24.1-Purity and Formulations

Formulations

• Formulation: a mixture that has been designed for a specific purpose to deliver a commercial product

• Formulations are complex mixtures that are made by following an exact recipe

• Each component of the formulation has a particular purpose

• Every part of the formulation is present in carefully measured quantities so that the final product has the required properties

• An everyday example of a formulation is paint

• Paint contains the following substances in exact quantities:

  • Pigment: gives the paint colour

  • Binder: forms a film that holds the pigment in place

  • Solvent: used to dissolve the other components and alter the viscosity

• Formulations are very important in the pharmaceutical industry where the exact ratios of each component must be precise

• By changing the formulation of a particular medicine chemists can make sure it delivers the drug to the correct part of the body at the right concentration, that it’s safe to consume and has an adequately long shelf life

• Other examples of formulations are: fuels, cleaning agents, alloys, fertilisers and foods

• The information on the packaging of some products tells you the formulation as a percentage composition

Melting and Boiling Points

• Melting and boiling points are unique. Pure substances each have a unique specific melting point and boiling point. The temperatures of the melting point and boiling point are specific to the substance.

• Sample purity can be tested. If we know the specific melting or boiling point for a substance, we can test the purity of a sample. If the sample has the same melting or boiling point as the substance, then we can confirm that the substance is pure.

• Impurities can be detected. By using the melting or boiling point of a sample, we can detect impurities. Impurities will change the melting and boiling point of a substance. Impure substances melt and boil over a range of temperatures.

24.2-Paper Chromatography

Paper chromatography

• Paper chromatography is used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Paper chromatography process

1. Water and ethanol solution is heated

2. As the paper is lowered into the solvent, some of the dye spreads up the paper

3.The paper has absorbed the solvent, and the dye has spread further up the paper

Chromatography phases

• Chromatography relies on two different ‘phases’:

  • the stationary phase, which in paper chromatography is very uniform, absorbent paper

  • the mobile phase is the solvent that moves through the paper, carrying different substances with it

• The different dissolved substances in a mixture are attracted to the two phases in different proportions.

• This causes them to move at different rates through the paper.

Interpreting a chromatogram

Separation by chromatography produces a chromatogram. A paper chromatogram can be used to distinguish between pure and impure substances:

• a pure substance produces one spot on the chromatogram

• an impure substance, or mixture, produces two or more spots

Practice Questions:

• What happens in descending paper chromatography?

  • The movement of the flow of solvent due to gravitational pull and capillary action is downwards

• In paper chromatography, paper sheets act as the

  • Stationary phase

• In paper chromatography, paper sheets are used as:

  • Adsorbent