Scientific--->Reef Conversion: Total Alkalinity and ΩAr/ΩCa

[Morganfbs]

Hey RC, I'm working at MBL/WHOI in Woods Hole, MA this summer, doing research on ocean acidification growth impacts on shellfish larvae. I'm working a lot with measuring total alkalinity, ph, and aragonite/calcite saturations in different treatment groups, and I'm trying to relate my work in the lab to what I see in reef keeping. Basically, I'm trying to convert the units of Total Alkalinity (TA) in the lab here, micro moles/kg of seawater, to meq/l, and also convert ΩAr and ΩCa to ppm CaCO3 or ppm Ca.

As a sample problem, I'd like to know Total Alkalinity of a normal seawater sample here (~2050 micromoles/kg) in meq/l

and a normal aragonite saturation of 2 Ω in ppm


Any input is most welcome!

 

[bertoni]

I'm sorry, but that's beyond my chemistry background. We'd need to know some precise definitions to have any hope of answering, but even that might not be enough for me.

 

[Morganfbs]

I'm sorry, but that's beyond my chemistry background. We'd need to know some precise definitions to have any hope of answering, but even that might not be enough for me.

Thanks for the response. I was afraid that this may be the case. As a marine biologist, not a chemist, I have been a little stumped recently.

 

[pisanoal]

Hey RC, I'm working at MBL/WHOI in Woods Hole, MA this summer, doing research on ocean acidification growth impacts on shellfish larvae. I'm working a lot with measuring total alkalinity, ph, and aragonite/calcite saturations in different treatment groups, and I'm trying to relate my work in the lab to what I see in reef keeping. Basically, I'm trying to convert the units of Total Alkalinity (TA) in the lab here, micro moles/kg of seawater, to meq/l, and also convert ΩAr and ΩCa to ppm CaCO3 or ppm Ca.

As a sample problem, I'd like to know Total Alkalinity of a normal seawater sample here (~2050 micromoles/kg) in meq/l

and a normal aragonite saturation of 2 Ω in ppm


Any input is most welcome!

What does the (ohm) before the Ca and At stand for? Never seen that in a chemistry context before. Is that what you are using to denote concentration? I can help with the math with some more info

 

[Dan_P]

Hey RC, I'm working at MBL/WHOI in Woods Hole, MA this summer, doing research on ocean acidification growth impacts on shellfish larvae. I'm working a lot with measuring total alkalinity, ph, and aragonite/calcite saturations in different treatment groups, and I'm trying to relate my work in the lab to what I see in reef keeping. Basically, I'm trying to convert the units of Total Alkalinity (TA) in the lab here, micro moles/kg of seawater, to meq/l, and also convert ΩAr and ΩCa to ppm CaCO3 or ppm Ca.

As a sample problem, I'd like to know Total Alkalinity of a normal seawater sample here (~2050 micromoles/kg) in meq/l

and a normal aragonite saturation of 2 Ω in ppm


Any input is most welcome!

If the specific gravity of seawater is 1.027 kg/liter, divide 2050 by 1.027 to obtain micromoles/kg.

What is "ΩAr and ΩCa"? Reference to solubility?

 

[pisanoal]

If the specific gravity of seawater is 1.027 kg/liter, divide 2050 by 1.027 to obtain micromoles/kg.

What is "ΩAr and ΩCa"? Reference to solubility?

+1. Unit conversion is 1 micromole/kg of alk is equal to .0205 meq/L. Mine was actually with sg of 1.025, so slight fluctuations in sg don't appear significant

 

[biom]

What does the (ohm) before the Ca and At stand for? Never seen that in a chemistry context before. Is that what you are using to denote concentration? I can help with the math with some more info

Ω (ohm) is Aragonite saturation state and equals to Ω =([Ca^2+] × [CO3^2-]) / [CaCO3] , where [Ca^2+] is the seawater concentration of dissolved calcium ions,[CO3^2-] is the seawater concentration of carbonate ions, [CaCO3] is the solubility of aragonite in seawater. So to back calculate the concentration of Ca in the sample from saturation state, you should know concentration of CO3 ion in the sample. It is far easy to measure dissolved Ca directly.

In your case Ω =2 that means the seawater is supersaturated with respect to aragonite and aragonite will precipitate.

And welcome Morganfbs in the shadows of marine biology, but don't forget that marine biology is half marine chemistry also.

 

[pisanoal]

Ω (ohm) is Aragonite saturation state and equals to Ω =([Ca^2+] × [CO3^2-]) / [CaCO3] , where [Ca^2+] is the seawater concentration of dissolved calcium ions,[CO3^2-] is the seawater concentration of carbonate ions, [CaCO3] is the solubility of aragonite in seawater. So to back calculate the concentration of Ca in the sample from saturation state, you should know concentration of CO3 ion in the sample. It is far easy to measure dissolved Ca directly.

In your case Ω =2 that means the seawater is supersaturated with respect to aragonite and aragonite will precipitate.

And welcome Morganfbs in the shadows of marine biology, but don't forget that marine biology is half marine chemistry also.

I guess in terms of the op's post, the only way to get [caco3] from the saturation state is to measure calcium concentration, then find your carbonate concentration. Id guess you could titrate your first alkalinity endpoint with pheno for bicarb, then titrate with say methyl orange for carb, and back out your carbonate concentration?

How is saturation state being measured?

 

[biom]

And [µmol/kg]= [mmol/kg]/1000, so you have 2050/1000=2.05 mmol/kg, since mmol/kg = meq/kg * 0.5, then 2.05/0.5=4.1 meq/kg, note is per kg since sea water is heavier than distilled water you can calculate more precisely.

 

[daveM100]

HUH? LOL this is why I didn't take chem in high school let alone after! Still lost after looking some of that stuff up rofl!

 

[pisanoal]

And [µmol/kg]= [mmol/kg]/1000, so you have 2050/1000=2.05 mmol/kg, since mmol/kg = meq/kg * 0.5, then 2.05/0.5=4.1 meq/kg, note is per kg since sea water is heavier than distilled water you can calculate more precisely.

I was off by a factor of 10 in my original conversion. Thanks for the correction. So the straight conversion factor is 1μmol/kg = .00205meq/liter. So just multiply your TA by .00205 to get meq/liter. Here is the unit analysis if that makes more sense for the op.

 

[Morganfbs]

I guess in terms of the op's post, the only way to get [caco3] from the saturation state is to measure calcium concentration, then find your carbonate concentration. Id guess you could titrate your first alkalinity endpoint with pheno for bicarb, then titrate with say methyl orange for carb, and back out your carbonate concentration?

How is saturation state being measured?

Good question. Using an algorithm called co2sys, we use pH and pCO2 (after water is equilibrated-bubbled with ambient air until it stabilizes at an air co2/water equilibrium) values to calculate other carbon system variables (total alk, Calcite, aragonite, HCO3, CO3)

 

[Morganfbs]

I was off by a factor of 10 in my original conversion. Thanks for the correction. So the straight conversion factor is 1μmol/kg = .00205meq/liter. So just multiply your TA by .00205 to get meq/liter. Here is the unit analysis if that makes more sense for the op.

Ah, very clear. Fantastic.

 

[Morganfbs]

Ω (ohm) is Aragonite saturation state and equals to Ω =([Ca^2+] × [CO3^2-]) / [CaCO3] , where [Ca^2+] is the seawater concentration of dissolved calcium ions,[CO3^2-] is the seawater concentration of carbonate ions, [CaCO3] is the solubility of aragonite in seawater. So to back calculate the concentration of Ca in the sample from saturation state, you should know concentration of CO3 ion in the sample. It is far easy to measure dissolved Ca directly.

In your case Ω =2 that means the seawater is supersaturated with respect to aragonite and aragonite will precipitate.

And welcome Morganfbs in the shadows of marine biology, but don't forget that marine biology is half marine chemistry also.

Thanks! You're all too correct about the chemistry :lolspin:

 

[bertoni]

Argh! I should have recognized the Ω. Seawater is supersaturated with respect to aragonite:

http://reefkeeping.com/issues/2005-07/rhf/index.htm#12

 

[pisanoal]

Argh! I should have recognized the Ω. Seawater is supersaturated with respect to aragonite:

http://reefkeeping.com/issues/2005-07/rhf/index.htm#12

Great article. I never would have guessed anything could stay in a state of supersaturation for long. It seems so delicate. I remember high school chemistry when we created a supersaturated solution by dissolving some salts in di water that was heated. The amount dissolved was more then the saturation point at room temp. We let it cool, dropped a grain of that salt in, and boom! A crystal lattice built off that grain and the solution came back to equilibrium. That was the last time I had anything to so with to any extent though... Haha. None of my college chem ever had much on supersaturated solutions.

That article looks like a lot of what the op was looking for.

 

[disc1]

The key is magnesium. It gets into the crystal lattice in place of calcium and frustrates the process of growing a crystal. Without the high level of magnesium, the calcium carbonate would come out of solution to very near the saturation point.

 

[disc1]

Be careful converting mmol/L directly to meq/L. It depends on what you have mmol of. If the measurement is mmol of acid to the equivalence point then the conversion is direct. But if your measure is mmol of carbonate, then 1mmol/L is 2 meq/L since one mol of carbonate has 2 equivalents of alkalinity.

 

[biom]

Yep, that's why we divide 2.05mmol by 0.5 =4.01meq

 

[sde1500]

This is why I didn't make it in Marine Bio. Freshman year it was "I can't wait to be a marine biologist and swim with fish and do cool stuff on the discovery channel" Then got my class schedule and it was all complex math, and chemistry, and only 1 Bio class and I got on out of that.