Standard Solution

Quantitative analysis depends on knowing one concentration exactly, and the standard solution supplies it. Two routes are recognised. A primary standard solution is prepared by weighing a substance that is pure, stable in air, non-hygroscopic and of high molar mass, dissolving it, and diluting to a calibrated volume; the concentration then follows from the mass and the volume alone. A secondary standard solution is made up approximately and its concentration determined afterwards by titration against a primary standard, a procedure called standardisation. Sodium hydroxide is the common example of the second type, since it absorbs atmospheric moisture and carbon dioxide and cannot be weighed reliably. Concentration is normally expressed in moles per litre, and the stated value carries an uncertainty inherited from the balance, the glassware and the endpoint determination.