What are primary standards in analytical chemistry?

Primary standards in analytical chemistry are defined as highly pure substances that can be used to prepare standard solutions with a known concentration. The key attribute of a primary standard is its high purity and stability, which allows for accurate quantitative analysis. This enables chemists to establish a consistent point of reference for various analytical procedures, particularly in titrations.

The requirement for high purity ensures that the mass of the primary standard used can be directly correlated to the amount of analyte present in the solution being studied. Common examples of primary standards include sodium chloride, potassium hydrogen phthalate, and silver nitrate, all of which possess stable compositions and known molecular weights.

In contrast, substances that produce a consistent color change are typically indicators used in titrations and do not serve as primary standards. Commonly used reagents for titration can include both primary and secondary standards depending on their purity and properties, while impure substances are generally not reliable for calibration since their lack of purity can introduce variability in the results. Thus, the focus on highly pure substances accurately defines what primary standards are in analytical chemistry.