Dear Kundan Kumar, 

I refer you to two links posted in QA (RG) containing good answers to similar questions as yours:

https://www.researchgate.net/post/How_does_the_addition_of_a_base_cause_Fe3O4_to_precipitate_in_coprecipitation

https://www.researchgate.net/post/What_is_the_effect_of_stirring_time_pH_on_the_synthesis_of_nanoparticle_by_Co-precipitation_method

Hoping this will be helpful,

Rafik

thank you sir

But there was not very definite discussion on the mechanism of ph affecting the co precipitation method? Please explain sir

 Dear Kumar,

For example, in the first link you have two answers which state:

"The use of a base is vital for co-precipitation of Fe2+ and Fe3+ in their mixed solution. It is important to note that the precipitation of Fe3+ occurs at pH 3-4, whereas Fe2+ get precipitated at pH 8-9. Therefore, adequate concentration of  OH- ion (and hence pH) is required for co-precipitation of both Fe3+ and Fe2+ ions in their salt solutions".

"Elimination of water from metallic hydroxides leads to oxides, often by heating. In your case, ferrous hydroxide and ferric hydroxide will be precipitated if we add a base to a solution containing Fe3+ and Fe2+. If we eliminate 4 water molecules from a mixed hydroxide  2Fe(OH)3.Fe(OH)2, one would get Fe3O4. 

For example the precipitation of Fe2+ and Fe3+   together to give 2Fe(OH)3.Fe(OH)2 is only possible at a certain pH as described in the first paragraph. These hydroxides eliminate water to form the oxide derivative Fe3O4. Please note, that there is no possibility to make this oxide from only Fe2+ or Fe3+; both ions should be together and precipitated to the hydroxides which are unstable and eliminate water to furnish the desired iron oxide. 

Hoping this will clarify the role of pH in coprecipitation.

Rafik

thank you sir

For co-precipitation synthesis of nano-particle other than Fe3O4, what should be the optimum pH value to obtain nano-sized particles ????

For example, Bismuth Borate nanoparticles