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.
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
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