如何製備標準溶液

The solid of KMnO4 cannot be stored pure for ong time, that means a standard solution of KMnO4 cannot be prepared by weighting a sample and dissolved it to a certain volume say 250 cm3. as the KMnO4 solid will contain impurity. A standard KMnO4 solution is prepared by making solution of rough concentration, [ by weighting a certain mass of KMnO4 and dissolve in a certain small amount of water and then make up to say 250 cm3. the accurate concentration is obtained by titration with a standard Fe2+ solution. Therefore, the KMnO4 solution can only be a secondary standard solution. It cannot be a primary standard solution.