Aeration and lowering alkalinity

[Water_man]

This is a continuation of this thread.

Where your error comes from is that carbonate hardness (what we call total alkalinity) is only the measureable part of the buffer system by definition, which is the bicarbonates (there is actually very little carbonate ions at normal pool pH). When we test TA we are actually only testing the bicarbonate part of the buffer system (at normal pool pH). Once you convert the bicarbonates to carbonic acid, by definition you have lowered the carbonate hardness or total alkalinity. The net result is that you have also lowered the pH. NOW, the tricky part is how to raise the pH without affecting the carbonate alkalinity. If you just introduce OH ions by adding a base you WILL convert some of the carbonic acid into bicarbonate and produce water but you are not shifting the equalibirum point but if you aerate you are actually REMOVING some of the carbonic acid and actually shifting the equalibrium point. This is about a deep as I will get without moving this to the "deep end" and it already probably should be there.
If you would like to continue this in the "deep end" please start a thread there and PLEASE, in the future, let's try and keep any theoretical chemistry out of the general forum.

You might want to read ta-what-is-it-really-t4979.html to get a better idea of the chemistry. Even that is pretty basic because I wrote it so people without a chemistry background could get a handle on it if they tried.

You're saying exactly what I'm saying : aeration removes the carbonic acid.
Actually it removes only one thing - carbon dioxide. The equilibria of the system do the rest of the job. The CO2 that you kicked out by aeration came from the HCO3-, hence you reduced your HCO3-, namely you reduced your alkalinity.

There are only three ways to reduce your alkalinity :
1. Dilution
2. Forming an insoluble salt with carbonate (for instance calcium carbonate.)
3. Getting read of CO2.
This is what aeration does. The process of doing it in the pool is lowering the pH first so that more carbonic acid is available and hence more free CO2 is dissolved in the water, but lowering the pH per se doesn't lower the alkalinity because the CO2 is still there .

If you still think I'm wrong, please show me how can aeration increase the pH without removing CO2 out, and how removing CO2 out doesn't effect alkalinity.
If you can do that, then you'll disprove the law of conservation of matter.

 

[Guest]

Your error comes from not understanding what the definition of Total alkalinity (carbonate hardness) is. It is the MEASURABLE part of the bicarbonate/carbonic acid buffer system which is the bicarbonates. When you test TA you are doing an acid titration which is only measuring bicarbonates. If you lower the pH of your sample BEFORE testing you will reduce the MEASURABLE amount of bicarbonate in the sample and when you test you will have a lower TA than if you had NOT lowered the pH first. The amount of carbonic acid (CO2) is NOT measured by the TA titration and is NOT part of the TA since by definition TA is measurable alkalinity and carbonic acid is an acid and not part of that alkalinity (but it is part of the bicarbonate buffer system).
Converting bicarbonate to carbonic acid IS lowering TA. The trick is to keep the carbonic acid from converting back into bicarbonate when you bring the pH back up. That is done by aeration which removes the carbonic acid from the system. In fact, the aeration step only speeds the process up. If no (OH)- ions are added to the system then the CO2 will gas off on it's own (and, in fact, this is the theory behind the 'slug method' of lowering TA).

 

[Water_man]

Your words are in blue bold

Your error comes from not understanding what the definition of Total alkalinity (carbonate hardness) is. It is the MEASURABLE part of the bicarbonate/carbonic acid buffer system which is the bicarbonates. When you test TA you are doing an acid titration which is only measuring bicarbonates.

I have no error here. I understand this part very well.

If you lower the pH of your sample BEFORE testing you will reduce the MEASURABLE amount of bicarbonate in the sample and when you test you will have a lower TA than if you had NOT lowered the pH first.

There's no debate here. However, if you do not aerate and you quickly bring the pH back to its original level, your alk will be the same

The amount of carbonic acid (CO2) is NOT measured by the TA titration and is NOT part of the TA since by definition TA is measurable alkalinity and carbonic acid is an acid and not part of that alkalinity (but it is part of the bicarbonate buffer system).

Correction : Carbonic acid is not CO2. It's H2CO3.
Although Carbonic Acid is not measured by the "alkalinity test", if you kick the CO2 out you kick the Carbonic Acid out and consequently you lower your bicarbonate, namely you lower your alkalinity. The chemical equlibria in the water "don't care" what you measure. But getting rid of the CO2 causes lowering your measured bicarbonate, because the bicarbonate was "deprived" of its CO2.

This is the chain of events caused by removing CO2:
HCO3- + H+ ----> H2CO3 -----> H2O + CO2
So when you remove the CO2 the two equilibria shift to the right.

Converting bicarbonate to carbonic acid IS lowering TA. The trick is to keep the carbonic acid from converting back into bicarbonate

Again we're saying exactly the same thing. But if all you do is aerate (as in the case which prompted this thread) and since the aeration removes the CO2 and hence removes the H2CO3, isn't this EXACTLY what I said before, i.e that aeration lowers alkalinity?

So, the bottom line is : If you aerate AND it causes a rise in pH - wouldn't you definitely also see lowered alkalinity?

 

[Guest]

You are missing the most important point... the difference between theoretical chemistry and practical applications. If you pH is high you can aerate until the cows come home and will not see any appreciable change in the TA. If you dump a bunch of acid in the pool you WILL see the TA drop. We are talking about practical applications in a swimming pool, let's not lose sight of that.

 

[Guest]

Your words are in blue bold


There's no debate here. However, if you do not aerate and you quickly bring the pH back to its original level, your alk will be the same
NO, you are in error. Aeration only speeds the process up. If you do not introduce (OH)- ions then the TA will NOT rise. CO2 will still outgas but it will happen more slowly
The amount of carbonic acid (CO2) is NOT measured by the TA titration and is NOT part of the TA since by definition TA is measurable alkalinity and carbonic acid is an acid and not part of that alkalinity (but it is part of the bicarbonate buffer system).

Correction : Carbonic acid is not CO2. It's H2CO3.
I never said it was. Carbonic acid can be considered to be just CO2 dissolved in the water for our purposes here. I am quite aware that is not totally correct but is it correct enough for what we are talking about.
Although Carbonic Acid is not measured by the "alkalinity test", if you kick the CO2 out you kick the Carbonic Acid out and consequently you lower your bicarbonate, namely you lower your alkalinity.
No, you simply raise the pH. You have already created the carbonic acid and 'destroyed' bicarbonate ions by dropping your pH


The chemical equlibria in the water "don't care" what you measure. But getting rid of the CO2 causes lowering your measured bicarbonate, because the bicarbonate was "deprived" of its CO2.

This is the chain of events caused by removing CO2:
HCO3- + H+ ----> H2CO3 -----> H2O + CO2
So when you remove the CO2 the two equilibria shift to the right.
Look at the left side of the equation. The act of adding the acid is what is lowering the TA and producing the carbonic acid. If you don't add excess hydrogen ions then nothing happens. You then have to look at gas equilibrium at the air/water interface on the right side of the equation which is what affects the outgassing of CO2. It becomes rather complicated and, IMHO, really serves no purpose to discuss this in the forum, even in the deep end since those that understand the chemistry will understand this and those that don't will really need a crash course in chemistry to understand the whole picture. This is why I try to keep it in simple terms.
Converting bicarbonate to carbonic acid IS lowering TA. The trick is to keep the carbonic acid from converting back into bicarbonate

Again we're saying exactly the same thing. But if all you do is aerate (as in the case which prompted this thread) and since the aeration removes the CO2 and hence removes the H2CO3, isn't this EXACTLY what I said before, i.e that aeration lowers alkalinity?
NO, aeration merely raises pH. It has NO impact on TA. Adding acid Lowers TA.
So, the bottom line is : If you aerate AND it causes a rise in pH - wouldn't you definitely also see lowered alkalinity?

In practical terms, no. If you drop the pH and then aerate yes but all you need to do is drop the pH to see a downward change in TA. I know it's counterintuitive.

 

[Water_man]

You're saying time and again that aeration just raises pH. I wonder if you can show me the process that can do it without lowering the bicarbonate level.
If you don't see how raising the pH by aeration is closely associated with and related to removing the bicarbonate out of the water solution (and thus lowering alkalinity) then you don't understand the chemistry of the process. What I tried to explain has nothing to do with "intuition". It's basic chemistry.

 

[Water_man]

You are missing the most important point... the difference between theoretical chemistry and practical applications. If you pH is high you can aerate until the cows come home and will not see any appreciable change in the TA. If you dump a bunch of acid in the pool you WILL see the TA drop. We are talking about practical applications in a swimming pool, let's not lose sight of that.

All "practical applications" are based on "theory". I never thought, nor proposed that in high pH, aeration will affect TA. Your "theortical chemistry" explains why you have to wait until the cows lay eggs for this to happen - there ain't gonna be a meaningful concentration of H2CO3 and hence neither CO2. No CO2 to kick out means there's no depletion of HCO3-. It's elementary, Dr Watson.
Finally, please remember that ALL the "practical applications" in the pool, including balancing, sanitation, etc. are just "applications" of those chemical principles and understanding how they work.

Going back to the "real life" - my system didn't have a high pH . It started at 7.5 Therefore, if someone suggested that the raising of the pH was caused by aeration, then that same aeration should have caused an associated change in alk. You can't get one without the other. It happens in theory, and in real life.

 

[chem geek]

Let's not get snippy here. Understanding why having carbon dioxide leave the pool (from aeration) only raises the pH and not the TA is not an easy thing to understand. It took me a while myself and I still have a hard time explaining it.

To understand Total Alkalinity, consider first pure water at a pH of 7.0. It would be a nice definition of TA to have this be zero for such pure water, but we'll see later on why this really does make sense. Now consider adding lye (sodium hydroxide) to the water so that the pH rises. Does the water have alkalinity? The definition of alkalinity is the sum of all substances that can accept a hydrogen ion though we'll see what we need to do with hydrogen ion itself in a moment. So after adding lye, the pH is high and there is a lot of hydroxyl ion in the water. If one adds acid, it takes a large quantity of acid to move the pH when the pH is higher -- 10 times more acid for each unit of pH that is higher (that is, 10 times more to go from a pH of 13 to 12 as from 12 to 11, etc.) -- because the pH is a logarithmic scale. Clearly, hydroxyl ion can accept a hydrogen ion and should be counted as alkalinity.

Now what happens as you continue to add acid (hydrogen ion)? It takes less and less acid as you approach a ph of 7.0 at which point a very small amount of acid causes a rather large drop in pH -- the fastest change in pH per unit of acid added occurs at this pH of 7.0. If you continue to add acid, the pH will continue to drop but it will take larger and larger quantities of acid to make the pH move lower for the same reasons as given above except now we are increasing hydrogen ion concentration by factors of 10 (we were also doing that before, but hydroxyl ion was dominant). So clearly the point at which the amount of hydrogen ion equals the amount of hydroxyl ion is a critical point which we define as having zero alkalinity since that's where the most rapid "falling apart" of buffering occurs.

So what definition can we use for TA? We can use TA = [OH^-]-[H⁺] for pure water. Technically, the TA is negative in pure water below a pH of 7.0. More generally, for any substances that can accept a hydrogen ion we have TA = [anything that can accept a hydrogen ion]-[H⁺] since the TA will be zero when the hydrogen ion concentration equals the total concentration of everything that can accept a hydrogen ion -- the point at which pH will move most dramatically with small changes in acid addition. In buffered water, this point is not usually at a pH of 7.0 -- in pool water with a carbonate buffer system, the pH at a TA of zero (i.e. where hydrogen ion concentration equals the bicarbonate ion plus CYA^- ion if present) is at around 4.5 which is why the indicator chosen for the TA test changes color at a pH close to 4.5. Also note that substances that can accept two hydrogen ions should count double towards TA, etc.

TA = 2[CO₃^2-] + [HCO₃^-] + [CYA^-] + 2[CYA^2-] +3[CYA^3-] + [OH^-] - [H⁺]

In practice, the bicarbonate ion and single charged CYA ion dominate and change the most along with hydrogen and hydroxyl ions (of the species composing TA) when the pH changes. Notice how TA is not just dependent on the level of carbonate and bicarbonate ions, but also on hydroxyl and hydrogen ions. This is why the TA moves with pH since pH is a measure (negative base 10 logarithm) of the hydrogen ion concentration (and the hydroxyl ion concentration also has a direct inverse relation to the hydrogen ion concentration). Even if there were no carbonates in the water at all, a higher pH results in a higher TA, but of course that was what we used in our first example starting off with pure water that had lye added to it. As we added acid, the pH dropped as did the TA.

When carbon dioxide leaves the pool, the carbonic acid shifts towards making more carbon dioxide and the bicarbonate shifts towards making more carbonic acid. This shift also consumes hydrogen ion which is what makes the pH rise. However, note that the quantity of removed bicarbonate ion exactly equals the quantity of removed hydrogen ion with the former lowering TA while the latter raises it by the exact same amount -- these cancel each other out. In the following, removal of hydrogen ion increases TA while removal of bicarbonate decreases it and removal of carbonate ion decreases it by two since it can accept two hydrogen ions. Everything nets out to 0 change in TA.

CO₂(aq) --> CO₂(g) ................ TA change from each species: -(0) +(0) = 0
H₂CO₃ --> CO₂(aq) + H₂O ..... TA change from each species: -(0) +(0) = 0
H⁺ + HCO₃^- --> H₂CO₃ ......... TA change from each species: -(-1 +1) +(0) = 0
H⁺ + CO₃^2- --> HCO₃^- .......... TA change from each species: -(-1 +2) +(+1) = 0
H₂O --> H⁺ + OH^- ................. TA change from each species: -(0) +(-1 +1) = 0

Yet one more way to look at this is from a charge balance perspective. In order to lower TA, one has to reduce the bicarbonate ion which is negatively charged. That species cannot be reduced without also reducing some positive charge or adding another negative charge. The outgassing of carbon dioxide is charge neutral and all chemical equilibrium equations are charge neutral so there can be no change in TA from that process because there is no removal of hydrogen or hydroxyl ions (and I mean complete removal from the water). When one adds a strong acid, then one is explicitly adding positive hydrogen ions to the pool which explicitly lower the TA and charge balance is maintained because negative chloride ions (that do not count towards TA) are added along with it. In fact, one can consider the definition of a "strong" acid to be one that adds hydrogen ions without adding any ions that accept hydrogen ions (for any practical pH). The hydrogen ions in effect shift bicarbonate ion to carbonic acid, but for acid addition, the hydrogen ions that are added are always "excess" and in the accounting always show up as lowering the TA regardless of whether this shows up as extra hydrogen ions or as fewer bicarbonate ions. The pH also drops -- more so if there are no carbonates -- less (but not zero) if there are carbonates. If there were a strong acid that outgassed from the water, then that would remove a source of hydrogen ions and would reduce TA, but this is practically impossible since a strong acid will have minimal concentration of a neutral molecule necessary to outgas (ions do not outgas).

Increasing TA is easy since you only need to add a chemical species that creates ions in water that can accept hydrogen ions, but you have to use a substance that doesn't have the ability to produce hydrogen ions itself. Therefore, adding carbonic acid to water doesn't change the TA (but would lower the pH), but adding sodium bicarbonate increases TA and adding sodium carbonate increases TA twice as much on a molecule for molecule basis. Injecting carbon dioxide into a pool makes no change in TA though lowers the pH -- the exact opposite of outgassing. Splash-out and backwashing can change the TA -- either increasing it or decreasing it depending on the TA level of the fill water.

It should be noted that Total Alkalinity only measures the pH buffering capacity of the water against a drop in pH, the acid neutralizing capacity. It does not measure the capacity of buffering against a rise in pH. Nevertheless, for buffer systems that are near their optimum (maximum buffering) point where the species with hydrogen and the one without are equal in concentration, there is an equal capacity in both directions. In the carbonate buffering system in the pH range of pools (7.0 to 8.0), there is more resistance to a drop in pH than a rise in pH. The base neutralizing capacity (sometimes called the Total Acidity) isn't measured in pool kits. The Borates buffer system has about 45 times the base neutralizing capacity as acid neutralizing capacity which is why it doesn't contribute very much to the TA (50 ppm Borates adds about 5 ppm to TA). This doesn't mean it won't contribute to slowing down a drop in pH when acid is added -- it does -- but that it has limited capacity in doing so (but the carbonates have lots of capacity so this is not a problem). So the Borates are well suited to pools that tend to rise in pH cutting down the amount of pH rise roughly in half at a level of 50 ppm Borates.

Richard

 

[Water_man]

Replying to Chem geek:

You’re implying that the buffering power of water is similar to that of fully dissociated salt of a week acid and a strong base, and this is not true because of the low dissociation constant of water.

Here’s a calculation that illustrates this point:

At pH 8 the hydroxyl ion concentration is 1 micro Molar. 120 ppm Bicarbonate alkalinity corresponds to approx 2 milli Molar bicarb. This is 2000 times more than the hydroxyl! If 20 ppm bicarb is taken out from the water by aeration it means a reduction by 0.4 milli Molar. If at the same time 0.4 milli Molar of OH- is produced, as you suggest, then you will increase your pH to 9. (use water dissociation constant equation). This doesn’t happen.

Also, If just by lowering the pH to 7.2 you cause alk to drop and aeration had no contribution , then if you lowered your pH to 7.2 followed by bouncing it back by adding a base without aeration, according to your argument the alk is supposed to be reduced because lowering the pH was supposed to do it , but it won’t happen!

Quoting what PoolDoc said here:


"You can monkey around with the alkalinity test results, simply by changing the pH . . . but not matter what you do, once you restore the pH, all the original alkalinity will reappear . . . UNLESS you get rid of some of it, somehow, and PHYSICALLLY REMOVE IT FROM THE POOL.

There are two . . . and ONLY two . . . ways to do this.

You can precipitate it as scale or calcium carbonate (which is what happens when you add cal hypo, and get a white cloud), but this is very messy, or even damaging. OR, you can turn it into carbon dioxide, and dump it into the air." (End of quote)

I was saying what PoolDoc was saying: no aeration --->no removal of bicarb ---- > no reducing alk.

If I'm wrong, at least I'm in a good company

 

[Guest]

Ben kept his chemistry to a minimum in his forum for the very same reason we try to keep it to a minimum here. It is over the heads of the majority of forum member. As far as his 'sticky' on lowering TA, he wrote that before a LOT of the discussion that took place over there about ph rise from outgassing (a lot of it was centered around pH rise from SWGs) took place. His method is correct. His explanation is simplified so it can be easily understood. Your quote of Ben Powell as your 'expert' to back up your opinion which indicates that you really don't understand the chemistry involved.

Once and for all it is NOT AERATION but the OUTGASSING OF CO2 that causes pH to rise. If you increase the concentration of CO2 in the water by lowering the pH you will increase the rate of outgassing, aeration or no! This is the part of the equation you are not looking at!
Further discussion on this is not going to be fruitful so I am locking this thread.