I use BTB solution for pH and CO2 levels measurement in my fish tank.
It is commonly used technique, try to search "Dropchecker Aquarium"
The solution is 4KH water + 1% BTB 1%inEth. I prepare the 4KH water by mixing 4950gr + 0,6gr NaHCO₃. For the final solution 50gr 1% BTB in ethanol is added to the 4KH water.
This final solution works fine until around a month or so, then the color starts to fade rapidly when stored in PET container and turns yellow.
I tried to aerate the freshly prepared solution which gave it beautiful blue color (honestly it is the best color for use in aquarium since it represents 7-8 pH giving it the best starting point) to remove CO2 and store it in a PET bottle with no air inside. This also did not work.
Do you have any idea ? Should I prime the PET with some base ?