Take 100 ml water sample in the conical flask of 250ml, add 2 drops of methyl orange indicator and use 0.1 mol/L  NaOH  standard solution for titration, until the solution turns to yellow from red, record  NaOH consumption volume a;
The calculation of acidity is as follows:
                           a*c
        Acidity=                 × 1000   mmol/L
                            V
        Where：
        a— volume of the consumed NaOH standard solution for tritration of acidity, mL.
        c—showing the molar concentration of NaOH,mol/L.
        V—showing the volume of the water sample，ml.