A primary standard is defined as which of the following?

• A. Secondary standard
• B. Primary standard
• C. Analytical standard
• D. Indicator

Answer: (B)

A primary standard is a substance of very high purity that can be weighed directly to prepare a solution with a precisely known concentration. Because its composition is well defined, its exact amount can be calculated from its mass and molar mass, allowing you to establish standard solutions without needing another standard for calibration. The best primary standards are stable in air, non-hygroscopic, and do not react with the container, so their mass and composition stay constant over time. This reliability is what makes them ideal for accurately standardizing solutions and calibrating instruments.

Secondary standards, by contrast, require calibration against a primary standard to determine their concentration. An analytical standard is used for calibration but is not inherently defined as a primary standard. An indicator is used to signal an endpoint in a titration, not to prepare standard solutions.