The Rf value is the retention factor used in the identification of organic compounds in a mixture. The Rf value is calculated by measuring the relative distance traveled by a particular organic compound with respect to the mobile phase. It is calculated in both paper chromatography and TLC.
Thin layer chromatography (TLC) is a separation method involved in the separation of two or more organic compounds by their distribution between solid and liquid phases. The stationary (solid) phase is a polar substance in TLC while the mobile (liquid) phase is a single or combination of solvents. The absorbent is coated as a thin layer in a glass slide. The organic compounds are separated by manipulating the mobile phase.
Key Areas Covered
1. What is TLC
– Definition, Principle, Uses
2. How to Calculate Rf Values for TLC
– Calculation of Rf Value
Key Terms: Mobile Phase, Mobility, Rf Value, Stationary Phase, Separation, Thin Layer Chromatography (TLC)
What is TLC
TLC is a chromatography technique responsible for the separation of organic compounds on a mixture based on their relative mobility. It uses a solid stationary phase made up of a polar absorbent and a liquid mobile phase composed of a single or a mixture of organic solvents. The polar absorbent can be either finely-ground alumina or silica particles. Separation of black ink by TLC is shown in figure 1.
Figure 1: TLC of Black Ink
The organic compounds and the mobile phase move over the stationary phase due to the capillary action. The differential mobility of organic compounds is achieved by the relative affinity of compounds towards the stationary phase and mobile phase. The compounds with a higher affinity towards the stationary phase move slowly as the stationary phase retain the organic compounds. Since stationary phase is polar in TLC, polar compounds move slowly. However, compounds with a higher affinity towards the mobile phase move faster through the stationary phase. The mobile phase is non-polar and, non-polar organic compounds, which have less affinity towards the stationary phase, move faster through the stationary phase. Individual compounds can be visualized as spots after the separation.
TLC is used in:
- The determination of the number of compounds in a mixture
- Verifying the composition of a mixture
- The determination of proper conditions for column chromatography
- Analyzing the fractions obtained from column chromatography
How to Calculate Rf Values for TLC
The Rf value is the relative distance traveled by a particular compound with respect to the mobile phase. It can be calculated by the following equation.
Figure 2: Calculation of Rf Value
Rf = Distance traveled by the compound (a)/Distance traveled by the solvent front (b)
The Rf value is also called the ratio-to front. Under defined conditions of stationary phase, mobile phase, and the temperature, the Rf value of a particular organic compound is a constant value. However, different organic compounds with similar polarity may have similar Rf values. Therefore, other properties of organic compounds such as color on the TLC plate can be used during the identification of the compound.
Conclusion
TLC is a chromatography technique used in the separation of organic compounds based on their polarity. The differential polarity of these compounds allows the differential mobility through the stationary phase of TLC. Rf value is the relative mobility of a particular organic compound with respect to the mobile phase. It is calculated by measuring relative distances traveled by the organic compounds.
Reference:
1. “Thin Layer Chromatography (TLC): Principle and Procedure.” Owlcation, 29 Dec. 2015, Available here.
Image Courtesy:
1. “TLC black ink” By The original uploader was Natrij at English Wikipedia – Transferred from en.wikipedia to Commons (CC BY-SA 3.0) via Commons Wikimedia
2.”Factor de retenció diagram” By Cathan – Own work (Public Domain) via Commons Wikimedia
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