Step 6 Filtration with Suction (Collection of the Solid)
The Erlenmeyer contains the desired solid and the solvent with a small amount of the desired solid still dissolved in the solvent. The liquid in contact with the solid is called the mother liquor and the solid—liquid mixture is called a slurry. This slurry is poured through a filter, which catches the solid and allows the mother liquor to pass through. If the desired solid contained a small amount of impurity, which is soluble in the solvent, that impurity will remain in the mother liquor. You will use either a Hirsh or Büchner funnel to collect the solid. If you have less than 0.5 g of solid, you will use the Hirsh funnel. If you have a half-gram or more, you will use the Büchner funnel. The instructor will explain how to set up and use these funnels. The solid is then washed. Washing a solid is a technique in which a small amount of cold liquid is poured over the solid, while the solid remains on the filter. Washing results in the loss of solid, so only very small amounts of cold liquid should be used in the washing process. As much liquid as possible should be squeezed out of the solid. This is accomplished by pressing down on the solid with a piece of clean filter paper. When as much liquid as possible has been removed from the solid, the solid is ready for the next step.