Question

4) When the excess iron (III) ions from the coagulant were precipitated as iron (III) hydroxide, an excess of hydroxide ion was added deliberately so that the solution was basic during precipitation. Why ?

Answer 1

When the excess iron (III) ions from the coagulant were precipitated as iron (III) hydroxide, an excess of hydroxide ion was added deliberately so that the solution was basic during precipitation because we want to shift the equillibrium in the forward direction that is to make more iron (III) hydroxide by providing more reagent . This follows Le Chatelier principle which state any change in the equillibrium will shift it into that direction which nulify the change occured.

at last we want that all Fe(III) should be precipitate as Fe(OH)3 thats why we provide excess hydroxide ion .