Measure adequate amount of water sample and put it in the conical flask, dilute it with demineralized water to 100 ml, add 5 ml NaOH solution of 2mol/L, add 0.05 g of calcium red indicator, shake the flask well. Then use 0.05 mol/L EDTA standard solution for titration, until the solution turns blue from wine red, the calculation is as follows:

                                              2a·M
        Calcium hardness =                     ×103   mmol/L
                                                 V
        where：
        a—showing the consumed volume of EDTA standard solution for titration，mL.
        M—showing the molar concentration of EDTA，mol/L
        V— showing the volume of water sample，mL.