Chromatography practical write up (Autosaved) PDF

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Summary

Lab write up for one of the practicals you will undertake....

Description

839772 The determination of Rf values from different nitrophenols using Thin layer chromatography, under different stationary and mobile phases to identify the nitrophenols present in an unknown solution.

Introduction

The purpose of this investigation was to determine the Rf values of nitrophenol solutions, using Thin layer chromatography. In pharmaceutics, Thin layer chromatography (TLC) can be used to separate non-volatile mixtures for analysis to help determine the identity of compounds based on the number of components in the mixture, and the purity of a compound (1). In our experiment, we aimed to use TLC to separate three different nitrophenols to determine their Rf values so that we could determine which Rf values were also present in our unknown nitrophenol mixture.

Method

We prepared two glass jars of each mobile phase (petroleum ether, dichloromethane and tetrahydrofuran). We swirled the jars around with the solvents and left them in the fume cupboard while we prepared our plates. We used Al2O3 and SiO2 stationary phase plates. We drew a line, in pencil, 1cm from the bottom and drew four lines vertically along the line of 0.2cm. Where the intersection of these four lines occurred, we placed our samples (ortho-nitrophenol, meta-nitrophenol, para-nitrophenol and unknown) and labelled them on each plate. We then placed each plate

839772 in a different jar of each mobile phase and left them until the solvent had travelled ¾ of the length of the plate and drew a line along the solvent front. We placed each sample under UV light and drew around the dots where the solutions had travelled. We measured from the 1cm line to the top of the dots for each solution and measured from the 1cm line to the solvent front for each solution to calculate Rf values. Then recorded these results in a table and drew our plates. Results In Figure 1, we can see the drawings of my SiO2 and Al2O3 plates. We can see that the orthonitrophenol travelled the furthest in all of the plates and that that the DCM had the best spread of data.

SiO2/PE

SiO2/DCM

SiO2/THF

Al2O3/PE

Al2O3/DCM

Al2O3/THF Figure 1

Key: Blue- Ortho-nitrophenol Black- Meta-nitrophenol Orange- Para- nitrophenol Green- Unknown solution From our reading of DCM, we can see that our unknown sample in SiO2 stationary phase had two spots that aligned with the ortho-nitrophenol and the para-nitrophenol. I compared this with the Al2O3 plates and saw that these dots were also closely aligned with the same nitrophenols.

839772 This meant our Rf values were also closely aligned. In Table 1 ortho-nitrophenol had a Rf value of 0.893 and our unknown had an Rf value of 0.891.

Ortho-nitrophenol Petroleum Ether 0.328 Dichloromethane 0.893 Tetrahydrofuran 0.926

Rf Values (SiO2) Meta-nitrophenol Para-nitrophenol 0.0408 0.245 0.327 0.125 0.925 0.943

Unknown 0.02 0.891, 0.126 0.925

Table 1

Ortho-nitrophenol 0.375 0.857 0.952

Petroleum Ether Dichloromethane Tetrahydrofuran

Rf Values (Al2O3) Meta-nitrophenol Para-nitrophenol 0.021 0.021 0.286 0.240 0.929 0.930

Unknown 0.146 0.760, 0.170 0.884

Discussion Overall, the Rf values that we obtained showed that our unknown solution had

Table 2

both ortho-nitrophenol and meta-nitrophenol since they both had similar Rf values to the two dots we obtained with our unknown. The DCM was the best solvent to use since it showed the best spread of our data due to the fact the polarity with the solutions meant there was good adsorption onto the stationary phase.

Question 1. Dichloromethane

Petroleum ether H x

H

x

C

H

H

x

H

O

x

C

C x

x

C l

X

C

C

H

X X

X

x

x

x

H

H

H

Tetrahydrofuran

X X

X X

x

H

x

H

H

X

C

x

H

X X X

H

C l

X X

X X

Figure 2

Question 2.

Figure 3

Figure 4

839772

d+

O H

d-

IIIIIIIIIIIIIIIIII

O

H

H Figure 5

Question 3. We needed to allow the chromatography solvent system to come to equilibrium, to ensure the atmosphere of the glass jar was saturated and in order to enable separation using capillary action across the stationary phase.

Question 4. Overall, I think that the solvent system which interacted the most strongly with nitrophenols was Tetrahydrofuran, due to the fact the solutions travelled the furthest along the plate. I decided this since it is the most polar mobile phase out of the three as it has oxygens with two lone pairs (figure 4). This increase in polarity means that the nitrophenols are more likely to be soluble in this mobile phase, meaning the adsorption onto the stationary phase is less likely (2).

Question 5. 2-Nitrophenol behaves differently to the other nitrophenols since it travels the furthest along the plates, this indicates that is it less polar than the others. The reason behind this is its structure. Since the nitro group (NO2) is closer to the hydroxyl group (OH) compared to the other nitrophenols, it means it can hydrogen bond intramolecularly. This means it is

839772 more soluble in the mobile phases, meaning it travels further along the plate since there is less adsorption onto the stationary phase (2). Appendix Rf Values calculated

e.g. SiO2 plate with PE mobile phase of ortho-nitrophenol. Distance from line to spot/distance from line to solvent front= Rf Value. 2.1/6.4=0.328

References

1. J. Clark. Chem guide Thin Layer Chromatography. Available at: http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html

2. Khan Academy on principles of chromatography. Available at: https://www.khanacademy.org/test-prep/mcat/chemical-processes/separationspurifications/a/principles-of-chromatography...