Purpose of lower TA

[xDom]

Pool shops I've been to have suggested a range of 125-150 ppm for TA. I've seen it mentioned a lot, on the internet, to keep the TA at 80-120 ppm.
This forum has suggested that there's nothing wrong with a TA of 60-70 pm for a SWG pool.
I actually suggested this a few months ago , to the pool testing man at the shop. My logic was that with going for a lower TA would require less Bicarb to be added. On top of this, my thought is that LESS acid would also be needed to bring the pH down. From what I read, the TA receives Hydrogen ions. This would mean if the pH needed to be reduced then any acid added would need to be " consumed? " by the TA before it had an effect on reducing pH. I know this flies in the face of advice from the pool shop man who said that a lower TA would mean MORE acid would be used ( I don't agree with this )

After reading the Orenda blogs I also learnt that a lower TA results in a lower pH "ceiling", another desirable outcome.

Is this reasoning valid? If I'm correct then is the only reasoning for shops to suggest a higher TA is to sell more chemicals?

 

[mknauss]

TA - Further Reading

www.troublefreepool.com

 

[xDom]

TA - Further Reading

www.troublefreepool.com

Thanks, I probably should’ve searched for that first. I’m new.

 

[woodyp]

Stay outta that pool store!

 

[PoolStored]

PH TA Relationship - Further Reading

www.troublefreepool.com

 

[xDom]

Stay outta that pool store!

They all seem to be the same! I’m of the belief now that it’s best to do your own research, own tests and just pop into the stores to buy whatever you know you need.

 

[Texas Splash]

just pop into the stores to buy whatever you know you need.

And that is key. With accurate home testing, in your are usually from a test kit from Clear Choice Labs, you do know exactly what to get with no distractions or pushy sales tactics.

 

[mgtfp]

I know this flies in the face of advice from the pool shop man who said that a lower TA would mean MORE acid would be used ( I don't agree with this )

Most pool shop men don't work in a pool shop because of their interest in chemistry.

As you have said, TA is quantifying how many molecules in the water can accept H⁺. This is what buffers the water against pH drops. This is particularly important when chlorinating with acidic sources of chlorine like Trichlor or chlorine gas. When using these sources of chlorine, TA keeps dropping (as does pH) with the addition and use of chlorine, which is why TA needed to be topped up regularly, and a higher TA level of 100-120 was advisable with these sources of chlorine.

When chlorinating with liquid chlorine or with a SWG, TA and pH remain constant across the complete chlorination cycle of adding/creating chlorine and chlorine getting depleted again (turned into chloride by UV and oxidation / sanitation processes). Therefore you don't have to keep adding "TA-increaser".

But why not only not having to keep adding "TA-increaser", and even maintain a lower level than the traditional one? The pool shop thinking is "more must surely be better because more buffer means more stable pH".

That's where it's important to understand that "TA-increaser" is plain old baking soda which is added to build up a carbonate buffer system. So you are basically oversaturating the water with dissolved carbon dioxide. Like in soda water. What happens when you open a bottle of soda water? The CO2 bubbles out and the water tastes less acidic, i.e. the pH rises.

The same happens with the dissolved CO2 in pool water. The pool water is over-saturated with CO2 (that means it contains more CO2 than you would have when leaving pure water exposed to the CO2 from the surrounding air). Therefore CO2 keeps outgassing, and the pH keeps rising in the process. That's the reason for the slow and steady rise of pH in liquid chlorine and SWG pools. It's not because of the high pH of liquid chlorine or the OH^- production of a SWG - yes, adding FC via liquid chlorine or a SWG increased pH, but with following decrease of FC back to the starting level, pH gets back to where it started. The upwards drift over time is from CO2 outgassing.

When chlorinating with Trichlor, this drift was welcome because it compensated the down-wards drift created by adding and using up chlorine. But Trichlor I'd not sustainable because of the CYA build-up.

With liquid chlorine and SWGs, we don't need nor want the pH-rise due to CO2-outgassing. We also don't have much need for high buffering capability against pH-drops, as we are not constantly adding acidic Trichlor. We want some buffering, but we don't need as much as a Trichlor user. Therefore TFP recommends lower TA levels than your pool shop man.

 

[xDom]

Most pool shop men don't work in a pool shop because of their interest in chemistry.

As you have said, TA is quantifying how many molecules in the water can accept H⁺. This is what buffers the water against pH drops. This is particularly important when chlorinating with acidic sources of chlorine like Trichlor or chlorine gas. When using these sources of chlorine, TA keeps dropping (as does pH) with the addition and use of chlorine, which is why TA needed to be topped up regularly, and a higher TA level of 100-120 was advisable with these sources of chlorine.

When chlorinating with liquid chlorine or with a SWG, TA and pH remain constant across the complete chlorination cycle of adding/creating chlorine and chlorine getting depleted again (turned into chloride by UV and oxidation / sanitation processes). Therefore you don't have to keep adding "TA-increaser".

But why not only not having to keep adding "TA-increaser", and even maintain a lower level than the traditional one? The pool shop thinking is "more must surely be better because more buffer means more stable pH".

That's where it's important to understand that "TA-increaser" is plain old baking soda which is added to build up a carbonate buffer system. So you are basically oversaturating the water with dissolved carbon dioxide. Like in soda water. What happens when you open a bottle of soda water? The CO2 bubbles out and the water tastes less acidic, i.e. the pH rises.

The same happens with the dissolved CO2 in pool water. The pool water is over-saturated with CO2 (that means it contains more CO2 than you would have when leaving pure water exposed to the CO2 from the surrounding air). Therefore CO2 keeps outgassing, and the pH keeps rising in the process. That's the reason for the slow and steady rise of pH in liquid chlorine and SWG pools. It's not because of the high pH of liquid chlorine or the OH^- production of a SWG - yes, adding FC via liquid chlorine or a SWG increased pH, but with following decrease of FC back to the starting level, pH gets back to where it started. The upwards drift over time is from CO2 outgassing.

When chlorinating with Trichlor, this drift was welcome because it compensated the down-wards drift created by adding and using up chlorine. But Trichlor I'd not sustainable because of the CYA build-up.

With liquid chlorine and SWGs, we don't need nor want the pH-rise due to CO2-outgassing. We also don't have much need for high buffering capability against pH-drops, as we are not constantly adding acidic Trichlor. We want some buffering, but we don't need as much as a Trichlor user. Therefore TFP recommends Lowe TA levels than your pool shop man.

Thank you for taking the time for this? Do you have a background in chemistry? Is this what you have learnt since getting a pool?

I’m going to let this sink in before I ask any follow up questions.

 

[UncleVanya]

Over time I found that maintaining the father-in-law's pool was easier at a TA around 70 with bleach and this also worked with the SWG last year. At 80 I had a slight rise in pH but eventually (backwashing) the TA level came down and the rise stopped. It was a lesson I took to heart.

 

[mgtfp]

Thank you for taking the time for this? Do you have a background in chemistry? Is this what you have learnt since getting a pool?

I’m going to let this sink in before I ask any follow up questions.

No worries. I'm a physicist by training, did some basic chemistry. Mostly, I just learnt about pool chemistry as I went along understanding what's going on in my pool. I went through many threads in TFP's Deep End and followed up on some of the papers quoted there. There is a lot to learn here. Pool chemistry seems to be something that gets largely ignored by the pool industry. Thanks to people like Chem Geek, a lot of (sometimes quite old) pool related research has been dug out and distributed here. More recently, there are people here like Joyful Noise and JamesW who have a really good understanding of chemistry in general. I feel like I just covered a fraction of what these guys know...

 

[xDom]

Most pool shop men don't work in a pool shop because of their interest in chemistry.

As you have said, TA is quantifying how many molecules in the water can accept H⁺. This is what buffers the water against pH drops. This is particularly important when chlorinating with acidic sources of chlorine like Trichlor or chlorine gas. When using these sources of chlorine, TA keeps dropping (as does pH) with the addition and use of chlorine, which is why TA needed to be topped up regularly, and a higher TA level of 100-120 was advisable with these sources of chlorine.

When chlorinating with liquid chlorine or with a SWG, TA and pH remain constant across the complete chlorination cycle of adding/creating chlorine and chlorine getting depleted again (turned into chloride by UV and oxidation / sanitation processes). Therefore you don't have to keep adding "TA-increaser".

But why not only not having to keep adding "TA-increaser", and even maintain a lower level than the traditional one? The pool shop thinking is "more must surely be better because more buffer means more stable pH".

That's where it's important to understand that "TA-increaser" is plain old baking soda which is added to build up a carbonate buffer system. So you are basically oversaturating the water with dissolved carbon dioxide. Like in soda water. What happens when you open a bottle of soda water? The CO2 bubbles out and the water tastes less acidic, i.e. the pH rises.

The same happens with the dissolved CO2 in pool water. The pool water is over-saturated with CO2 (that means it contains more CO2 than you would have when leaving pure water exposed to the CO2 from the surrounding air). Therefore CO2 keeps outgassing, and the pH keeps rising in the process. That's the reason for the slow and steady rise of pH in liquid chlorine and SWG pools. It's not because of the high pH of liquid chlorine or the OH^- production of a SWG - yes, adding FC via liquid chlorine or a SWG increased pH, but with following decrease of FC back to the starting level, pH gets back to where it started. The upwards drift over time is from CO2 outgassing.

When chlorinating with Trichlor, this drift was welcome because it compensated the down-wards drift created by adding and using up chlorine. But Trichlor I'd not sustainable because of the CYA build-up.

With liquid chlorine and SWGs, we don't need nor want the pH-rise due to CO2-outgassing. We also don't have much need for high buffering capability against pH-drops, as we are not constantly adding acidic Trichlor. We want some buffering, but we don't need as much as a Trichlor user. Therefore TFP recommends lower TA levels than your pool shop man.

When you talk about the pH rising in a pool, you're saying that its due to CO2 outgassing, equalising with the atmosphere. Correct?
Where does the CO2 in the pool come from for starters? Is it all from the TA increaser?
Also when CO2 dissolves in the water it becomes Carbonic acid? Is all the CO2 in the water Carbonic Acid? The Carbonic acid converts back to CO2 as it outgasses, right?

 

[mgtfp]

The CO2 is actually mostly dissolved CO2 in water, often written as CO2(aq). The CO2 is in an equilibrium reaction with Carbonic acid (H2CO3), but at pool pH it's actually mostly CO2(aq). The H2CO3 is in an equilibrium reaction with bicarbonate ion (HCO3-), which is in another equilibrium with carbonate ion (CO3--).

By adding TA increaser (bicarbonate of soda), all those equilibriums have to reshuffle, but at pool pH you end up with mostly bicarbonate ion. But the little dissolved CO2 you get in equilibrium is still above the concentration that would be in equilibrium with atmospheric CO2, written as CO2(g), and therefore wants to outgas.

I'm on kids sport duty this morning, will write down the equations and show some graphs when I have more time.

 

[JamesW]

The Y axis is the percentage of baking soda that converts into carbon dioxide.

The X axis is the pH.

If the pH is 6.8, about 26% of the carbonate alkalinity converts to carbon dioxide.

HCO₃^- + H⁺ --> H₂CO₃ --> H₂O + CO₂

This process raises the pH because the hydrogen ions are picked up.

The CO₂ offgasses and more carbonate alkalinity converts to CO₂ and the pH rises even more.

y = 100-(100/(1+10^(6.35– x))), x from 4 to 8 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[mgtfp]

I thought that James would be quick at that one

 

[JamesW]

Carbonic acid has 2 pKas for the hydrogen ions; one at 6.3 and one at 10.3.

Below is the graph of how much bicarbonate converts to carbonate based on the pH.

Because 8.3 is the midpoint, you get the same amount of carbonic acid and carbonate when you add bicarbonate.

You get about 1% carbonate ions and about 1% carbonic acid or carbon dioxide at a pH of 8.3.

This is why baking soda always pushes the pH towards 8.3.

If the pH is above 8.3, adding baking soda produces more carbonate by releasing hydrogen ions, which lowers the pH.

If the pH starts at 10.3 and you add baking soda, half of the added baking soda would become carbonate.

The Calcite (Calcium Carbonate) Saturation Index depends on the amount of calcium and the amount of carbonate.

As you can see, the amount of carbonate increases exponentially when the pH goes above 8.3.

You have 50 times more carbonate at a pH of 10.3 than at a pH of 8.3.

100HCO₃^- --> 98HCO₃^- + H₂CO₃ + CO₃^2- (pH = 8.3).

100HCO₃^- --> 50HCO₃^- + 50CO₃^2- + 50H⁺ (pH = 10.3).

y = (100/(1+10^(10.3– x))), x from 7.8 to 12 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

 

[xDom]

Thanks mgtfp and JamesW, once again I'll think about this for a while. Good one

 

[xDom]

I have a question relating to pH. In a pool do H+ and OH- exist at the same time?
So does a pH lower than 7 indicates a certain concentration of H+ and zero OH- or does it mean that the concentration of H+ is higher than OH-?

Edit: in thinking about this after I posted it I guess the answer is in the previous responses!
I’m trying to wrap my head around how H+ and OH- CAN exist simultaneously, why don’t they instantaneously bond?

 

[mgtfp]

They mostly do bond. But they also have a tendency to fall apart again, just much slower than bonding with each other. It's an equilibrium reaction, just like the equilibrium reaction between chlorine and CYA bonding with each other and falling apart again. Because the reaction rates are proportional to the concentrations of the reaction partners, an excess on one side of the equation gets depleted until there is an equilibrium where the same number of bonding and falling apart reactions occur per second.

In case of water, the propensity for bonding between H⁺ and OH^- is much higher than for H₂O falling apart, the equilibrium is reached at a level where there are many, many more H₂O molecules than H⁺ and OH^- molecules. But there are some left, just not many.

In water, the product of the H⁺ and OH^- concentrations is always 10^-14:
[H⁺] [OH^-] = 10^-14

In other words, K_W=10^-14 is the equilibrium constant in the equilibrium reaction
H₂O <-> H⁺ + OH^-

An equilibrium constant that is much, much smaller than 1 (like 10^-14) means that the equilibrium is very far on the left-hand side of the reaction (remember, "equilibrium" doesn't mean that there are the same number of molecules on both sides, it means that the number of reactions per second from left to right equals the number of reactions per second from right to left).

Defining
pH = -log([H⁺]),
pOH= -log([OH^-]) and
pK_W = -log(K_W)
and taking the logarithm of above equation, turns that into
pH + pOH = pK_W = 14

Chemists prefer to work with "p" values to avoid all these annoying 10^-....

At pH 7, you have for example an H⁺ concentration of 10^-7 mol/L and an OH^- concentration of also 10^-7 mol/L.

At pH 6, you have an H⁺ concentration of 10^-6 mol/L and an OH^- concentration of 10^-8 mol/L.

At pH 8, you have an H⁺ concentration of 10^-8 mol/L and an OH^- concentration of 10^-6 mol/L.

And so on. Even in full strength muriatic acid (HCl concentration of about 31-32%) there is some OH^- left: The pH is about -1 (yes, pH can be negative) and pOH = 15. That means an H⁺ concentration of 10¹ mol/L and an OH^- concentration of 10^-15 mol/L.

 

[xDom]

They mostly do bond. But they also have a tendency to fall apart again, just much slower than bonding with each other. It's an equilibrium reaction, just like the equilibrium reaction between chlorine and CYA bonding with each other and falling apart again. Because the reaction rates are proportional to the concentrations of the reaction partners, an excess on one side of the equation gets depleted until there is an equilibrium where the same number of bonding and falling apart reactions occur per second.

In case of water, the propensity for bonding between H⁺ and OH^- is much higher than for H₂O falling apart, the equilibrium is reached at a level where there are many, many more H₂O molecules than H⁺ and OH^- molecules. But there are some left, just not many.

In water, the product of the H⁺ and OH^- concentrations is always 10^-14:
[H⁺] [OH^-] = 10^-14

In other words, K_W=10^-14 is the equilibrium constant in the equilibrium reaction
H₂O <-> H⁺ + OH^-

An equilibrium constant that is much, much smaller than 1 (like 10^-14) means that the equilibrium is very far on the left-hand side of the reaction (remember, "equilibrium" doesn't mean that there are the same number of molecules on both sides, it means that the number of reactions per second from left to right equals the number of reactions per second from right to left).

Defining
pH = -log([H⁺]),
pOH= -log([OH^-]) and
pK_W = -log(K_W)
and taking the logarithm of above equation, turns that into
pH + pOH = pK_W = 14

Chemists prefer to work with "p" values to avoid all these annoying 10^-....

At pH 7, you have for example an H⁺ concentration of 10^-7 mol/L and an OH^- concentration of also 10^-7 mol/L.

At pH 6, you have an H⁺ concentration of 10^-6 mol/L and an OH^- concentration of 10^-8 mol/L.

At pH 8, you have an H⁺ concentration of 10^-8 mol/L and an OH^- concentration of 10^-6 mol/L.

And so on. Even in full strength muriatic acid (HCl concentration of about 31-32%) there is some OH^- left: The pH is about -1 (yes, pH can be negative) and pOH = 15. That means an H⁺ concentration of 10¹ mol/L and an OH^- concentration of 10^-15 mol/L.

Thanks