Transition Points in Salts

Ammonium nitrate exists in four different forms, all of which are enantiotropic the change of white phosphorus into the red (or violet) variety is monotropic. Mercuric iodide exhibits a striking example of an enantiotropic transition. Above 126.3°, it is obtained in yellow rhombic crystals while below that temperature, a scarlet tetragonal modification appears. 

In crystallizing salts from solution, it frequently happens that it is possible to obtain more than one hydrate. In all such cases, a perfectly definite temperature can be found above which the one hydrate will deposit, and below which the other one with a larger number of molecules of water of hydration appears. Thus, above 38°, zinc sulfate deposits crystals of the composition ZnS04 6H20, while crystals deposited below that temperature have the formula ZnS04-7H20. This is also called a transition point. 

Hydrates are merely one form of a double salt, and it often happens that other double salts as well as hydrates exhibit transition or decomposition points. Salts, whether simple or double, that have transition points at which color changes occur are of special interest. Mercuric iodide is a striking example of such a simple salt and is easily prepared by double decomposition. The preparation of a few other examples will be described. 

References on Transition Points Textbooks on physical chemistry Findlay, The Phase Rule, 32, 227, 279, 6th ed. (1927) Friend, I, 67 Mellor, I, 52. 

Prepare a solution of 6.8 g. of mercuric chloride in about 50 cc. of water and a second solution of 18.3 g. of 

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