The acid may be either hydrochloric or sulphuric, but the former is usually preferred, since it is by far the best solvent for iron and its compounds.  The reaction in the presence of hydrochloric acid is as follows: 

6FeCl_{2} + K_{2}Cr_{2}O_{7} + 14HCl —­> 6FeCl_{3} + 2CrCl_{3} + 2KCl + 7H_{2}O.

NORMAL SOLUTIONS OF OXIDIZING OR REDUCING AGENTS

It will be recalled that the system of normal solutions is based upon the equivalence of the reagents which they contain to 8 grams of oxygen or 1 gram of hydrogen.  A normal solution of an oxidizing agent should, therefore, contain that amount per liter which is equivalent in oxidizing power to 8 grams of oxygen; a normal reducing solution must be equivalent in reducing power to 1 gram of hydrogen.  In order to determine what the amount per liter will be it is necessary to know how the reagents enter into reaction.  The two solutions to be employed in the process under consideration are those of potassium bichromate and ferrous sulphate.  The reaction between them, in the presence of an excess of sulphuric acid, may be expressed as follows: 

6FeSO_{4} + K_{2}Cr_{2}O_{7} + 7H_{2}so_{4} —­> 3Fe_{2}(so_{4})_{3} + K_{2}so_{4} + Cr_{2}(so_{4})_{3} + 7H_{2}O.

If the compounds of iron and chromium, with which alone we are now concerned, be written in such a way as to show the oxides of these elements in each, they would appear as follows:  On the left-hand side of the equation 6(FeO.So_{3}) and K_{2}O.2CrO_{3}; on the right-hand side, 3(Fe_{2}O_{3}.3So_{3}) and Cr_{2}O_{3}.3So_{3}.  A careful inspection shows that there are three less oxygen atoms associated with chromium atoms on the right-hand side of the equation than on the left-hand, but there are three more oxygen atoms associated with iron atoms on the right than on the left.  In other words, a molecule of potassium bichromate has given up three atoms of oxygen for oxidation purposes; i.e., a molecular weight in grams of the bichromate (294.2) will furnish 3 X 16 or 48 grams of oxygen for oxidation purposes.  As this 48 grams is six times 8 grams, the basis of the system, the normal solution of potassium bichromate should contain per liter one sixth of 294.2 grams or 49.03 grams.

A further inspection of the dissected compounds above shows that six molecules of FeO.So_{3} were required to react with the three atoms of oxygen from the bichromate.  From the two equations

3H_{2} + 3O —­> 3H_{2}O 6(FeO.So_{3}) + 3O —­> 3(Fe_{2}O_{3}.3So_{3})

it is plain that one molecule of ferrous sulphate is equivalent to one atom of hydrogen in reducing power; therefore one molecular weight in grams of ferrous sulphate (151.9) is equivalent to 1 gram of hydrogen.  Since the ferrous sulphate crystalline form has the formula FeSO_{4}.7H_{2}O, a normal reducing solution of this crystalline salt should contain 277.9 grams per liter.