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Paco
10/14/2009, 08:20 AM
Hi everyone. I have the same problem that most reefers have with low pH. What I'm curious about is whether there's a way to calculate indoor CO2 levels by the pH of the tank. If we've had many visitors in our home, the pH will often drop into the 7.6 range overnight. One particular early morning it was in the 7.5s!

I understand atmospheric CO2 levels are around 380 or 390 PPM now. Some climate experts are saying that anything over 350 is going to kill us all. I don't want to get into that debate -- what I'm interested in is what's my indoor CO2? Yes, I could go buy a monitor but then I wouldn't have any money for frags :lolspin:.

Thanks!

Mark

PS I keep my Alk around 9 DKH.

Randy Holmes-Farley
10/14/2009, 08:26 AM
Not readily done with a working tank, but certainly with a cup of tank water:

All you need to know is alkalinity and pH in seawater, and it is possible to calculate CO2 levels.


Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm

High pH: Causes and Cures
http://reefkeeping.com/issues/2005-03/rhf/index.htm

from the second one:

Figure 2. The relationship between alkalinity and pH for seawater with normal carbon dioxide levels (black), excess carbon dioxide (purple) or deficient carbon dioxide (blue). The green area represents normal seawater.

http://www.reefkeeping.com/issues/2005-03/rhf/images/Figure2sm.jpg

Boomer
10/14/2009, 10:35 AM
What I'm curious about is whether there's a way to calculate indoor CO2 levels by the pH of the tank.

No, as you do not now how much CO2 the tank itself is generating.


7.6 and dKH 9 = ~3.15 ppm CO2

NSW @ pH 8.15 and 9dKH = ~0.8 ppm CO2

NSW @ pH 8.30 and 9dKH = ~0.5 ppm CO2

So, you have ~ 6 x to much CO2 form the air and tank generated CO2. And most of that will be an air issue.

I understand atmospheric CO2 levels are around 380 or 390 PPM now.

Yes, but your house can be double that.

Indoor CO2 Problems
http://www.advancedaquarist.com/issues/apr2002/short.htm




Some climate experts are saying that anything over 350 is going to kill us all.

I find that very amusing :lol:

Right now in our geologic history we are at some of our lowest CO2 levels. Over the last 600 million years there have been 3 geologic points with very low CO2, 450 MYA, 300 MYA and now. During the days of the Dinosaurs and the flourish of plant life CO2 avg 2,000 ppm. And during the explosion of the mammals, its avg 1,500 ppm. And during the rise of man is was ~ 500 PPM. Now it is in the mid 300's and we are all going to due :)

Billybeau1
10/14/2009, 02:53 PM
due or die, Boomer ? :blown:





:D

Boomer
10/14/2009, 04:00 PM
:lolspin:

TheH
10/14/2009, 06:56 PM
Randy, suppose we take a cup of saltwater and thoroughly aerate with air from the house and allow it to come to equilibrium. As you have mentioned, we can then readily determine the CO2 level in that water sample given the carbonate alkalinity and pH (salinity, temperature?).

But how can we now compute the concentration of CO2 in the house air? It won't be equal to the CO2 level in the water, will it? What formula can we use to relate the CO2 level in the water with the CO2 level in the air?

Boomer, let's make a calculator for this it sounds novel.

Randy Holmes-Farley
10/14/2009, 07:03 PM
One just needs to solve the equation which I used to generate the graph, for ppm CO2. It is equation 2 in this article:

The Relationship Between Alkalinity and pH
http://www.advancedaquarist.com/issues/may2002/chem.htm

One would need to control temperature to match the stated temp for the values of K1, K2, and KH from the scientific literature.

To be really accurate, you'd want to make sure that borate alkalinity was not above NSW levels.

TheH
10/14/2009, 07:20 PM
Randy, I have the formulae to calculate K1 and K2 for H2CO3 (each as a function of salinity and temperature).

Can you point me in the right direction for a formula to calculate KH for CO2 as a function of salinity and temperature as well? Do air-pressure/elevation and air-temperature affect KH here?

Boomer
10/14/2009, 10:19 PM
Pressure and temp affect the pCO2 not the KH. One would just go with surface water pH, atm, temp and solubilty. At 35 ppt, 24 C, Solubility = 297 x 10^-4 moles / l /atm

Randy' s equation 2 has pCO2 which means you must be able to measure the pCO2. Such, as from this link

Indoor CO2 Problems
http://www.advancedaquarist.com/issues/apr2002/short.htm

The only other way to solve it with out measurement is to pull the 2 pCO2 from the right hand side of the equation, putting them on the left side and move the Ac (Carbonate Alk) to the right side side. Then plug in the K<sub>1</sub>, K<sub>2</sub>, K<sub>H</sub> and pH.


or the short version

pCO2 = Ac x pH/ [K<sub>1</sub> x s (1 +2K<sub>2</sub>/pH)] = atm CO2 ( ppm)

s = solubilty of CO2 in seawater, in moles / l / atm

formula to calculate KH for CO2 as a function of salinity


That is what Randy's equation is, Ac ( Carboante Alk) = meq / l total Alk - meq/ l Borate.

Flipper62
10/14/2009, 10:35 PM
AAAAAAAAAAAAAA..........http://i297.photobucket.com/albums/mm238/flipper62_photos/spinninghead.gif Head spinning again

My tank looks good....My tank looks good.

I like pickels....I like pickels.

Need to go clean the tank.....YA....need to clean the tank

SORRY......Couldn't help myself.......LOL..LOL..LOL......:dance:

Boomer
10/14/2009, 10:42 PM
So how I spaced out the Ac equation till Randy jarred my head :lol:

Flipper62
10/14/2009, 11:04 PM
So how I spaced out the Ac equation till Randy jarred my head :lol:

LOL...LOL..

So at first read......My brain went in to Over Load SHUTDOWN.

However now you & Randy have forced me into NOT going to my office tomorrow. I will be rereading......& rereading both of your explanations until it makes sence to me.

I'm always up for researching & learning more. PLUS, I dont really want to go in anyway.......Hell, I own the Company. I can do what ever I want.

My wife said I can.....LOL...LOL...:lol:

TheH
10/15/2009, 09:04 AM
Sorry, by KH I mean Henry's Law Coeffecient for CO2. I have a formula to estimate it as a function of temperature, but not as a function of temperature and salinity. I believe you are referring to it as solubility?

Randy Holmes-Farley
10/15/2009, 10:46 AM
Yes, it is the Henry's Law value you need. I can't recall where I got it from for the calculation, but probably from Pankow's "Chemical Oceanography". I can check later some time.

Boomer
10/15/2009, 11:41 AM
H

Yes, as solubility. There are tables for seawater I have some. Your are solving for pCO2 in my equation but Randy's you are not and still need to measure or solve for pCO2 and solve K<sub>H</sub>. For some dumb *** reason, as I did not READ Randys text, I was looking at or thinking K<sub>H</sub> = K<sub>w</sub>


CO2(g) + H2O = H2CO3*

K<sub>H</sub> = {H2CO3*} / pCO2

Randy

I do not see how such a calculator could work in a tank, a closed system, which are rarely in equilibrium. With pH x and Alk y how are you going to know if x pH is from tank generated CO2 or air generated CO2 or a combo of each.

Randy Holmes-Farley
10/15/2009, 12:05 PM
I agree that it is not suitable for a functioning tank, unless you are just asking what CO2 levels "Would" this be in equilibrium with. :)

Boomer
10/15/2009, 12:27 PM
I figured that :)

I found the stuff in Pankow's " Aquatic Chemistry Concepts". It may be in Millero or Sarmiento or Emerson also but I did not look.

TheH
10/15/2009, 04:18 PM
I have the Henry's Law value for CO2 at standard temperature and 35psu. What I am trying to find is an equation that can determine the Henry's Law value for CO2 at any given temperature and given salinity.

The best I have been able to do is account for temperature with the van't Hoff equation:

KH(T) = 29.41 * exp(2400 ( (1/T) - (1/298) ) )

Paco
10/16/2009, 10:40 AM
Thanks everyone, for answering my question, in great detail. Short answer seems to be: Leave a Window Open Overnight :)