Chemical analysis, Year 11: formulations and chromatography

A formulation

HintThe word shares its opening with 'formula', as in a recipe to be followed.

The whyPaints, medicines, fuels, cleaning agents, alloys, fertilisers and foods are all formulations. What makes a mixture a formulation is the design: the ingredients and their amounts are chosen so that the product does its job.

So that the product has the properties it needs

HintThink what would happen to a paint or a medicine if the recipe changed from batch to batch.

The whyChanging the proportions changes the properties: a paint with too little binder flakes off, and a tablet with too much of the active drug could be harmful. So every batch is made to the same recipe.

Each one is there for a particular purpose

HintAsk what job every ingredient is doing.

The whyA formulation is not a random mix: every component is chosen because the product needs what it does. You are not expected to know the ingredients of real branded products, only to recognise this idea.

The household cleaning spray

HintThree of them are single substances; look for the one made by mixing several.

The whyDistilled water is a single compound, and oxygen and copper are single elements. The cleaning spray is a designed mixture of several chemicals, each with a job, which is what a formulation is.

Its ingredients are only mixed, not chemically joined

HintCompare how salt and water are held in sea water with how sodium and chlorine are held in salt.

The whyIn a compound the elements are bonded together in proportions fixed by the chemistry. In a formulation people choose the proportions and each component keeps its own properties, so it is a mixture.

The mobile phase (in chromatography)

HintOne of the two parts of the system stays still; this is the other, named for the fact that it travels.

The whyChromatography always has two phases. In paper chromatography the paper is the stationary phase, which does not move, and the solvent is the mobile phase, which carries the substances along.

0.45

HintDivide the smaller distance by the larger one.

The whyRf = distance moved by the substance ÷ distance moved by the solvent = 3.6 ÷ 8.0 = 0.45. Both distances are measured from the start line. An Rf value has no unit, because it is one length divided by another. The expression is given in the specification content for this section.

It is more soluble in the solvent and less attracted to the paper

HintEach dye is pulled two ways: by the liquid that is rising and by the sheet it is rising through.

The whyEach substance is shared between the two phases. One that spends more of its time dissolved in the moving solvent is carried further; one that clings to the paper is held back. Different substances are shared differently, so they separate.

Two dyes can travel together in one solvent; a pure substance gives one spot in every solvent

HintA mixture can hide if its components happen to move the same distance.

The whyA single spot shows only that nothing separated in that solvent. If the colouring still gives one spot in several different solvents, the evidence that it is a single substance is much stronger.

No — an Rf value depends on the solvent used

HintThink about whether the dye dissolves equally well in both liquids.

The whyAn Rf value belongs to a substance in a particular solvent. To identify an unknown by comparing its Rf value with known values, both must have been measured in the same solvent.

centre

HintSpots are blobs, not points, so one agreed place on the blob has to be used.

The whyA spot can be several millimetres tall, so measuring to its top or its bottom would give different Rf values. Measuring to the middle every time makes results comparable. The solvent distance is measured from the same start line to the solvent front.