Standard solutions are prepared in their approximate concentrations and are then standardised against recognised primary standards. The primary standards are all very pure chemicals that can be further purified by drying in an oven and cooling down in a vacuum desiccator over phosphorus pentoxide. Primary standards for acids, alkalis and oxidising agents are anhydrous sodium carbonate, potassium hydrogen phthalate anf potassium iodate.
In industry you use these so frequently that you get to know their molecular weights by heart. Once you have standardised solutions of acid, alkali and sodium thiosulfate you use these to standardise other working solutions, so that you have a hierarchy of standards, with the primary standards being the most reliable.
