How does Acid reduce TA?

[tomfrh]

I'm trying to get my head around pH/acid/Total-Alkalinity.

What is happening chemically to reduce the total alkaline substances? How are the alkaline substances (primarily bicarbonates?) eliminated when you add the acid?


Is there a good reference that covers all this stuff? I need to understand it better.

 

[Arizonarob]

Acid drops the PH so you can aerate the water to bring down TA, but at the same time it brings PH back up.
You lower one at the expense of the one you lower to drop another.

 

[cfherrman]

I'm curious too, I imagine that the bicarbs get outgassed when eaten on from the acid

 

[JoyfulNoise]

No, aeration does not consume alkalinity, it’s the acid that does that. How it does that depends on what alkaline species you’re looking at.

For bicarbonate, the primary pH buffer in pool water, this chemical equation is operative -

CO2 (aq) + H2O <——> H+ + HCO3-

When you add acid to pool water with bicarbonate ions (HCO3-) in it, you are increasing the H+ concentration. That shifts the equilibrium of the reaction to the left and more dissolved CO2 is created. The reduction in bicarbonate is the reduced TA that you measure. The water is now at a lower pH and there is a larger amount of CO2 in the water.

The same equation also explains why aeration raises pH. If you consider the partial pressure of CO2 in the air above the water and compare that to the amount of dissolved CO2 in pool water, you’ll find that pool water is over-carbonated relative to the atmosphere. When you agitate the water (“aeration”), you release that dissolved CO2 into the air and the amount of CO2 in the water decreases. Because there is still sufficient bicarbonate buffer in the water, the reaction equilibrium shifts to the left and the bicarbonate combines with a proton to form more dissolved CO2. The bicarbonate is trying to restore the CO2 lost to the air. When a proton is consumed, pH goes up.

There are other alkaline species in pool water as well - cyanurates and borates (if you add borates). Those alkaline species have their own equilibrium reactions that consume a proton and convert cyanurate to cyanuric acid or borate to boric acid. In those cases, the concentrations are conserved as there is no way for those chemicals to diffuse into the atmosphere. It’s only because bicarbonate can be converted to a soluble gas phase that it causes pH drift.

 

[mconnell]

Matt,

That's good stuff...as a chemistry minor and MD...the fundamentals are so important. For me personally, understanding why things happen is so much more important than plugging in numbers into a calculator and doing what it says. I always want to know the WHY.

Great job and thank you

 

[tomfrh]

The same equation also explains why aeration raises pH.... The bicarbonate is trying to restore the CO2 lost to the air. When a proton is consumed, pH goes up.

If you're losing more bicarbonates as protons are consumed how come TA doesn't go down further as pH comes back up? Or does it?

 

[JamesW]

The proton (hydrogen ion) is like negative TA. So, when they combine, the effect is no change in TA.

-1 + 1 = 0.

 

[mconnell]

The TA is going down at the same time the pH is rising. It will reach an equilibrium point based on the amount of acid added and aeration/CO2 concentration. In my limited experience, the pH rises faster than the TA drops.

 

[JamesW]

TA does not drop as pH rises due to aeration. TA remains the same.

Some people use carbon dioxide to lower pH by injecting it into the water. This lowers the pH with no effect on TA.

 

[JoyfulNoise]

tomfrh

Rather then re-invent the wheel...or post as it may be ... have a look at this thread by Richard Falk (chem geek). He explains a lot of the basic chemistry with actual calculations -

TA--What is it really?

Concepts like pH and TA can start to get complicated because we often talk about them as if they are independent variables. They are not. Even the idea of pH can get very complicated for anything other than pure water. Single buffer chemistry can lead to solving two-variable equations and, when you get to a situation where you have water plus two weak acid/conjugate base buffer systems (carbonates and cyanurates, etc), you can easily get into a situation where you are solving for the roots of a quartic or quintic equation ... and that doesn't even account for ionic strength variations of the equilibrium constants.

 

[mconnell]

You add acid and consume HCO3 which decreases the TA by converting it to CO2 dissolved in the water.

You aerate the water to drive off the CO2 created and the H+ again shifts to the left to restore CO2 equilibrium driving up the pH.

The TA drops because of the acid (h+) consuming HCO3. The pH rises by blowing off the excess CO2. These reactions are occurring simultaneously.

The acid pushes the reaction to the left increasing the CO2 and aeration pulls it to the left blowing off the CO2. The result is a decrease in the HCO3 (decreased TA) and a decrease in the H+ (increase pH).

 

[DeanP66]

My TA always stays the same when I aerate. It does not go down. Adding muriatic acid causes both my pH and TA to go down. When I aerate, only the pH comes back up without affecting TA. I always test both pH and TA before I add muriatic acid and after I bring pH back up after I aerate.

 

[mconnell]

Dean you are correct.

If you only aerate you will increase the pH by blowing off CO2 and the TA should remain relatively stable. You're not adding acid to consume the HCO3 and push the reaction to the left.

Adding acid will decrease the TA by consuming HCO3 and decrease the pH by adding H+ as you stated.

If the goal is to decrease the TA because it is too high (which I believe was the question), you add acid which will drive down the TA and the pH initially(you added acid so the pH should decrease); but, the pH will ultimately increase as you aerate the water. That is the goal to decrease a high TA. These reactions are not isolated yet occurring simultaneously. For a high TA this process may need to be repeated. It is a race at first then a slow dance as things are fine tuned with the help of TFP.

I still have two left feet and can't dance. Still learning and enjoying the process.

 

[tomfrh]

TA does not drop as pH rises due to aeration. TA remains the same.

This is where I'm getting confused. Joyful Noise says that adding acid convert more bicarbonate to CO2, and the reduction in bicarbonate is the reduction in TA.

And he say that aqueos CO2 that releases into the air gets replaced when more bicarbonate coverts to aqueos CO2. So why wouldn't that too reduce TA?

 

[tomfrh]

The proton (hydrogen ion) is like negative TA. So, when they combine, the effect is no change in TA.

-1 + 1 = 0.

So you have to count protons when calculating TA? now I'm really confused.

 

[mconnell]

Tom,

Let me put both my left feet in my mouth, but I believe it is because of the CYA stability in water and the ability of the CO2 the blow off. The more volatile nature of the CO2 allows it to diffuse into the atmosphere and be the driving force in the pH drift. See last paragraph of joyful's post.

Perhaps Joyful can educate us both a little more on this.

Excellent topic. I want to know if I should add borate to my pool after this thread. I love my water fall and worry now about my pH and TA even more.

Mark

 

[JohnT]

This might help:



Key:

H₂CO₃=carbonic acid which is CO₂ in water. Dark Blue line
HCO₃^-=Bicarbonate. Magenta Line
CO₃^2-=Carbonate. Green Line

The chart above shows the relative fractions of the various carbonate forms in water versus pH. If you've ever fought pH rise, that middle area with no CO₂ and no carbonate is where your pH wants to settle at 8.3 or so. When the pH moves down toward 6, the carbonic acid fraction starts to increase. So as you move pH down towards 6, some of the carbonate changes to a form that can leave the water as a gas. Aeration speeds up the process.

A key point is that the chart (actually the alkalinity buffering process, the chart is just a visual aid) above works both ways. Changing the fractions changes the pH and changing the pH changes the fractions. If you increase the carbonic acid fraction by injecting CO₂ into the water, the pH will move down. If you increase the carbonate fraction by adding something like sodium carbonate (pHup) then the pH rises.

So when aeration or natural outgassing of CO₂ happens, the pH moves toward the point on the graph that matches the new CO₂ fraction. But alkalinity doesn't change.

This is only the carbonate portion of alkalinity. Alkalinity is also affected by silicates, borates, hydroxide, phosphates and cyanurates. These other ions have their own characteristics and their own pH charts. When you add two buffer systems to your pool, it's possible to put pressure on the pH to stay in a narrower range. This is what happens when you use borates for buffering. The carbonate buffering is much better at keeping the pH from dropping out of our ideal range than it is at keeping it from climbing out of our ideal range, while the borate buffering system helps keep pH from climbing. The combination does a better job of keeping pH close to where we want it than just the carbonate buffer.

 

[tomfrh]

What do you mean by "carbonic acid is CO2 in water". I thought carbonic acid is in equilibrium with CO2 and H20, but not actually the same thing?

 

[JamesW]

Technically, carbonic acid is different from dissolved carbon dioxide. However, they are often used interchangeably.

When aerating, a hydrogen ion and a bicarbonate ion are lost. Hydrogen ions are acid. When you remove acid, you raise the TA just like when you add acid, it lowers TA.

Alternatively, think about it like this.

HCO₃^- + HCO₃^- --> CO₂ + H₂O + CO₃^2-

The carbonate counts 2x as much as bicarbonate towards TA.

 

[tomfrh]

Technically, carbonic acid is different from dissolved carbon dioxide. However, they are often used interchangeably.
.

Ok.

I'm reading that carbonic acid only exists in trace amounts, in equilibrium with C02. That would mean that the carbonic acid produced when pH is lowered will quickly convert to C02 and H20, ie carbonic acid is an intermediary, yes? Is that why they're used interchangeably?