FeSO4 is not considered a primary standard due to its instability and lack of inertness, which are essential characteristics for primary standards. A primary standard must be stable, inexpensive, and easily obtainable. The discussion also touches on ferrous ammonium sulfate, which is questioned as a primary standard as well. Understanding the specific characteristics of primary standards is crucial for evaluating substances like FeSO4 and ferrous ammonium sulfate. Overall, the stability and reactivity of these compounds disqualify them from being classified as primary standards.
