A standard solution is a solution whose concentration is accurately known.
It can be prepared by dissolving an accurately weighed solid in a solvent and making this up to an exact volume with a volumetric flask.Standard solutions are normally used in titrations to determine unknown information about another substance. Volumetric flasks are very accurate.A 100cm3 volumetric flask has a tolerance of ± 0.20cm3 and a 250cm3 volumetric flask has a tolerance of ± 0.30cm3.Primary standards should ideally be:• solids with high molar masses• be available to a high degree of purity• be chemically stable• not absorb water from the atmosphere• be soluble in water• react rapidly with other substances used in titrations.
Hydrochloric acid is not suitable because hydrogen chloride is a gas that escapes from the solution, so its concentration changes. Sodium hydroxide is hygroscopic (absorbs water from the atmosphere) and absorbs carbon dioxide from the atmosphere.