This double salt of ferrous sulfate and ammonium sulfate is a laboratory reagent. Ferrous ammonium sulfate, not ferrous ammonium sulphate as it is sometimes referred to, dissolves in water and possesses an octahedral molecular geometry when formed as an aquo complex. It derives its name from the German chemist Karl Friedrich Mohr, a 19th century pioneer in the development of titration methodology. Mohr’s salt is used in analytical chemistry as the preferred source of ferrous ions due to the solid’s extended shelf life and resistance to oxidation. Sulfuric acid is often added to solutions using ferrous ammonium sulfate in order to reduce the oxidation of the ferric iron. The most common method of preparation for ferrous ammonium sulfate is to dissolve an equimolar mixture of hydrated ferrous sulfate and ammonium sulfate in water with a small amount of sulfuric acid, then subjecting the combined solution to crystallization. Ferrous ammonium sulfate is typically comprised of light green crystals in solid form.