I have done a solution of 13% potassium dichromate, and for unknown reason after 2 weeks some christal developed in the bottle, then i have to dissolve them again making the solution warm. Quite annoying, I am not even at saturation point and it's stored in the dark. do you know what is happening and how to prevent it ? could it be the bottle material? mine is a plastic one, for chemicals (made of PET i suppose) thank you !