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Retention factor formula is related to chromatography technique that is widely used for the separation of components of a mixture. Such components are distributed between two phases, one is stationary phase and other is mobile phase. The separation of components depends upon the affinity towards the stationary phase and mobile phase. The movement of solute is monitored by the help of retention factor formula.
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Key Terms: Chromatography, Solute, Solvent, Solution, Stationary and Mobile Phase, Gas, Solid, Liquid, Adsorption, Distance
What is Chromatography?
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Chromatography is a separation technique in which a mixture sample gets separated into its individual components. It includes- thin layer chromatography (TLC), high performance thin layer chromatography (HPTLC), high performance liquid chromatography (HPLC) and gas chromatography (GC) etc.
- The type of stationary and mobile phase is different for different techniques. For example-in TLC, the stationary phase is silica gel (solid) and the mobile phase is a mixture of solvents (liquid) whereas in GC, the stationary phase is solid and the mobile phase is gas.
- There are two major principles followed in chromatography:
- Partition Chromatography: In Partition Chromatography, separation depends upon the partition of components between stationary and mobile phase.
- Adsorption Chromatography: In Adsorption Chromatography, the stationary phase is a solid and the mobile phase is usually a liquid. The separation depends upon selective adsorption of components of a mixture on stationary phase.
Chromatography
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What is the Retention Factor?
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The term retention factor is represented by Rf. and related to the chromatography technique. It is defined as the ratio of the distance travelled by solute to the distance travelled by a solvent. It is a unitless term andis written as:
| Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent |
The value of retention factor depends upon the affinity of solute towards stationary and mobile phase. For example, if a component has more affinity towards the stationary phase, slower will be its movement, which means the less the retention value and vice versa. For example, if a component has less affinity towards the stationary phase, faster will be its movement. This means that solute has less retention value. Similarly, if a component has more affinity towards the stationary phase, slower will be its movement. This means that solute has high retention value. Even if the same mobile phase is used then also Rf values of different compounds vary. Also, in different solvents, the Rf value of a compound may be different.
How to Find the Retention Factor of a Component?
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We know that,
Retention Factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent
Let us understand with the help of an example. Refer to the given figure below:
TLC Chromatogram
In the above figure:
- A mixture sample (red spot) is spotted at the point of origin on the TLC plate. It is coated with silica G which acts as a stationary phase.
- Then spotted TLC is dipped into the mobile phase below the point of origin and allowed to run till 6 cm from point of origin. This is known as the solvent front.
- The individual solutes present in the sample will move on the stationary phase through the mobile phase.
- In the given figure two different solutes are separated from a mixture sample. One is yellow and the other is orange.
- The distance travelled by the yellow and orange spots are 3cm and 4.5 cm respectively.
Using the retention factor formula, we can calculate that,
Rf of yellow spot = 3/6 = ½
Rf of orange spot = 4. 5/6 = 0.75
Factors Affecting Retention Factor
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Below mentioned are some of the factors affecting retention factor:
- Thickness layer of stationary phase
- Moisture on the TLC plate
- Saturation of solvent (mobile) chamber
- Temperature
- Volume of mobile phase
- Nature of the TLC plate
- Sample size
Things to Reme
- The retention factor formula is used in chromatography techniques like TLC, HPTLC etc.
- It measures the distance covered by the individual component out of mixture through the stationary and mobile phase. There are two major principles followed in chromatography-partition chromatography and adsorption chromatography.
- Retention factor is the ratio of the distance travelled by solute to the distance travelled by a solvent.
- The value of retention factor depends upon the affinity of solute towards stationary phase and mobile phase. More the affinity, lesser the value and vice versa.
- It is a unitless quantity.
- Retention Factor is determined by various properties such as thickness layer of stationary phase, moisture, saturation of solvent, sample size etc.
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Sample Questions
Ques. The distance travelled by solute and solvent is 3cm and 7cm respectively. Find the retention factor. (3 marks)
Ans. Given,
Distance travelled by solute = 3 cm
Distance travelled by solvent = 7cm
We know that,
Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent
Put the given values of solute and solvent in above equation, we get,
Retention factor (Rf) = 3 / 7 = 0.42
Hence, the retention factor of solute is 0.42.
Ques. The distance travelled by solute is 7cm and retention factor is 0.42. Find the distance travelled by solvent. (3 marks)
Ans. Given,
Distance travelled by solute = 7cm
Retention factor = 0.42
We know that,
Rf = Distance travelled by a solute / Distance travelled by a solvent
Hence,
Distance travelled by a solvent = Distance travelled by a solute / Retention factor (Rf)
Put the given values in formula provided above, we get,
Distance travelled by a solvent = 7 / 0.42 = 16.66 cm
Hence, the distance travelled by solvent will be 16.66 cm
Ques. The distance travelled by solvent is 10 cm and retention factor is 0.42. Find the distance travelled by solute. (3 marks)
Ans. Given,
Distance travelled by solvent = 10 cm
Retention factor = 0.42
We know that,
Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent
Distance travelled by a solute = Distance travelled by a solvent x Retention factor (Rf)
= 10 x 0.42 = 4.2 cm
Hence, distance travelled by a solute is 4.2 cm.
Ques. Which of the following is the unit of retention factor? (3 marks)
a. cm
b. m
c. km
d. None of the above
Ans. d. None of the above
Explanation: Retention factor is unitless and thus has no units. This can be illustrated from an example below:
We know that,
Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent
The value of both, that is, distance travelled by a solute and distance travelled by a solvent is in cm. Hence, both will cancel out each other’s standard form. Hence become unitless.
Ques. The retention factor for the solute represented by the following figure is
a. 0.1
b. 0.2
c. 0.7
d. 0.6
Ans. c. 0.7
Explanation: Given,
Distance travelled by solvent = 18.8 cm
Distance travelled by a solute (pink spot) = 15cm
We know that,
Retention factor (Rf) = Distance travelled by a solute / Distance travelled by a solvent
= 15 cm / 18.8 cm
= 0.7
Hence, the correct answer is 0.7
Ques. Find the retention factor of yellow solute from the given TLC plate.
a. 0.1
b. 0.2
c. 0.7
d. 1.3
Ans. c. 0.7
Explanation: Given,
Distance travelled by solvent = 16 cm
Distance travelled by a solute (yellow) = 11.5 cm
We know that,
Retention factor (Rf) = Distance travelled by a solute (yellow) / Distance travelled by a solvent
= 11.5 cm / 16 cm
= 0.7
Hence, the correct answer is 0.7.
Ques. Read the Assertion (A) and Reason (R) given below. (3 marks)
Assertion: The separation of solute depends upon the affinity of solute towards the stationary phase and mobile phase.
Reason: Greater the affinity of solute towards the stationary phase, higher will be its retention value and vice versa.
Choose the correct answer from the given options.
a. Both A and R are correct and R is the correct explanation of A.
b. Both A and R are correct but R is not the correct explanation of A.
c. A is correct and R is false.
d. A is false and R is correct.
Ans. c. A is correct and R is false.
Explanation: In chromatography, the separation of solute depends upon the affinity of solute towards the stationary phase and mobile phase. The greater the affinity of solute towards the stationary phase less will be its retention value (as solute will be retained by stationary phase). Similarly, the lesser the affinity of solute towards the stationary phase higher will be its retention value (as solute will not be retained by stationary phase).
Hence, A is correct but R is false.
Ques. What are the two major principles of chromatography? (3 marks)
Ans. There are two major principles followed in chromatography-
- Adsorption chromatography- In this, the stationary phase is a solid and the mobile phase is usually a liquid. The separation depends upon selective adsorption of components of a mixture on stationary phase.
- Partition chromatography- In this, separation depends upon the partition of components between stationary and mobile phase.
Ques. What is paper chromatography? (2 marks)
Ans. Paper chromatography is a technique that uses paper sheets as its stationary phase. These paper sheets are used as adsorbent and a solution is passed through it. In paper chromatography, dissolved chemical substances are separated by their different migration rates. The basic principle of paper chromatography is partition chromatography or adsorption chromatography.
Ques. Which of the following is marked incorrectly? (3 marks)
a. Solvent front
b. Distance travelled by solute
c. Both a and b
d. None of the above
Ans. b. Distance travelled by solute
Explanation: In the given figure, the lowest green colour spot is the point of origin where the sample is spotted. This is not the distance travelled by solute. Hence, the correct answer is option b.
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