An element, which can be used to store pure phosphorus to keep it from reacting is ARGON.
Phosphorus exist in three different forms, white, red and black phosphorus. The red and the black phosphorus are quite stable but the white phosphorus is unstable and can explode when it comes in contact with oxygen. Because of this, it is usually stored under water. Argon also can be used to store the white phosphorus, because it an inert element and will not react with the phosphorus.