Diesel Exhaust Fluid is a dilute urea solution made with 32.5% urea and 67.5% deionized water.  It is used in modern diesel engines to reduce nitrogen oxides that are produced during combustion. 

DEF is a clear, colorless solution that should be stored in a cool, dry place out of  direct sunlight.  Diesel Exhaust Fluid will freeze at approximately 12 degrees F.  It weighs about 9 pounds per gallon.  Heat will shorten the shelf life of DEF.

High Density Polyethylene (HDPE) is a great choice for storing Diesel Exhaust Fluid.  Be sure to keep Diesel Exhaust Fluid sealed to keep airborne contaminants out of the solution.  Fittings can be made from Polypropylene, CPVC or 316 Stainless Steel.  For gaskets, EPDM and Viton are good choices.