[kodls]

Hello guys,

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 ?

[kodls]

Here is picture showing (from the left) 3month old solution, 1,5month old solution and fresh solution

[Corribus]

What is BTB?

PET is permeable to gasses (it is why carbonated beverages stored in PET bottles will eventually go flat).

[kodls]

BTB = Bromothymol Blue

Oh, I did not think of that.
Can somebody tell what would be ideal packaging? Must be plastic, glass is not possible 

[kodls]

Is HDPE better option? Does anyone have an experience ?

[Corribus]

HDPE would be worse. PET actually has pretty good barrier properties for a polymer. But if you want something basically impermeable, try glass.

[kodls]

HDPE would be worse. PET actually has pretty good barrier properties for a polymer. But if you want something basically impermeable, try glass.

Just found out some companies make fluorinated barriers inside HDPE container, for example: https://www.ipackchem.com/in-mould-fluorination/

Will try soon and post results, I also tried PVDC (heat shrink around the PET bottle) foil which is almost impermeable according to the table below



Performed same as PET with no foil because it is almost impossible to air seal it.

[kodls]

Update on the test: I have tried to stabilize the solution by simply leaving RO water for days on open air so it catches needed CO2 to be in equilibrium with the surrounding air, that way when it is stored in PET it should not have need to "catch" more CO2 and acidize the solution.
It performs better and does not turn yellow rather it is bright green. But still it is not blue as it should be.
Do you think the Ethanol in the solution (see 1st post) would somehow break down after some time and cause the solution to ged bad ?