Basicity refers to the equivalent percentage of OH and Al in polyaluminum chloride. B=〔OH〕/〔Al〕× 100%。 Polyaluminum chloride can be understood as the hydrolysis product between normal salt Alcl3 and Al(OH)3. Therefore, in an ideal situation, the alkalinity of polyaluminum chloride is 16.7% to 83.3%, but there are impurities in industrial products, which affect the determination of alkalinity, so that the determination of alkalinity can reach more than 90%, even 100%, which is the error of the determination method and the actual situation is impossible.