I am actually trying to use the KH (carbonate hardness) test solution of API by adding 4 drops of aquarium water to make it 4dkh
. I do not have that bromothymol blue. So it seems that the solution is not appropriate and will have to buy what you are recommending.
Yes, this is incorrect. The kH test kit does not have anything to do with this.
You need a reference solution that will measure 4 dkH. This can either be bought, or made yourself with sodium bicarbonate.
Once you have this reference solution, using the kH test solution, it should measure 4 dkH.
However, in order to properly use the 4 dkH reference solution in a drop checker, you will need bromothymol blue (the API regular pH test kit is this chemical).