Evaporating Liquids

 It is often necessary to reduce the volume of a solution that contains a nonvolatile solute. Figure 1. illustrates how this procedure is accomplished. The ribbed cover glass permits vapors to escape and protects the remaining solution from accidental contamination.

Evaporation is frequently difficult to control because of the tendency of some solutions to overheat locally. The bumping that results can be sufficiently vigorous to cause partial loss of the solution. Careful and gentle heating will minimize the danger of such loss. Glass beads may also minimize bumping if their use is permissible. Some unwanted substances can be eliminated during evaporation. For example, chloride and nitrate can be removed from a solution by adding sulfuric acid and evaporating until copious white fumes of sulfur trioxide are observed (this operation must be performed in a hood). Urea is effective in removing nitrate ion and nitrogen oxides from acidic solutions. Ammonium chloride is best removed by adding concentrated nitric acid and evaporating the solution to a small volume.

Ammonium ion is rapidly oxidized when it is heated. The solution is then evaporated to dryness. Organic constituents can frequently be eliminated from a solution by adding sulfuric acid and heating to the appearance of sulfur trioxide fumes (in a hood). This process is known as wet ashing. Nitric acid can be added toward the end of heating to hasten oxidation of the last traces of organic matter.

Figure 1. Arrangement for the evaporation of a liquid.