Chemistry of colour: Not a “Pigment” of Your Imagination

Soumya Satheesan
12 January 2018

Above: Image © MicrovOne,iStockPhoto.com

What’s your morning routine like? Here’s mine: My alarm clock rings and I drag myself out of bed. I head to the bathroom and I brush my teeth with toothpaste. Then I take a shower using a glossy bar of soap. I get dressed, untangle my hair with a plastic comb, and maybe put on some makeup, like lipstick.

What do these things – toothpaste, soap, plastic, and lipstick – often have in common? Colourful pigments!

What are pigments?

The word “pigment” refers to compounds that are intensely coloured. They’re mainly used to colour other materials and make things look more attractive. If you looked around you, you’d see all kinds of examples where pigments have been used: in the paint on the walls or on cars outside your window, light-reflecting signs, printing inks, and even various electronics - like your TV, radio or phone.

Pigments are separated into different categories. They may be organic, meaning they contain the element carbon, or inorganic, meaning they do not contain carbon. Inorganic pigments can be man-made or they can be found naturally, like in minerals. Organic pigments from natural sources, such as plants, have been used for centuries, including during prehistoric times for cave paintings. Today synthetic (or man-made) organic pigments from coal tars and other petrochemicals are more commonly used.

Cobalt (II) phosphate, Co3(PO4)2, is an inorganic pigment that is violet in colour.
Above: Cobalt (II) phosphate, Co3(PO4)2, is an inorganic pigment that is violet in colour.
Pigment Red 144 is an organic pigment used for its bright red color.
Above: Pigment Red 144 is an organic pigment used for its bright red color.

Did you know? Museum workers analyze the pigments and dyes in works of art to determine how old the art is, where it comes from, and whether it’s real or fake!

Pigments vs. Dyes

You might have heard the word dye used to describe a substance used for colouring things. Pigment and dye are often used interchangeably, but they have specific meanings when it comes to colour chemistry.

Your clothes, food colouring, and paper appear a certain colour thanks to dyes. Dyes are soluble colouring materials, so they can dissolve into the liquid that carries them. They are used as a solution and they hold tightly onto whatever they are applied.

Unlike dyes, pigments are insoluble in their liquid carrier. This means that they do not dissolve in the liquid to form a solution. Instead, they mix as tiny solid particles with a liquid. The solubility depends on the structure of the compound. For example, compounds with a phosphate group generally have a low solubility. Because pigments are insoluble materials, they have to be mixed with substances called binders or vehicles before they can be used for colouring.

Did you know? In biology, the term "pigment" has a different meaning than in chemistry. In biology, it refers to any coloured molecule found in a cell. It does not matter whether it is soluble.

Why do I see pigments?

White light, like sunlight, is made up of different colours. When you shine white light onto pigments they appear a certain colour because they absorb some parts/colours of the light while reflecting others. Similarly, pigments absorb some of colour of light, while the rest of the light bounces off. So, everytime I look at my blue toothbrush, I don’t see absorbed colours. I see only the blue light that is reflected.

Pigments Today

Today, chemists who work with pigments look for various ways to improve their uses. They may try to come up with ways to use less expensive ingredients or to help pigments hold their colours better. Thanks to all this work with pigments, our world remains beautifully colourful!

Did you know? Paint is basically composed of a binder and a pigment. For a long time, paints have been used for artwork and decorative purposes. Since the 20th century, paint technology has improved a lot, and paint gained additional characteristics like protection from fire, corrosion resistance, and heat stability.

Learn more!

Soumya Satheesan

I’m currently working towards a BScE in Chemical Engineering at the University of New Brunswick.







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