In the determination of
silicon content in an organic sample, a small and accurately-measured quantity of a substance is added to the large amount of
sulfuric acid, then heated in an evaporating dish. The dish is heated with a
Bunsen burner, until only stable precipitate remains, which contains the silica content. The dish is then closed and heated at high temperature until completely clean, fused silica is produced. Comparison of the initial weight of the substance and that of the fused silica allows the content of silicon in the sample to be determined. The shape of the evaporating dish encourages evaporation in two ways: • The shell is relatively flat. A relatively large liquid surface promotes evaporation. • If heated in a flask or beaker, a part of the evaporated liquid condenses on the vessel walls and flows back into the solution. This does not happen in a dish. When heating liquid in an evaporating dish, the low walls encourage splashes and so stirring or swirling of evaporating liquids is considered bad practice, owing to the risk of spillage. Evaporation, especially in production quantities rather than merely for analysis, is now mostly performed in a
rotary evaporator. This is preferred because it works much faster and may be used under vacuum, avoiding unwanted reactions with the atmosphere and allowing control of noxious fumes. Evaporation under vacuum also reduces the severity of
bumping and violent ebullition. == See also ==