How to Calculate RF Value

Chromatography is a scientific collection of laboratory techniques for separating and analyzing a mixture of substances or chemicals. Separating the substances takes place with a solution seeping through an absorbent product. These techniques involve passing the mixture from a mobile phase to a stationary phase. Each substance in the mixture does this at a specific rate causing it to become separated as it moves into the stationary phase. The retardation factor (Rf) is the relative amount of time that a particular substance spends in the mobile phase.

Define Retardation Factor

The first step is to refine the retardation factor mathematically. It is the ratio of the time that a substance spends in the stationary phase to the time that it spends in the mobile phase. Therefore in Rf= Ds/Df ,where RF is the retardation factor, Ds is the migration distance of the substance and Df is the migration distance of the solvent front. Retardation factor values are helpful in the identification in comparing unknown and known compounds. Compounds can have the same Rf value for a singular solvent but not likely for multiple solvents. Using more solvents helps to obtain more Rf values for more accurate identification.

Stationary and Mobile Phases

Next, dip a strip of filter paper into a solution containing the mixture of substances to be analyzed and a solvent. As the solvent is absorbed through the filter, the various test substances will drop out of the solution. The period of time that a test substance remains in solution is its stationary phase. The period of time in which the solvent is moving through the filter is the mobile phase.

Examine Test Results

After examining the results of the chromatography performed in step 2, measure the distance that the test substance traveled before dropping out of the solution (Ds) and then the distance that the solvent traveled(Df). The retardation factor, Rf, is then given by the expression Ds/Df.

Interpret Retardation Factor

The next step is to interpret the retardation factor. The retardation factor is only meaningful if the test substance moves some measurable distance and it can never move farther than the solvent. Therefore, a meaningful retardation factor will always be between 0 and 1.

Compare Retardation Factors

Finally, find a known substance with a retardation factor similar to that of the test substance. Chemists typically combine this procedure with other chromatography techniques in order to identify an unknown substance.