M9 medium is a minimal medium commonly used in the fermentation laboratory for growing bacteria such as Escherichia coli. It is easy to prepare since it only consists of several mineral salts and sugar. However, one problem of this medium is the salt precipitation.
The precipitation is from calcium sulfate formed when calcium chloride reacts with magnesium sulfate when the calcium is added to the solution containing sulfate ions. To avoid this, calcium chloride should be added to demineralized water before adding the other salts. This allow the calcium to be dissolved homogeneously at a very low concentration; thus when sulfate ions are added later on the formed calcium sulfate concentration is still below its solubility at the room temperature.