Procedure for calculation:

R_f value: It is a characteristic parameter called retardation factor and abbreviated as Rf. That represents the position of an ion or a substance along with respect to solvent phase. R_f of a solute is defined as the ratio of the rate of movement of the solute to the rate of movement of the solvent. R_f is most generally used within paper and thin layer chromatography and is considered as a characteristic of nature of the solute sample which may, of course, change with the solvent phase. It describes relative migration of the solute with respect to the solvent and may be represented as

R_f = Distance traveled by the solute/Distance traveled by the solvent

= d _s/d _m                                                                                                                         

Where, d_s and d_m are linear distances measured from the line of origin whereas spots are put as describes in Figure. By definition, R_f   value cannot exceed 1.0. Ideally, R_f values must be in the range of 0.1 to 0.9 with a minimum separation of 0.05. In order to have better separation, two spots have to not overlap along with each other and these must be symmetric without any tailing. In sequence to prevent tailing, different solvent mixtures, mixed in proper ratio, must be tried. If a spot of the solute is not symmetric, then d_s are measured from the position of maximum intensity or the centre of the spot.

Figure:  Procedure for calculation of R_f value in paper chromatography