I have some ferric oxalate for Pt/Pd printing that is approaching the dreaded 3-month expiration. Except for the days I have been using it, I have stored this in a brown bottle inside another brown bottle in my refrigerator. By the calander, I am about 2 weeks under 3 months. So far I am not aware of any change in its effectiveness. When this stuff is past its prime, how do I know it is time for a change?