How Can an
Impure Solid Substance Be Purified and How Can Purity Be Measured?
G. General Overview of the Experiment:
Group members collaborate to choose various mixtures of hexane and dichloromethane developing solvents to test for separating fluorene from the impurities in the crude preparation that is available.
Each group member does a TLC analysis of the crude fluorene preparation in a different solvent system.
Each group chooses its best solvent mixture for separating fluorene from its impurities.
Each group member separately purifies a sample of the crude fluorene preparation using the group’s chosen solvent mixture, recovers the purified fluorene, analyzes the purity of the recovered fluorene by TLC and calculates the percent recovery fluorene from the chromatography.
Group members compare results and present the effectiveness and reproducibility of purifying fluorene from the crude preparation with their solvent system.
H. Procedure:
Thin Layer chromatography
As we discussed in the pre-lab session, you will first use thin layer chromatography (TLC.) to estimate the degree of impurity of the fluorene and select the best solvent or solvent system to be used in a column chromatography separation to obtain purified fluorene.
Group members collaborate to decide on appropriate solvent mixtures to use for assessing the effectiveness of their solvent system in separating fluorene from its impurities and then consult with the instructor on their choices.
Manager: Assign a solvent or solvent mixture to each group member.
Each group member: Using acetone as solvent (why???), prepare ~ 0.2-0.3 ml of a dilute solution of crude fluorene and pure fluorene in separate small test tubes.
Your instructor will have demonstrated the proper technique for spotting plates. Practice spotting the solutions on scrap TLC plates.
Each group member: Prepare a developing chamber using assigned solvent or solvent system.
Obtain a good TLC plate from your instructor. Note that the plates should be handled only along the edges and that the surface should not be touched.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 1 depicts a TLC plate, which has been marked at the bottom with a pencil to show the location at which the sample will be applied (point of origin) and where development begins and at the top where solvent movement has ended.
Figure 2 depicts that same slide under a UV lamp at 254 nanometers.
Figure 3 depicts a slide that has been spotted and viewed under a UV lamp at 254 nanometers.
Figure 4 depicts the same slide that has been developed in the appropriate solvent and is now viewed under the UV lamp. It shows one spot suggesting that only one component is present in the sample analyzed.
By means of a ruler and pencil, LIGHTLY draw a horizontal line across the plate at about 1 cm from the bottom. (See Figure 2 above)
Along this line and about 0.3 cm from the edge, mark off two "columns" at 1-cm intervals. These will be the locations at which you will spot your samples.
On each of the three plates, spot the crude fluorene in the first column and the pure fluorene in the second column. (See Figure 5). Your instructor will have demonstrated the procedure.
View the slide under a UV lamp before development to check spots.
Figure 5
By means of tweezers, place the spotted TLC plate into the chamber, cap the chamber, leave it in a fixed position and allow the slide to develop to 0.5 cm from the top. (top line on the plate)
By means of tweezers, remove the developed slide from the chamber and mark the actual position of the solvent front by means of a pencil.
Examine the dried slides under a short-wave (254 nm) UV lamp; lightly circle the observed spots with a pencil.
Collaborate with your group to determine the group’s the best solvent or solvent system for separation of the crude fluorene (Figure 6). If there is a
