High Alk-high Co2...ideal?

[thackray]

Randy,

You recently referred me to Fig #1 from your article:
The Relationship Between Alkalinity and pH.
http://www.advancedaquarist.com/issues/may2002/chem.htm.

This is the graph of alkalinity versus pH for CO2 = 350ppm (the equilibrated value). My own tank which is operated exclusively with kalkwasser and aerated by two means swings between pH 8.1 in the morning and pH 8.4 in the evening with the alkalinity holding at about 3.5meq/l (10dkh). As expected, this swing is almost perfectly centered on the graph of Fig#1.

We have added SPS corals and clams from a smaller tank that was being overgrown. As we are reaching the limits of kalkwasser addition via make up water, my partner dosed the tank with Seachem Reef Builder. This moved the alkalinity to 4.5meq/l (12.5dkh) narrowed my diurnal pH swing and shifted the center of my swing to a slightly more acid reading. The net effect was to move the operating point of the tank well to the left of the curve of Fig #1 a region that implies high alkalinity and high CO2 concentration. Anecdotally, my partner (she is the reef keeper) noted a marked improvement in the plating of coralline algae after this addition.

Anecdotes aside, I note that the typical recommendations for tank chemistry are to the left of the curve of Fig #1 since they all basically recommend the pH of normal sea water (8.2) but then suggest enhancing the alkalinity above that of normal sea water (2.5meq/l) to values as high as 5meq/l. The principal reason listed for enhancing the alkalinity is that the alkalinity changes a lot in a closed system which is certainty true. But could the enhanced CO2 levels be a more subtle reason to elevate the alkalinity? I know that the process of calcification as proposed by Tom McConnaughey lists CO2 in several of the reactions.

Furthermore, except for a well aerated kalkwasser system, all of the other Ca/Alk systems, operate to the left of the curve of Fig #1. This includes most two part systems, calcium reactors and calcium reactor/kalkwasser combos.

Is it possible that maintaining CO2 levels higher than 350ppm and keeping the alkalinity higher than normal to lift pH levels to say 8.2 is the ideal situation? In other words, operating to left of the curve of Fig #1 but maintaining normal pH levels.

[Boomer]

Is it possible that maintaining CO2 levels higher than 350ppm and keeping the alkalinity higher than normal to lift pH levels to say 8.2 is the ideal situation? In other words, operating to left of the curve of Fig #1 but maintaining normal pH levels.

Yes to a point:D

First, that CO2 of 350 ppm, is not that in water but ambient air above water, with such plots given IF the water and air where in equilibrium, which they rarely are, especially in a tank. The actual level of CO2 in most reef tanks is on the order of 0.35 - 1.5 mg / l CO2. pH is dictated buy the ratio of CO2 to Alk, which means if you know your Alk and pH you can mathematically calculate CO2. Since CO2 by itself will lower pH if it increases and increase pH if it decreases most of the pH problems is from CO2. In order to increase your pH and Alk and get a higher pH you would want a slightly higher CO2. All of these plots are based on a steady state of CO2 where it never changes, which is the only way to really explain it in a simple form. You can have a pH of 8.3, with a low Alk and low CO2 and have the same pH with a high CO2 and High Alk. If either of these change in a non-proportional way then the pH will shift up or down depending on the change in CO2 and Alk. For example, some ruff calculations from Buch-Davis equations.

Normal Seawater;

pH 8.3 (not corrected for seawater)

carb Alk = 2.15 (no borate)

CO2 = 0.36 mg / l



Examples;

1.. pH =8.1 (same)

Alk (same) = 2.15

CO2 = 0.63 mg /l


2. pH = 8.1 (same)

Alk = 3.15 (same)

CO2 = 0.92 mg / l


3. pH = 8.1(same)

Alk = 1.15 (same)

CO2 = 0.33 mg /

So if you wanted to raise the pH up to 8.3 and have a high alk of say 3.85 the CO2 = 0.65 . However, that would also be a steady and as soon as some CO2 entered the system the pH would go down. Alk has no control of CO2 and CO2 has no control over Alk, they are both independent, meaning no matter what your Alk is, the pH will drop as soon as more CO2 enters the system by any means. Your tank could have a Alk of 5 meq / l and a pH of 8.3 and as soon as some CO2 enters the pH will go down but the Alk will remain the same. The key to keeping a good pH and higher Alk is to have a certain CO2, as constant as possible.

If I confused you Randy will fix it :lol:

[thackray]

Boomer, Randy,

I took the 350ppm CO2 concentration from the legend on the referenced graph which apparently refers to the concentration in air. I mistakenly referred to this as the level in equilibrated seawater, that was wrong. Your numbers are the correct level for the CO2 in seawater.

However, I understand the relationship between alkalinity, pH and CO2. My question was/is if we set the alkalinity purposefully high and force the CO2 to a certain (above normal) level so that the pH is controlled in the area of 8.2 would the elevated CO2 in combination with elevated alkalinity enhance calcification? I was also noting that most systems seem to run this way i.e. to the left of the curve of figure #1.

As I page through Mike Paletta’s book Ultimate Marine Aquariums I am amazed by how far to the left of the curve most of these very successful tanks are.

You mention that a tank that has a non-equilibrated CO2 level should eventually return to equilibration but that does seem to happen easily. Sighting my tank example from the original post, its been three days since we put in the Seachem reef builder and I’m still to the left of the curve. (I’m running a skimmer and an air stone.) Just to restate that example, prior to putting in the Seachem I was right on the curve running a kalkwasser only with aeration system.

Anyone who is running high alkalinity and keeping their pH normal is running with “elevated� CO2.

If this condition aids in calcification then it would be useful to talk about setting levels of alkalinity and CO2 high, consistent with suitable pHs.

Phil Thackray

[Boomer]

Phil

You mention that a tank that has a non-equilibrated CO2 level should eventually return to equilibration but that does seem to happen easily.

You mention that your pH swings from 8.1-8.4, that shows you right there it is not in equilibrium with air. If it was really in equilibrium there would be no pH shift. The pH is being lowered at night, this does not happen in the ocean. Before your tank can get to equilibrium that CO2 increase would have to diffuse out, but its diffusion rate can't keep up with the increase. Buy the time days end comes the "plants" use up that CO2 and things go back to more normal. If this continued, on a 24/7 lights on, the pH would rise due to the loss of CO2, as the CO2 would not be able to diffuse fast enough into the water, the opposite, in which case you would have to inject CO2 to keep up with the demand. The reason behind FW planted tank and Maine planted tanks to use CO2 injection.

Anyone who is running high alkalinity and keeping their pH normal is running with “elevated� CO2.

That is correct they would have to be. With out that extra CO2 and that high Alk the pH would go up, it has to. The CO2 is keeping it in balance. In general, if you want a set pH and a high Alk you must have a higher CO2 to keep it there.

One can control the lower pH night drop by dripping kalk to reduce that CO2, as the CO2 combines with the kalk to get bicarbonate. Many do it that way. One could also tweak a controller to do this for you.

If this condition aids in calcification then it would be useful to talk about setting levels of alkalinity and CO2 high, consistent with suitable pHs.

Yes, it should. I believe Randy or Habib had posted some abstracts awhile back that show that.

Here is something on a quick search

http://www.reefkeeping.com/issues/2004-05/rhf/index.htm

from the above

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in Porites porites .4 In this case, doubling the bicarbonate concentration resulted in a doubling of the calcification rate. Uptake of bicarbonate can apparently become rate limiting in many corals.5 This may be partly due to the fact that both photosynthesis and calcification are competing for bicarbonate, and that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration).

[thackray]

Boomer,

I’m afraid a typo completely reversed my point. I meant to say that “a tank that has a non-equilibrated CO2 level should eventually return to equilibration but that does NOT seem to happen easily�. Your example makes that point as well.

I realize that there have been suggestions that high alkalinity may increase calcification rates but what I’ve seen does not talk about the CO2 level. If CO2 is really significant then the design of ideal systems should be so influenced but I have seen no evidence of that (but my time in grade and practical experience in the reefing field is very limited.)

Phil Thackray

[Boomer]

Phil

I realize that there have been suggestions that high alkalinity may increase calcification rates but what I’ve seen does not talk about the CO2 level.

It really doesn't need to be talked about :D If you want to keep a high Alk at any pH there must be a higher CO2 to accompany it. If there is not a higher CO2 the pH will change. If they/you are running a pH of 8.3 and a high ALk there must be a higher CO2, than is normally found in seawater. If we lowered the CO2, with that high Ak, the pH would go up, lets say to 8.5, now your pH is to high. If on the other hand if we raised the CO2, with that high Ak, the pH would go down, to say 8.1, now it is to low. It is an exact balance.

xpH + yCO2 = z CO2.

I think I know what you are trying to get at. Can I have a pH of 8.3 , Alk of 3.5 and a low CO2 and/or could I have the same pH of 8.3, a Alk of 3.5 and a high CO2 and the answer is no. And if we could would the corals do better at one of these as far as calcification. There is no chemical way of doing that, so the question can not really be answered. The corals just have to deal with what the chemistry dictates.


Anyone who is running high alkalinity and keeping their pH normal is running with “elevated� CO2.

That is correct they would have to be. With out that extra CO2 and that high Alk the pH would go up,


Also, not all corals, just like marine plants, don't use CO2 but HCO3, which is converted to CO2

[thackray]

Boomer,

I realize that specifying the Alk and pH defines the CO2 level. The studies like the one you referenced generally refer to increased levels of Alk and/or Alk and Ca. I’m certain that pH is recorded in these experiments but if the real guns are Alk and CO2 why not discuss that. If I’m going to design a Ca, Alk ,CO2 (??) delivery system for my tank I’d like to know what levels of what are really important. If someone tells me that increased alkalinity will increase calcification I’m probably not going to think about CO2 enhancement but maybe I should. Even if that study lists a pH that implies significant CO2 enhancement above equilibrium conditions I’m probably just going to let my pH roam within the normal “ok� zone.

If CO2 is important then I think it’s important to emphasis that not just imply a value by listing the pH.

Phil Thackray

[thackray]

Boomer,

To the second part of your reply – no I’m not trying to achieve impossible combinations. I know that we can control the Ca, Alk, pH, and CO2 within the following limitations:
1)The product of the Ca and Alk concentrations cannot exceed a certain value which is pH dependent,
2)The Alk, pH and CO2 are interrelated by the Buch-Davis equations and,
3)The total Alk is unaffected by the CO2 levels (principle of conservation of alkalinity.)

However, that leaves a lot of room to play around in!

Phil Thackray

[Boomer]

but if the real guns are Alk and CO2 why not discuss that

Becasue you can not measure CO2 in seawater buy any normal means. There is no such thing as a test kit that will work in seawater, so we are stuck with pH and Alk. It also makes no difference, as I tried to explain. As I pointed out, @ xpH + yAlk = zCO2. If you give me a Alk and pH I can calculate what the CO2 is. As far as pH goes, you could have a pH 6 and still have a high Alk, normal Ca and high CO2 . The body chemisty needs a certain pH. Try reading this on water parameters to shoot for

http://www.reefkeeping.com/issues/2004-05/rhf/index.htm

If someone tells me that increased alkalinity will increase calcification I’m probably not going to think about CO2 enhancement but maybe I should

As I'm sure you know that if you add a buffer to the water the Alk increases but did you know the CO2 also increases when you add that buffer ? Any time you increase the Alk, by any means, to keep or stay at a specifc pH, there must be an increase in CO2. If you add to much buffer then all three change, pH, Alk and CO2. They will all increase.

[thackray]

Boomer,

I understand the chemical interrelationships and the difficultly of measuring CO2. Clearly if the Alk goes up and the pH stays the same this means that the CO2 has gone up.

I’m just trying to look at the water balance/coral growth issue from a little different perspective that might alter the way we approach the system design.

By the way I was in Duluth for Grandma’s marathon. My tank partner ran in the ladies division. What a nice place.

Phil Thackray

[Randy Holmes-Farley]

Unfortunately, there have been relatively few good studies of what conditions are optimal for growth for corals (or for calcification generally). The above reference does show that raised alkalinity increases calcification. But what is the optimum level? At what pH? At what calcium level?

Other studies have convinced most oceanographers that decrease pH (via increased atmospheric CO2) will decrease calcification by wild corals, with clacium and alkalinity at the usual levels of the ocean. But in an aquarium where calcium and alkalinity are not constrained, I do not think we understand what is optimal.

FWIW, other issues than calcium and alkalinity and pH may also come into play. Phosphate and organics, for example may impact calcification variably as a function of pH, calcium, and alkalinity levels.

To many questions, and not enough experiments to say anything definitive, IMO.

[thackray]

Dear Randy, Boomer,

Thank you for your time and input. Not every question has a simple, complex (or extant) answer. Perhaps foolishly, for the time being I’ll remain interested in the locus of points north of “normal seawater� in Fig #1.

Phil Thackray

[Boomer]

Phil

By the way I was in Duluth for Grandma’s marathon. My tank partner ran in the ladies division. What a nice place.

I"m glad you enjoyed yourself. Yes, it is a great place, reason I can't get myself to move anywhere else :lol: