Carbonic Acid is a Weak Acid

Help me out here folks...

Carbonic Acid, the acid formed in a solution of CO2 and water, is a weak acid which means that it only partially dissociates in water. This is, I think, a huge advantage under some circumstances.

I dose two part to maintain alkalinity. There is a direct linear relationship between the dose rate and the alkalinity in my tank. At best, I can reach an unstable, temporary equilibrium.

Because carbonic acid is weak, it has reserves of carbonic acid that will, when an excess of H+has been consumed by the dissolution of aragonite, give off more H+. It is, essentially, a buffered reaction. This means a calcium reactor forms a stable equilibrium with the equilibrium point shifting slightly as variables change.

Because carbonic acid is a weak acid, calcium reactors are inherently more stable than other methods of maintaining alkalinity and calcium, correct?

 

Yes, because you are not trying to keep up or get ahead as when dosing. You are meeting the need as it occurs.

 

I'm not sure I quite follow. What do you mean by "inherently more stable? I think it is inherently less stable as it is easier to buffer a rise in pH, then a drop in pH. I'm not a chemist, and it's been a while since I did any significant work in chemistry, but I think this should still be pretty a pretty reasonable summary.

In seawater there is always a mixture of carbonic acid, bicarbonate, and carbonate, which is defined by pH.
H2CO3 <--> H+ + HCO3- <--> 2H+ + CO3--

When we look at carbonate and bicarbonate, as you add base, bicarbonate shifts to carbonate and pH goes up buffering a rise.

HCO3- + OH -> CO3--

as you lower the pH though carbonate shifts to bicarbonate, buffering a drop.

CO3-- + H20 -> HCO3- + OH-

The limiting factor, at lower pHs, is the amount of carbonate and eventually you run out of carbonate and bicarbonate does the buffering.

So, then HCO3- + H+ -> H2CO3

And
H2CO3 -> CO2 +H2O

Where the amount of carbonic acid is dependent purely on the partial pressure of CO2 though (i.e. the amount in the air), it has nothing to do with pH. In the reactor, you can adjust pCO2 in the reactor, but in your tank, it is based on the level of CO2 in your home.

CO2 from the air/reactor becomes carbonic acid, and is essentially only dependent on the amount of CO2 in the air.
CO2 + H2O -> H2CO3

In the reactor, when pCO2, is increased, carbonic acid then looses H+ adding alkalinity as bicarbonate and disassociating CaCO2 becoming calcium + bicarbonate.
H2CO3 -> HCO3- + H+
CaCO3 + H+ -> Ca++ + HCO3-

This dissolution raises the pH of the reactor limiting the drop via CO2 addition.

At low pH, there will be very little carbonate, the tank will have higher pH though, and slightly more carbonate. The bicarbonate, when entering the tank, will then depress the pH until CO2 is gassed off.
2HCO3- -> CO3-- + CO2 + H2O

In theory, this could raise the pH, and this rise in pH, if substantial enough, calcium and carbonate would precipitate.
Ca++ + CO3-- -> CaCO3

In reality though, out-gassing, isn't perfectly efficient, and the pH of the aquarium tends to drop lower. Therefore, I would consider this less much stable than other methods. It tends to pull down pH, which is easier to do than to raise pH. So, methods that inherently raise pH, will be more stable in my opinion as it is more difficult to overly increase pH. Also, the calcium and alkalinity in the tank becomes a weighted average of the calcium and alkalinity in the tank and the effluent of the reactor. Because slight changes in CO2, result in large changes in this effluent concentration; this also makes it tough to dial in the correct dosage. With a 2-part, the concentration is fixed and a weighted average can be easily calculated, therefore there is less effort trying to "dial-in" the correct dosage. With reactors, many people give up because it is tough to "dial-in" the dosage and maintain a reasonable pH. This is much more strait-forward with 2-part.

 

Sorry I cannot speak about the exact chemistry, only experience. I have ran both calc reactor and 2 part/3 part. Ultimately I don't see a difference as long as proper alk/cal/mag levels are reached. With that being said, the advantage of calc reactor does seem to be trace elements that may not be present in the 2 part. The disadvantage of calc reactor, as mentioned before is a slightly lower PH. A lot of successful large tank owners run both calc and kalk reactors to maintain a good level of PH. The calc reactor is also harder to "dial in" at first, it's a steep learning curve, but the upside is that once it's dialed in, it's can maintain proper levels for long periods of time without having to readjust. For large tanks (150gal or larger) with heavy sps load, it is much easier to run a calc reactor, otherwise you will be going though many many gallons of 2 part, this usually is not very economical.

 

Yeah, for sure, on a big tank it makes lots of sense. I just didn't get the question and claims of "stability".

Also, by the way, before someone asks I just noticed two typos.

on the 17th line, where I have "CO3-- + H20 -> HCO3- + OH-" that should be
"CO3-- + H+ -> HCO3-" I re-wrote that section before posting and should have changed that... Also, above, where I had "and pH goes up buffering a rise." "pH goes up, but buffers a further rise".

 

I'm referring to the stability of the alkalinity. If something causes the uptake to increase or decrease, dosing will just drive down or up the alkalinity as a result. You seem to be talking about whole tank stability which is not my point.

 

A calcium reactor is just a form of dosing. If something drives uptake to increase or decrease, you need to adjust the reactor, or else it goes up or down. Really no different than having 2-part set on a drip, with an automatic doser.

The only real difference is the reactor produces a "balanced additive" that is the calcium and carbonate are produced in the same proportions as used for calcification by corals. You can't raise one, without raising the other, as with 2-part. So, it is said to be useful to "maintain stable calcium and alkalinity", it's not usually used to raise calcium and alkalinity.

No comprende, if we are not talking about calcium and alkalinity in the tank, were are we talking about?

 

You tell me. I was trying to understand why you brought up pH maintainable when I was talking about keeping alkalinity stable. As for the rest, I have clearly communicated my point badly, because I'm trying to talk about how the weakly dissociative properties of carbonic acid allow it to buffer alkalinity dosing to help maintain more stable alkalinity.

 

First, just to straiten out the terminology a bit, you don't buffer alkalinity. Alkalinity is "buffering" capacity. More specifically, alkalinity is a measure of the amount of acid required, to lower the pH of the water, to the point where all of the carbonate and bicarbonate are converted to carbonic acid. Above, I defined parameters of this system. So anyways, what you do do is add more base. In an aqueous solution, bicarbonate is the conjugate base of carbonic acid and the conjugate acid of carbonate, this is a "buffering system" as it "buffers" against changes in pH.

Now, you said, "There is a direct linear relationship between the dose rate and the alkalinity in my tank. At best, I can reach an unstable, temporary equilibrium." However, there is not really a direct linear relationship between the dose rate and alkalinity in your tank, as the equilibrium described, also applies to dosing. There are factors that can increase or decrease alkalinity, more, or less, than you might expect, such as the dissolution or precipitation of calcium carbonate. There is always however an equilibrium, this equilibrium point though, may not be where you want it to be. However, whether you dose 2-part, or ca reactor effluent, you can maintain this equilibrium point, after setting it to the desired balance.


Now, you also said, "carbonic acid that will, when an excess of H+has been consumed by the dissolution of aragonite, give off more H+. It is, essentially, a buffered reaction."

When aragonite comes into play, we are now talking about a heterogeneous equilibrium, as calcium carbonate is a different phase. Therefore, the rate is different from the equilibrium rate of the aqueous forms of inorganic carbon.
Also, when we are talking about a ca reactor, we now have two, more or less, separate equilibrium systems. Keep in mind, without the reactor, we have do still have the same equilibrium system though, it's just one system, not two.

So, then you mentioned "This means a calcium reactor forms a stable equilibrium with the equilibrium point shifting slightly as variables change."

That's fine, but this happens with 2-part as well, if we dose continuously, for example, with a dosing pump. The chemistry doesn't care where the alkalinity is coming from.

Finally, you said "Because carbonic acid is a weak acid, calcium reactors are inherently more stable than other methods of maintaining alkalinity and calcium, correct?"

Acids and bases will all reach an equilibrium in the aquarium. Carbonic acid is the conjugate acid of bicarbonate and the ability of CO2 to disassociate into carbonic acid, therefore makes CO2 fairly soluble. The amount of CO2 though, is dependent on the partial pressure, which is artificially inflated in the ca reactor. We can use this to our advantage and create an "on-demand" calcium/bicarbonate additive, that can be dosed to the aquarium to maintain calcium and alkalinity, once the drip rate is adjusted to match the needs of our system. It also adds to calcium and alkalinity in the proportions used by organisms for growth (although there are factors that can shift these proportions).

Now what I think you are trying to get me to say, is that there is some sort of inherent self-regulation in the system and that as alkalinity is consumed, more carbonic acid results and more bicarbonate to offset this shift. In reality though, as described above, this happens with 2-part as well, as there are sources and sinks for calcium carbonate precipitation and dissolution. I.e. rocks, sand other precipitate. The set-point may be inherently lower, but
it is more stable than with a ca reactor, as the system is more simplified there are less variables, many of which, with regards to a ca reactor have non-linear and unpredictable effects. Adding an acid to the system, in artificially high proportion also skews the equilibrium of the system decreasing the stability. Further adds an additional complication in that it has a negative effect on pH, which has feedback effects, such as decreased carbonate uptake rates by organisms for example. The results of this may require further complexity to truly overcome, such as timers to regulate Co2, or the use of kalkwasser etc...

I think this is sort of getting away from your point, but with that regards, if I understand correctly, the important point is there is actually an equilibrium system in place regardless of the dosing method.

 

With all due respect, I'm fairly sure it was clear I used buffer in the broader sense. A person can quite reasonably say their excess fat is a buffer against starvation. There is no alkalinity whatsoever involved there. I get that as a result I'm saying I have a buffer for my buffer, but its the easiest way to explain
It quickly. Sadly, not as clearly as I had hoped.

You say two part produces a stable equilibrium. I have not been able to achieve this experimentally. Actually, I get the opposite result. Increases and decreases in dosing produce a predictable, continuous increase or decrease. If i dose too little and dont adjust daily, it will eventually drop to about 118ppm while if i dose too much, it will eventually wander off to the precipitation point. if there is an equilibrium, its ina very narrow band. I don't think the "sinks" you refer to have an appreciable effect on the system except near the end points.