.

I would think this would drive down the TA in a predictable fashion. However, TA (or bicarb as I think of it) seems to be stable for long periods of time.

So question one is: why?

Question two is: why do you have to drive the pH down to 7.0 to get the TA to get lower? I guess the answer depends on the first question as well.

Thanks!

 

[linen]

Welcome to tfp, Kidneydoc

So, you always add HCL to the pool 1 or 2 times a week.

If you get your TA down lower, you won't have to do it that often. I have dosed mine a total of twice this season.

I would think this would drive down the TA in a predictable fashion. However, TA (or bicarb as I think of it) seems to be stable for long periods of time.

Yes, but it does not move it much. For example going from 7.8-7.2 ph when TA is ~100 ppm, will only lower your TA by 10 ppm. It will lower even less if you are using borates. Additionally, if you adding water (either manually or with an autofill), then if you fill water is high in TA, you will conteract the Muriatic acid additions.

Question two is: why do you have to drive the pH down to 7.0 to get the TA to get lower? I guess the answer depends on the first question as well.

No need to drive it down to 7, we recommend only going down to 7.2 typically. It is not how low you go, but the change in ph that determines how much TA is reduced.

How are you measuring for TA?

 

[dschlic1]

I correct the pH on my SWCG pool with HCL. Over long periods of time (months, even year) I see no downward change in TA.

 

[Kidneydoc]

Thanks all!
I use the TFP-100 test kit for chemistry readings.

 

[Kidneydoc]

Welcome to tfp, Kidneydoc

So, you always add HCL to the pool 1 or 2 times a week.

If you get your TA down lower, you won't have to do it that often. I have dosed mine a total of twice this season.

I would think this would drive down the TA in a predictable fashion. However, TA (or bicarb as I think of it) seems to be stable for long periods of time.

Yes, but it does not move it much. For example going from 7.8-7.2 ph when TA is ~100 ppm, will only lower your TA by 10 ppm. It will lower even less if you are using borates. Additionally, if you adding water (either manually or with an autofill), then if you fill water is high in TA, you will conteract the Muriatic acid additions.

Question two is: why do you have to drive the pH down to 7.0 to get the TA to get lower? I guess the answer depends on the first question as well.

No need to drive it down to 7, we recommend only going down to 7.2 typically. It is not how low you go, but the change in ph that determines how much TA is reduced.

How are you measuring for TA?

I use the TFP 100 test kit

Thanks for the answer....but it would still seem like one would need to add bicarb more often if the TA dropped even by 10 ppm from the pH changed as you described above. It seems there must be other buffer systems at work the buffers the HCL and keeps the TA higher than I would expect.

 

[RobbieH]

Borates at 50ppm are a buffer for pH and TA.

 

[Kidneydoc]

I agree.....we just moved and my current pool is new to me, but I had a pool a few years back and did add the borates. I plan to do the same to my current pool soon.

But no borates in pool now. I can't recall all my chemistry from college, but I suspect the amount of bicarb (TA) @ 80 to 100 is much larger than the amount of H+ ions from the usual amount of HCL we add from week to week. Perhaps that is why it seems to take a long time to move the TA down with HCL in the usual amounts we use to adjust pH?

 

[linen]

What is your makeup water TA and ph?

What chemicals are you using in your pool regularly?

 

[Kidneydoc]

pH changes day to day, but the pH is around 7.5 to 7.8
I have only added HCL to the pool.
The TA is 100
The CYA is 80
The CA=290
FC has been around 8 to 10.
No CC
salt about 3300

I did bring a sample to a pool store just for fun, and they tested the phos at 1000, but I have not done anything about that.

 

[linen]

FC has been around 8 to 10.

Why so high on the FC level? Is you swg producing all of that? Your FC minimum is 4 ppm for 80 ppm cya and a swg.

I did bring a sample to a pool store just for fun, and they tested the phos at 1000, but I have not done anything about that.

Ignore phosphates. In a properly chlorinated pool, phosphates do not matter.

 

[HouTex]

FWIW, I add over a gallon of MA to my pool each month to keep pH in check and I don't see a decrease in TA. The only way I see a decline is if I follow the procedures in pool school for lowering TA. I'm pretty sure it's because my fill water is around 220 TA. If anything, my TA slowly creeps up to around 80 or 90 and eventually I'll bring it back down to 60-70.

 

[Kidneydoc]

FC has been around 8 to 10.

Why so high on the FC level? Is you swg producing all of that? Your FC minimum is 4 ppm for 80 ppm cya and a swg.

I agree...just moved in last week and I have been turning down the SWC %...I suspect the prior owner did some kind of shock treatment before moving out, and I am now seeing the tail end of what may have been a higher FC level.

 

[linen]

Ahhh, makes sense! If you have only been there a week, it probably is too early to determine ph and TA trends. Post your fill water (makeup) water TA and ph.

 

[Kidneydoc]

Ahhh, makes sense! If you have only been there a week, it probably is too early to determine ph and TA trends. Post your fill water (makeup) water TA and ph.

....good idea....i will check when I get home

 

[mas985]

High TA fill water is usually the culprit. This has been the case for me as well. My TA fill water is over 200 ppm and I cannot get the pool TA below 100 ppm even though I am constantly adding acid. It also doesn't help that the PH of the fill water is above 7.8.

I tried borates and lowering TA which worked for a couple of months until TA rose again but it is much easier to just work with what you have. Adjust the PH to keep the CSI slight negative (-0.5 to -0.25) and let the TA settle where it will.

 

[chem geek]

If you have a SWG, the pH is always rising up due to Hydrogen gas production (and then gassing off) with SWG. So, you always add HCL to the pool 1 or 2 times a week.

The pH is not rising because hydrogen gas outgasses. The net chemistry from an SWG after accounting for chlorine usage/consumption is pH neutral as described in this post. Though the pH would rise from the SWG when it is generating chlorine, the usage/consumption of chlorine is an acidic process and these two counteract each other.

The rise in pH from SWG pools is due to the carbon dioxide outgassing which is why lowering the TA level often helps. There may also be some undissolved chlorine gas that outgasses and that will also cause the pH to rise.

As others have noted, the TA would normally drop from acid addition, at least if the pH rise were due to carbon dioxide outgassing. When the pH rise is from chlorine gas outgassing, there is no net change in TA after acid addition restores the pH. This is because with carbon dioxide outgassing the pH rises with no change in TA but when chlorine gas outgasses both the pH and TA rise in the same way that acid lowers both. With evaporation and refill, the TA in the fill water is added to the pool. This is shown below:

Effect ................. pH . TA
CO₂ Outgassing ... + ... 0
Cl₂ Outgassing ..... + ... +
Acid Addition ........ - ... -