Crystallization, recrystallization, melting point analysis and TLC.

Crystallization

  • Crystallization is the formation of a solid in crystalline form from the gas or melt phase.
  • The process occurs by either supersaturating a concentrated solution, lowering the temperature, or evaporating the solvent.
  • In this method, most contaminants usually remain in the solution.
  • Crystallization is often used as a purification step, e.g. in sugar production.

Recrystallization

  • Cleansing of crystalline matter.

    • Redissolution and recrystallization.
    • The purpose is to increase the purity of an already crystallized substance.
    • Increasing the purity, the impurities remain in the mother liquor (i.e. the remaining solution).

  • Redissolve crystal by heating and subsequently restore by cooling.

  • The yield percentage decreases, as the mother liquor has a higher saturation of the desired substance.

  • Evaporation and cooling can crystallize out if recrystallization, i.e. that more of the desired substance comes in, but the impurity increases.

  • The crystals from the recrystallization can be dissolved again and returned to the primary crystallization.

Melting point analysis

  • Melting point analysis is a method of determining the temperature point at which a substance changes from solid to liquid.
  • The analysis is used to determine the purity of a sample.
  • Impurities in the molecular lattice lower the melting point.

Thin Layer Chromatography (TLC)

Stationary: Stationary plate/paper

Mobile phase: movement running fluid

  • TLC can be used on many different substance mixtures such as colour mixtures, amino acids, lipids, alkaloids and mycotoxins.
  • Chromatographic separation technique where a plate is used either of glass or strong aluminium foil.
    • The plate on one side is covered with a powder which has different properties in relation to which substance classes you want to separate.
    • On the lower part of the plate, attachment points are marked, usually with a soft pencil, where a few drops of the substance mixtures are then placed with a micropipette.

TLC method (simplified).

  • Apply a solution of the substances to be separated into the lower part of a plate.
  • Also, apply standard solutions of known substances.
  • Place the plate upside down in a beaker with a suitable solvent at the bottom.
  • Wait for the solvent to move up the plate.
  • Remove the plate when the solvent is approximately 1 cm from the top edge of the plate.
  • Mark the position of the liquid front with a pencil.
  • Use a beaker with a lid and filter paper to saturate the air with the solvent vapours.
  • Analyze the coloured spots that have moved differently up the plate.
  • Identify the different dyes in the mixture using the location of the standard solutions on the plate.

Result processing of TLC.

  • To compare the running length of the spots, RF values ​​are used. (Relative to Front or retardation factor).

    • The RF value is determined by measuring the running distance of the spots.
    • The running distance is measured by measuring from the individual spot to the centre and then dividing this length by the length of the liquid front.

  • 2D-TLC is used for more complex mixture systems.

    • First, the TLC plate is developed in a solvent mixture which separates the polar components, and then the plate is dried.
    • The plate is then turned 90 degrees and developed in a solvent mixture which separates the non-polar components of the mixture.