Ferron dosage(［Ferron］) has a significant effect on the accurate determination of Al3+ concentration and its speciation by Ferron colorimetric method.However,the importance of this most critical factor has not been clearly realized.Different Ferron concentrations have been used in Al-Ferron system by researchers from different laboratories.Various molar absorption coefficient ε values for different Ferron concentrations have been obtained,which have affected the accuracy for determination of Al speciation in polyaluminum chloride(PAC) solution unavoidably.Systematic and comprehensive studies were conducted in this work.It is found that the molar absorption coefficient ε increases in exponential form with the increase of ［Ferron］ in mononuclear Al solution.ε-could be regarded as the maximum value at a plateau from “ε-［Ferron］” curves when ［Ferron］ reaches to 2.0×10-3 mol/L.The suitable ［Ferron］ range is from 2.0×10-3 mol/L to 3.0×10-3 mol/L,and 2.0×10-3 mol/L is the recommended optimal dosage achieving the most sensitive and accurate results.