An IV bag including a bladder containing a first substance and a storage cap containing a second substance operable to release the second substance into the bladder to mix with the first substance without exposing the first substance to outside contamination, the storage cap for use with an IV bag including an inner storage chamber for storing a first substance, a first annular opening having a first annular convex surface facing toward the storage chamber, and a plunger element located within the storage chamber having a domed-end with a convex surface facing to the annular opening, wherein the domed end is moveable to make and break contact with the annular opening, and wherein when the domed end is in contact with the first annular convex surface, a seal is formed between two convex surfaces along an annular path to seal the storage chamber.
