What are examples of polarity in chemistry

Department of biology, chemistry, pharmacy

Substances that you want to examine using thin-layer chromatography can be solid or liquid. Liquids in particular may only have a low volatility because they have to remain on the thin-layer card and must not have evaporated until they are detected.

Chromatographic methods always work on the principle that a mobile phase, which must be gaseous or liquid for this purpose, runs past a stationary phase and the substances dissolved in the mobile phase are retained by the stationary phase to different degrees, which leads to it a separation of the substances occurs. In thin-layer chromatography, a solvent is used as the mobile phase, which rises solely through capillary forces in the stationary phase, which must be porous for this purpose. The speed decreases with increasing height, because gravity counteracts the capillary forces. Because the substances "run" on the thin-layer card, the solvent used, which is responsible for this fact, is logically called "solvent".

Thin-film cards are usually coated with silica gel as the stationary phase. Silica gel is a condensed polysilicic acid that contains polar (Si-OH) groups on its surface.

As with most other chromatographic methods, selection is made according to the polarity of the substances to be separated. Polar substances adsorb more firmly on the silica gel surface and therefore migrate less quickly than non-polar substances. The strength of this adsorption can be influenced by the choice of solvent:

  • A more polar mobile solvent, for its part, binds adsorptively to the silica gel and thus competes with dissolved polar substances.

To Egon Stahl the following relationship triangle applies:

Most of the time, especially in the practical course, only silica gel plates are used, so that the stationary phase is invariant. The relationship is therefore simplified as follows:

  • If a substance runs too little, the polarity of the solvent must be increased.
  • If a substance runs with the solvent front, the polarity of the solvent must be lowered.

Refrain from all other half-baked explanations such as:

Similis similibus solvuntur - So, for example, non-polar substances would actually have to run particularly far in non-polar solvents and, on the other hand, very little in polar solvents.

Again and again it is in the mind and again and again it is botch!

In fact, solubility only plays a subordinate role.

Very often, mixtures of hexane and ethyl acetate are used for chromatography, whereby the polarity can be continuously increased by increasing the proportion of ethyl acetate. Mixtures of these two solvents are particularly suitable because their boiling points of 69 ° C and 77 ° C are very similar and the composition hardly changes when the solvent evaporates as intended in the TLC chamber.