Formalin is a chemical and very effective.It use for storage of our sample.It not use when large time storage require.It only use for short time storage.Because formalin is too reactive and damage the sample.It acidic in nature and denature the sample.Formalin not use for all animals preservation.It just use for vertebrates. It not easy to handle because of their nature.

First kill the animal relax for some time after relaxation put in to bottle or jar and add formalin 100% and 40% water.Fill the bottle two or third part with formalin.Than add some amount of sodium bicarbonate.