Formulations and Testing Purity

Useful mixtures are designed, not accidental

Many useful products are mixtures rather than single substances. A mixture contains two or more substances that are not chemically combined with one another. Each substance retains its own chemical properties, although the mixture can have useful overall properties.

A formulation is a mixture designed as a useful product. Its components have particular purposes and are added in carefully measured proportions. Changing those proportions can change how well the product works. Food and drink products, medicines, sunscreens, perfumes and paints are all examples of formulations.

Mixtures are therefore not necessarily undesirable. A manufacturer may deliberately make a formulation, while also checking that each ingredient is sufficiently pure. Chemical analysis helps with this quality control and can identify substances in forensic investigations or environmental samples.

What chemists mean by pure

Scientifically, a pure substance is a single element or compound, with no other substance mixed into it. A pure compound can contain more than one element: what matters is that those elements are chemically combined in one compound. Pure water, for example, contains only the compound water.

Everyday language uses pure differently. It often means natural or having nothing added. Milk described as ‘pure milk’ is still a mixture of substances, so it is not chemically pure. The everyday description tells us about additions to the product, not whether it contains just one element or compound.

An impurity is an unwanted substance mixed with the substance of interest. Whether a mixture is wanted matters: the deliberately chosen ingredients of a formulation are not automatically impurities.

Testing purity using melting points

A pure substance has a sharp, characteristic melting point. At a given pressure, pure water melts at $0\,^{\circ}\mathrm{C}$. Impurities generally lower a substance’s melting point and broaden its melting range: melting starts at one temperature and finishes at a higher temperature.

To use melting-point data, compare the sample with the known melting point of the pure substance. Consider both where melting occurs and how wide the range is.

Suppose a compound has a reference melting point of $80\,^{\circ}\mathrm{C}$. A sample melting sharply at $80\,^{\circ}\mathrm{C}$ is consistent with a pure sample. Another sample melting from $75$ to $79$ is likely to contain impurities: its melting temperature is lower and melting occurs over a range.

In a melting-point test, a small solid sample is heated slowly in melting-point apparatus. Record the temperature when melting begins and when the last solid disappears. Slow heating makes these changes easier to observe. Wear eye protection and avoid touching hot apparatus. This type of analysis is useful when checking the purity of substances such as medicines.

How paper chromatography separates substances

Chromatography separates substances because they distribute differently between two phases. In paper chromatography, the stationary phase is the paper and the mobile phase is the solvent moving through it.

As the solvent rises through the paper, it carries dissolved substances with it. A substance that spends more time dissolved in the moving solvent travels further. A substance held more strongly by the stationary phase spends less time moving and travels a shorter distance. Separation therefore depends on both attraction to the stationary phase and solubility in the mobile phase, not simply on the original colour of the sample.

Different components of an ink can move at different rates and form separate spots. The resulting pattern is called a chromatogram.

Making a readable chromatogram

Draw a pencil baseline near the bottom of chromatography paper. Place a small spot of the sample on it and let it dry. Known substances can be spotted beside the sample for comparison.

Stand the paper in a container holding a shallow layer of solvent. The solvent must touch the paper but remain below the baseline, so the samples do not dissolve directly into the solvent reservoir. Pencil is used because an ink line could dissolve and separate, adding unwanted spots.

Allow the solvent to rise. Remove the paper before the solvent reaches the top, immediately mark the solvent front in pencil, and leave the paper to dry. The solvent front is the furthest point reached by the solvent; marking it promptly preserves the position before the solvent evaporates.

Wear eye protection. When using an organic solvent, keep it away from flames and use suitable ventilation; follow the precautions for the particular solvent.

Reading spots and calculating Rᶠ

In the example chromatogram, the unknown sample produces two spots, so it contains at least two substances. Its lower spot lines up with reference A and its upper spot with reference C. These matches suggest that A and C are present because the samples were run together under the same conditions.

The $R_f$ value compares how far a substance travels with how far the solvent front travels:

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$

Measure vertically from the baseline to the centre of the spot, and from the same baseline to the solvent front. In the constructed example, the upper unknown spot travels $6.0\,\mathrm{cm}$ and the solvent front travels $8.0\,\mathrm{cm}$. Its $R_f$ is therefore $6.0÷8.0=0.75$: the substance travels three-quarters of the solvent-front distance.

An $R_f$ value has no unit because it is a ratio of two distances. Values lie between 0 and 1. Compare an unknown’s value with reference values obtained under the same conditions. Changing the solvent can change the value, so an $R_f$ is not a universal identifying number.

Choosing solvents and revealing invisible spots

An aqueous solvent is water-based. A non-aqueous solvent is not water-based; ethanol is one example. Choose a solvent that dissolves the substances being tested and allows their components to separate.

A pure substance should give one spot in each suitable solvent. More than one separated spot shows that a sample is a mixture. However, two components may travel together and overlap in one solvent. Repeating the test with a different suitable solvent can separate those components and give stronger evidence about purity.

Not all substances produce coloured spots. A blank-looking chromatogram does not necessarily mean that nothing is present. Locating agents, such as ninhydrin or iodine, can make colourless spots visible. Suitable substances may also be detected under ultraviolet light. Use locating chemicals under teacher supervision, avoid inhaling vapours, and do not look directly into a UV source.

When suggesting a purity test, bring these decisions together: use paper chromatography with a suitable solvent, reveal colourless spots if necessary, and consider a second solvent if one spot could conceal overlapping substances.

Formulations and purity

• Formulation: a useful mixture made with carefully measured proportions; each component has a purpose.
• Scientifically pure: one element or compound only.
• Everyday ‘pure’: often natural or with nothing added; it may still be a mixture.

Melting-point evidence

• Pure: sharp melting point matching the reference value.
• Impure: generally a lower melting point and a broader melting range.
• Compare both temperature and range.

Chromatography

• Stationary phase: paper. : moving solvent.