基准物质Primary standard

组成与化学式完全相符（含结晶水的数目也须恒定）；纯度高；在空气中稳定，不吸湿、不风化、不与 CO₂ 或 O₂ 反应；摩尔质量大，可减小称量的相对误差；反应按确定的计量关系定量进行。不满足这些条件的物质（如 NaOH 易吸水吸 CO₂、HCl 易挥发、KMnO₄ 含杂质会分解、Na₂S₂O₃ 不稳定）只能先配成近似浓度，再用基准物质标定。

Its composition must match its formula exactly, including a constant amount of water of crystallisation; it must be pure; stable in air, neither hygroscopic nor efflorescent, nor reacting with CO₂ or O₂; of large molar mass to reduce relative weighing error; and react by a definite stoichiometry. Substances that fail these tests, such as NaOH (which takes up water and CO₂), HCl (volatile), KMnO₄ (impure and decomposing) and Na₂S₂O₃ (unstable), are made up approximately and standardised against a primary standard.

酸碱滴定：邻苯二甲酸氢钾（KHP，标定 NaOH）、无水碳酸钠和硼砂（Na₂B₄O₇·10H₂O，标定 HCl）；氧化还原滴定：K₂Cr₂O₇、草酸钠（标定 KMnO₄）、KIO₃ 和 KBrO₃（标定 Na₂S₂O₃）、As₂O₃；配位滴定：金属锌、碳酸钙、氧化锌（标定 EDTA）；沉淀滴定：NaCl（标定 AgNO₃）。使用前通常需在规定温度下干燥至恒重。

For acid–base titration: potassium hydrogen phthalate (KHP, for NaOH), anhydrous sodium carbonate and borax (Na₂B₄O₇·10H₂O, for HCl). For redox: K₂Cr₂O₇, sodium oxalate (for KMnO₄), KIO₃ and KBrO₃ (for Na₂S₂O₃) and As₂O₃. For complexometry: zinc metal, calcium carbonate and zinc oxide (for EDTA). For precipitation: NaCl (for AgNO₃). They are usually dried to constant mass at a specified temperature before use.

现代计量体系中，各国计量院（如中国计量科学研究院、美国国家标准与技术研究院 NIST）提供有证标准物质（CRM），具有可溯源到国际单位制的定值和不确定度，用于校准仪器、验证方法和保证检测结果的可比性。

In modern metrology, national institutes such as China's National Institute of Metrology and the US National Institute of Standards and Technology (NIST) supply certified reference materials (CRMs) with values and uncertainties traceable to the SI, used to calibrate instruments, validate methods and keep results comparable.