What Determines the Colour of Compounds?

[CrimpJiggler]

I've come across the concept of complementary colours where if a compound absorbs one wavelength then to the eye, it will appear as its complementary colour. Heres a chart of complementary colours:

so for example if a compound absorbs violet radiation, the compound will appear yellow. In college I did an experiment involving the ferroin (an iron-phenanthroline complex) and determined that its maximum absorption occurs at 511 nm. This observation coincides with this complementary colour theory because 511 nm lands in the cyan region of the visible spectrum and ferroin is orange (cyans complementary colour).

What I don't understand is how does this work when a compound has more than one significant visible absorption? What colour would it appear then? Is it determined by the compounds most intense absorption?

 

[DrDu]

Just look at it as a subtractive mixture of colors who absorb each only at one frequency.

 

[CrimpJiggler]

I used to think that the colour that we see a compound as is what's left over after all the wavelengths of that compounds absorption spectrum have been subtracted but I've since concluded that this theory is false and that the complimentary colour theory is actually how it works. Ferroin for example. It appears dark orange. Does that mean it absorbs all wavelengths except for a narrow band in the orange/red range? I've seen ferroins absorption spectrum and that's not the case. It actually has a relatively narrow absorption band in the cyan region.

EDIT: Both theories appear to hold for azure B.
[URL]http://i00.i.aliimg.com/photo/421553239/Azure_B.summ.jpg[/URL]
[URL]http://www.scielo.br/img/revistas/jbchs/v16n5/26457f1.gif[/URL]
as you can see it absorbs most visible wavelengths but doesn't absorb blue and violet very strongly.

Heres methylene blue:
[URL]http://omlc.ogi.edu/spectra/mb/mb-water.gif[/URL]
it only really starts absorbing at 550 nm so there should still be a fair bit of green in there:

http://www.cbu.edu/~seisen/Photosynthesis_files/image001.gif

In this case the complementary colour theory seems to work a lot better: methylene blue has its maximum absorbance in the orange region, oranges complementary colour is blue. Something that can't be explained by complementary colours though is brown or gray compounds. They can only be produced by mixing colours. Even black compounds can't be explained by thinking that they absorb all the wavelengths because there are clearly varying intensities of black. Tar is about the blackest thing I've seen. Potassium permanganate doesn't look as black as tar to me. Now that I think of it, if something were to completely absorb all incident radiation, you wouldn't be able to see it at all, it would appear as a black void lol.

 

[DrDu]

To add to your confusion I'd like to mention that strongly colored substances reflect that color best which they absorb best. That's why KMnO4 crystalls reflect green-yellow light.

 

[LudusRex]

The color of a compound is determined by its molecular structure and the way it interacts with light. When light shines on a compound, some of the wavelengths are absorbed while others are reflected or transmitted. The wavelengths that are absorbed correspond to the color that we see. This is why different compounds can have different colors, as they absorb different wavelengths of light.

The concept of complementary colors is based on the fact that when a compound absorbs a certain wavelength, it appears as its complementary color to our eyes. This is because the complementary color is the one that is not absorbed and is therefore reflected or transmitted. In the case of the ferroin complex, it appears orange because it absorbs blue-green light (around 511 nm) and reflects or transmits orange light.

When a compound has more than one significant visible absorption, it can appear as a combination of colors. This is because each absorption will contribute to the overall color that we see. The color will be determined by the relative intensities of each absorption and their corresponding complementary colors. So, it is not necessarily determined by the most intense absorption, but rather by the overall combination of absorptions.

It is important to note that the color of a compound can also be affected by factors such as concentration, pH, and the presence of other molecules. These can alter the way the compound interacts with light and therefore change its color. This is why some compounds may appear different colors in different conditions.

In summary, the color of a compound is determined by its molecular structure and the way it interacts with light. The concept of complementary colors helps explain why compounds appear certain colors, but the overall color can be influenced by multiple factors. The color of a compound with multiple significant absorptions is determined by the combination of these absorptions and their complementary colors.