In my lab we make a stock solution of 0.2MPB at pH 7.4 (adj using NaOH) and dilute later according to our needs. We typically make batches of 4 liters at a time and store them at 4c in the fridge.
From time to time the salts seem to come out of solution and crystallize at the bottom of the container. This doesn't happen every time. Could it have something to do with the age of the reagents? What's going on?