practical techniques

• Filtration under reduced pressure apparatus:

  • Buchner flask
  • Buchner funnel
  • Pressure tubing
  • Filter paper
  • Access to filter or vacuum pump
• Filtration under reduced pressure

  STEP 1: Connect one end of the pressure tubing to the vacuum outlet or to the filter pump whilst attaching the other end of the rubber tubing to the Buchner flask.
• Filtration under reduced pressure

  STEP 2: Fit the Buchner funnel to the Buchner flask ensuring that there is a good tight fit. This is usually obtained using a Buchner ring or a rubber bung.
• Filtration under reduced pressure

  STEP 3: Switch on the vacuum pump, or the tap, to which your filter pump is attached
• Filtration under reduced pressure

  STEP 4: Check for good suction by placing your hand across the top of the funnel
• Filtration under reduced pressure

  STEP 5: Place a piece of filter paper inside the Buchner funnel and wet this with the same solvent used in preparing your solid. You should see the paper being sucked down against the holes in the funnel
• Filtration under reduced pressure

  STEP 6: To filter your sample, slowly pour the reaction mixture from a beaker into the centre of the filter paper
• Filtration under reduced pressure

  STEP 7: Rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel
• Filtration under reduced pressure

  STEP 8: Rinse the crystals in the Buchner funnel with more solvent and leave them under suction for a few minutes so that the crystals start to dry
• The solid product obtained after filtration will contain impurities which can be removed by carrying out recrystallisation.
• Purification by recrystallisation depends upon the desired product and the impurities having different solubilities in the chosen solvent.
• Recrystallisation

  STEP 1: Pour a quantity of the chosen solvent into a conical flask. If the solvent is flammable, warm the solvent over a water bath. If the solvent is water, place the conical flask on a tripod and gauze over a Bunsen and warm the solvent.
• Recrystallisation

  STEP 2: Tip the impure sample into a second conical flask or beaker
• Recrystallisation

  STEP 3: Slowly add the solvent to the impure sample until it dissolves in the solvent. You should add the minimum volume of solvent needed to dissolve the solid.
• Recrystallisation

  STEP 4: Once the solid has dissolved, allow the solution to cool. Crystals of the desired product should form in the conical flask or beaker. When no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid.
• Chemists determine the melting point of solids to identify whether a solid compound is pure.
• A pure organic substance usually has a very sharp melting range of one or two degrees
• The melting range is the difference between the temperature at which the sample starts to melt and the temperature at which melting is complete
• If the compound contains impurities, the solid melts over a wide range of temperatures
• An impure sample also has a lower melting point than a pure sample
• Melting point determination

  STEP 1: Before taking the melting point of a solid you should ensure that the sample is completely dry and free flowing
• Melting point determination

  STEP 2: Take a glass capillary tube or melting point tube. Hold one end of the capillary tube in the hot flame of a Bunsen burner. Rotate the tube in the flame until the end of the tube is sealed.
• Melting point determination

  STEP 3: The capillary tube is allowed to cool, and is then filled with crystals to about 3 mm depth. This is usually carried out by pushing the open end of the capillary into the solid sample to force some of the solid into the tube.
• Melting point determination

  STEP 4: Once you have prepared your sample you will need to take its melting point. In schools and colleges, one of two methods will be available depending on the apparatus available.
• Using electrically heated melting point apparatus

  STEP 1: Place the capillary tube containing the sample into a sample hole and a 0-300°C thermometer in the thermometer hole of the melting point apparatus.
• Using electrically heated melting point apparatus

  STEP 2: Using the rapid heating setting, start to heat up the sample whilst observing the sample through the magnifying window.
• Using electrically heated melting point apparatus

  STEP 3: Once the solid is seen to melt, record the melting point. Allow the melting point apparatus to cool.
• Using electrically heated melting point apparatus

  STEP 4: Prepare a second sample in a new capillary tube and place in the melting point apparatus and again heat up the sample.
• Using electrically heated melting point apparatus

  STEP 5: As the melting point is approached, set to low and raise the temperature slowly whilst observing the sample. An accurate determination of the melting point can then be obtained.
• Using an oil bath or Thiele tube method

  STEP 1: Set up the Thiele tube or oil bath

  STEP 2: Attach the capillary tube containing the sample to a thermometer using a rubber band
• Using an oil bath or Thiele tube method

  STEP 3: Insert the thermometer through a hole in the cork if using a Thiele tube or clamp the thermometer if using an oil bath. The end of the thermometer and the end of the capillary tube should dip into the oil.
• Using an oil bath or Thiele tube method

  STEP 4: Using a micro-burner, slowly heat the side arm of the Thiele tube or the oil bath whilst observing the solid.

  When the solid starts to melt, remove the heat and record the temperature at which all of the solid has melted.

  It is important to heat the oil slowly when approaching the melting point, and it is advisable to repeat the melting point determination a second time to ensure that you obtain an accurate value.