Approximately 18.5 g (± .5 g) of Na2C10H14N2O8 • 2 H2O (disodium EDTA) and 3 or 4 drops of 3.0 M MgCl2 are placed in a clean 400-mL beaker. The masses here need not be analytical balance accuracy since the resulting solution will be standardized. (About 200 mL of deionized water are added to dissolve the edta salt.