Criterion of purity

Pure solid covalent organic compounds and many inorganic complexes incorporating organic ligands have definite melting points. The pure solid will melt reproducibly over a narrow temperature range, usually less than 1°C,and this melting range is known as the melting point. If the compound is not pure, the melting range will increase significantly and the upper end of the melting range will be lowered. Thus the melting point (m.pt.) of a compound is a measure of its purity. Other methods are used routinely to estimate purity of solid compounds, such as NMR, and the presence of a single 'spot' or a single peak on a chromatogram from thin-layer, gas-liquid or high-performance liquid chromatography, but melting point remains the standard measure of purity.

*Note: The term melting point really means the melting range of a chemical and in your laboratory report you should always quote the measured melting range under the heading 'melting point'.