Which term best describes a primary standard?

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

Answer: (C)

The key idea is what makes a standard usable as an exact reference in a titration or calibration: a primary standard is a reagent of exceptionally high purity that can be weighed accurately, has a stable composition, and a known formula. Because you can determine its mass precisely and know exactly how many moles that mass corresponds to, you can prepare standard solutions with very well defined concentrations. That reliability is what makes the term best describe this role.

An analytical standard is a broader term for standards used in analysis to calibrate instruments or validate methods; they may be solutions or materials derived from primary standards, but the essential property that defines a primary standard—precise, weighable quantity leading to a known concentration—belongs to the primary standard itself. A secondary standard is a standard whose concentration has been determined by comparison to a primary standard, not the primary reference. An indicator is a substance used to signal the end point of a reaction, not a standard for preparing standard solutions. Potassium hydrogen phthalate, for example, is a classic primary standard because it meets the purity, stability, and weighability criteria needed for accurate standardization.