Question

Answer the questions –- Print so I can read it in PeN 1) Assume that a solvent having more isopropanol than solvent G was used in the experiment. Predict whether dye A would migrate a smaller or larger distance in this solvent than in solvent F or G. Why?

2) TLC can be used to help identify unknown compounds. Explain how this might be accomplished using the dyes as an example.

3) READ Harris page 604-623 – define the following terms –DO NOT quote from the textbook word from word but paraphrase using your own words to show understanding!!! Definitions should not use/show equations! Expect 1-2 well-crafted sentences for each! STAPLE typed/printed pages NO HANDWRITTEN answers will be accepted for question 3! a. Retention factor b. Resolution c. Diffusion d. Plate e. Plate height f. Longitudinal diffusion g. Adsorption chromatography h. Partition chromatography

Answer 1

1) If dye A is organic in nature , then it will be more soluble in non-polar isopropanol than any polar solvent .So it will migrate larger distance in solvent with more isopropanol.

2) TLC can be used to help identify unknown compounds. Explain how this might be accomplished using the dyes as an example.

TLC or thin layer chromatography, has a thin layer of solid adsorbent like SiO2(silica ) or alumina (Al2O3) applied on glass or plastic,called the stationary phase.A solvent generally non-polar is allowed to flow through the stationary phase(the mobile phase), carrying the components of the mixture with it.Mostly, non –polar components of the mixture is separated by the non-polar mobile phase and the polar components are adsorbed by the polar stationary phase.

A small amount of the mixture to be analyzed is put near the bottom of the TLC and then the plate d in placed in the solven.The eluent or the solvent slowly rises up the plate by capillary action.

As the solvent moves past the mixture an equilibrium is established for each component of the mixture with its molecules adsorbed on the solid adsorbent and the molecules which are in solution.

As the components differ in solubility in the solvent ,some components will move farther up the plate than others.

The plate is removed from the developing chamber, dried, and the components of the mixture are separated.

3) a. Retention factor =It is the ratio of the distance migrated by the sample to be analysed to the distance moved by the solvent.

b. Resolution=The resolution of a elution in HPLC is the difference in retention times between two given peaks, divided by the combined widths of the peaks.

RS=2[(tR)B−(tR)A]/WB+WA

Where B has longer retention time, tR and W is the elution peak width .If the resolution is greater than one, the peaks can be differentiated.

c. Diffusion =Fluid or solvent travels from the region of high concentration of solute dissolved in it to the region of low concentration spontaneously.This process is called diffusion.

d. Plate=Glass or plastic or metal surface used for chromatography, where solid adsorbent is applied for running an eluent or solvent.The mixture to be separated is placed at the bottom of the plate.

e. Plate height =

The chromatographic column is divided into discrete sections called theoretical plates. There exists an equilibrium partitioning of the solute between the stationary phase and the mobile phase in these plates.The column efficiency in defined terms of the number of theoretical plates, N and plate height.

                N = L / H

Where, L is the total length of the column and H is the height of each theoretical plate. Smaller theoretical plates defines more Column efficiency.

f. Longitudinal diffusion =

It is well-known that molecules disperse due to the diffusion. The longitudinal diffusion is the diffusion along the column longer or longitudinal axis .This leads to the band broadening of the chromatographic zone.

g. Adsorption chromatography =

Adsorption chromatography is the chromatography in which a mobile solvent or gaseous phase is adsorbed on a stationary solid phase. The equilibrium between the mobile and stationary phase accounts for the separation of different solutes , as the solute is distributed between the phases based on their affinity for solid phase or solubility in the mobile solvent.

h. Partition chromatography=

Partition Chromatography is based on a thin film formed on the surface of a solid support by a liquid stationary phase. Solute is distributed between the mobile phase and the stationary liquid. Eg column chromatography