The latest in the Colourful Chemistry series looks at the inorganic compounds that give many paints their colours. This shows a limited selection of the most common compounds, and there are many others; there are also a large range of organic based pigments, which aren’t discussed here (although could possibly be the topic for a future graphic).
Generally, paints are composed of a binder, a solvent, and a pigment. The binder is a polymer, either dissolved in the paint or suspended in it by emulsifiers, the role of which is to hold the pigment to the surface. For emulsion paints, the solvent used is just water, whilst for others organic compounds such as turpentine are utilised.
The pigment can be organic or inorganic. Organic pigments are actually preferred, as they tend to be brighter and more stable; however, inorganic pigments are cheaper, and are heat and light stable. Also, it isn’t possible to obtain jet black or brilliant white pigments.
Typically, inorganic pigments are crystals of metal compounds (often oxides). Transition metals have their bonding electrons in d orbitals – when the metal is bonded to groups of atoms, this can cause the d orbitals to ‘split’ into two levels of energy. The electrons in these orbitals can then absorb energy to move between the two levels – the energy gap between the levels determines the wavelength of light absorbed. As not all of the light hitting the compound is reflected, this means we perceive them to be coloured.
Due to the presence of toxic heavy metals in some of the inorganic compounds used as pigments, a number of them have limited or entirely prohibited use. For example, chromate pigments are rarely used, due to environmental problems associated with chromium; similarly, cadmium compounds have been mostly phased out to their high toxicity. Clearly, this rules out a number of the pigments featured in the graphic for use – hence the utility of organic pigments!