Rf Value: Calculation, Significance and Affecting Factors

In this article we will learn about Rf value also known as Retention Factor. Here we will discuss calculation of Rf values and its importance in thin layer chromatography.  

Key words: Rf value or Retention factor, Thin layer chromatography (TLC), stationary phase, mobile phase. 

Table of Contents

1. Definition of Rf value
2. Thin layer chromatography
3. Calculation of Rf value
4. Importance  of Rf value
5. Factors affecting Rf value
6. Conclusion

1. Definition of Rf value

Rf value or Retention factor is defined as it is ratio of distance travelled by solute to the distance travelled by solvent. Since it is ratio there is no unit for Rf value. (Fig 1)

Figure 1: Calculation of Rf value or Retention factor

The Rf value is reported in Paper chromatography or Thin layer chromatography (TLC) to identify organic compounds. TLC is very primary method to identify organic compounds and it is often used in organic chemistry laboratory to monitor various chemical reactions and purifications. 

2. Thin layer chromatography

It is the chromatographic method to separate the compounds based on Rf value. Here different compounds appears as spots. It is performed on silica gel plate which is also known as stationary phase. Commonly the silica is coated on glass plate or aluminum plate. The sample is deposited on TLC plate and dipped in to a glass chamber which has solvent. This is also known as eluent or mobile phase. Then the sample loaded TLC is placed into the chamber. The solvent moves towards top end due to capillary action. The sample or solute also moves towards top end along with the solvent. Some compounds are having more affinity towards silica whereas some compound has grater affinity towards solvent. Due this difference in affinities compound they moves towards the top end with different speed. Thus, different compounds from a mixture get separated on TLC as spots. (Fig 2) 

Figure 2: Thin Layer Chromatography

TLC is the convenient and quick method to monitor chemical reactions. It is also used on preparative scale to purify small amount of compounds.

3. Calculation of Rf value

See the following TLC,

Figure 3: Thin layer chromatography plate

The TLC shows three different spots which corresponds to compounds A, B and C (Fig 3). Here the solvent travelled up to 15cm distance. This is also known as solvent front. The distance is usually calculated from base line. The compound A, B and C are referred as solute. For the calculation of Rf value, we need to consider distance travelled by solute and distance travelled by solvent. The distance should be calculated from base line to the center of the spot. 

Therefore; Rf value for spot A can be calculated as

Rf value of A = [Distance travelled by spot A] / [Distance travelled by solvent]

                        = 5.1 / 15

                        =   0.34

Hence Rf value of spot A is 0.34.

Rf value of B = [Distance travelled by spot B] / [Distance travelled by solvent]

                        = 7.5 / 15

                        =   0.50

Hence Rf value of spot B is 0.50.

Similarly, for spot C Rf value is calculated as;

Rf value of C = [Distance travelled by spot C] / [Distance travelled by solvent]

                        = 2.0 / 15

                        =   0.13

Hence Rf value of spot C is 0.13.

4. Importance  of Rf value

• Rf value is a physical constant of the compound. 
• Each organic compound has specific Rf value.
• Rf value is used for identification of organic compound.
• Rf value is useful in monitoring chemical reactions and purifications.

4.1 Monitoring Chemical Reactions

There are many methods available for identification of organic compounds such as UV spectroscopy, IR spectroscopy, Mass spectrometry and  NMR spectroscopy. In addition to this reactions can be monitored by identification of compounds produced in reaction mixture, such as HPLC, LCMS, Gas chromatography etc. These methods are highly advanced and accurate. But it take reasonable time and instruments to perform above analysis.

TLC is very quick method to identify organic compounds. It takes lesser time and efforts to monitor chemical reactions. For example, consider the reaction of benzophenone with sodium borohydride to benzhydrol. (Fig 4)

Figure 4: Reduction of Benzophenone by Sodium borohydride

The reaction can be monitored by TLC;

See below TLC showed for 0 min, 30 min and 1h after stating the reaction. There are three spots shown. "S" for Benzophenone (Starting material); "R" for reaction mixture and "C" for Co-spot means here both spots given "S" and "R". At 0 min there is only one compound with Rf value 0.55. After 30 min reaction proceeds forward therefore it showed spot at Rf 0.55 and Rf 0.22. This confirms that partial conversion of starting material in to product. After 1 h there is only one spot with Rf 0.22. Hence from the last TLC it is confirm that reaction completed and there is new compound formed in the reaction mixture. (Fig 5)

Figure 5: Monitoring of reaction by TLC

4.2 Monitoring Purification by Column Chromatography

Sometimes there will be formation of multiple compounds in the reaction mixture. Therefore multiple spots can be seen on TLC. These compounds needs to be isolated by column chromatography. Identity of compound is confirmed by Rf value. During the column chromatography purification, usually the compound with same Rf value are considered as single compound. 

Preparative TLC : 

In this method the sample is loaded on TLC. The spots are given as many as possible. Then the TLC dipped in respective solvent which provides good separation in the spots. The required spot then carefully scratched along with the silica in a glass tray. The solid powder then dissolved in suitable solvent and filtered. The filtrate then concentrated to provide pure compound.  

5. Factors affecting Rf value

Rf value is a physical property of compound and it get affected by following factors.

5.1 Solvent (Mobile phase)

Since Rf value is ratio of distance travelled by solvent and solute, therefore solvent is the most important factor which affect the Rf value. In polar solvent the solute get solubilize easily whereas non-polar solvent solubilize solute with less extent. Hence respective solvent takes the solute towards top with faster rate. Therefore it is observed that Rf value of compound is higher in polar solvent as compared to non-polar solvent. Usually mixture of solvents is used for TLC to tune the Rf value.

5.2 Solute (Sample)

Solute or compounds are also responsible for Rf value. The compounds which consists of polar groups such as hydroxyl (OH) or amine (NH2) are able to bind with silica of TLC plate. Therefore its travel towards top end becomes slower as compared to solvent. Therefore Rf value to polar compound is lower. Conversely non-polar compounds binds to the silica with lesser extent. Hence solvent and solute travel faster towards top end. Thus Rf value of non-polar compounds is higher than polar compounds.

5.3 Temperature

At higher temperature solubility of compounds is more therefore temperature affects the Rf values of compound.

5.4 Thickness of silica layer

Silica of the TLC plate binds with solute / compound, thus thicker the silica layer stronger the binding. And this results in lowering the Rf value. 

6. Conclusion

To summarize this topic, 

• Rf value or Retention factor is a physical property of compounds.
• It is ratio of distance travelled by solute to the distance travelled by solvent.
• Rf value denoted by 3 digit number and it does not have unit.
• Rf value is useful for monitoring chemical reactions and purification of organic compounds.
• There are factors which affects Rf values of compounds. For example, solvent or mobile phase, nature of compounds, temperature and thickness of silica layer.

That's all for this topic, see you in the next blog.