Need help with some chemistry math - CO2, HCO3, CO3, pH, kH

[karimwassef]

Ok.. for personal edification, I'd like to be able to calculate my CO2 components and after some research, found this paper...

http://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring2012/CO2pH.pdf

AWESOME! right?

Ok... this is why I need help because after running through some math, I think I got an answer, but I need some validation that I'm doing this right.

Based on the paper (page 4) and using a pH of 8.3, salinity of 35, P at 1atm and T of 25C:

pK1 = 5.8401
pK2 = 8.9636

So K1 = 1.44e-6, K2 = 1.09e-9. and H+ = 5.01e-9

Using page 9, I get that

[CO2] = 0.0035 [HCO3-]
[CO3 2-] = 0.22 [HCO3-]

so that the

Sum (CO2) = 1.22 [HCO3-]

Assuming sum (CO2) = 2.3 mM, then

[HCO3-] = 1.88mM

with a molar weight of 61 g/mol, that is 0.115g/L or 115ppm for bicarbonate.

Is this correct?

If it is, then [CO2] = 0.4ppm, [CO3 2-] = 25ppm, and sum(CO2) = 140ppm

The big assumption here is that sum(CO2) = 2.3mM. I used this from page 9 again, but I don't know how this compares to my system.

full paper:
https://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf

and another reference (Exam with answered questions :D ):
http://www.ocean.washington.edu/courses/oc520/MT1_09_key.pdf

[karimwassef]

By the way, my Alkalinity is 8.5dKH

The freshwater tank calculators that use pH and Alk to calculate CO2 give an answer of 1.3ppm. I realize this is freshwater and doesn't apply directly, but I couldn't find a simple formula for salt water.

[karimwassef]

One more question - I would also like to be able to relate my room CO2 (~600ppm) to my tank CO2. Maybe that's a different set of formulae, but ultimately, I'd like to connect all those CO2 dots.

[LX20000]

They got CO2 ppm = 1.1 in the paper, right?

Where you get Sum(CO2) = 2.3mM ??

Your math works out fine if all your assumptions are correct... I just don't understand a few of the assumed values.... and again, they get CO2 ppm = 1.1, which is close to what the other paper you mention gets for freshwater.

[LX20000]

Found this online

With CO2 atmospheric of 387 PPM, dissolved CO2 is 1.32 x 10-5 M. Assuming simple Henrys Law, guess value at 600 PPM is 2.046 x 10-5 M


START OF TEXT FOUND ONLINE
Carbon Dioxide in the Ocean The levels of carbon dioxide in the atmosphere has been increasing since the beginning of the industrial revolution due to increased combustion of fossil fuels. Much of the carbon dioxide that has been released dissolves in the ocean. We can use Henry's Law to calculate the concentration of dissolved carbon dioxide in an aqueous solution. The current, average concentration of CO2 is 387 ppm, that is 387 x 10-6 atm. [CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

[LX20000]

Remember... you won't have any tank CO2 at your PH.... you will have carbonates... right?

[karimwassef]

I used their assumption for sum CO2. That's the big unconnected dot for me.

I dont expect CO2, I do expect HCO3-.. that's why CO2 came out at 0.4ppm or 6.5e-6 M. That's half of what you get at 387ppm atmospheric CO2.

At least we're in the ballpark ( x2).. :D

my atmosphere CO2 is actually 450ppm.

[karimwassef]

I don't understand this :

[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

where does 29.41 atm/M come from?

If I use 450ppm, does that convert to 1.53e-5 M?? If that's true, I can use that and work backwards to calculate the constitutive elements and the correct sum (CO2).

[karimwassef]

ok.. if it is, then:

atm CO2 = 450ppm
=> 0.015mM [CO2] = 0.9ppm

that works is sum(CO2) = 5.4 mM

following through:

atm(CO2) = 450ppm
[CO2] = 0.9mm
[CO3 2-] = 59ppm
[HCO3 -] = 270ppm
sum(CO2) = 330ppm

for a pH of 8.3

Is there a way to relate this to dKH?

[Dan_P]

ok.. if it is, then:

atm CO2 = 450ppm
=> 0.015mM [CO2] = 0.9ppm

that works is sum(CO2) = 5.4 mM

following through:

atm(CO2) = 450ppm
[CO2] = 0.9mm
[CO3 2-] = 59ppm
[HCO3 -] = 270ppm
sum(CO2) = 330ppm

for a pH of 8.3

Is there a way to relate this to dKH?

[CO3]*2 + [HCO3] is the total bicarbonate alkalinity = 270 + 118 ppm HCO3

388 ppm HCO3 is 388 mg/kg of water. 388 mg/Kg / 61 mg/meq HCO3 is about 6.5 meq/Kg. I don't use dKH, so, I will let you do the conversion.

[LX20000]

I don't understand this :

[CO2] = P/KH = 3.87 x 10-4 atm/29.41 atm M-1 = 1.32 x 10-5 M

where does 29.41 atm/M come from?

If I use 450ppm, does that convert to 1.53e-5 M?? If that's true, I can use that and work backwards to calculate the constitutive elements and the correct sum (CO2).

1.53e-5 * 44 gm/mole = 67.3e-5 gm/L = 673 ppm. Right?. Not 900

[karimwassef]

I was using the 29.41 atm/M you used going from 387ppm to 1.32e-5M .

I didn't know where the 29.41 came from.

[karimwassef]

1.53e-5 * 44 gm/mole = 67.3e-5 gm/L = 673 ppm. Right?. Not 900

also not sure where the 900 comes from :)

Sorry - this is a topic I need more help with fundamentals on.

[LX20000]

OK... try this one.... I think this is a good overview, and quotes current CO2 conc. globally, and you could subst. in your house 450 ppm

http://ion.chem.usu.edu/~sbialkow/Classes/3650/Carbonate/Carbonic%20Acid.html

Concentrate on this, 3/4 of the way down

"Since CO2 makes up 0.0355% of the atmosphere (on the average) and KCO2 =2x10-3............"

You can use same equation and subst. whatever you want for household CO2 level

[karimwassef]

Where does that KCO2 come from?

[karimwassef]

aha: Henry's Law

http://butane.chem.uiuc.edu/pshapley/GenChem1/L23/web-L23.pdf

That's where 29.41 atm/M comes from!

So [CO2(aq)] = P(CO2) / K = 450ppm x 1 atm / 29.41atm/M = 15.31 x 10-6 M

and [CO2(aq)] = 1.53 x 10-5 M

[karimwassef]

ok. I'm much closer. I can calculate total alkalinity as ~ carbonate alkalinity = 2 [CO3 2-] + [HCO3 -]

but that returns a result in moles (M)

How do I convert that to meq/L or ppm or dKH?

[LX20000]

Bingo!. Give that man an emerald crab.

[LX20000]

Moles/L x 44 gm/mole = xxx gms/L

Xxx gms/L x 1,000 = ppm

Wait.... Make that 1000

[karimwassef]

I found it!!

http://comp.uark.edu/~ksteele/hc2004/ALKALINITY-%20CALCULATION.htm

So to calculate the alkalinity, the formula must be done in g/L (not moles) and then divided by 2!

alkalinity = ( 2 [CO3 2-] + [HCO3 -] ) / 2 = 151 ppm = 8.4dKH

[karimwassef]

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zpsg82h992i.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zpsg82h992i.jpg" border="0" alt=" photo 0_zpsg82h992i.jpg"/></a>

[karimwassef]

that captures all the variables and actually lines up with my measured data
pH = 8.3
dKH = 8.4
CO2 (atm) = 384 ppm
my temperature, salinity and pressure may not be exactly what is there but it should be close enough.
This basically is my carbon calculator.

<a href="http://s1062.photobucket.com/user/karimwassef/media/E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg" border="0" alt=" photo E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg"/></a>

yes - my garage is very hot and dry.. desert conditions in there... it can get to 50F in the winter, but I'm running very hot now.. so, not 10C :D ...

[karimwassef]

to make this better, pK1 and pK2 need to be functions of temperature and salinity --- next steps.

this also says that the CO2 at the bottom of a 3' foot tank will be different, as will the alkalinity..?

wait --- so does this mean that measuring pH and atmospheric CO2 would be sufficient to measuring Alkalinity ?? Whoaaw!:hmm5:

[karimwassef]

before I get carried away -... please validate or correct my formulas above

[karimwassef]

https://www.gfdl.noaa.gov/bibliography/related_files/millero0201.pdf

found fits for those variables..

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zps6npgbmvm.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zps6npgbmvm.jpg" border="0" alt=" photo 0_zps6npgbmvm.jpg"/></a>

moved a little, but basically, it still aligns.

[karimwassef]

so.. experimenting with different environmental conditions..
temperature at 84F, Salinity at 35, pH at 8.4 and CO2 at 450ppm... would give a dKH of 15!

My tank wouldn't run there, and the 450ppm is usually a temporary state when the garage door is open.. but still.

It might explain why my reef seems to do better at lower salinities? (30-31, not 35)

Is this real? What I am not accounting for?

[karimwassef]

Here's the excel - I hope that works.

[karimwassef]

Here's the spread over the 5 variables: Temp, Salinity, Pressure, pH, and CO2

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/1_zpsf8ljxsma.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/1_zpsf8ljxsma.jpg" border="0" alt=" photo 1_zpsf8ljxsma.jpg"/></a>

For each chart where one variable is being changed vs. CO2 to determine Alk, the fixed variables are based on the base case:
<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zps6npgbmvm.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zps6npgbmvm.jpg" border="0" alt=" photo 0_zps6npgbmvm.jpg"/></a>

If this is right, it would be possible to make an alkalinity monitor that measures air pressure, water temperature, CO2 in the air, salinity and pH and constructs the dKH...

This would be a continuous monitor over time... All variables except for air pressure and CO2 are already being measured by my Apex.

[jccaclimber]

I'm not awake enough to check the math, but how ar you getting from CO2 concentration in the air to the concentration in the water?

[karimwassef]

These are the must-reads:

http://www.soest.hawaii.edu/oceanography/courses/OCN623/Spring2012/CO2pH.pdf
http://comp.uark.edu/~ksteele/hc2004/ALKALINITY-%20CALCULATION.htm
http://butane.chem.uiuc.edu/pshapley/GenChem1/L23/web-L23.pdf

Henry's Law - There's a formula that converts between atmosphere CO2 and CO2 in the water. The third paper talks through it for different gases.

Actually - that parameter KH is probably also a function of temperature. I'll have to correct that factor too...

[karimwassef]

Here are the readings... for reference, all tank water is tied to garage air with the doors closed and outside air with doors open.

House (no tank contact)
<a href="http://s1062.photobucket.com/user/karimwassef/media/B379A3D4-D38D-4162-A7E2-ACC6E46597C2_zpsytt4dbsi.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/B379A3D4-D38D-4162-A7E2-ACC6E46597C2_zpsytt4dbsi.jpg" border="0" alt=" photo B379A3D4-D38D-4162-A7E2-ACC6E46597C2_zpsytt4dbsi.jpg"></a>

Outside
<a href="http://s1062.photobucket.com/user/karimwassef/media/2E40D3F8-10A1-4E1F-8173-B99A4A5E1794_zpsiyjmrx3d.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/2E40D3F8-10A1-4E1F-8173-B99A4A5E1794_zpsiyjmrx3d.jpg" border="0" alt=" photo 2E40D3F8-10A1-4E1F-8173-B99A4A5E1794_zpsiyjmrx3d.jpg"></a>

Garage (afternoon, door closed all day)
<a href="http://s1062.photobucket.com/user/karimwassef/media/E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg" border="0" alt=" photo E0740EDC-8A6C-4513-B914-8EF731E4B80B_zpsgpzti86e.jpg"></a>

[jccaclimber]

Other than temperature it looks like exchanging house air with garage air would be good.

[karimwassef]

Why? You want to actively inject CO2 based on the depletion of CO2 in the garage?

As long as my Alk is over 8.5dKH though, does it matter?

[jccaclimber]

On your theory that outside air is better (tank does better with the garage door open), you would want slightly more CO2 in the garage.

The CO2 in the house is too high, the CO2 in the garage is low, but the tank does better when raised.
The relative humidity in the house is on the high side, the relative humidity in the garage is on the low side.

[karimwassef]

That's because I'm running the dehumidifier in the garage. Raises the temp and reduces humidity.

There is no way to open it up to the house per the wife's constrains... so fresh air is the better part.

What's your take on deriving alkalinity from atm CO2, pressure, temp, salinity and pH?

Based on this, and assuming little variance in pressure (or using the weather reports), and measuring CO2, I should be able to plot out a continuous dKH with the data generated by my Apex- pH, salinity, temp.

[karimwassef]

If this isn't correct - please stop me :)

[jccaclimber]

That's because I'm running the dehumidifier in the garage. Raises the temp and reduces humidity.

There is no way to open it up to the house per the wife's constrains... so fresh air is the better part.

What's your take on deriving alkalinity from atm CO2, pressure, temp, salinity and pH?

Based on this, and assuming little variance in pressure (or using the weather reports), and measuring CO2, I should be able to plot out a continuous dKH with the data generated by my Apex- pH, salinity, temp.

I can see your wife's view, abating the noise crossover would be a significant challenge. Maybe leave the small door open with a fan when she isn't home? Just a short term (8 hours?) experiment to see what happens.

You say your tank does better with the big door open, and we know from your APEX data that kalk dosing to maintain pH increases during this time. If the benefit in the door being open is the result of more CO2 from the outside air, would crossover with the house (even more CO2) have an increased benefit? I would expect that unlike all the other components of "fresh outdoor air", CO2 is the only one likely to be available from house air. It also seems (from that data) that you could afford to turn the dehumidifier down slightly, although given the risk of mold I can see keeping it where it is.

I'm just now reading through your previous couple posts. One thing I have observed in the past is that CO2 in water tends to lag changes in the air levels of CO2 by several hours, although you have air injection whereas I do not.

You can get a decent measure of CO2 in water with a drop checker ($20), although since I assume you are in the 3-6 PPM range you would need a very low dkH solution to get a visual indication. I have a gallon or so of 40 dkH and some distilled water we could make a low concentration from, but I'm not sure we would get a lot in the way of meaningful data.

[karimwassef]

the dissolved CO2 aq is 1ppm.. I don't expect to get confirmation from that.

The largest target here is carbonate alkalinity. I can gather the CO2 and pressure data, as well as the Alkalinity and compare measured vs. calculated...

I just create work for myself :)

[karimwassef]

thinking through this.. my Alkalinity is a function of my Kalkwasser dosing... for CO2 to be a direct indicator of this, the Kalk dosing should be connected to my CO2 reading somehow?

It's indirect, but it would go something like this..

dosing Kalk would introduce the OH- in the water that pulls CO2 from the atmosphere to create HCO3- but reducing the CO2 in the air. As that carbonate is consumed, more kalk is added, and more CO2 is pulled in..

Also, photosynthesis is constantly consuming carbonate in the water, driving more demand on CO2.

If I STOP kalk dosing, the CO2 in the air would stop declining.

Also, if I turn the lights off, the CO2 in the air would also stop declining.

I'm not going to do either of those :) but I will collect the data on photoperiod and kalk dosing from my Apex and superimpose that with my measured CO2.

If the hypothesis is correct, the data should show it.. even with a lag.

[karimwassef]

Here's an excerpt from the University of Hawaii publication that is confusing me:

CaCO3 Precipitation/Dissolution Precipitation/Dissolution

A tricky subject when discussing “CO2” (or, more properly, PCO2)

Ca2+ + CO3 2- → CaCO3

Does this reduce the CO2 (PCO2) level of the seawater?

Hint: when pH is between 7.5 and 8.5: CO2(aq) + CO3-2 + H2O ↔ 2HCO3-

No! Lost CO3 2- will be replaced:

HCO3- → CO3 2- + H+

But this H+ release causes:

HCO3- + H+ → CO2 + H2O

Thus, CaCO3 precipitation causes a decrease in Total CO2, but an increase in PCO2

So creating coral skeletons would increase CO2 in the air??

[bertoni]

No, creating coral skeletons will remove carbon dioxide from the tank (and air, eventually). I think he means the relative presence of carbon dioxide will be increased (temporarily) due to the action of the final equation (conversion of bicarbonate and H⁺ to carbon dioxide and water).

[karimwassef]

so would I measure a reduction or increase in CO2 ppm ?

[bertoni]

Yes, over enough time, assuming there's minimal air exchange with the outside.

[karimwassef]

no - I meant which is it.. increase or reduction?

[bertoni]

Oops, sorry, carbon dioxide would drop over time if that's all that were happening.

[karimwassef]

ok. that's what I'm experiencing. When I open the door to the outside air, the CO2 equilibrates at 450ppm. When I close the door and during the photoperiod, it drops down to 380ppm.

I'm assuming that that implies more O2 in the air (and water)?

Thanks

[bertoni]

I'm not sure how the oxygen level would change, but I'd expect it to increase with photosynthesis. So, to the degree that the drop occurs due to photosynthesis, you might be able to measure some increase in the oxygen content

[LX20000]

Is that a spreadsheet you are filling out online ??

[karimwassef]

yes. posted it above #27

[LX20000]

Did you make the SS or found it online?

[karimwassef]

What's SS?

[jccaclimber]

SpreadSheet?

[karimwassef]

I made it based on the formulae I got out of the different papers I found.

[karimwassef]

Once I was able to get the fundamental relationships right, the rest was just algebra and putting it into excel is pretty basic.

I've since added data tables for the graphs over different variables. That's a little higher level excel, but not too bad. I've attached it here.

It took some manipulating to get it to fit under the 102KB file max in reefcentral.

[SchyffS]

Fantastic...out of my chemistry understanding. I think I see the breakthrough but not sure. However, can we get a summary?


Sent from my iPhone using Tapatalk

[karimwassef]

Basically, CO2 takes different forms in saltwater. At the pH, temperature, pressure and salinity ranges we have, it's carbonate and bicarbonate with a minuscule amount of dissolved gas. Those carbonates are what we get when we measure alkalinity (dKH).

However, the ultimate source for all these carbonates and ultimately alkalinity is CO2 in the air. So- if you measure this CO2 along with the elements that control diffusion of the CO2 into the water and the variables that control the proportions of the carbonates, you can go from gas CO2 to Alkalinity.

The big assumption is that the air is being aggressively mixed up with the water quickly. In my case, I use several methods to do this- primarily a double injected high pressure penductor (used to be my skimmer). Temporarily offline for upgrades...

But it says that if you measure pH, salinity, temperature & pressure, you can go from CO2 to dKH.

I first noticed this effect when I was curing concrete in the tank with tap water and noticed that the skimmer (in fresh water) was able to push the pH down from 11 to 8... that was a few years ago but this finally connects the dots for me.

It basically says that the primary & natural building blocks for coral are Ca and CO2 (in the air). Real reefs use atmospheric CO2 and we do the same.

This also explains why I run at 8.5 dKH and have massive growth while conventional tanks call out Alk of 10.5... I don't need an "alkalinity store" in my tank. The gas exchange is sufficient to pull in whatever is needed as long as there's access to fresh air.

Unlike most tanks where adding fresh air is actually to reduce CO2, I need fresh air to bring in more CO2 to the tank. It's basically a giant plant living in my garage and without outside air, the CO2 drops as it photosynthesizes.

[disc1]

OK, let's clear up a little chemistry here. Yes, pH and alkalinity and CO2 concentration are inextricably mathematically linked. If you know any two you can calculate the third.

But your thought that you can replenish alkalinity by taking on CO2 from the water is not right. It doesn't work like that. As you dissolve CO2, the alkalinity stays the same.

CO2 + H2O --> H2CO3.

Now if I dissolve that H2CO3 into my tank, it makes carbonate but that also leaves those two protons. Those are going to neutralize their equivalents of carbonate into bicarbonate.

H2CO3 + CO3-- --> HCO3- + HCO3-

There's a carbonate on the left, so two units of alkalinity. After the reaction there are two bicarbonates, again 2 units of alkalinity.

The truth is that for every equivalent of alkalinity that you create by dissolving CO2 you also take one away because you've just added acid.

And that's why we always say that the only way to correct pH without affecting alkalinity is to remove CO2. The addition or elimination of CO2 is an alkalinity neutral process. It neither creates nor removes any alkalinity.

[karimwassef]

They're in equilibrium, if carbonate is consumed and there's CO2 in the atmosphere, it will balance out. The carbonate doesn't come from the CO2 in the water, it comes from the CO2 in the air being converted into carbonate (since it can't exist as a dissolved gas in any large concentration).

That's what I got from the first paper (Univ of Hawai).

Do you agree with the spreadsheet? Knowing the CO2 & pH determines Alk.

[disc1]

Yes, I could have given you that equation without your spreadsheet. That's a well known and documented fact that anyone with some math skills could work out ab initio.

But you're missing something in your theory about CO2 form the air creating all the carbonates. Where do those two protons go? Where does the acid go? It neutralizes a base is where it goes. And that decreases the alkalinity by the same amount you just increased it by.

Think of it this way if that doesn't do it for you. Alkalinity is the same as long as you don't add anything or take anything away. You can convert bicarb to carbonate or even take your bicarb to carbonate and put the protons on sulfate to make bisulfate. Either way, at the end you have the same number of titratable groups present. The CO2 doesn't represent any alkalinity by itself, so anything you do to make it into something basic MUST involve neutralizing another base.

It's like when you have pure water. A proportion of that water hydrolyses into H+ and OH- So you're making OH-. That's a base. So why doesn't the pH or the alkalinity change? Why doesn't a cup of pure RODI water have at least that much alkalinity then? It's because you also made acid in the same step. And when you make acid and base in the same proportion, then you haven't affected alkalinity at all.

I'm not guessing or making this up. This is not an opinion. This is chemistry 101 stuff.

[jccaclimber]

OK, let's clear up a little chemistry here. Yes, pH and alkalinity and CO2 concentration are inextricably mathematically linked. If you know any two you can calculate the third.

But your thought that you can replenish alkalinity by taking on CO2 from the water is not right. It doesn't work like that. As you dissolve CO2, the alkalinity stays the same.

CO2 + H2O --> H2CO3.

Now if I dissolve that H2CO3 into my tank, it makes carbonate but that also leaves those two protons. Those are going to neutralize their equivalents of carbonate into bicarbonate.

H2CO3 + CO3-- --> HCO3- + HCO3-

There's a carbonate on the left, so two units of alkalinity. After the reaction there are two bicarbonates, again 2 units of alkalinity.

The truth is that for every equivalent of alkalinity that you create by dissolving CO2 you also take one away because you've just added acid.

And that's why we always say that the only way to correct pH without affecting alkalinity is to remove CO2. The addition or elimination of CO2 is an alkalinity neutral process. It neither creates nor removes any alkalinity.
In his case he can raise pH by adding kalkwasser. If the pH goes down due to more CO2, his kalk dosing will increase to hold it. Almost like a giant CO2 scrubber.

[disc1]

SO, if I have a solution of Na2CO3, lets say at 10mM. SO that has an alkalinity of 20meq/L. Now I start adding CO2. And yes, that CO2 becomes H2CO3 and then HCO3-. And that proton neutralizes one equivalent of carbonate. So for every CO2 that goes in and makes a bicarbonate, it also converts a carbonate into bicarbonate. So let's say I add enough to convert the whole thing. Let's say I add exactly 10mmol of carbonic acid. That all converts to bicarbonate and converts all my carbonate to bicarbonate. So two things happen, the pH goes down and now I have 20mM bicarbonate which is still 20meq/L of alkalinity.

*** Actually you won't ever have completely bicarbonate, they'll be in equilibrium but for every carbonate that is left over in this experiment there will also still be 1 carbonic acid present.

[disc1]

In his case he can raise pH by adding kalkwasser. If the pH goes down due to more CO2, his kalk dosing will increase to hold it. Almost like a giant CO2 scrubber.

Yes, the key here is the kalk. Now you're adding base and you're not adding acid with it. So now the OH- in the kalk can take up the acid from the carbonic acid and convert the carbonic into carbonate.

But let's look at that in the same light.

Let's start with a liter of pure water. Let's first add 10mmol of hydroxide. Now the pH is higher and the alkalinity is 10meq/L. Now let's start adding 5mmol of CO2 (half the amount because it is diprotic) and we get 5mM of carbonic acid. The protons go neutralize the hydroxide to water, so that alkalinity is now gone but we've created 5mM of carbonate or 10meq/L. Again, the alkalinity stayed the same through the addition of the CO2.

[disc1]

So what I'm getting at is that it is perfectly fine to say that the carbonate comes from atmospheric CO2. But you CANNOT say that the alkalinity came from there. The CO2 neutralizes exactly as much alkalinity as it creates.

[jccaclimber]

I think we're on the same page.

[karimwassef]

That ties in the last part of the math... pH (as OH- moves it up or down).

So, pH, CO2 & Alk are all part of the equilibrium math. The CO2 is from the air. The OH- is from the kalk and the resulting measures are pH and Alkalinity.

If the air is low in CO2, the alkalinity in the water would drop (at the same pH) ...

The formula is very basic - I got it from a chemistry class notes online (well, a few sources of notes), but I've never seen the calculator on a reef forum. I hope it's useful to others.

[karimwassef]

The formula also implies that pH can be used as a proxy for Alkalinity if atmospheric CO2 ppm is constant (and the other variables too - salinity, pressure, temperature).

[disc1]

If the air is low in CO2, the alkalinity in the water would drop (at the same pH) ...


No, if the CO2 is low there's less carbonic acid in the water so the pH goes UP. The alkalinity WOULD NOT CHANGE. CO2 cannot change the alkalinity by itself. As the CO2 leaves the water we also convert bicarbonate to carbonate and the alkalinity balances out at the same value you started with.

[disc1]

The formula also implies that pH can be used as a proxy for Alkalinity if atmospheric CO2 ppm is constant (and the other variables too - salinity, pressure, temperature).

Yes, if CO2 concentration stays constant then pH moves with alkalinity.

[karimwassef]

No, if the CO2 is low there's less carbonic acid in the water so the pH goes UP. The alkalinity WOULD NOT CHANGE. CO2 cannot change the alkalinity by itself.

That's why I said "at the same pH".

[disc1]

That's why I said "at the same pH".

But if you reduce the amount of CO2 you cannot have the same pH. It's the alkalinity that stays the same. The CO2 and pH are the ones moving here.

[karimwassef]

Yes, if CO2 concentration stays constant then pH moves with alkalinity.

Ok. This explains why I've been able to trigger my kalk reactor on my pH and maintain Alkalinity. It does mean that I need to monitor my CO2 so that the linkage is consistent.

[karimwassef]

But if you reduce the amount of CO2 you cannot have the same pH. It's the alkalinity that stays the same. The CO2 and pH are the ones moving here.

That's where I'm using other controls - like Kalk - to control pH.

I'm working to control both CO2 and pH using Kalk and controlled venting. This formula should then predict the Alkalinity in the water.

[disc1]

No. That situation is a fantasy. You aren't going to have a situation where CO2 is constant in the system.

It is much easier to measure pH and alkalinity than to measure the CO2 concentration. So if you are going to measure two and infer the third then the inference needs to be on CO2.

[disc1]

That's where I'm using other controls - like Kalk - to control pH.

I'm working to control both CO2 and pH using Kalk and controlled venting. This formula should then predict the Alkalinity in the water.

OK, so you're adding alkalinity through the kalk. Not as you tried to claim by the carbonate. Yes, it has been well known for years and years that you can keep your carbonate alkalinity up by dosing kalk.

But the alk is coming from the kalk, not the CO2. CO2 by itself does not affect alkalinity.

[karimwassef]

No. That situation is a fantasy. You aren't going to have a situation where CO2 is constant in the system.

It is much easier to measure pH and alkalinity than to measure the CO2 concentration. So if you are going to measure two and infer the third then the inference needs to be on CO2.

Why? I can measure atmospheric CO2 in a closed system. I'm actually doing that (check earlier posts)

[disc1]

That's where I'm using other controls - like Kalk - to control pH.

I'm working to control both CO2 and pH using Kalk and controlled venting. This formula should then predict the Alkalinity in the water.

Why in the world would you want to do that? You can measure the alkalinity. There's no need to predict it.

[karimwassef]

Kalk doesn't have any carbonate... it's Calcium and OH-

The OH- pulls the CO2 from the air to make carbonate...

[karimwassef]

Why in the world would you want to do that? You can measure the alkalinity. There's no need to predict it.

Because I can monitor CO2 with sensors... not titration.

[disc1]

Kalk doesn't have any carbonate... it's Calcium and OH-

The OH- pulls the CO2 from the air to make carbonate...

Exactly. But OH- is an equivalent of alkalinity. You're adding the alkalinity with the kalk. You're not adding the alkalinity with the CO2. You're just changing the form.

[disc1]

Because I can monitor CO2 with sensors... not titration.

What are you using for measuring CO2?

[karimwassef]

Two channel sensor greenhouse monitor.

[disc1]

Two channel sensor greenhouse monitor.

And when you propagate the error from that sensor through the math is this more or less accurate than an alk test?

[karimwassef]

Exactly. But OH- is an equivalent of alkalinity. You're adding the alkalinity with the kalk. You're not adding the alkalinity with the CO2. You're just changing the form.

Ok. Doesn't matter though. Carbonate Alkalinity is the dominant form and the one we reef keepers (and our corals) care about.

The useful purpose of OH- is in pulling CO2 from the air to make carbonate for our corals to use.

[karimwassef]

Here's the spread over the 5 variables: Temp, Salinity, Pressure, pH, and CO2

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/1_zpsf8ljxsma.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/1_zpsf8ljxsma.jpg" border="0" alt=" photo 1_zpsf8ljxsma.jpg"/></a>

For each chart where one variable is being changed vs. CO2 to determine Alk, the fixed variables are based on the base case:
<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zps6npgbmvm.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zps6npgbmvm.jpg" border="0" alt=" photo 0_zps6npgbmvm.jpg"/></a>

If this is right, it would be possible to make an alkalinity monitor that measures air pressure, water temperature, CO2 in the air, salinity and pH and constructs the dKH...

This would be a continuous monitor over time... All variables except for air pressure and CO2 are already being measured by my Apex.

I'll check the error, but 50ppm is ~ 1dKH

[disc1]

Yes, but here:

Basically, CO2 takes different forms in saltwater. At the pH, temperature, pressure and salinity ranges we have, it's carbonate and bicarbonate with a minuscule amount of dissolved gas. Those carbonates are what we get when we measure alkalinity (dKH).

However, the ultimate source for all these carbonates and ultimately alkalinity is CO2 in the air. So- if you measure this CO2 along with the elements that control diffusion of the CO2 into the water and the variables that control the proportions of the carbonates, you can go from gas CO2 to Alkalinity.

The big assumption is that the air is being aggressively mixed up with the water quickly. In my case, I use several methods to do this- primarily a double injected high pressure penductor (used to be my skimmer). Temporarily offline for upgrades...

But it says that if you measure pH, salinity, temperature & pressure, you can go from CO2 to dKH.

I first noticed this effect when I was curing concrete in the tank with tap water and noticed that the skimmer (in fresh water) was able to push the pH down from 11 to 8... that was a few years ago but this finally connects the dots for me.

It basically says that the primary & natural building blocks for coral are Ca and CO2 (in the air). Real reefs use atmospheric CO2 and we do the same.

This also explains why I run at 8.5 dKH and have massive growth while conventional tanks call out Alk of 10.5... I don't need an "alkalinity store" in my tank. The gas exchange is sufficient to pull in whatever is needed as long as there's access to fresh air.

Unlike most tanks where adding fresh air is actually to reduce CO2, I need fresh air to bring in more CO2 to the tank. It's basically a giant plant living in my garage and without outside air, the CO2 drops as it photosynthesizes.

you seem to indicate that you don't need to add anything and the CO2 takes care of all your alkalinity needs. That's the falsehood I was trying to clear up.

[disc1]

I'll check the error, but 50ppm (which is gross error) is ~ 1dKH

50ppm equates to 1dkh. But you have to propagate that error through the math. You're doing mathematical operations on that number that make the error larger than 50ppm by the time you come out the other end.

https://chem.libretexts.org/Core/Analytical_Chemistry/Quantifying_Nature/Significant_Digits/Propagation_of_Error

[karimwassef]

Accuracy is 50ppm or 5% (whichever is greater). It will be off by 1dKH absolute but it should capture relative change...

This is a cheap $100 monitor from Amazon, but the ability to continuously monitor all the variables means that an Alkalinity alarm is viable.

The next step would be a more accurate sensor on an arduino that captures and calculates...

[karimwassef]

50ppm equates to 1dkh. But you have to propagate that error through the math. You're doing mathematical operations on that number that make the error larger than 50ppm by the time you come out the other end.

https://chem.libretexts.org/Core/Analytical_Chemistry/Quantifying_Nature/Significant_Digits/Propagation_of_Error

It's linear. I plotted it out in the graphs too.

[disc1]

And you have to consider the error involved in the other calculations. The temperature has an error, so does the pH measurement. And the pressure. And many of the constants have some degree of error associated. You've got an awful lot going into this since you are trying to correlate solution CO2 from atmospheric levels. All that error has to be accounted for.

[disc1]

It's linear. I plotted it out in the graphs too.

If that was how it really worked my job would be a LOT easier. You've got to figure the errors in the other measurements in there.

[karimwassef]

Yes, but here:



you seem to indicate that you don't need to add anything and the CO2 takes care of all your alkalinity needs. That's the falsehood I was trying to clear up.

I did oversimplify because I'm focusing on the end result: CO2 in the air is converted into carbonates. Yes: The working ion that does that is the OH- in the Kalk, but Kalk in an environment devoid of CO2!is worthless... so I skipped to the end :)

[disc1]

I did oversimplify because I'm focusing on the end result: CO2 in the air is converted into carbonates. Yes: The working ion that does that is the OH- in the Kalk, but Kalk in an environment devoid of CO2!is worthless... so I skipped to the end :)

Yeah but you skipped something important, the fact that the kalk is adding the alkalinity. Without the kalk the CO2 is equally as worthless.

[karimwassef]

If that was how it really worked my job would be a LOT easier.

Great! Are you volunteering to help make it better? I value the expertise :)

I really think this is a worthwhile effort that would help other reef keepers in the end.

[karimwassef]

Yeah but you skipped something important, the fact that the kalk is adding the alkalinity. Without the kalk the CO2 is equally as worthless.

Like a great marriage... OH- and CO2.. equal partners in coral propagation.

[disc1]

Great! Are you volunteering to help make it better? I value the expertise :)

I really think this is a worthwhile effort that would help other reef keepers in the end.

If you had a sensor for solution CO2 I might agree. I think that by the time we calculate all the error in here you're going to be a lot more than 1dKH of play.

[karimwassef]

Let's try? I was planning on collecting Alk and CO2 at the same time and plotting the results.

[karimwassef]

If this results in just an "Alk alarm" for excursions out of bounds, I still think it's worthwhile.

[disc1]

Let's try? I was planning on collecting Alk and CO2 at the same time and plotting the results.

Don't forget temperature, pH, and pressure. And salinity.

[karimwassef]

Don't forget temperature, pH, and pressure. And salinity.

My Apex grabs all that already except for pressure.

I was planning on using weather data for pressure as I don't expect it to vary by much and even when it does, it has a weak effect.

[bertoni]

I agree that there are likely to be severe problems with accuracy, but the project still might be interesting. We haven't even gotten to the issue of photosynthesis, for example.

[karimwassef]

Found another nice paper :

http://www2.ca.uky.edu/wkrec/interactionsphetc.pdf

Wouldn't the algae and zooxanthele just consume the dissolved CO2 or would they absorb carbonates and convert prior to photosynthesis? I'd assume that an acid is needed for that and that bacteria play a role?

I'd expect the respiration of fish and other animals to constantly reintroduce dissolved CO2 into the water. Beyond that, the air and water equilibrium will keep the water CO2 tied to the atmospheric CO2. Right?

In a closed system, I see the atmospheric CO2 drop by 80ppm during my photoperiod (460ppm to 380ppm). I'm assuming that this is because CO2 in the water is being consumed and it's pulling more from the air.

I don't know if this is consumed as carbonates to build coral skeletons or CO2 for photosynthesis in my algae scrubber and corals.

I'll get to the heart of it : are photosynthesis and coral skeleton production in competition for CO2? Can one limit the other (assuming sufficient OH- available to pull CO2 into carbonates? - last comment for David to be sure I got the point :D)?

[karimwassef]

Wow...

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4034600/#!po=60.5263

Love these guys!!!

Another great source:

http://www.mdpi.com/1996-1073/6/11/6060/pdf

[jccaclimber]

Found another nice paper :

http://www2.ca.uky.edu/wkrec/interactionsphetc.pdf

Wouldn't the algae and zooxanthele just consume the dissolved CO2 or would they absorb carbonates and convert prior to photosynthesis? I'd assume that an acid is needed for that and that bacteria play a role?

It's been a while since I've read it and it was freshwater, but I'm pretty sure Dianna Walstad's "Ecology of the Planted Aquarium" mentions something about this with plant species that can thrive in higher KH water. If I remember correctly they were making a slight pH gradient across the leaves resulting in slight precipitation on one side, but freeing up CO2 for use by the plant. No idea how this translates to coral, but they are masters of changing their immediate environment.

Found another nice paper :

I'd expect the respiration of fish and other animals to constantly reintroduce dissolved CO2 into the water. Beyond that, the air and water equilibrium will keep the water CO2 tied to the atmospheric CO2. Right?

Taking decomposition into account as well, but yes.
Note: Walstad had an example in the form of a lake where CO2 production was constant from decomposition. This resulted in CO2 above equilibrium in the morning when plants started using it, then below equilibrium as they used it up (and growth slowed).

Found another nice paper :

In a closed system, I see the atmospheric CO2 drop by 80ppm during my photoperiod (460ppm to 380ppm). I'm assuming that this is because CO2 in the water is being consumed and it's pulling more from the air.

I don't know if this is consumed as carbonates to build coral skeletons or CO2 for photosynthesis in my algae scrubber and corals.

I'll get to the heart of it : are photosynthesis and coral skeleton production in competition for CO2? Can one limit the other (assuming sufficient OH- available to pull CO2 into carbonates? - last comment for David to be sure I got the point :D)?

I think there is an Advanced Aquarist (or similar) article discussing competition between building skeleton and feeding zoox.

[jccaclimber]

Regarding last night's exchange, I think the end goal may be falling out of sight in the conversation. No, it isn't perfect, yes, there is error, yes, it may still have value.

At the root of this: Coral happiness (inferred from polyp extension) increases when fresh air is allowed in. Kalk dosing to maintain pH increases with increased fresh air (by tracking through doser and pH data logged on this system combined with knowing what days the garage door was open all day). The question is why.

While you may not be able to hold CO2 constant in the real world, you can use the equations to (generally) determine how the system will trend when different knobs are turned.

[karimwassef]

Regarding last night's exchange, I think the end goal may be falling out of sight in the conversation. No, it isn't perfect, yes, there is error, yes, it may still have value.

At the root of this: Coral happiness (inferred from polyp extension) increases when fresh air is allowed in. Kalk dosing to maintain pH increases with increased fresh air (by tracking through doser and pH data logged on this system combined with knowing what days the garage door was open all day). The question is why.

While you may not be able to hold CO2 constant in the real world, you can use the equations to (generally) determine how the system will trend when different knobs are turned.

Yes :beer:

I think it was very constructive and clarifying. It's all about growing happy corals faster. The rest is details :D

[bertoni]

Algae often take up bicarbonate or carbonate ions, but even if they're taking up only dissolved carbon dioxide, they are affecting the pH of the tank and the total amount of carbon dioxide, so you'll need to subtract that amount from the total carbon dioxide consumption. If the algae are not being harvested, the consumption averaged over time will be zero after any initial growth is done, but the flux will change during the day vs the night.

[disc1]

Does it matter which form they take up? The three are in equilibrium. So no matter which one they use, the end result is the same.

[karimwassef]

ok, but as they grow and create biomass, they (both algae and coral zooxanthele) consume carbonates and are in competition with coral skeleton growth ... correct?

as they consume carbonates, they reduce alkalinity ... correct? (I'm being specific to be sure we're not disconnected)

and they reduce alkalinity and consume carbonate and/or CO2, they increase pH ... correct? (again, just being sure)

In my case, I feed the algae back to my fish and they consume the algae, they release the CO2 back into the tank ... correct?

proof I actually do this: :D

http://i1062.photobucket.com/albums/t496/karimwassef/654C0AD9-0D6F-47C4-B757-78F5AF321B3F_zpsie2iyrbz.jpg

[bertoni]

The net effect of algae (non-calcareous) on alkalinity is zero. When they perform photosynthesis, they will consume carbon dioxide, carbonate, or bicarbonate, but they release any alkalinity they consume soon after stripping the carbon that they need, or even as part of the uptake process. When they aren't performing photosynthesis, they will consume oxygen and release carbon dioxide. The net effect over time is effectively zero, assuming no algal mass is exported. Photosynthesis will raise the pH of the tank.

I wouldn't say that the photosynthesis demands generally are in conflict with calcification, on average, because there's plenty of carbon dioxide for both processes, in actual reef tanks. I've never seen the pH rise to dangerous levels in a tank with reasonable aeration. The pH would have to be very high for carbonate or bicarbonate to become limited that way.

[karimwassef]

I'm confused again. So let's take it one piece at a time?

The net effect of algae (non-calcareous) on alkalinity is zero. When they perform photosynthesis, they will consume carbon dioxide, carbonate, or bicarbonate, but they release any alkalinity they consume soon after stripping the carbon that they need, or even as part of the uptake process.

6CO2 + 6H2O ------> C6H12O6 + 6O2

where is the hydroxide release?
If the left hand side is carbonates and hydroxide instead of CO2 and water, the output is still just sugar and oxygen, right?

When they aren't performing photosynthesis, they will consume oxygen and release carbon dioxide. The net effect over time is effectively zero, assuming no algal mass is exported.

My day cycle is very long.. 18 hours and my night cycle is very short ... 6 hours. I don't think they're cancelling each other out? The algae's mass, as it grows, is consuming carbon and sequestering it in its tissue. Right?

Photosynthesis will raise the pH of the tank. I observe that during my photoperiod - CO2 level in the air drops and pH increases (even without Kalk).

I wouldn't say that the photosynthesis demands generally are in conflict with calcification, on average, because there's plenty of carbon dioxide for both processes, in actual reef tanks. I've never seen the pH rise to dangerous levels in a tank with reasonable aeration. The pH would have to be very high for carbonate or bicarbonate to become limited that way. I didn't mean that high pH was the issue. I meant that if there is a shortage of CO2 (such as in a closed environment), then the photosynthesis and calcification processes are both pulling on carbonates and therefore in conflict?

Jonathan, can you check off my questions above? I need to walk through the logic. :D

1. as they grow and create biomass, they (both algae and coral zooxanthele) consume carbonates in photosynthesis and are in competition with coral skeleton growth ... correct?

2. as they consume carbonates, they reduce alkalinity ... correct? (I'm being specific to be sure we're not disconnected)

3. and they reduce alkalinity and consume carbonate and/or CO2, they increase pH ... correct? (again, just being sure)

4. In my case, I feed the algae back to my fish and they consume the algae, they release the CO2 back into the tank ... correct?

[bertoni]

6CO2 + 6H2O ------> C6H12O6 + 6O2

where is the hydroxide release?
That equation covers the part in which the alga is using carbon dioxide directly. If the alga is taking up bicarbonate, the equation will include the release of alkalinity. The total effect would be something like:

HCO₃^- -> OH^- + CO</sub>2</sub>

I don't know the actual pathway for this conversion. The hydroxyl ion is released, or balanced by the uptake of H⁺. Otherwise, the cell becomes charged. :)


Pretty much. As the alga grows, and as it performs photosynthesis in excess of respiration, it will sequester carbon, lowering the pH. Some of the excess carbon can be stored as food, which might or might not count as "growth", depending on your personal definition.

[quote]I observe that during my photoperiod - CO2 level in the air drops and pH increases (even without Kalk).
Yes, that's correct.

I didn't mean that high pH was the issue. I meant that if there is a shortage of CO2 (such as in a closed environment), then the photosynthesis and calcification processes are both pulling on carbonates and therefore in conflict?
Yes, there is some conflict. In most tanks, though, and in fact in every tank I've seen, the pH doesn't rise far enough that carbon (in any form) is limiting. I think our tanks would die, otherwise, as the pH would reach lethal levels for most of the organisms.

1. as they grow and create biomass, they (both algae and coral zooxanthele) consume carbonates in photosynthesis and are in competition with coral skeleton growth ... correct?
To some extent, yes, but there is a lot of inorganic carbon available.

2. as they consume carbonates, they reduce alkalinity ... correct? (I'm being specific to be sure we're not disconnected)
Calcification does consume alkalinity, but not algal growth (unless the alga is calcareous) or photosynthesis.

3. and they reduce alkalinity and consume carbonate and/or CO2, they increase pH ... correct? (again, just being sure)
Calcification won't have much effect on pH. It'll drop it to some extent, by consuming alkalinity, but the effect is small in our tanks.

4. In my case, I feed the algae back to my fish and they consume the algae, they release the CO2 back into the tank ... correct?
Yes, they release the carbon back into the tank, some of it in the form of carbon dioxide. Some might be in other waste products.

[disc1]

I'm confused again. So let's take it one piece at a time?



6CO2 + 6H2O ------> C6H12O6 + 6O2

where is the hydroxide release?
If the left hand side is carbonates and hydroxide instead of CO2 and water, the output is still just sugar and oxygen, right?



If the left side was carbonate and hydroxide then the equation wouldn't balance. Balance it and you'll see where the alk comes from. You'd have to take up acid. Taking up acid is increasing alkalinity.

You have to get straight in your head what alkalinity really is. It's NOT just the total concentration of carbonate species. Sure we can measure it that way because all of that is in equilibrium. But any strong base you add counts and any acid you remove counts.

[bertoni]

Oops, good point, I didn't double-check the equation. Sigh! Thanks!

[karimwassef]

David- I get that even though you seem to insist that I don't :D

You both said the same thing, Jonathan with words and David with the equation... but what drives the extraction of the OH- to release the CO2?

There's HCO3- available (ok)
Photosynthesis requires CO2 (ok)
The HCO3- converts to OH- and CO2 (?) why?? What drives this reaction to the right?

Is there an acid in play ?

By the way, without any kalk addition, my pH would go from 7.9 to 8.2 during photosynthesis on my day cycle. At night, it would go back down to 7.9... It's not dangerous, but it is substantial and I expect it is the result is substantial CO2 uptake during my long photoperiod.

[bertoni]

The exact mechanism might be fairly complicated. I did some searching, and I suspect that details are going to require some research. I have a book on aquatic photosynthesis, if I can find it. It's an older edition of this:

https://www.amazon.com/Aquatic-Photosynthesis-Paul-G-Falkowski/dp/0691115516/ref=sr_1_1?s=books&ie=UTF8&qid=1489638265&sr=1-1&keywords=aquatic+photosynthesis

The disassociation will happen spontaneously as a normal part of the equilibrium processes, but there might be other drivers involved.

Here's a sample article:

http://www.pnas.org/content/112/23/7315.full.pdf

That discusses only one species.

[karimwassef]

Ok. So not being a biologist or chemist, here's what I got out of that paper:

Photosynthetic cells create a demand for CO2 & use specialized proteins to harvest and maintain CO2 and inorganic Carbon from carbonates. So the driving energy source is photosynthesis (indirectly), the mechanism is specialized proteins, and the outcome is hydroxide ions.

Is that it in a nutshell?

What isn't mentioned is an acid to pull out the OH- or a supporting agent like a bacteria, etc... it's just the limiting of CO2 in water that has driven these cells to create biological transport mechanisms to do this work.

[karimwassef]

Taking it one step further...

With OH- released by the uptake of CO2 out of HCO3-, atmospheric CO2 (that needs to be in equilibrium) is pulled in by the hydroxide to create replacement carbonate... so, photosynthesis (through the action of the hydroxide) pulls CO2 out of the water and it's equilibrium in air... and replenishes the HCO3- yes??

(I know I said it twice - just to be sure)

So while the middle is complicated.. the start of the journey is atmospheric CO2 and the end is photosynthesis into O2 & sugars... everything in the middle is just a transport mechanism.

I'll stop there before taking the next step.

[karimwassef]

Is my walk above correct?

If so, that explains why photosynthesis and coral calcification are not in true competition, but they are in temporary competition...

Photosynthesis may consume carbonates temporarily in the process but ultimately it consumes atmospheric CO2. If the CO2 is limited, then the pH goes up due to an abundance of OH-. In that case, there is a true demand on carbonate Alkalinity that can limit calcification. Likewise, if CO2 is not limited, but the time that it takes for CO2 to be pulled into the water is long, then there is a temporary increase in pH (rather than a permanent one in the case of true CO2 limitation). In that case, the non-photosynthesis dark period would create the opportunity to rebalance and carbonates return to the water bringing pH back down and repopulating the HCO3-. That means that there can be a lag during which photosynthesis "borrows" carbonate Alkalinity as a temporary path to the CO2 it needs. Net net, with access to normal fresh air and enough mixing, it'll even out.

On the other hand, calcification consumes the carbonates directly. I'm not sure about this next step: but this consumes Alkalinity without releasing OH- so the pH doesn't change due to calcification. I'm not sure if this is the whole story since this might depend on the speciation of the carbonate? If it's CO3 2-, I think what I said is true. But since most is HCO3-, there's an H+ that would be released? Wouldn't that reduce pH?

So would photosynthesis temporarily release OH- while calcification permanently release H+?

I realize this is Chem 101 but I appreciate the slow walk with me on this. :D

[bertoni]

With OH- released by the uptake of CO2 out of HCO3-, atmospheric CO2 (that needs to be in equilibrium) is pulled in by the hydroxide to create replacement carbonate... so, photosynthesis (through the action of the hydroxide) pulls CO2 out of the water and it's equilibrium in air... and replenishes the HCO3- yes??
Yes, carbon dioxide from the atmosphere replaces the carbon dioxide consumed by photosynthesis.

[bertoni]

If the CO2 is limited, then the pH goes up due to an abundance of OH-. In that case, there is a true demand on carbonate Alkalinity that can limit calcification.
It'd take a very high pH to end calcification, but it's possible, although the mechanism more likely is the death of the coral due to the high pH.

Likewise, if CO2 is not limited, but the time that it takes for CO2 to be pulled into the water is long, then there is a temporary increase in pH (rather than a permanent one in the case of true CO2 limitation). In that case, the non-photosynthesis dark period would create the opportunity to rebalance and carbonates return to the water bringing pH back down and repopulating the HCO3-. That means that there can be a lag during which photosynthesis "borrows" carbonate Alkalinity as a temporary path to the CO2 it needs. Net net, with access to normal fresh air and enough mixing, it'll even out.
Algae will respire and produce carbon dioxide. I don't remember whether the dark reactions would do that directly, but other processes will.

On the other hand, calcification consumes the carbonates directly. I'm not sure about this next step: but this consumes Alkalinity without releasing OH- so the pH doesn't change due to calcification. I'm not sure if this is the whole story since this might depend on the speciation of the carbonate? If it's CO3 2-, I think what I said is true. But since most is HCO3-, there's an H+ that would be released? Wouldn't that reduce pH?
The process of calcification reduces the pH by reducing the alkalinity, as you've noted. The effect is very small. This article has a chart:

http://www.advancedaquarist.com/2002/4/chemistry

The calcification step actually releases two H⁺ ions.

[karimwassef]

I didn't mean that high pH would limit calcification. I meant that the drop in carbonate alkalinity would.

algae would only release CO2 in the dark - same as plants, right?

[karimwassef]

wow.. awesome article... and it's in 2002... I miss Randy :)

He says "In part this may be due to the fact that both photosynthesis and calcification are competing for bicarbonate, and the fact that the external bicarbonate concentration is not that large to begin with"

[karimwassef]

He also goes where I was going.. and that's the release of the H+ actually helps in the availability of CO2 for photosynthesis.. So calcification actually helps photosynthesis, but photosynthesis can also rob calcification of the carbonate alkalinity in needs, temporarily... but then calcification needs energy (ATP) that is provided by the photosynthesis (I added a little bit here)...

It's actually beautiful...

[karimwassef]

I'll need to dig in and read it slowly... but a quick question here.

Has anyone tested Carbonic anhydrase enzymes in a reef?

[bertoni]

I didn't mean that high pH would limit calcification. I meant that the drop in carbonate alkalinity would.

algae would only release CO2 in the dark - same as plants, right?
The drop in carbonate alkalinity could limit calcification, as you stated. I suspect that the effect is small in our tanks, though. Most of us live in houses that have even more carbon dioxide in the air than the outdoors. Algae would release net carbon dioxide when the photosynthesis rate is low or zero, which typically means the night. Some reactions will release carbon dioxide during the day, but the alga is consuming more than it would release, in a typical situation, anyway.

[karimwassef]

Ok. That aligns.

My setup is a little different since the tank is in the garage which is sealed (for temperature) and noone spends any time there. The net effect is a measurable decline in CO2 and increase in pH.

Since I'm using pH as an indicator to add Kalk, this means that less Kalk could be used unless the tank has access to fresh air to compensate.

What I find is that opening the garage doors during the day stimulates an increase in Kalk consumption and a very positive response on coral polyp extension (not sure why the second one happens).

This connects the dots on both the photosynthetic and calcification fronts and potentially introduces a few interesting options to accelerate growth even further.

I will be moving and taking this tank down (most likely). I'm trying to understand the basis of the successful growth I'm seeing here before the change so I can replicate it.

[karimwassef]

so.. has anyone tested Carbonic anhydrase enzymes in a reef?

[karimwassef]

https://www.hindawi.com/journals/ijce/2013/813931/

https://www.hindawi.com/journals/ijce/2013/813931.fig.001.jpg

https://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme_Catalytic_Mechanism/Carbonic_Anhydrase

Here's a reference I found for the use of CA (Carbonic Anhydrase). Looks like Zinc is the key mineral here... more oysters?? :D

https://www.chem.tamu.edu/rgroup/marcetta/chem489/Presentations/Carbonic-Anhydrase.pdf

coincidentally, there just happens to be a lot of research on these topics because of the interest in CO2 sequestration in photosynthesis (into algal biomass) and calcification (for concrete) as mechanisms.

[bertoni]

I don't know of any papers on the relevant enzymes, but I'm not a marine biologist, so I'd be a poor source in any case. I agree that a sealed garage might produce some different effects than the typical tank. How high does the pH get?

[karimwassef]

without kalk additions, it can get just above 8.25. But my concern is not in CO2 limiting driving a high pH. I'm more concerned with it limiting carbonates.

During the day, with full on photosynthesis, the "borrowing" of HCO3- due to the a limiting CO2 could limit calcification... basically, the carbonate alkaliity is not uniform. It can drop during the day.

I add kalkwasser and that compensates, but since it's triggered on pH, the same mechanism depleting CO2 would limit my Kalk drip by keeping pH high.

To compensate for THAT, I have a sinusoidal pH threshold that my Kalk triggers on. So, during the day, it's 8.4 and at night, it's 8.1... During the intermediate times, it's set to 8.2.

I did this without really understanding what was happening but the results were great! I simply turned off kalk for a few days and captured the pH swings that were naturally occurring without any kalk. I then added 0.2 to the naturally occurring pH swings and used that as my varying threshold.

I now think I understand. By setting my pH threshold higher during the day, I was compensating for the limiting of CO2 that was raising pH during the day. And that kept my Alkalinity up.

When I opened the garage door, I allowed more CO2 back in, again raising Alk.

[karimwassef]

Here's another paper that walks the same systems in nature:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3932737/

In this case, the open air basically eliminates the issue of CO2 limitation. And in that case, photosynthesis actually helps calcification by raising pH and making it easier to corals to create skeletons.

Their conclusion is that carbonate sediment (sand) provide the necessary carbonate alkalinity and the macroalgae provide photosynthesis to raise pH to help calcification.

This could actually support the idea that an ATS and a sand bed both help captive reefs grow... I happen to have both, also - accidental. :D

[karimwassef]

This one talks about flow improving calcification
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4202238/

I also saw more growth when I set up my surge for very high flow.

So:

1. High air/water exchange creates opportunity for rapid CO2 absorption.
2. Kalk on a sinusoidal pH threshold injects carbonate to pull in the CO2.
3. Long photoperiod + Large ATS = high photosynthesis raising pH.
4. Sand bed provides an additional carbonate sediment source.
5. High flow surge improves calcification.

Those are the elements that I would need to replicate in my new system - Fresh air, Kalk, light, ATS and flow.

Everything else is offline - GFO, Skimmer, UV, etc...

I might try a few other experiments with my current tank - since this year is going to be the frag out and tear down year -

Collect the data to track alkalinity and CO2.
Maybe try a heavy oyster diet to add Zinc (intent to support carbonate anhydrase?)
Maybe try to inject CO2 like planted aquariums and compensate the pH drop with Kalk. I run an Alk of 8.5, so this might increase it to the 12 or 13 levels... but I'll take it slow.
I have Randy's three parts and can do that too, but I prefer the Kalk.
Maybe change the light, adding a lot more UV.
Maybe allow a much higher temperature... push 85 or 86... to see if the system is resistant to temperature bleaching.

I'm satisfied with the chemistry here. I'll collect data and get back.

[bertoni]

pH at 8.4 should be excellent for aiding calcification. That seems like a very good target.

[karimwassef]

ok.. taking data points and the biggest source of error is the pH sensor. No surprise there, but even a 0.1 pH error can generate an additional 1 dKH error in reading. The CO2 would need to be off by 50 ppm to generate that large of an error.

Having said that though, I think that the data can be used to calibrate the formula.

One question - I'm using Salifert. Does it matter if I take the water sample indoors to run the test? Does it matter if it sits for a few minutes?

[bertoni]

It shouldn't matter where you run the test, and letting it sit for a few minutes will be fine.

[SchyffS]

@karimwassef, can you explain how you monitor and dose kalk using ph controller.


Sent from my iPhone using Tapatalk

[karimwassef]

I use an Apex controller. I have two pH probes and I use one of them to trigger my kalk reactor. For example, if pH is < 8.4, I turn on a solenoid to gravity feed RODI water into my kalk reactor and it drips kalk water into my sump. When the pH is greater than 8.4, my Apex turns off the solenoid.

[karimwassef]

still haven't had enough time to run the tests without interruption, but here is an interesting vid I thought was interesting on the topic.

<iframe width="560" height="315" src="https://www.youtube.com/embed/kmpzDfrqliU" frameborder="0" allowfullscreen></iframe>

I am a little confused about the idea that excess CO2 would reduce the CO3 in the water. Isn't HCO3 the natural form at our pH and corals can use it just as they can use CO3?

Are they just saying that the excess CO2 would reduce the pH and it would take time for that to stabilize and return into usable CO3 at the pH needed for calcification?

I also wondered where the Ca and CO3 in real reefs come from and this basically says that the total is in balance and the total consumption by carbonate users is small enough that the dissolving CaCO3 in deeper waters is sufficient to keep an abundant supply of Ca and CO3 for reefs. Is my understanding correct? Obviously, there's no kalk reactor on the reefs, but the deeper waters are basically acting as a constant resupplying system.

[bertoni]

Carbon dioxide dissolves to form carbonic acid, which releases an H⁺ ion, lowering the pH. That H⁺ could combine with carbonate to form bicarbonate, and raise the pH a bit. Those are the kinds of changes that are happening.

Corals take up carbonate in some form, but the details aren't clear. In any case, calcium carbonate is more soluble at a lower pH, and thus the energy cost to form it should be higher. At least that's my limited understanding.

[karimwassef]

so is the video accurate?

[bertoni]

I haven't watched it. I'll try to make some time later.

[karimwassef]

Anyone seen the video?

I'm especially curious about the comment that excess CO2 reduces carbonates and that the primary equilibrium mechanism for calcium and carbonates is deeper water pressure dissolving coral skeletons.

[bertoni]

The video seems accurate to me.

[karimwassef]

Ok. At what depth does CaCO3 dissolve?

[bertoni]

I'll try to look up the depth vs dissolution rates, but I've been a bit busy lately. It might take a while. Please ping me back if I don't answer in a few days.

[karimwassef]

Ok. Anyone else know? :D

[karimwassef]

ok. Sorry for the long delay but I needed a long weekend so I can collect enough data to make a conclusion.

First, I'll share the results:

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zps1ico3jno.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zps1ico3jno.jpg" border="0" alt=" photo 0_zps1ico3jno.jpg"/></a>

The bottom chart shows three different test kit results over time. API1 and API2 are both API Alkalinity but one is over a year old (API1) and one is brand new (API2). The Salifert is also a year old, but I thought it would be more accurate, so I used it too.

The orange markers are the calculated result as a function of the measured parameters: pH, temperature, salinity and CO2 ppm.

I was unhappy with the range of measured test results and I could have just shown the new API that lines up best with the calculated result, but in the interest of sharing everything so that we can all benefit, I'm putting it all out there.

The top graph shows the measured CO2 (ppm) level and the measured pH (black dotted line).

I'll get to the yellow dots (back calculated pH) in a minute.

So - the new API comes closest to the expected calculated result. I looked at the error in the input variables to the formula, but none (except for pH) were significant enough to explain the large gaps in the evening (9pm and 11pm). The gaps were very large (1.4 - 1.6dKH) but really constrained in time to the late evening.

I believe I have an explanation... my pH probe is in my sump downstream from my kalk drip (that is in the first chamber from the overflow. This means that during periods of high kalk dosing, the pH probes are reading the "future" pH of the tank, and not the present. When I take a sample for Alkalinity testing; however, I take it from the DT in the center. This means that during high dosing times, my pH is not uniform and neither is my Alkalinity.

My doser kicks into a higher pH threshold in the evenings (needing to push to a pH of 8.3) and that coincides with the large error during that time.

I went back and calculated what the pH would need to have been so the formula would have correctly predicted the measured Alk_API2 result. That's the plotted yellow markers in the top graph.

So - this shows that the formula is significantly sensitive to pH measurement error. All other variables are relatively small in impact. At the largest error, the pH is only off my 0.05! but drives an error of ~1.5dKH...

My conclusions:

1. I need to rerun the test with my pH probe in the same region as I take samples for the Alk test.
2. The high quality pH probe needs to be calibrated frequently to be effective.
3. It should be possible to calibrate the error out. In my case, the error was actually very small except for a couple of datapoints explained above.

I would like to invite others to replicate (please) and post your results too? :D

Open to all feedback, even and especially the negative stuff :) but I would like it to be constructive please?

[karimwassef]

On a different and sad note... I had the unfortunate experience of the CO2 in my garage rising to 900ppm... this has never happened before to this tank since I have always fed my tank outside air which maintained a constant and level CO2 input...

I took it down and hadn't gotten around to putting the new injected in place, when this event happened.

Based on the formula and the timing of the event, this pushed my Alkalinity above 15... and my Acropora almost completely died overnight (most of my SPS were hit hard too). I can't help but wish I had put an alarm system in place or at least fixed the injector sooner. A simple catch would have been to disengage the doser if CO2 goes above 500... the harm of less Alk is much easier to fix than too much.

I realize that most people don't use pH to dose, so this wouldn't normally cause a crash, but in my case it was catastrophic. I share this so that others can benefit from my failure.

I'm still grieving so I haven't uploaded the images yet, but I will, in time.

[karimwassef]

I guess saying it was cathartic..

<a href="http://s1062.photobucket.com/user/karimwassef/media/karimwassef001/IMG_0054_zpsod6rb9rm.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/karimwassef001/IMG_0054_zpsod6rb9rm.jpg" border="0" alt=" photo IMG_0054_zpsod6rb9rm.jpg"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/karimwassef001/IMG_0058_zpsd04zxvyc.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/karimwassef001/IMG_0058_zpsd04zxvyc.jpg" border="0" alt=" photo IMG_0058_zpsd04zxvyc.jpg"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/karimwassef001/IMG_0057_zpsqbrehtzm.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/karimwassef001/IMG_0057_zpsqbrehtzm.jpg" border="0" alt=" photo IMG_0057_zpsqbrehtzm.jpg"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/karimwassef001/IMG_0056_zps8nloxhin.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/karimwassef001/IMG_0056_zps8nloxhin.jpg" border="0" alt=" photo IMG_0056_zps8nloxhin.jpg"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/karimwassef001/IMG_0055_zpsrznjv7ux.jpg.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/karimwassef001/IMG_0055_zpsrznjv7ux.jpg" border="0" alt=" photo IMG_0055_zpsrznjv7ux.jpg"/></a>

ok.. fixing it.

[bertoni]

I hope your corals recover well! I'm not sure what to add.

I did check on the calcium carbonate solubility question. Solubility begins about 2 km deep, and increased to about 4 km. Not of interest in our systems. :) Aragonite dissolves more easily than calcite, so it goes first.

[karimwassef]

But that does mean that a high pressure could be used to dissolve calcium carbonate... good input.

Thanks for your well wishes. My tank has survived a lot before. I was lucky to have shared frags before this event so hopefully all the coral lineages survive elsewhere.

On the data above, I'm convinced that the method is predictive enough to be useful with the caveat that pH must be measured as accurately as possible to be useful. What do you think?

I've also reconsidered my view on photosynthesis competing with calcification- with enough gas exchange, photosynthesis raises pH that actually aids calcification. Any CO2 or bicarbonate consumed is replenished from the atmosphere.

Finally, I am surprised in the real variability in Alkalinity. Does anyone know what the real natural reef variability is? We usually target one value: 8.5dKH or 11.5dKH, etc... but it looks like the real conditions yield a pretty wide range instead. But then - how much Alk swing promotes growth or does it actually hinder growth?

[bertoni]

If your pH meter is accurate enough, you probably can get some idea of the carbon dioxide level.

I don't know how much the alkalinity level might change in natural reefs. For lagoons that get flow primarily during tides, the level might change some, but I haven't seen any data. The water volume to coral ratio probably is fairly high, though.

[agv180]

Woow Karim. excellent analysis I have a question, that such a kit to assemble or a cheaper equipment to those who plan to market in the coming months.

[karimwassef]

Not sure I understand.

I use an Apex with two pH probes, temp, conductivity and a separate CO2 monitor.

I haven't made it into an automatic sensor, but I put a camera to monitor the CO2 reading and use an app on my iPhone where I can enter the Apex parameters and CO2 and it returns the calculated Alk.