The highest-quality standard in metrology or chemistry
A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working standards.[1][2] See Hierarchy of Standards.
^Skoog, Douglas A., Donald M. West and F. James Holler. "Fundamentals of Analytical Chemistry 8th ed." Harcourt Brace College Publishers. 1995 ISBN0-03-035523-0
^Holt Science and Technology: Physical Science. Ed. Rinehart and Winston, Inc. Holt. Holt McDougal (July 2000). ISBN978-0-03-051957-4.