primary standard
A primary standard is the gold-standard chemical you can trust so completely that you build everything else on it — like a master key cut so precisely that all other keys are checked against it. Because you can rely on it absolutely, you can make a standard solution simply by weighing it out, with no further checking needed.
To earn the title, a substance must be exceptionally well-behaved: extremely pure and of known composition, stable so it does not change while sitting on the balance or the shelf, free of water it might gain or lose, and ideally fairly heavy per mole so that small weighing errors matter less. Few chemicals qualify; well-known examples include potassium hydrogen phthalate and sodium carbonate.
It matters because a chain of measurements is only as trustworthy as its first link. The primary standard is that first link — the anchor that ties laboratory numbers back to a defined amount of a known substance. Everything downstream, including secondary standards and routine reagents, ultimately traces back to one.
Potassium hydrogen phthalate (KHP) is dried, weighed exactly, and dissolved to make a standard acid solution — which is then used to standardize a sodium hydroxide solution that cannot be weighed reliably.
A primary standard is pure and stable enough to trust by weight alone.
Sodium hydroxide and hydrochloric acid are not primary standards: NaOH absorbs water and carbon dioxide from the air, and concentrated HCl loses gas. Both must be standardized against a true primary standard before their concentration can be trusted.