K2Cr2O7 + 7H2SO4 + 6FeSO4 = K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H20.
This reaction is often employed in the estimation of iron in iron ores.
Potassium chrome alum. It will be noticed that the oxidizing action of potassium dichromate leaves potassium sulphate and chromium sulphate as the products of the reaction. On evaporating the solution these substances crystallize out as potassium chrome alum, which substance is produced as a by-product in the industries using potassium dichromate for oxidizing purposes.
Chromic anhydride (CrO3). When concentrated sulphuric acid is added to a strong solution of potassium dichromate, and the liquid allowed to stand, deep red needle-shaped crystals appear which have the formula CrO3.This oxide of chromium is called chromic anhydride, since it combines readily with water to form chromic acid:
CrO3 + H2O = H2CrO4.
It is therefore analogous to sulphur trioxide which forms sulphuric acid in a similar way:
SO3 + H2O = H2SO4.
Chromic anhydride is a very strong oxidizing agent, giving up oxygen and forming chromic oxide:
2CrO3 = Cr2O3 + 3O.
Rare elements of the family. Molybdenum, tungsten, and uranium are three rather rare elements belonging in the same family with chromium, and form many compounds which are similar in formulas to the corresponding compounds of chromium. They can play the part of metals and also form acids resembling chromic acid in formula. Thus we have molybdic acid (H2MoO4), the ammonium salt of which is (NH4)2MoO4. This salt has the property of combining with phosphoric acid to form a very complex substance which is insoluble in nitric acid. On this account molybdic acid is often used in the estimation of the phosphoric acid present in a substance. Like chromium, the metals are difficult to prepare in pure condition. Alloys with iron can be prepared by reducing the mixed oxides with carbon in an electric furnace; these alloys are used to some extent in preparing special kinds of steel.