How to Calculate RF Values for Dyes
Retention factor, or RF, represents a parameter in a separation technique known as paper chromatography. Many high school and university chemistry students use paper chromatography to separate a mixture of dyes, such as those found in candy or ink pens.
Paper chromatography, like all forms of chromatography, consists of a stationary phase and a mobile phase. In general, the technique involves a piece of cotton-fibre paper with a line drawn about a centimetre from the bottom. An experimenter places a small drop of a dye mixture on the line and then places the paper in a developing chamber containing a solvent less than 1cm deep. As the solvent travels up the paper, it carries the dyes in the mixture with it, but different dyes travel different distances. The experimenter then calculates the RF of each dye by dividing the distance travelled by each "spot" on the chromatogram by the total distance travelled by the solvent.
Place the paper chromatogram flat on a table and trace the position of the "solvent front" with a pencil. The solvent front represents the distance travelled by the solvent and can usually be discerned by a noticeable stain left on the paper.
Measure the distance from the starting line drawn along the bottom of the paper to the line traced along the solvent front using a metric ruler. Chemists normally state this distance in millimetres, or mm.
Circle the spots left by each dye in the mixture, then measure the distance from the starting line to the centre of each circle using a metric ruler.
Calculate the RF for each component of the dye by dividing the distance travelled by each component by the distance travelled by the solvent front. For example, if the mixture contained two dyes that travelled 24 and 35mm and the solvent front travelled 57mm, then the RF values for the two dyes would be (24 / 57) = 0.42 and (35 / 57) = 0.61, respectively.