Decolouration
Pure benzoic acid is colourless, so a coloured solution indicates that treatment with decolourizing carbon is necessary.
Caution: Do not add decolourizing carbon to a boiling solution! Cool the solution slightly, add about 0.5 g of carbon, and reheat to boiling for a few minutes.
Hot filtration and crystallization
1. If there are insoluble impurities or decolorizing carbon in the solution, perform a hot filtration using a 100-mL beaker to receive the filtrate.
2. Rinse the empty flask with about 1 mL of hot water and filter this rinse into the original filtrate. If the filtrate remains colored, repeat the treatment with decolorizing carbon.
3. Cover the opening of the flask with a piece of filter paper, an inverted beaker, or loose-fitting cork to exclude airborne impurities from the solution, and allow the filtrate to stand undisturbed until it has cooled to room temperature and no more crystals form.
4. To complete the crystallization, place the flask in an ice-water bath for at least 15 minutes.
Isolation and Drying
1. Collect the crystals on a Büchner or Hirsch funnel by vacuum filtration and wash the filter cake with two small portions of cold water. Press the crystals as dry as possible on the funnel with a clean cork or spatula.
2. Spread the benzoic acid on a watch-glass, protecting it from airborne contaminants with a piece of filter paper, and air-dry it at room temperature or in an oven.
Caution: Be certain that the temperature of the oven is below the melting point of the product!
Analysis: Determine the melting points of the recrystallized benzoic acid, the weight of the latter material, and calculate your percent recovery using the following equation.
Percent recovery = Amount recovered (mg)/Amount dissolved (mg) X 100%