* High purity: It must have a known composition with very few impurities. The level of impurities should be less than 0.01-0.02%, and ideally, their impact on the analytical result should be negligible or correctable.
* Stability: It should be stable under normal laboratory conditions (air, light, moisture). It shouldn't decompose, react with the atmosphere, or absorb moisture over time, thus maintaining a constant composition.
* High molar mass: A high molar mass minimizes the effect of weighing errors on the concentration of the prepared solution. A small weighing error will have less of an impact on the overall concentration of a solution made from a substance with a high molar mass.
* Readily available and inexpensive: While purity is paramount, it's also important that the substance is accessible and affordable for widespread use.
* Easy to dry and purify: The substance should be easily purified and dried to a constant weight to ensure accurate measurements.
* Reacts stoichiometrically (completely and in a known manner): It should participate in the analytical reaction in a well-defined, predictable way. This means that its reaction with the analyte is complete and proceeds according to a balanced chemical equation.
Examples of common primary standard substances include:
* Potassium hydrogen phthalate (KHP): Often used to standardize solutions of strong bases.
* Sodium carbonate (Na₂CO₃): Used to standardize solutions of strong acids.
* Benzoic acid (C₇H₆O₂): Used to standardize solutions of strong bases.
* Potassium dichromate (K₂Cr₂O₇): Used to standardize solutions of reducing agents.
* Sulfamic acid (H₃NSO₃): Used to standardize solutions of strong bases.
It's important to note that even primary standards are not perfect, and their purity and stability should be verified before use, often by appropriate analytical techniques. Secondary standards, on the other hand, are substances whose concentration is determined by comparison to a primary standard.