High-purity stable compounds, such as potassium hydrogen phthalate, used to prepare standard solutions.
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A primary standard is a highly pure compound, usually at least 99.9 percent pure, whose composition is known exactly and which can be weighed accurately to prepare a solution of known concentration. Good primary standards are stable in air, do not absorb water or carbon dioxide, resist oxidation and have a high molar mass to minimize weighing error. Examples include potassium hydrogen phthalate, anhydrous sodium carbonate, potassium dichromate and oxalic acid dihydrate. Sodium hydroxide and potassium permanganate are not primary standards, because the first is hygroscopic and the second is hard to obtain pure and reacts slowly with water. Primary standards are used to standardize other solutions by titration.
Key ideas: Requirements: high purity, stable composition, non-hygroscopic, high molar mass, soluble, and reacts rapidly and completely. Common standards: KHP for bases, Na2CO3 for acids, K2Cr2O7 and Na2C2O4 for redox, NaCl and AgNO3 for halide titrations, and zinc or CaCO3 for EDTA. Dry the solid in an oven first and cool it in a desiccator.
Common mistakes and connections: Students treat NaOH, HCl or KMnO4 as primary standards because they are common reagents, but they absorb CO2 or water, are volatile or decompose. A solid with a low molar mass magnifies weighing error, so the high molar mass of KHP (204.2 g/mol) helps. Related to standard solutions, titrimetric analysis and uncertainty.
Primary standards are pure, stable and weighable; a high molar mass such as KHP at 204.2 g/mol makes the relative weighing error small. The interactive lesson shows this as a live animation or chart.
Potassium hydrogen phthalate (204.2 g/mol) is weighed out to standardize sodium hydroxide.
High-purity stable compounds, such as potassium hydrogen phthalate, used to prepare standard solutions.
Primary Standards is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Analytical, Lab & Spectroscopy first: Theoretical and Percent Yield, Concentration Calculations, Standard Solutions.
The next topics in Analytical, Lab & Spectroscopy are Volumetric Glassware and Filtration.
Yes. The interactive lesson includes a self-check question on Primary Standards, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Primary Standards is topic 19 of 99 in Analytical, Lab & Spectroscopy.