Primary standard 2026-10-18
A primary standard supplies a reliably known reacting inventory from which a titrant concentration can be determined.
The defining advantage is known high purity: the weighed portion can be assigned a known amount of substance. A titrant whose concentration is established against that inventory is a secondary standard solution in IUPAC terminology.[ref]
standard solution. IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry; Gold Book, standard solution, S05924; definitions of primary standard and secondary standard solution; underlying source Orange Book, second edition, p. 48
An accurate balance determines total mass, including any unwanted water or other material. To infer reacting inventory, one also needs known composition, stability during storage and weighing, complete transfer and dissolution, and a sufficiently complete chemical reaction of known stoichiometry. A finely resolved balance cannot establish these chemical facts. Potassium hydrogen phthalate is useful because a suitably prepared portion provides a known acid inventory for sodium hydroxide titrant standardization; the KHP standardization protocol below shows how that claim is put into practice.
Preparation belongs to the particular material and method. For example, the MIT handout specifies drying potassium hydrogen phthalate at for hours or overnight, cooling in a desiccator, and weighing by difference; its footnote identifies removal of superficial moisture as the purpose. This is a condition of that potassium hydrogen phthalate procedure, not a universal instruction to heat every primary standard in the same way.[ref]
The Potentiometric Titration of an Acid Mixture (MIT 5.310 laboratory handout). Printed Titration-5, §IV.2 and footnote 5: drying, cooling, weighing by difference, and superficial moisture