Jar Test:
Hydroxide [e.g. Al(OH)3, Fe(OH)3] is the desired end product of coagulation. It is insoluble, floc-forming and heavier than water and it carries the positive electric charge necessary to neutralize the negative charges of the colloidal particles. Further reason for the addition of some form of alkali are to establish the optimum pH value at which coagulation can take place and to raise the final pH value after treatment to reduce corrosiveness.
Where floc formation is poor, coagulants aids are added. By producing a heavier, faster setting floc, this allows smaller basins to be used and smaller doses of the main coagulant may also be possible. The experiment to find out optimum dose of coagulant is known as Jar Test.