Equilibrium between ferric chloride, hydrochloric acid, and water

Equilibrium between Ferric Chloride Hydrochloric Acid, and Water, 

The distinction between the relations for equilibrium in this case and that previously described, and consequently also the special interest of the phenomena, lies in the noteworthy possibilities attending complete fusion. This has already been pointed out in the case of calcium chloride. 

Here we have to study the same thing in the more complicated instance of a triple system. It is convenient to start in such a case with combinations of two at a time. Previously these were combinations of water and potassium sulphate on the one side, and water and magnesium sulphate on the other here they are water and ferric chloride on one side, and water and hydrochloric acid on the other. 

Let us represent the equilibrium between water and ferric cliloride in Fig. 38, in the upper half, as the solubility curve for ferric chloride. It consists of five parts, relating to Fe,Cle. I2H,0, Fe Cle. 72 0, Fe Cl. 5H,0, Fe.Cle. 4H2O, and anhydrous FCgCIg respectively. Each part consists of two branches leading backwards, whose common point has a vertical tangent, and corresponds to a composition of the solution which is identical with that of the hydrate 

The solubility of the hydrates of hydrochloric acid existing at low temperatures may be drawn in the under part of Fig. 38. We may here also start from the point 0° for pure water, and going along the curve of freezing points of solutions of hydrochloric acid reach the cryohydric point for HCI.3H2O at b then follow the solution of that hydrate, b c d, which at c has the same composition as the solid hydrate and ends at d with separation of the hydrate HCl. 2H2O then the curve for saturation 

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