Preparation and Analysis of Multiple Hydrates of Simple Salts

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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.


Originally published as:

Schaeffer, R. W., Chan, B., Marshall, S. R., Blasiole, B., Khan, N., Yoder, K. L., Trainer, M. E., & Yoder, C. H. (2000). Preparation and analysis of multiple hydrates of simple salts. Journal of Chemical Education, 77(4), 509. https://doi.org/10.1021/ed077p509