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Retardation Factor (Rf)
In chromatography, a retardation factor (Rf) (also known as retention factor) describes the ratio of time spent in the stationary phase relative to time spent in the mobile phase .The Rf value is defined as the ratio of the distance moved by the solute (i.e. the dye or pigment under test) and the distance moved by the solvent (known as the Solvent front) along the paper, where both distances are measured from the common Origin or Application Baseline, that is the point where the sample is initially spotted on the paper
- Rf value = distance from the baseline travelled by solute.
- Distance from baseline travelled by solvent (solvent front)
In paper chromatography there is the stationary phase which is the absorbent Chromatography paper and the mobile phase which is a liquid solvent (or mixture of solvents) used to carry the sample solutes under analysis along the paper. Usually, one uses chromatography to find out the components of a sample which are seperated depending how much soluble these are in particular solvents and hence how far they travel along the chromatography paper. Samples are usually of organic matter (not ionic salts) which dissolve in certain polar solvents (namely water) or non-polar (organic) solvents.
Principle
A particular compound will travel the same distance along the stationary phase by a specific solvent (or solvent mixture) given that other experimental conditions are kept constant. In other words, every compound (dye, pigment, organic substance etc) have a specific Retention Value (Rf) for every specific solvent and solvent concentration. Rf values come very handy for identification because one can compare Rf values of the unknown sample (or its consituents) with Rf Values of known compounds.
Due the fact that the solvent front is always larger from the distance travelled by the solute, Rf values are always between 0 – 1
Rf values do not have units since it is a ration of distances and are usually written as the following examples:
- Rf= 66 (60% Ethanol) – if % is given it is assumed that the mixture is in water hence 60% ethanol 40% water.
- Rf = 0.78 (Ethanol-Methanol mixture {1:2}) – a mixture of 1 part Ethanol and 2 parts Methanol
- Rf= 25 (Ethanol-Methanoic Acid-Acetone mixture {4:3:1}) – a mixture of 4 parts Ethanol, 3 parts Methanoic Acid and 1 part Acetone. Note that mixture compounds with larger proportions are placed first in the mixture sequence.
Environment Conditions
The Rf value of a particular pure dye or analyte in a particular solvent (or mixture) is constant if the following experimental conditions are kept constant:
- Temperature
2.Chromatography medium, ie same type and grade of Chromatography Paper
3.Solvent concentration and purity
- Amount of sample spotted on Chromatography medium