Alkalinity vs. pH

[rglauson]

I've only had a saltwater aquarim for about one year and started with 30 gallon tank. A month after starting I decided to build my own acrylic 75 gallon tank, black acrylic stand and hood. Since I'm new I've been trying to understand the relationship betwee Alkalinity and pH. The literature appear to indicate a direct relationship between pH and alkalinity for a given CO2 level. That being said, I have seen posting for aquarium that state pH's of 8.0 to 8.4 with alkalinity levels of 12 dKh which would be way off any graph that shows a steady state relationship between the two. Why is that? Is the alkalinty measured of a differnet type (not just carbonate and bi-carbonate)? Can these stated apparent imbalances occurs long term without significant precipitate?

 

[Anonymous]

There is no apparent inbalance.

As you stated for a given co2, carbonate, and pH are interrelated.

But your assumption of "a given co2" is not correct. at low co2 values you can have high pH with relativily low kh values. You may also have a greater pH drop under lights out conditions.

As long as the co2 from the fish is being consumed such that the tank becomes a net consumer of co2 and producer of oxygen in a 24 hour period, IME things will be fine.

 

[rglauson]

Thanks for the response. I am assuming than that for the conditions or normal pH (8.0 to 8.4) that an alkalinity of 11 or 12 dKh would required elevated CO2 levels compared to normal atmospheric conditions.

 

[Anonymous]

Thanks for the response. I am assuming than that for the conditions or normal pH (8.0 to 8.4) that an alkalinity of 11 or 12 dKh would required elevated CO2 levels compared to normal atmospheric conditions.

From what I understand and from what I have observed in my tanks co2 must be less then atmospheric.

my .02

 

[rglauson]

Just reviewing the pH vs Alkalinity curves that have been published in Advanced Aquarist among other places, higher CO2 will result in a higher alkalinity for the same pH level. The data suggests that a normal pH of 8.3 would have an alkalinity of only 3 meq/L (8.4dKH) with normal CO2 levels. At elevated CO2 levels the alkanility would be significantly higher. Have lower CO2 for the same pH would result in a lower alkalinty value. Based on this I assumed that tanks that have reported alkalinty levels of 10 to 12 dKH and pH levels of around 8.2 to 8.4 would have to have artificially elevated CO2 levels via CaCO3/CO2 reactors. Being new to the hobby suggests that there may be flaws to my logic/understanding and would appreciate any corrections to my thinking.

 

[Anonymous]

Just reviewing the pH vs Alkalinity curves that have been published in Advanced Aquarist among other places, higher CO2 will result in a higher alkalinity for the same pH level. The data suggests that a normal pH of 8.3 would have an alkalinity of only 3 meq/L (8.4dKH) with normal CO2 levels. At elevated CO2 levels the alkanility would be significantly higher. Have lower CO2 for the same pH would result in a lower alkalinty value. Based on this I assumed that tanks that have reported alkalinty levels of 10 to 12 dKH and pH levels of around 8.2 to 8.4 would have to have artificially elevated CO2 levels via CaCO3/CO2 reactors. Being new to the hobby suggests that there may be flaws to my logic/understanding and would appreciate any corrections to my thinking.

There is also the interplay between alk (carbonate) and calcium. With low calcium (250ppm), and aa alk of 4 meg/l the, addition of macro algaes rasied my pH from 7.6-7.8 to 8.4-8.8 (api test kit) in a week. But when I started my initial buffering of calcium with oyster shells calcium did rise to 400ppm. But alk slowly dropped to 1.5-2 meg/liter. pH remained at 8.4-8.8. So I started adding baking soda (the diy 2 part system) and got calcium and alk to 400ppm and 8-10 dkh. Meanwhile pH remained at 8.4-8.8.


my .02