Precipitation Starting
Homogenous Solution

In this process, the precipitating ion is not included into the solution but rather is brought about all through the solution by a homogenous chemical reaction. This is the definitive in precipitation procedure. In view of the fact that the precipitating agent is brought about in an even way throughout the solution, local excess of the precipitating agent are steered clear of. In precipitation from homogenous solution the supersaturation is maintained very low at all times and this results in a very pure, dense precipitate. Frequently substances that form only as unstructured solids will precipitate as well formed crystalline solids by means of this method. The most important techniques of homogenous precipitation can be classified in this manner;

• Increase in pH.
• In most cases the pH is made more alkaline by hydrolysis of urea in boiling aqueous solution. The ammonia that is little by little set free increases the pH of the solution in a consistent way, making metal ions that appear insoluble hydroxides or hydrous oxides to precipitate. In the precipitation of aluminium from homogenous solution, urea is placed into an acidic solution of aluminium that has some sulphuric or succinic acid. No precipitation takes place until the solution has been heated long enough for the ammonia to increase the pH to the required value. In this course of action aluminium precipitates as the basic sulphate or the basic succinate and not as aluminium hydroxide. The precipitate that is attained in this form is a great deal denser and has less impurities than are aluminium precipitates produced by the normal adding of ammonia to aluminium solutions.

One other instance is the precipitation from homogeneous solution of barium chromate. Chromate is included to barium in a solution that has enough acid in order to put off precipitation. Urea is included and the solution is then boiled. The ammonia that is let out increases the solution pH and barium chromate little by little precipitates out.

• Release of anion
• Release of cation
• Precipitation from mixed solvents
• Change in valency