I just brought myself 4dKH reference solution. Now my problem is that I can't find my CO2 indicator solution. I have an API PH test kit lying around. Can I use that as an substitute for the CO2 indicator? If yes, how many drops should I use?
If you are using the standard API pH test kit (the one that has a range of up to 7.6 or 7.8, I believe), it will work fine as a replacement for the "CO2 indicator solution." Both reagents are simply bromothymol blue, and will work.
The number of drops is arbitrary. You add enough so that it is dark enough to see, but not so much that it becomes dark blue. I usually add ~6-8 drops.