Using tank water with the bromothymol blue indicator solution in a dry checker just measures the pH level of the aquarium.
But, using the dKH solution that is carbonate based with the bb solution not only measures the pH, but it lets you get a good estimate of the CO2 level.
Chuck’s CO2 calculator said, “NOTE: This calculator will only work if your water is using is a carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations.”
If you have read Chuck's CO2 chart/calculator and he mentions that this chart is only valid if the only buffers are carbonate based. This means that you really
can't use aquarium water because it also contains other buffers like phosphate plus other buffers, bases, and acids.
It was hoppy/Vaughn, T. Barr and others that worked this out.
They put 2 and 2 together.
CO2 indicators and Chuck's CO2 chart have been around for many years.
CO2 indicators' directions say to use aquarium water, but they come with the Bromothymol Blue freshwater low range pH indicator.
Chuck's CO2 chart says that it is only valid if you have carbonate based buffers and it also says that with a 4 dKH and a pH of 6.6 that you have 30 ppm of CO2.
Bromothymol Blue works in a pH range from 6.0 to 7.6. It's a yellow color at a pH of 6.0 and lower, a blue color at a pH of 7.6 and higher, and halfway it is a green color.
API's freshwater low range pH test kit uses Bromothymol Blue and it's color card shows a greenish color with a pH range from 6.4 to 6.8.
Then it was decided to make a known 4 dKH carbonate based solution using distilled water. This indicates a green color in the CO2 indicator at a pH of 6.6 for a 30 ppm CO2 level. (There's a little ± in this.)
Presto! They have invented a easy and cheap method to determine the CO2 levels in aquariums reliably with just a simple glance at the aquarium.
I thank them for their hard work. This is a great idea!!
I made my own 4dKH solution by following Bill's directions:
http://www.plantedtank.net/forums/general-planted-tank-discussion/42429-kh-standard-how.html
Here's some baking soda/KH/pH calculators and info:
http://dataguru.org/misc/aquarium/calKH.asp
http://www.cnykoi.com/calculators/calckh.asp
http://fins.actwin.com/aquatic-plants/month.200612/msg00109.html
http://jdieck1.home.comcast.net/~jdieck1/chemcalc.html
http://americanaquariumproducts.com/AquariumKH.html