Figuring TA Drop

[smackdab]

When I opened my pool my CYA was a bit low and my TA was 110 because my fill water is high. I had some leftover 99% Trichlor tabs leftover from years ago so I used them to bump up my CYA some and they are almost dissolved. I thought I could figure what my TA would drop down to in PoolMath in the "effects section" but I was mistaken. It only shows -TA for the acids. I guess I could just restest but I kind of like to predict whats going to happen. This isn't my fault but because I'm a member here.

If 53 fl oz of 31.45 Muriatic Acid drops the pH by 0.8 and lowers the TA by 13 ppm, then could I expect the exact same thing from Trichlor?
In other words if 35 oz of Trichlor lowers the pH by 0.8 then would it drop the TA by 13 ppm too or are the acids different?
I see Dry Acid is slightly different for it lowers TA by 14 for a 0.8 pH drop.

 

[Patrick_B]

Well, you may be sorry you asked. It's kind of strange....and it's controversial.

Cyanuric Acid actually contributes a bit of Total alkalinity to the pool water, albeit not much. Not Carbonate, or Bicarbonate alkalinity, but Cyanurate alkalininty.

 

[chiefwej]

This conversation could quickly slide into the "deep end".

 

[Patrick_B]

It could, but I can't take it that far. :lol:

 

[jblizzle]

Patrick has already exceeded what I could have said

 

[smackdab]

Well, you may be sorry you asked. It's kind of strange....and it's controversial.

Cyanuric Acid actually contributes a bit of Total alkalinity to the pool water, albeit not much. Not Carbonate, or Bicarbonate alkalinity, but Cyanurate alkalininty.

Ahh. I didn't know that. I've searched and can't find anything on it. Put her in the deep end and lets find out. May as well have something to talk about. If it gets too technical, I'll just have to leave you guys with it.

 

[chem geek]

Though it's pH dependent, roughly one-third of the CYA level contributes to the TA level. The reason we don't concern ourselves with this is that PoolMath automatically accounts for all components of TA, not only carbonates and CYA, but also borates. This only matters in calculating the calcite saturation index and PoolMath does that for you.

Trichlor Addition

As for the Effects adding chemicals in PoolMath, the TA level doesn't change when adding Trichlor because it's not itself a strong acid nor is it a salt. It does dissolve in water to form other compounds but they all counteract each other with regard to changing the TA.

Trichlor + Water ---> Cyanuric Acid (CYA) + Hypochlorous Acid (HOCl)

The pH drops because both Cyanuric Acid and Hypochlorous Acid are weak acids. However, adding a weak acid to water does not change the TA. The reason is that the lower pH from the hydrogen ion that lowers the TA is exactly compensated by the corresponding salt ion that increases the TA. I show this below for each of these acids where CYA is represented by H₃CY.

............. -TA ... +TA
H₃CY ---> H⁺ + H₂CY^-
Cyanuric Acid ---> Hydrogen Ion + Cyanurate Ion

............. -TA .. +TA
HOCl ---> H⁺ + OCl^-
Hypochlorous Acid ---> Hydrogen Ion + Hypochlorite Ion

So while it is true that acid lowers (counts negatively towards) TA, it is also true that a salt ion of a weak acid that can accept acid (hydrogen ion) counts positively towards TA.

Contrast the above with what happens when you add either a strong acid or a salt:

Strong Acid Addition

.......... -TA
HCl ---> H⁺ + Cl^-
Hydrochloric Acid ---> Hydrogen Ion + Chloride Ion

With a strong acid as shown above the hydrogen ion counts against TA (i.e. lowers it) but the chloride ion does not accept a hydrogen ion at any reasonable pH and certainly not at the pH transition of the TA test which is at a pH of around 4.5. Also, the dissociation shown above goes completely to the right.

Sodium Bicarbonate (Baking Soda) Addition

............................ +TA
NaHCO₃ ---> Na⁺ + HCO₃^-
Sodium Bicarbonate ---> Sodium Ion + Bicarbonate Ion

With a weak acid salt as shown above the bicarbonate ion counts towards TA (i.e. raises it) because the sodium ion is like the chloride ion not affecting TA while carbonic acid is a weak acid so bicarbonate ion can accept a hydrogen ion at a reasonable pH well above the pH of the TA test (it's roughly 50/50 carbonic acid and bicarbonate ion at a pH of 6.3). You can see this below. Also note that in the above the dissociation goes completely to the right.

+TA .... -TA
HCO₃^- + H⁺ <---> H₂CO₃ <---> CO₂(aq) + H₂O
Bicarbonate Ion + Hydrogen Ion <---> Carbonic Acid <---> Carbon Dioxide + Water

The above shows why the outgassing or the injection of carbon dioxide into the water does not change the TA. When carbon dioxide outgases and the above reaction shifts towards the right, any reduction in bicarbonate ion that counts towards TA is exactly compensated by a reduction in hydrogen ion that counts against TA. The lowering of the hydrogen ion concentration is what raises the pH.

 

[smackdab]

I see what you have is one cancelling the other out. So using Trichlor or Dichlor wouldn't actually drop the TA level or be enough to even worry about and my TA is still 110 after 2 lbs of Trichlor dissolved. It isn't a big deal if it didn't drop because I can take care of that. I just thought I would kill two birds with one stone but I guess I was wrong.

 

[chem geek]

Two points. First is that though the TA doesn't change when adding Trichlor, the resulting lowering of pH DOES lower the carbonate alkalinity because of the drop in pH and it is that change in carbonate ion concentration that directly affects the calcite saturation index, though it is computed from the other factors of (mostly) pH and TA. So the reality is that you really aren't trying to target a TA level, but rather a carbonate alkalinity level since that (along with pH and calcium) is what is protecting plaster surfaces and preventing scaling (and also related to the rate of pH rise from carbon dioxide outgassing). This is all put together in the calcite saturation index. The Recommended Levels in the Pool School give rough ranges that have that index be (mostly) reasonable (exception being for SWCG pools it's too low at the low end and on average).

Second is that Dichlor is not the same as Trichlor and DOES change the TA by increasing it. This is shown below:

Dichlor Addition

Dichlor + Water ---> Cyanuric Acid (CYA) + Hypochlorous Acid (HOCl) + Sodium Hydroxide (NaOH)

Unlike Trichlor, Dichlor produces a strong base in addition to the two weak acids. A strong base raises the TA (as well as the pH). This is shown below:

........................ +TA
NaOH ---> Na⁺ + OH^-
Sodium Hydroxide ---> Sodium Ion + Hydroxide Ion

The overall pH from Dichlor addition is close to pH neutral (only slightly acidic) because the strong base largely counteracts the two weak acids. As for the carbonate alkalinity, it only changes a little from the slightly lowered pH.

Sodium Hypochlorite (Chlorinating Liquid or Bleach) Addition

As for the hypochlorite sources of chlorine, they raise both the pH and TA upon addition. This is shown below for bleach or chlorinating liquid:

........................ +TA
NaOCl ---> Na⁺ + OCl^-
Sodium Hypochlorite ---> Sodium Ion + Hypochlorite Ion

The pH rises because about half of the hypochlorite ion becomes hypochlorous acid so lowers the quantity of hydrogen ions:

-TA .. +TA
OCl^- + H⁺ <---> HOCl
Hypochlorite Ion + Hydrogen Ion <---> Hypochlorous Acid

Chlorine Usage/Consumption

Now all of the above is fine and dandy, but it neglects the fact that chlorine doesn't keep going up and up in the pool, but instead gets consumed/used and this is an acidic process that changes both the pH and the TA. More details about this are in this post, but using the breakdown from sunlight as an example the following is what happens to TA:

........................... -TA
2HOCl ---> O₂(g) + 2H⁺ + 2Cl^-
Hypochlorous Acid ---> Oxygen Gas + Hydrogen Ion + Chloride Ion

+TA
2OCl^- ---> O₂(g) + 2Cl^-
Hypochlorite Ion ---> Oxygen Gas + Chloride Ion

So you can see that to the degree that the pH and TA rose from adding a hypochlorite source of chlorine because it produced hypochlorous acid, when that same hypochlorous acid gets used/consumed, the pH and TA drop back to where they started. Even for the hypochlorite ion that remained as is, it's addition increased the TA but when it gets used/consumed it decreases TA. The only net change is a small pH and TA increase from the "excess lye" in chlorinating liquid or bleach (I don't show that above).

The bottom line is that when you are trying to adjust your TA you do so by increasing it by adding some form of carbonates, usually sodium bicarbonate (baking soda), though using sodium carbonate (pH Up or washing soda) will also increase carbonates (but increases pH more). When you want to lower your TA, you do so by adding acid and increasing aeration of the water usually at lower pH. One should generally ignore the TA changes that occur when the pH is going up and down -- just focus on the TA level when you are at your desired pH.

 

[pooldv]

Good thing he ^^^^ is in charge of the chems! sheesh!

 

[smackdab]

Yea. I'm not a scientist but I can see what is happening and understand. If the red and green TA markers weren't in there then I would probably still be staring at it this time next week.

Thanks for taking time to type all that and explain it. I don't see what is controversial about it.

 

[chem geek]

(I added some sub-titles in the posts since they were pretty long so hopefully that helps make it a little easier to read. I'm glad the TA colored text helped identify what is going on.)

It's only controversial in the same way that some people swear that adding carbon dioxide to a pool increases TA and that outgassing of carbon dioxide decreases TA. This isn't true, but it's what people believe because, after all, carbon or carbonates are going into or leaving a pool. What people miss is that hydrogen ions count against TA so pH changes affect TA. People do tend to understand this latter point of pH changes affecting TA since they know that a strong acid lowers TA, but they don't put the two ideas together to see that with weak acids (like carbonic acid that comes from carbon dioxide) these cancel each other out nor do they look at the chemistry to see that these exactly cancel.

So perhaps it would be better to characterize this as confusing or non-obvious, but when so many people make incorrect statements it becomes "controversial" to point out the truth. Also, when it comes to pH, the industry doesn't account for the acidity and salt creation from chlorine usage/consumption so they make claims that bleach and chlorinating liquid raise the pH a lot or that Dichlor is near pH neutral -- these are half-truths since they only apply to the chlorine addition and not to the net effect of what happens after that chlorine is used/consumed which is what really matters. And the industry also tends to be hopelessly confused about carbon dioxide outgassing (let alone the chlorine/CYA relationship).

The following is a summary of what happens with various types of chlorine addition and its associated usage/consumption for each initially adding 10 ppm FC and then getting consumed:

...................................... Addition ........................ Usage/Consumption ..... Net Result (excluding CYA, CH)
Chlorine Type ... FC .... TA .. pH .. CYA .. CH .. Salt ........ FC .. TA .. pH .. Salt ........ FC .. TA .. pH ... Salt

Bleach ........... +10 . +7.3 .. + ..... 0 ..... 0 . +8.25 ..... -10 . -7.1 .. - .. +8.25 ........ 0 . +0.2 . 0* . +16.5

Cal-Hypo ....... +10 . +7.5 .. + ..... 0 ... >+7 . +4 ........ -10 . -7.1 .. - .. +8.25 ....... 0 . +0.4 . 0* .. +12

Trichlor .......... +10 .... 0 .... - ... +6.1 ... 0 ..... 0 ......... -10 . -7.1 .. - . +8.25 ....... 0 . -7.1 ... -- .. +8.25

Dichlor .......... +10 .. +3.5 . 0* . +9.1 ... 0 ..... 0 ......... -10 . -7.1 .. - . +8.25 ....... 0 . -3.5 ... - .... +8.25

*The pH (and TA) rises a little from the "excess lye" in bleach and chlorinating liquid and the calcium hydroxide in Cal-Hypo; Dichlor is slightly acidic upon addition.

You can see from the above that both Trichlor and Dichlor addition and usage combined are like adding acid to the pool; Trichlor twice as much as Dichlor (for the same FC amount).

 

[smackdab]

I meant to let you know that my TA didn't drop any that I could tell. I did have to add water a time or two during but maybe a total of 1.5-2" water added.