secondary standard
A secondary standard is a solution whose concentration you know well, but only because you checked it against a primary standard — not because you could simply weigh it out. Think of a wristwatch set precisely against the national atomic clock: the watch is now trustworthy, yet its accuracy is borrowed, traced back to a better reference.
Concretely, you prepare a solution of a substance that cannot be weighed reliably — perhaps it absorbs moisture, slowly decomposes, or comes as a concentrated liquid — so its exact concentration is unknown at first. You then standardize it by reacting it against a primary standard, which fixes the true value. From then on, the solution serves as a known reference.
It matters because many of the most useful reagents in a lab — sodium hydroxide, hydrochloric acid, many titrants — are not primary-standard-quality. The secondary standard lets you still work with them quantitatively, with confidence, by anchoring their value to something more fundamental. The catch: that value can drift over time, so secondary standards often need restandardizing.
A sodium hydroxide solution is made up to roughly 0.1 mol/L, then titrated against weighed potassium hydrogen phthalate. The titration fixes its true concentration at 0.1023 mol/L, making it a secondary standard.
Its concentration is trustworthy because it was tied back to a primary standard.
Secondary standards inherit, but never improve on, the accuracy of the primary standard they were checked against — and they can degrade afterward. Solutions like dilute NaOH should be restandardized periodically rather than assumed unchanged.