Question

Describe how the rate of movement of a substance compared to the rate of movement of the solvent can be used to identify a substance in a chromatography experiment

Answer 1

Chromatography is a great analytical method, used for the separation, purification and identification of compounds. This is a physical method of separation in which the individuals to be separated are distributed between two phases, one is stationary (TLC plate) while the other moves in a specific direction (mobile phase). As the mobile phase passes through the spot on which sample has been adsorbed, it dissolves the components more or less readily; depending upon the solubility and carries them along with it while moving on the support.

For a given temperature and for a specific solvent, we can determine the characteristic rate of movement of each substance on the chromotographic plate, as the mobile phase moves. This is represented by relative front or retardation factor also called R_f value. R_f values of different compounds are different even if the mobile phase (solvent) is same. Furthermore,R_f value of a compound may be different in different solvents. R_f values can be calculated by using the following expression:

R_f = distance travelled by the substance from the reference line / distance travelled by the solvent from the reference line. solvent front moves faster than the compounds, the R_f value of a substance will always be less than one.

Polar compounds will not move faster on TLC plate compare to non-polar compounds.

Polarity order: carboxylic acids & bases > alcohols > amines > aldehydes > ketones > halides > ester > ether > hydrocarbon.

This is a general trend on TLC plate what i have observed.

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