主题：【求助】哪个高人按照美国药典做过EDTA-2Na0.05M的标定请帮帮我?!

最近一直按照美国药典做EDTA-2Na0.05M的标定,可是由于其指示剂的问题滴定结果总是偏高,请做过这个实验高人给点指导,先谢谢了!
以下是美国药典29版中这一部分的滴定方法:
Edetate Disodium, Twentieth-Molar (0.05 M)
C10H14N2Na2O8·2H2O, 372.24
18.61 g in 1000 mL
Dissolve 18.6 g of edetate disodium in water to make 1000 mL, and standardize the solution as follows.
Accurately weigh about 200 mg of chelometric standard calcium carbonate, previously dried at 110 for 2 hours and cooled in a desiccator, transfer to a 400-mL beaker, add 10 mL of water, and swirl to form a slurry. Cover the beaker with a watch glass, and introduce 2 mL of diluted hydrochloric acid from a pipet inserted between the lip of the beaker and the edge of the watch glass. Swirl the contents of the beaker to dissolve the calcium carbonate. Wash down the sides of the beaker, the outer surface of the pipet, and the watch glass with water, and dilute with water to about 100 mL. While stirring the solution, preferably with a magnetic stirrer, add about 30 mL of the edetate disodium solution from a 50-mL buret. Add 15 mL of sodium hydroxide TS and 300 mg of hydroxy naphthol blue, and continue the titration with the edetate disodium solution to a blue endpoint.