分别配制0.2mol/L的Na2HPO4溶液和0.2mol/L的NaH2PO4溶液(具体步骤:称取Na2HPO4.12H2O 晶体7.16g溶解,用100ml容量瓶定容,得0.2mol/L的Na2HPO4溶液,称取NaH2PO4.2H2O晶体3.12g溶解,用100ml容量瓶定容,得0.2mol/L的NaH2PO4溶液)取0.2mol/L的Na2HPO4溶液81ml,再取0.2mol/L的NaH2PO4溶液19ml混合,即得pH7.4的0.2 mol/L磷酸缓冲液不需要PH计