Analysis of Alum

This insoluble salt can be readily isolated by filtration, dried without decomposition by heating in a Bunsen flame, and weighed. Even though other negative ions such as the carbonate ion,, also form insoluble barium 

To reduce errors arising from incomplete precipitation, an excess of BaCl2 is added, and the solution is maintained near the boiling point for sufficient time to permit the precipitation to reach equilibrium. Then, cooling of the mixture leads to the quantitative formation of BaS04. The complete equation for the reaction of an aluminum alum solution with a barium chloride solution is 

and Purcell, K. F., chemistry and Chemical Reactivity. Saunders College Publishing, Philadelphia, 1987, sections 2.11, 2.12, 4.1, and 4.2. 

After the mixture has cooled to room temperature and the precipitate has settled, carefully decant (Laboratory Methods H) the clear solution, and discard this clear solution. Next, use ashless filter paper, and filter the remaining mixture of BaS04 and remaining clear solution using either gravity filtration or vacuum filtration (Laboratory Methods I). Use distilled water from your wash bottle to assist transfer of all BaS04 from the beaker onto the ashless filter paper. 

During the filtration, heat a clean crucible and cover, held by a wire triangle (Laboratory Methods D), as hot as possible for approximately 5 minutes. Permit the crucible to cool to room temperature and then weigh it on an analytical balance. Record the mass in TABLE 14.1. 

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Weigh the dry crystals and the watch glass on a beam balance, and record the mass in TABLE 13.1. The crystals of alum can be used for the analysis of alum (Experiment 14). 

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