- A primary standard is a substance of high purity, stable in air, not hygroscopic, with a precisely known formula, and a high equivalent weight, used for standardizing solutions. - Potassium permanganate (KMnO₄) is a strong oxidizing agent but is not a primary standard because it is difficult to obtain in high purity, is not completely stable in solution, and can be reduced by organic matter. - Sodium Thiosulphate (Na₂S₂O₃) is typically used as a secondary standard, requiring standardization against a primary standard like potassium iodate or potassium dichromate, as its solutions are not highly stable. - EDTA (Ethylenediaminetetraacetic acid) is also commonly used as a secondary standard, standardized against primary standards like calcium carbonate or zinc. - Potassium dichromate is stable, non-hygroscopic, and readily available in high purity, making it suitable as a primary standard, particularly in redox titrations.