A standard solution is prepared by dissolving a definite weight of substance (a primary standard), in a definite volume. A substance is classified as a primary standard if it has following characteristics:
(i) It is easily available in state of high purity.
(ii) It is neither hygroscopic nor deliquescent.
(iii) It shows high solubility in water.
(iv) It does not dissociate or decompose during storage.
Substances whose standard solutions cannot be prepared directly are called secondary standards. For example, KMnO4, NaOH, KOH, etc. The solutions of such substances are standardized by titrating against solution of some appropriate primary standard.