3 Purifying organic solid / PAG6

Step Reason 1. Dissolve the impure compound in a minimum volume of hot (near boiling) solvent. An appropriate solvent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold. The minimum volume is used to obtain saturated solution and to enable crystallisation on cooling 2. Hot filter solution through (fluted) filter paper quickly. This step will remove any insoluble impurities and heat will prevent crystals reforming during filtration 3. Cool the filtered solution by inserting beaker in ice Crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated. Ice will increase the yield of crystals 4. Suction filtrate with a buchner flask to separate out crystals The water pump connected to the Buchner flask reduces the pressure and speeds up the filtration. 5 Wash the crystals with distilled water To remove soluble impurities 6. Dry the crystals between absorbent paper Purifying an organic solid: Recrystallisation Loss of yield in this process • Crystals lost when filtering or washing • Some product stays in solution after recrystallisation • other side reactions occurring buchner flask. Used for purifying aspirin. f the sample is very pure then the melting point will be a sharp one, at the same value as quoted in data books. One way of testing for the degree of purity is to determine the melting “point”, or melting range, of the sample. Heat Heating oil- needs to have boiling point higher than samples melting point and low flammability Thermometer with capillary tube strapped to it containing sample Measuring melting point. Comparing an experimentally determined melting point value with one quoted in a data source will verify the degree of purity. Sometimes an error may occur if the temperature on the thermometer is not the same as the temperature in the actual sample tube. Melting point can be measured in an electronic melting point machine or by using a practical set up where the capillary tube is strapped to a thermometer immersed in some heating oil. In both cases a small amount of the salt is put into a capillary tube. If impurities are present (and this can include solvent from the recrystallisation process) the melting point will be lowered and the sample will melt over a range of several degrees Celsius. Thermometer with capillary tube strapped to it containing sample. Heating oil- needs to have boiling point higher than samples melting point and low flammability