Preparation and Analysis of Multiple Hydrates of Simple Salts

We have developed a laboratory project in which the student prepares a series of hydrates of simple salts and then determines the extent of hydration of the product(s). We believe this provides a good introduction to the concepts of solubility, saturation, recrystallization, relative compound stability (e.g., a dihydrate vs tetrahydrate at elevated temperature), and simple gravimetric analysis. Moreover, the project lends itself to many variations. For example, a student could be given a "starting" hydrated salt and asked to prepare another hydrate within a specified temperature range. Or students could be given the formulas of several hydrates stable over different temperature ranges and be asked to "discover" a method of preparation. If it is deemed desirable to extend the project, the cation and/or anion could be determined quantitatively.

The preparation of ionic hydrates is accomplished by four methods: (i) slow evaporation of the solvent from a near saturated solution of the starting hydrate at a temperature within the stability range of the desired hydrate, (ii) crystallization within the temperature range of the target hydrate from a saturated solution prepared at higher temperatures, (iii) crystallization within the temperature stability range from mixed solvents, and (iv) heating a higher hydrate to the temperature range of the desired lower hydrate. Analysis for water of hydration content is performed gravimetrically by gently heating the sample in a Bunsen flame. Students were able to produce results generally within 1-5% of the theoretical.