. Preparation of 0.05 M Standard Calcium Solution. - Primary standard calcium carbonate (CaCO3) is dried at 100 C for 1 hr and allowed to cool to room temperature in the desiccator. Approximately 2.5 g ( 0.1 mg) of the previously dried CaCO3 is weighted out by difference into a 400-mL beaker. The CaCO3 is dissolved in a minimum amount of 6 M HCl, transferred quantitatively to a 500-mL (+ 0.20 mL, for class A) volumetric flask, and diluted to exact volume with deionized water. The exact [Ca2+ ] may be calculated from the exact mass of CaCO3 used.