9.02.2 Separation and Chromatography

Syllabus

Chromatography can separate mixtures and can give information to help identify substances.
Chromatography involves a stationary phase and a mobile phase.
Separation depends on the distribution of substances between the phases.
The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its Rf value:

Rf = distance moved by substance / distance moved by solvent

Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Required Practical 6

Investigate how paper chromatography can be used to separate and tell the difference between coloured substances.
Students should calculate Rf values.

What does this mean?

How do we separate different types of mixtures?

Impure substances are mixtures - some parts are different to others.

So different parts may behave differently and we can use these different properties to separate them.

Insoluble solids from liquids - Filtration

Liquids (and dissolved solids) have particles small enough to go through the tiny holes in filter paper.

Solids don't.

We call this filtering

So the liquid (solvent) and dissolved substances go through the filter - and are called the filtrate

The Insoluble solid stays in the paper - and is called the residue.

Don't call a filter a sieve in an exam!

Sieves have very large holes too large to remove sand from water, for example

Soluble solids from solutions - Crystallisation

Soluble solids (solutes) dissolve in liquids (solvents) to form solutions.

We could just leave the solution and the solvent would slowly evaporate, leaving the solute to crystalise.

Though we usually warm the solution to speed up evaporation.

We don't evaporate to dryness (until no water is left) because the solute may be damaged and crystals may "spit" across the room.

Liquids from solutions - Simple Distillation

As we saw above, heating a solution will cause the solvent (liquid) to evaporate and will leave solid solutes behind.

This means that we lose the liquid unless we trap the vapour and cool it until it condenses (turns back to a liquid).

Evaporation followed by condensation is called Distillation.

You may see the equipment (right) in class but we can distil using much more basic equipment if we need to (see below)

On a desert island with no fresh water you should avoid drinking seawater because eventually it will kill you.

You probably wouldn't have a bunsen burner and you certainly wouldn't have a convenient gas supply but some useful plastic waste may wash up on the shore.

And you could distil sea-water using nothing but a big container, a sheet of plastic and a rock.

The sun provides the energy, the water evaporates until it hits the plastic sheet, when it condenses and drips into a different container - Genius!

One liquid from another liquid - Fractional Distillation

You can separate liquids from each other by distillation too.

All that is different is the addition of a fractionating column to the still.

This helps vapours to condense and means that liquids are separated by their boiling points - lowest first.

You wouldn't be asked about details in an exam but you should know the difference between simple and fractional distillation.

Videos (basic separation)

Chromatography - separating tiny amounts of a mixture

You may have separated inks using Chromatography before.

This form of chromatography is more useful for analysing what is in a colour rather than actually separating useful amounts of the inks.

The usual order of events in the process...

Draw a pencil line around 2cm above the bottom of the paper - (pencil won't smudge, a 2cm gap allows the bottom of the water to touch the paper without touching the ink spots.
Add the spot of ink and place the bottom of the paper in the water (or other solvent) - (don't let the spot touch the water or it will run.)
Wait for the solvent to rise most of the way to the top of the paper - (not all the way or calculating an accurate Rf value will be impossible.)
Remove the paper and dry.

This allows you to see how many colours were in the original ink, and w

hich colours are most soluble - (the ones which move furthest.

The further the solvent is allowed to rise the better the separation of the colours.

But if two colours had very similar solubility in water they would be very difficult to separate like this.

But if we make another chromatogram, using a different solvent we may find that the results are very different.

In this case (right) the inks are clearly not very soluble in pure water.

We can tell this because they didn't move much and so didn't separate at all.

But in a 5% alcohol solution they move much more and separate well.

A pure ink - one made of only one colour - won't separate in any solvent.

So, so an ink that seems to be made of only one colour when tested in water may actually be made up of several colours.

And we can show this by finding a solvent that it dissolves in more easily

Calculating Rf values.

Rf values are a way of measuring solubility - how well a substance dissolves

Examiners love asking you to calculate these values so you should learn how to do so.

Using ruler, measure from the pencil-line to the solvent front.
Then measure from the pencil-line to the centre of the spot.
Use the formula:

Rf = distance moved by substance / distance moved by solvent

In our cases (right) the yellow colour has a higher Rf than the blue because it is more soluble

In the case above, the distances have been measured to 1 decimal place, so the Rf value shouldn't be recorded to lots of decimal places.

Rf values are useful for identifying a particular substance - providing that someone has done the same experiment before to establish the Rf value before.

But it's quite likely that many substances will have almost exactly the same Rf value in water.