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Retention Factor

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Retention Factor formula is one of the formulas used in the chromatography technique for separating various components of the mixtures. In paper chromatography when a solution is placed at the base level, the solute and solvent travel some distance. The components of the solutions are retained on the paper. The amount of retention can be expressed using a factor called the Retention Factor.

In this article, we will learn about Chromatography, the Retention factor, its formula and others in detail.

Chromatography Definition

Chromatography is the technique used to separate various substances into their individual components. There are various techniques of chromatography that are used for separating various substances that include:

  • Thin Layer Chromatography (TLC)
  • High-Performance Thin-Layer Chromatography (HPTLC)
  • High-Performance Liquid Chromatography (HPLC)
  • Gas Chromatography (GC)

There are two basic principles for performing chromatography which include:

  • Partition Chromatography
  • Adsorption Chromatography

Now, let’s learn about them in detail.

Partition Chromatography: Partition chromatography is the technique of the separation of the compound depending on stationary and mobile phases of the components.

Adsorption Chromatography: Adsorption chromatography is the technique of the separation of the compound depending upon the selective adsorption of components of the mixture.

Retention Factor Definition

Retention Factor also called the Rf value is defined as the ratio of the distance travelled by the solute to the distance travelled by the solvent. As it is the ratio of two similar quantities it is a unitless quantity. It is related to Chromatography.  We can find the Rf value of any substance using the formula added below:

Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent

Rf value of any compound depends upon the affinity of the solute towards the stationary and mobile phases, i.e. if the component of the substance has an affinity toward the stationary phase then its movement is slower and if the component has a lesser affinity toward the stationary phase, its movement is faster.

Retention Factor Formula

Retention Factor can be defined as the ratio of the distance travelled by solute to the distance travelled by solvent. It is denoted by Rf. The formula is given by:

Retention Factor (Rf) = (Distance Travelled by Solute Particle)/(Distance Travelled by Solvent Particle)

These Retention Factors of various solutions are used to make comparisons between them with different combinations of solute and solvents. The Rf values vary from one solution to another solution. Interactions of individual components with each other and the concentration of the component in the sample will affect the Retention factor value.

How to find Retention Factor (Rf value) of a Component?

We can easily find the Rf value or the retention factor of the component is calculated using the Retention Factor Formula added below,

Retention Factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent

We can understand this using the example discussed below in the image added.

Retention Factor of a Component

Now from the above figure,

A mixture taken is placed TLC plate (shown as the red spot) is the point of origin. A coating of silica gel is introduced that acts as the stationary phase.

A solvent front is marked at the 6 cm mark allowing the mobile phase to run through the origin.

The above image added showed two different solutes present in the mixture where one is yellow and the other is orange.

In the test, the distance travelled by the yellow spot is  2 cm and the distance travelled by the orange spot is 3 cm.

Now we can easily calculate the Rf value of the orange and the yellow spot as,

  • Rf of yellow spot = 2/4.5 = 0.444
  • Rf of orange spot = 3/4.5 = 0.667

Factors Affecting Retention Factor (Rf value)

There are various factors that affect the retention factor or the Rf value of the solute or solvent of the mixture that is:

  • Thickness of the stationary phase layer.
  • The moisture content on the TLC plate
  • The nature of the TLC plate
  • Saturation of the mobile phase
  • Volume of the mobile phase
  • Temperature of the environment
  • Size of the sample taken

Read More,

Solved Examples on Retention Factor Formula

Example 1: Calculate the retention factor if the solute and solvent moved from 5cm and 10cm respectively from the baseline which is at 0cm.

Solution:

Given,

  • Distance travelled by Solute = 5cm
  • Distance travelled by Solvent = 10cm

Retention Factor (Rf) = (Distance Travelled by Solute Particle)/(Distance Travelled by Solvent Particle)

Rf = 5/10

Rf = 1/2 = 0.5

Thus, the retention factor for the given solution is 0.5

Example 2: Calculate the retention factors for solutes A and B if they moved from 2cm and 5cm from the base level and the solvent moved to 10cm up from the baseline which is at 0cm.

Solution:

Given,

  • Distance travelled by Solute A = 2cm
  • Distance travelled by Solute B = 5cm
  • Distance travelled by Solvent = 10cm

Retention Factor of A (Rf) = (Distance Travelled by Solute A Particle)/(Distance Travelled by Solvent Particle)

Rf = 2/10 = 0.2

Retention Factor of B (Rf) = (Distance Travelled by Solute B Particle)/(Distance Travelled by Solvent Particle)

Rf = 5/10 = 0.5

Thus, the retention factor of solute A, B are 0.2, 0.5

Example 3: Calculate the retention factor if the solute is moved to 23% above the base level and the solvent is moved up to 50%.

Solution:

Given,

  • Distance travelled by Solute = 23% = 23/100 = 0.23 units
  • Distance travelled by Solvent  = 50% = 50/100 = 0.5 units

Retention Factor (Rf) = (Distance Travelled by Solute Particle)/(Distance Travelled by Solvent Particle)

Rf  = 0.23/0.5

Rf  = 0.46

Thus, the retention factor for the given solution is 0.46

FAQs on Retention Factor

Q1: What is Rf  Value?

Answer:

The Rf value is also called the retention factor and it is defined as the ratio of distance travelled by the solute particle to the ratio of distance travelled by the solvent particle.

Q2: What is Retention Factor Formula?

Answer:

The formula used to calculate the retention factor is added below,

Retention Factor (Rf) = (Distance Travelled by Solute Particle)/(Distance Travelled by Solvent Particle)

Q3: What does a High Rf value mean?

Answer:

The high value of the retention factor signifies that the interaction between the compound and the surface is very strong. We can also deduce by the high Rf value that the compound has a high solubility in the mobile phase.

Q4: Can the value of the Retention Factor be greater than 1?

Answer:

No, the Rf can never exceed the 1 value as the Rf value of close to one signifies that the spot and the solvent front are close enough and therefore the data is unreliable.



Last Updated : 19 Dec, 2023
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