Composition of the Saturated Solutions of Ferric Chloride and its Hydrates.
(The name placed at the head of each table is the solid phase.)
| Ice. | |
| Temperature. | Composition. |
| ±-55° | ±2.75 |
| -40° | 2.37 |
| -27.5° | 1.90 |
| -20.5° | 1.64 |
| -10° | 1.00 |
| 0° | 0 |
| Fe2Cl6,12H2O. | |
| Temperature. | Composition. |
| -55° | ±2.75 |
| -41° | 2.81 |
| -27° | 2.98 |
| 0° | 4.13 |
| 10° | 4.54 |
| 20° | 5.10 |
| 30° | 5.93 |
| 35° | 6.78 |
| 36.5° | 7.93 |
| 37° | 8.33 |
| 36° | 9.29 |
| 33° | 10.45 |
| 30° | 11.20 |
| 27·4° | 12.15 |
| 20° | 12.83 |
| 10° | 13.20 |
| 8° | 13.70 |
| Fe2Cl6,7H2O. | |
| Temperature. | Composition. |
| 20° | 11.35 |
| 27·4° | 12.15 |
| 32° | 13.55 |
| 32.5° | 14.29 |
| 30° | 15.12 |
| 25° | 15.54 |
| Fe2Cl6,5H2O. | |
| Temperature. | Composition. |
| 12° | 12.87 |
| 20° | 13.95 |
| 27° | 14.85 |
| 30° | 15.12 |
| 35° | 15.64 |
| 50° | 17.50 |
| 55° | 19.15 |
| 56° | 20.00 |
| 55° | 20.32 |
| Fe2Cl6,4H2O | |
| Temperature. | Composition. |
| 50° | 19.96 |
| 55° | 20.32 |
| 60° | 20.70 |
| 69° | 21.53 |
| 72.5° | 23.35 |
| 73.5° | 25.00 |
| 72.5° | 26.15 |
| 70° | 27.90 |
| 66° | 29.20 |
| Fe2Cl6 (ANHYDROUS). | |
| Temperature. | Composition. |
| 66° | 29.20 |
| 70° | 29.42 |
| 75° | 28.92 |
| 80° | 29.20 |
| 100° | 29.75 |
The lowest portion of the curve, AB, represents the equilibria between ice and solutions containing ferric chloride. It represents, in other words, the lowering of the fusion point of ice by addition of ferric chloride. At the point B (-55°), the cryohydric point (p. [117]) is reached, at which the solution is in equilibrium with ice and ferric chloride dodecahydrate. As
has already been shown, such a point represents an invariant system; and the liquid phase will, therefore, solidify to a mixture of ice and hydrate without change of temperature. If heat is added, ice will melt and the system will pass to the curve BCDN, which is the solubility curve of the dodecahydrate. At C (37°), the point of maximum temperature, the hydrate melts completely. The retroflex portion of this curve can be followed backwards to a temperature of 8°, but below 27.4° (D), the solutions are supersaturated with respect to the heptahydrate; point D is the eutectic point for dodecahydrate and heptahydrate. The curve DEF is the solubility curve of the heptahydrate, E being the melting point, 32.5°. On further increasing the quantity of ferric chloride, the temperature of equilibrium is lowered until at F (30°) another eutectic point is reached, at which the heptahydrate and pentahydrate can co-exist with solution. Then follow the solubility curves for the pentahydrate, the tetrahydrate, and the anhydrous salt; G (56°) is the melting point of the former hydrate, J (73.5°) the melting point of the latter. H and K, the points at which the curves intersect, represent eutectic points; the temperature of the former is 55°, that of the latter 66°. The dotted portions of the curves represent metastable equilibria.
As is seen from the diagram, a remarkable series of solubility curves is obtained, each passing through a point of maximum temperature, the whole series of curves forming an undulating "festoon." To the right of the series of curves the diagram represents unsaturated solutions; to the left, supersaturated.
If an unsaturated solution, the composition of which is represented by a point in the field to the right of the solubility curves, is cooled down, the result obtained will differ according as the composition of the solution is the same as that of a cryohydric point, or of a melting point, or has an intermediate value. Thus, if a solution represented by x1 is cooled down, the composition will remain unchanged as indicated by the horizontal dotted line, until the point D is reached. At this point, dodecahydrate and heptahydrate will separate out, and the liquid will ultimately solidify completely to a mixture or "conglomerate" of these two hydrates; the temperature of
the system remaining constant until complete solidification has taken place. If, on the other hand, a solution of the composition x3 is cooled down, ferric chloride dodecahydrate will be formed when the temperature has fallen to that represented by C, and the solution will completely solidify, without alteration of temperature, with formation of this hydrate. In both these cases, therefore, a point is reached at which complete solidification occurs without change of temperature.
Somewhat different, however, is the result when the solution has an intermediate composition, as represented by x2 or x4. In the former case the dodecahydrate will first of all separate out, but on further withdrawal of heat the temperature will fall, the solution will become relatively richer in ferric chloride, owing to separation of the hydrate, and ultimately the eutectic point D will be reached, at which complete solidification will occur. Similarly with the second solution. Ferric chloride dodecahydrate will first be formed, and the temperature will gradually fall, the composition of the solution following the curve CB until the cryohydric point B is reached, when the whole will solidify to a conglomerate of ice and dodecahydrate.