Why is Ph climbing?

[waterbear]

Then explain to me why the TA increases when CO2 injection is used for pH control (and also why it causes an increase in TA in planted aquariums). Also if this is the case and the TA is lowered when the acid is added that would mean the 'slug method' works'. It would seem that by outgassing CO2 you have actually lowered the amount of the buffer system in the water since you have removed one of it's components. It would seem in the example that it was accounting for a slight increase or decrease in CO2 due to respiration and the effect is what would be expected with a buffer system. I do not believe in a stiuation where you have a forced continuous gassing off would be the same thing at all. YOur comments?

[chem geek]

The "slug method" does not work. If you add acid you lower both the pH and the alkalinity so if you are trying to just lower the alkalinity by itself with no change in pH, then you can't do so by just adding acid. Those that have claimed that the slug method works or "could work" claim that you outgas CO2 when you add the acid, and technically in the localized region where they are dumping the acid the pH is in fact so low that it does force the carbonate in that area to more rapidly outgas, but this area is so small that even if all of the carbonate was removed, it would have a negligible effect. By the time the acid is dispersed through more of the pool so that a larger area could be affected, the acid effect is weakened so that the increase in outgassing isn't very high and is so slow anyway that the total release of CO2 is small. Somebody somewhere probably carefully added acid in such a way that it spread near the surface and did in fact slightly lower alkalinity more than normal and hence this procedure was "born", but it is extremely ineffective (especially compared to Ben's sure-fired approach).

When you inject CO2 for "pH control" you are implying that there is some sort of pH drift (demand) already that you are trying to counteract. Since injecting CO2 makes the pH drop, the implication is that you have something else in the aquarium that is trying to make the pH rise (i.e. is basic or alkaline). So the combination of adding CO2, which just makes the pH drop, plus this basic source, causes the pH to be stable (or to drop less) and for the alkalinity to increase.

It is true that by outgassing CO2 you are removing carbonate from the system which is the primary component of the buffering system, but the very nature of the pH shift that is caused by this outgassing increases the buffer "start point" by having less hydrogen ion that needs to be "neutralized". Think of hyrdrogen ion (which is measured by pH) as being "anti-alkalinity" or "negative alkalinity". When you outgas CO2 you lose the alkalinity from carbonate but also lose the "negative alkalinity" from hydrogen ion so there is no net change in alkalinity (and the pH has risen). [EDIT] If you don't like the concept of "negative alkalinity", then consider that H+ and OH- are always in equilibrium (with water) so that when one goes up the other goes down. So when the pH rises, you get more OH- and clearly OH- is alkaline and increases alkalinity.

TA = 2*[CO3(2-)] + [HCO3-] + [OH-] - a(H+)

Even ignoring the a(H+) for the moment, the first two carbonate-like species decline with outgassing, but the pH rises so the OH- increases by exactly the same amount (and H+ decreases -- [OH-]-[H+] is a "net" excess of alkalinity from pH; the reason a(H+) activity is used is that the titration pH test measures the activity of H+, not its concentration, though they are pretty close to the same in pool water). It is as if each CO2 that leaves is the same as removing H2CO3 (CO2 + H2O) from the system and this takes away 2H+ and one CO3(2-), the latter counting double for alkalinity so there is no net change.
[END-EDIT]

Believe me, this is one of the most counter-intuitive things I have encountered in chemistry and it still doesn't "feel" right to me, but it seems to be true as measured by the titration alkalinity test.

Richard

[Rangeball]

N=1, but in my and my pool's experience when I was aerating ALK rose along with PH until I got the ALK below 200. Even then, ALK only drops after I add the acid and let PH drop THEN aerate, and ALK remains constant.

[chem geek]

N=1, but in my and my pool's experience when I was aerating ALK rose along with PH until I got the ALK below 200. Even then, ALK only drops after I add the acid and let PH drop THEN aerate, and ALK remains constant.

Did you phrase that correctly? When you are aerating, your alkalinity rose along with pH? Or did you see it drop?

Before you got started with the aeration, you may have had a tendency to drift in pH due some acidic or basic process in your pool. It's hard to isolate the different pieces since it is an open system.

Richard

[Rangeball]

If I recall correctly, when my alk was very high as PH rose, alk tended to rise as well. It may very well have been from the process you stated occuring simultaneously, but when my ALK is below 200 it isn't as noticeable.

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

[chem geek]

In any event, simply aerating has never lowered my alk. I had to reduce it with acid first, and try to hold it there as I aerated to bring PH back up.

My point exactly. Simply aerating just increases the pH (ignoring the increase in alkalinity you saw at high alkalinity levels). It is the adding of acid that lowers the alkalinity (and also lowers the pH). The reason you add the acid first is that the outgassing of CO2 is more rapid at lower pH so by adding acid first you make the process more efficient (occur more rapidly). It would still work if you aerated first and then added acid -- it would just take a lot longer.

[waterbear]

When you inject CO2 for "pH control" you are implying that there is some sort of pH drift (demand) already that you are trying to counteract. Since injecting CO2 makes the pH drop, the implication is that you have something else in the aquarium that is trying to make the pH rise (i.e. is basic or alkaline). So the combination of adding CO2, which just makes the pH drop, plus this basic source, causes the pH to be stable (or to drop less) and for the alkalinity to increase.

CO2 injection is used in aquariums (usually freshwater live plant tanks to promote growth) and also used in commercial pools for pH control. The net effect in both cases is a rise in TA

Richard

just for clarification here are some links of commercial equipment
http://www.brockent.com/CategoryView.asp?CategoryId=377
http://www.poolequip.com/Merchant2/m...egory_Code=CO2
http://www.aquasol.com/co2_systems.htm

If you read the info on the links they blame the rise in TA on the size of the CO2 bubbles injected.

[chem geek]

The only explanation I can come up with that is consistent with what these guys are saying is that injecting CO2 into the water increases the Carbonate Alkalinity, which is a true statement. However, there is a difference between carbonate alkalinity and Total Alkalinity. The former does not include [OH-] while the latter does. For the pool alkalinity tests, you measure Total Alkalinity, not Carbonate Alkalinity. For practical purposes, the two are very close in a pool (except at high pH), unless there is CYA present (in which case there is the CYA adjustment that can be made).

What can I say? Perhaps you can do an experiment if you've got one of these CO2 injectors. Just be sure to do the experiment in water that doesn't have other things going on (i.e. living things doing photosynthesis and respiration, etc.).

Anyway, I'm off on vacation. Talk to y'all after next week!

Richard

[waterbear]

The only explanation I can come up with that is consistent with what these guys are saying is that injecting CO2 into the water increases the Carbonate Alkalinity, which is a true statement. However, there is a difference between carbonate alkalinity and Total Alkalinity. The former does not include [OH-] while the latter does. For the pool alkalinity tests, you measure Total Alkalinity, not Carbonate Alkalinity. For practical purposes, the two are very close in a pool (except at high pH), unless there is CYA present (in which case there is the CYA adjustment that can be made).

What can I say? Perhaps you can do an experiment if you've got one of these CO2 injectors. Just be sure to do the experiment in water that doesn't have other things going on (i.e. living things doing photosynthesis and respiration, etc.).

Anyway, I'm off on vacation. Talk to y'all after next week!

Richard

I have started a new thead in the China shop to continue this side discussion since it is getting a bit too technical for the general forum.
http://www.poolforum.com/pf2/showthr...9517#post29517