Abstract: Based on dissociation equilibrium of the chromic acid and the re-lationship between the transformation of the chromic acid and dichromic acid ions, combined with the requirements of the endpoint of titration for color change, the pH’s upper limit of the acidity in Mohr method was worked out to be6. 36. However, considering the impact of various factors such as error of each parameter and activity, the pH’s upper limit of the acidity in Mohr method is usually taken as6. 5. On this basis, the specific dosage of indicator under different acidity conditions was determined by the relation between chromate and acidity. And this can provide elaborate theoretical basis for titration operation.