The picric acid solution always precipitates crystals of picric acid when it gets cold outside. I have never seen it actually freeze. No matter how cold it gets out here the solution remains completely liquid. The coldest winter nighttime temperature in upstate South Carolina in is about -10C/14F although most of the time it is not THAT cold. There are many days in winter where I can comfortably sit outside in jeans and a t-shirt during the afternoon. Water that is left out overnight does not freeze at all most of the time. That's not to say the winters here are balmy; it usually gets down into the 30s or 40s at night (F). We almost never get any snow but we do get cold, dreary rain come through every few days. That's always fun. It's too cold to run a dehumidifier in the shed so I have to wrap up EVERYTHING I don't want corroding in well sealed plastic.