Primary Standard
Definition and meaning of Primary Standard in chemistry.
A primary standard is a chemical substance of known high purity that is used to prepare solutions of exactly known concentration for use in titrations and other quantitative analyses.
In more detail
Primary standards must meet strict criteria: they should be at least 99.9% pure, stable during storage, non-hygroscopic (they should not absorb water from the air), and have a relatively high molar mass to minimize errors when weighing. Examples include potassium dichromate (K2Cr2O7) for redox titrations and potassium hydrogen phthalate (KHC8H4O4) for acid-base titrations. Unlike secondary standards, primary standards can be directly weighed and used to prepare a standard solution without first being standardized against another substance, making them essential for establishing the accuracy of all subsequent analyses.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Purity Requirement | Minimum 99.9% |
| Key Properties | Stable, non-hygroscopic, high molar mass |
| Common Examples | Potassium dichromate (K2Cr2O7), potassium hydrogen phthalate (KHC8H4O4), sodium carbonate (Na2CO3) |
In an acid-base titration, potassium hydrogen phthalate (KHC8H4O4), a primary standard, is dissolved in water to prepare a solution of precisely known concentration. This solution is then used to standardize (determine the exact concentration of) a sodium hydroxide solution of unknown concentration.
Frequently asked questions
How is a primary standard different from a secondary standard?
A primary standard is of certified high purity and can be directly weighed to prepare a solution of known concentration. A secondary standard must be standardized (calibrated) against a primary standard first.
Why is purity so critical for primary standards?
High purity ensures that the measured mass corresponds directly to the actual substance rather than impurities, so any calculated concentration will be accurate.