Show an absorption spectrum of iron and explain.

Question

In class, we were talking about the red waterfalls in Antarctica and how and why the iron content and air made it red. But we also need to show an example of an absorption spectrum of iron and explain it but I don't know what to do for that. Please help!!!

Answer 1

Two different things get called "the absorption spectrum of iron," and for your Blood Falls project you want the first one.

1. The one you need: aqueous iron(III), a broad band spectrum

Plot absorbance on the y-axis against wavelength across 350 to 700 nm. You get one broad, featureless shoulder that is highest in the near-UV and violet and dies away toward the red. Here are points you can plot directly:

   wavelength    colour seen there    relative absorbance
   ----------    -----------------    -------------------
    350 nm       near-UV               1.00  (very high)
    400 nm       violet                0.85
    450 nm       blue                  0.60
    500 nm       blue-green            0.30
    550 nm       yellow-green          0.12
    600 nm       orange                0.05
    650 nm       red                   0.02
    700 nm       deep red              0.01  (negligible)

Drawn roughly, it looks like this:

   A
   |***
   |*****
   |*******
   |  *******
   |    *******
   |      ********
   |          *********
   |               **********________________
   +-------------------------------------------------
    350   400   450   500   550   600   650   700  nm
    |------- strongly absorbed ------|              
         violet / blue / blue-green                 
                                 |-- transmitted --|
                                    orange and red

2. Why that shape makes the water red

A solution shows you the colour it does NOT absorb. This spectrum removes violet, blue and blue-green and passes orange and red, so the transmitted light is red-orange. That is the complementary-colour rule, and it is the whole explanation of the colour in one sentence.

3. Why iron(III) absorbs so strongly there, and iron(II) does not

This is the part that will earn you the marks.

The brine emerging at Blood Falls is iron(II), Fe2+, and it comes out essentially colourless to faintly greenish. Fe2+ in water only has d-d transitions available, and those are Laporte-forbidden, so their molar absorptivity is tiny - single digits. Weak absorption means almost no colour.

The moment that brine meets air, oxygen oxidizes Fe2+ to Fe3+, which immediately hydrolyzes and precipitates as hydrated iron(III) oxide-hydroxide, essentially rust. Those Fe3+ species absorb through a ligand-to-metal charge transfer transition: an electron jumps from oxygen to iron. Charge transfer is fully allowed, so its molar absorptivity is in the thousands - a factor of roughly a thousand stronger than the d-d bands of Fe2+.

So the sequence is: colourless Fe2+ brine, contact with air, oxidation to Fe3+, an intense allowed charge-transfer band eating the blue end of the spectrum, and red-orange light coming back out. The iron was always there; the air is what created the colour. That is exactly the point your class was making.

4. The other kind, in case your instructor means this one

In atomic absorption spectroscopy, iron vapour absorbs at sharp discrete lines rather than in a broad band, because free gaseous atoms have no vibrational or solvent broadening. The analytical line is 248.3 nm, with others at 248.8, 252.3, 302.1, 372.0 and 386.0 nm. Sketched, it is a set of thin vertical spikes on a flat baseline - a completely different picture from the smooth hump above.

If you show both and say in one line why they differ - free atoms give lines, ions in solution give broad bands - you will have covered the question no matter which one was intended.

One practical note on the earlier comment in this thread: you can absolutely present a spectrum in text form. A labelled table of wavelength against absorbance, like the one above, is a legitimate way to communicate a spectrum, and your instructor will accept it as readily as a drawing. If you want a real published curve, search for the absorption spectrum of ferrihydrite or of iron(III) oxyhydroxide rather than "iron" - that is the actual compound doing the absorbing.