A primary standard is a highly pure chemical that can be used to determine the concentration of a solution through titration without requiring calibration against another standard. In contrast, a secondary standard is a solution whose concentration has been determined by comparison to a primary standard. Secondary standards are useful for practical applications because they can be easier to handle and prepare than primary standards.
To elaborate, primary standards are characterized by their stability, non-hygroscopic nature, and ability to be weighed accurately. Common examples include sodium carbonate for acid-base titrations and potassium hydrogen phthalate. These substances remain unchanged under normal laboratory conditions, making them reliable for precise quantitative analysis.
On the other hand, secondary standards are often solutions made from primary standards. They can degrade or change over time and are generally less pure. Because these solutions must be calibrated against a primary standard, their use is generally considered less reliable than using a primary standard alone.