1. the precipitated material must have a very low solubility in water; i.e., its Kap must be a very small number.
2. it must precipitate in a high state of purify or be capable of reprecipitation for further purification.
3. it must be capable of drying or of ignition at temperatures above 100 to a definite compound of fixed composition.
4. it should not be hydroscopic at room temperature.