Definition - What does Primary Standards mean?
A primary standard is a measurement that is used in the calibration of working standards. A primary standard is chosen owing to its accuracy and its stability when exposed to other substances. Primary standards can be measured in terms of metrics including length, time or mass.
Safeopedia explains Primary Standards
Primary standards are widely used in analytical chemistry. In this instance, a reagent that is easily weighed, has low reactivity with other substances, is pure and unlikely to change in weight when exposed to humid conditions and has a high equivalent weight is chosen. Less important factors that are taken into account include the availability of the substance chosen as primary standard and low toxicity levels to reduce risk. Titration acids and redox titrations are examples of primary standards.