Regardless of crystallization method, the purity of the solid can be verified by taking the melting point.Ī good (suitable) recrystallization solvent will dissolve a large amount of the impure compound at temperatures near the boiling point of the solvent. In the real world, this will never happen and recrystallization is a technique that has to be practiced and perfected. In the former case, the impurity could be filtered off at high temperature, while in the latter case the impurity would completely stay in solution upon cooling. In addition the impurity either would be completely insoluble in the particular solvent at the high temperature, or would be very soluble in the solvent at low temperature. In the ideal case, the solvent would completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in that solvent at room temperature or at zero ✬. The solubility of the compound in the solvent used for recrystallization is important. This is of course not a perfect process, but it does increase the purity of the final product. The crystallization process itself helps in the purification because as the crystals form, they select the correct molecules, which fit into the crystal lattice and ignore the wrong molecules. Compounds, which are less soluble, will crystallize first. The most basic of these techniques for the purification of organic solids is recrystallization, which relies on the different solubilities of solutes in a solvent. Several techniques for purifying these compounds have been developed. When organic substances are synthesized in the laboratory or isolated from plants, they will obviously contain impurities. (If there are any impurities that have the same solubility as the main component, then a different solvent needs to be chosen.) There are two types of impurities: those more soluble in a given solvent than the main component and those less soluble. Recrystallization is a method of purifying a solid. Liquids are customarily purified by distillation, while solids are purified by recrystallization (sometimes called simply ' crystallization'). Purification of your products must be performed to remove by-products and impurities.

Recrystallization - The products of chemical reactions can be impure. The observation is made using a magnifying lens and either a digital or mercury thermometer. Melting point 'machines' work on the principle of externally heating a small amount of sample in a small diameter glass tube called a capillary. The method of measurement is necessary due to the impracticality of melting the substance, and then observing it freezing. The smaller the range, the purer is the substance. The range is taken from the first instant melting starts, to the time that the sample is completely liquid. The melting point is measured using a thermometer that is at the same temperature as the heated solid, and is recorded as a range. Observing a small quantity of the solid as it is heated carries out the measurement of melting point. Impurities found in the solid will tend to change the melting point by causing it to 'spread' out into larger range. A pure solid material melts at specific temperature. Melting points are generally constant at pressures different from ambient atmospheric pressures, unlike the effect of changes in pressure on boiling points. The melting point is the temperature at which the solid and liquid states of a substance are in equilibrium at atmospheric pressure.

The purity of a solid may be determined by measuring its melting point.

Filtration and drying will separate the material from the suspension containing the impurities The impurities will be left in the solution. Crystal growth is a process that occurs with a dramatic increase in purity of the substance, as only those substances that have the exact crystal structure will come out of the solution. Crystals of the formerly dissolved solid will grow from the solution. Once the solute is dissolved producing an unsaturated solution a 'condition' of the solution is change to cause supersaturation. Changing the pH of solution may accomplish the same end if the solute is acidic or basic. When heat is involved a solvent is chosen in which the solute has a low solubility at room temperature, but a higher solubility at an elevated temperature. The dissolution process is achieved heating or changing the pH.

As the name implies a solid sample with suspect purity is dissolved into an appropriate solvent. Recrystallization is a common method used to purify a sample. CHM220 RECRYSTALLIZATION AND MELTING POINT

Melting point measure apparatus