co2 questions

plankton

Premium Member
Do you mean why does tank pH drop with the absorption of CO2 from the air? Or, why we add CO2 to a Calcium Reactor to drive the pH down to dissolve the media back into the water?

Read this nice article by Randy :

http://reefkeeping.com/issues/2007-05/rhf/index.php

Carbon Dioxide and pH



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Marine aquarium water's pH is intimately tied to the amount of carbon dioxide dissolved in the water and to its alkalinity. The reason that carbon dioxide impacts pH is because when it enters the water, it rapidly turns into carbonic acid. Acids lower pH, so more carbon dioxide means more carbonic acid, which means lower pH.

If seawater is fully aerated with normal air (that is, it is in full equilibrium with the air), then its pH is exactly determined by its carbonate alkalinity: the higher the alkalinity, the higher the pH. There is, in fact, a simple mathematical relationship between alkalinity, pH and carbon dioxide that I have discussed previously. Figure 2 shows this relationship graphically for seawater equilibrated with normal air (350 ppm carbon dioxide), and equilibrated with air having extra carbon dioxide, as might be present in certain homes (1000 ppm) or when the carbon dioxide is deficient (as may happen in aquaria using limewater, also known as kalkwasser).

Understanding the overall relationship between carbon dioxide, alkalinity and pH (Figure 2) is a key principle in solving most pH problems encountered in coral reef aquaria.

Why Does pH Fall?



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As mentioned above, low pH becomes a problem when it falls below about 7.8; that is, when the pH drops below 7.8 for any portion of the day. Of course, if the pH reaches a low value of pH 7.9, aquarists may still want to raise it, but the need is not so immediate. Several things commonly reduce pH, and each has its own unique solution. Finally, there's nothing to prevent a tank from having all of these problems simultaneously!

The first step toward solving a low pH problem is to determine why it exists in the first place. Some possibilities include:

A calcium carbonate/carbon dioxide reactor (CaCO3/CO2 reactor) is in use on the aquarium.
The aquarium has low alkalinity (substantially below 2.5 meq/L, or 11 dKH).
The aquarium contains more CO2 than the surrounding air due to inadequate aeration. Don't be fooled into thinking that an aquarium must have adequate aeration because its water is very turbulent. Equilibrating carbon dioxide is MUCH harder than simply providing adequate oxygen. There would be NO diurnal pH swing if carbon dioxide were perfectly equilibrated. Because most aquaria's pH is lower during the night than during the day, they are demonstrating incomplete aeration.
The aquarium contains excess CO2 because the air that it is being equilibrated with contains excess CO2. This is the most common cause in cases that I have discussed that involve more than a thousand aquarists mentioning pH problems.
The aquarium is still cycling and excess acid is being produced by the nitrogen cycle and the degradation of organics to CO2.

Scott
 

Boomer

Bomb Technician (EOD)
Premium Member
What test kit is that ? CO2 test kits DO NOT work in seawater :) And in seawater CO2 is < 1 ppm and more like 0.5 - 0.75 ppm
 

Patrick12

Crypt Assassin
CO2 is supposed to be low in a marine sample. Also, not really sure what would make a CO2 read low in a sample of seawater? Additionally, the pKa value of carbonate tends to be much lower in a seawater than freshwater....like 1-2 pH units lower.....the CO2 test kit you would be able to afford would just convert HCO3 to CO3 and you should get a falsely elevated reading.....not a low reading. In any event, it really has no use in a marine aquarium and will not read accurately.
 

Boomer

Bomb Technician (EOD)
Premium Member
Pat

You are to high on the pKa's it is from ~ 0.25 - 1 pH units :)

It is the salts in seawater itself that cause the interference just like they do for many other tests for seawater. You just can't get a reagent for CO2 to work right in seawater. The only way to measure CO2 in seawater is with a CO2 meter and a pCO2 probe. There are none really for us as those we could use measures only CO2 to +/- 1 ppm in water and they are ~ $2,000

You really do not even need a meter if using Buch-Park equations, which are a function of pH, CA, Salinity, fH, pK1 and pK2, which will calculate CO2 very accurately. I had a math algorithm expert make me one you using one of the latest accepted pKa sets.

Seawater CO2 Calculator
http://www.hamzasreef.com/Contents/Calculators/CO2LevelSalt.htm
 

rennne39

New member
Pat

You are to high on the pKa's it is from ~ 0.25 - 1 pH units :)

It is the salts in seawater itself that cause the interference just like they do for many other tests for seawater. You just can't get a reagent for CO2 to work right in seawater. The only way to measure CO2 in seawater is with a CO2 meter and a pCO2 probe. There are none really for us as those we could use measures only CO2 to +/- 1 ppm in water and they are ~ $2,000

You really do not even need a meter if using Buch-Park equations, which are a function of pH, CA, Salinity, fH, pK1 and pK2, which will calculate CO2 very accurately. I had a math algorithm expert make me one you using one of the latest accepted pKa sets.

Seawater CO2 Calculator
http://www.hamzasreef.com/Contents/Calculators/CO2LevelSalt.htm

according to the calculator that the link above goes to my co2 is 2.4 so is this ok
 

Randy Holmes-Farley

Reef Chemist
Premium Member
pH and alkalinity are the only things you need to measure to see if you have an appropriate amount of CO2 in a seawater aquarium. If they are OK, CO2 is OK. If the pH is too low, you have excessive CO2.
 

Boomer

Bomb Technician (EOD)
Premium Member
rennne

I agree with Randy. We really do not need to know what the CO2 is, only if we have to much and really only if the pH is low. I had that calculator made as many ask "how much CO2 do I have"

What numbers did you put in to get that 2.4. That is to high unless you have a low pH and high Alk. Most reef tanks are not above 1.5 ppm unless the pH is to low. Your pH looks like it is round 7.8
 
Pat

You are to high on the pKa's it is from ~ 0.25 - 1 pH units :)

It is the salts in seawater itself that cause the interference just like they do for many other tests for seawater. You just can't get a reagent for CO2 to work right in seawater. The only way to measure CO2 in seawater is with a CO2 meter and a pCO2 probe. There are none really for us as those we could use measures only CO2 to +/- 1 ppm in water and they are ~ $2,000

You really do not even need a meter if using Buch-Park equations, which are a function of pH, CA, Salinity, fH, pK1 and pK2, which will calculate CO2 very accurately. I had a math algorithm expert make me one you using one of the latest accepted pKa sets.

Seawater CO2 Calculator
http://www.hamzasreef.com/Contents/Calculators/CO2LevelSalt.htm


Thanks for the calculator. I'm on the high side with 2.14
 

rennne39

New member
my
temp 76
ph 8 ppm
kh 142.9 ppm
spec 1025.
calcium 470
nitrate .01mg
nitrite 25mg
ammonium .05mg
this test was done on 11-17 i have done a 10 water change and will do the test tonight
 

Boomer

Bomb Technician (EOD)
Premium Member
Nope, you have ~ 1 ppm and are fine. Here is your error

The calculator only has dKH and Meq/ l and you have Alk ppm

142.9 ppm Alk = 142.9 / 50 = 2.858 meq / l and 2.858 x 2.8 = 8 dKH = 1.09 ppm CO2
 
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