3.9 Separation of Solutions and Mixtures Chromatography

Chromatography provides a method of:

Separating a mixture of solutions

Chromatography separates a mixture of solutions based upon:

Polarity differences

Polarity differences between solutions are sometimes considered:

Solubility

Types of chromatography:

Paper, Thin layer, Column

Most common type of chromatography in AP Chemistry:

Paper

Methodology for paper chromatography:

A line is drawn (typically in pencil) near the end of the strip of paper. A drop of the sample to be separated is placed on that line. The paper is placed in an (ideally) sealed container with a shallow layer of solvent where the bottom of the paper is touching the solvent, but the line is not. Time passes and the solvent and dyes will separate. Remove the paper before the solvent reaches the top of the paper. Measure the height of the leading edge of the solvent and measure the leading edge of each dye.

Diagram showing methodology for paper chromatography:

Rf=

Distance travelled by dye / Distance travelled by solvent

Rf stands for:

Retention factor

Rf is same/unique for all solutes

Unique

d ink (subscripted) =

Farthest distance the dye travelled

d ink (solvent) =

Farthest distance the solvent travelled

In paper chromatography, the same compound will / will not move at the same rate relative to ______ on different trials.

Will, The same solvent

In paper chromatography, does Rf factor always differ between compounds?

Yes

Why does Rf factor always differ between compounds in paper chromatography?

Different compounds will have an at least slightly different polarity to other compounds

In paper chromatography, the more similar in polarity the sample is to the solvent the farther / less farther it will travel

Farther

In paper chromatography, polar water will have polar samples travel far / a short distance and nonpolar samples travel far / a short distance.

Far, a short distance

In paper chromatography, nonpolar benzene will have polar samples travel far / a short distance, and nonpolar sample travel far / a short distance.

A short distance, far

How to identify a sample using chromatography?

By comparing Rf values

Is identification of a sample using chromatography possible by comparing distances?

No

The stationary phase is the:

Paper

The mobile phase is the:

Solvent

In the image, does the mixture contain dye 2?

No

In the image, how do you know that the mixture does not contain dye 2?

There is not a yellow dot in the mixture column

In the image, the mixture contains:

Dye 1 and 3

Is it uncommon for dyes to have multiple parts which would look like multiple dots different distances from the base line

No

Is thin layer chromatography similar to paper chromatography?

Yes

How is thin layer chromatography different from paper chromatography?

Separation occurs on a layer that is on top of a thin sheet of plastic

Examples of layers used in thin layer chromatography:

Silica or alumina

Are layers used in thin layer chromatography polar or nonpolar?

Polar

Are layers used in thin layer chromatography always polar?

No

Thin layer chromatography can be used to separate:

Some samples that are not colored to the naked eye

How to see separated samples not colored to the naked eye when performing layer chromatography?

By using UV light

How does UV light help to see separated samples not colored to the naked eye when performing layer chromatography?

It helps to fluoresce solvent to see substances

When using UV light to see separated samples not colored to the naked eye when performing layer chromatography, the dark spots represent:

Location of components of the mixture

Thin layer chromatography is frequently used with:

Amino acids

In thin layer chromatography, the same compound will move at the same / a different rate relative to the _____ on different trials.

Same, solvent

Is Rf factor same for different compounds in thin layer chromatography?

No

Why is Rf factor not the same for different compounds in thin layer chromatography?

Different compounds will have at least slightly different polarities

In thin layer chromatography, typically, the more nonpolar the sample is, the more/less farther it will travel.

More

In thin layer chromatography, typically, the more polar the sample is, the more/less farther it will travel.

Less

Must leading edge be identified in thin layer chromatography?

Yes

Must leading edge be allowed to reach the top in thin layer chromatography?

No

In thin layer chromatography, identification of a sample is possible by:

Comparing Rf values

In thin layer chromatography, identification of a sample is not possible by:

Comparing distances

Job of column chromatography:

Separate a mixture in solution

Methodology for column chromatography:

Steel wool is placed at the bottom of a burette to prevent the gel from escaping. A burette is filled usually with very polar silica or alumina gel. The mixture that is to be separated is placed at the top of the burette and is flushed time and again with nonpolar solvent (usually). (pour solvent, wait and watch, repeat). The mixture separates and each phase is collected in a beaker or flask one at a time.

Diagram of column chromatography:

In column chromatography, usually, the most polar parts of the mixture will travel the fastest/slowest and the least polar will travel the fastest/slowest

Slowest, fastest

What should you do after separating one part in column chromatography?

Speed up the movement of the remaining part(s)

How to speed up the movement of the remaining part(s) after separating one part in column chromatography?

By using a new solvent

In column chromatography, using a new solvent to speed up the separation of parts is for:

Separation, not analysis

How to identify the component that you separated in column chromatography for which you used a new solvent to speed up the process?

By using a different method of chromatography

Distillation is a method of:

Physically separating mixtures

Distillation separates substance based on:

Differences in boiling point and intermolecular forces

Process of distillation with diagram:

The starting liquid 15 in the boiling flask 2 is heated by a combined hotplate and magnetic stirrer 13 via a silicone oil bath (orange, 14). The vapor flows through a short Vigreux column 3, then through a Liebig condenser 5, is cooled by water (blue) that circulates through ports 6 and 7. The condensed liquid drips into the receiving flask 8, sitting in a cooling bath (blue, 16). The adapter 10 has a connection 9 that may be fitted to a vacuum pump. The components are connected by ground glass joints (gray).

Techniques for separating mixtures:

Chromatography (paper, thin-layer, column) and distillation

Applications of distillation include:

Distillation of fermented beverages, Desalination of salt water, Separation of crude oil into fuels and other petroleum products

The separation of crude oil into fuels and other petroleum products often occurs through:

Fractional distillation

Typically, if the boiling points are close together, simple/fractional distillation is a better technique to separate substances

Fractional

Fractional distillation leads to:

Better purification

Why does fractional distillation lead to better purification?

Allows the vapor to condense and revaporize several times

With all types of chromatography, you will need to recognize:

Which part of the system will interact with the materials being tested

With all types of chromatography, you have to consider:

Intermolecular forces for all components