To prepare very pure, formula-weight borax as a primary standard, good quality commercial product is recrystallized three times from water and dried to constant weight in a vacuum desiccator over a desiccant with suitable vapor pressure. Recent studies have shown that the best desiccant consists of solid NaCl, sucrose and a saturated sucrose solution. The correct water content (47.21%) is achieved by drying a sample in a Pt crucible, first on a steam bath, then at 200°C, and finally between 700-800°C. To check for any further impurities, cf. I. M. Kolthoff, Gravimetric Analysis, II (pp. 97-98).
