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Author: Subject: LUMINOL
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cool.gif posted on 4-8-2006 at 10:29

What exactly catalizes the reaction that causes LUMINOL to glow? is it the hemaglobin itself? or is it the Iron? if it's the IRON in hemeglobin, then i suppose the luminol blood splatter test wouldn't work on iron railings that were unpainted, the whole thing would light up?

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[*] posted on 5-8-2006 at 05:25

IIRC it's the heam part of the heamoglobin that does it. It would hardly be a useful test it it responded to iron since that's about 5% of the earth's crust.
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[*] posted on 5-8-2006 at 06:08

It's the Fe(II) in heam. The reaction with alkaline peroxide is catalysed by many transition metals ions including Co(II) and Cu(II), as well OCl-. However the pattern of light output differs, 'naked' cations give a brighter light that quickly fades, while blood is less bright but longer lasting. I suspect that weak chelates of metals might also give longer lasting glows, but they are not commonly found around typical murder scenes.

I get the impression that if the area had been sprayed with inorganic Fe(II), the longer duration glow from blood would be noticed and at least serve as a flag to do more detailed study of those spots.

Note that inorganic iron tends to go to Fe(III) fairly quickly, rust is mostly Fe(III) hydroxide and hydrated oxides. The Fe(III) affects the luminol reaction in a concentration dependent manner.
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[*] posted on 6-11-2006 at 17:19

The classic laboratory demonstration uses potassium ferricyanide (K3[Fe(CN)6]). Here Fe is in the 3+ oxidation state (as will any Fe2+ that is present with peroxide). the oxidation occurs as a Fe3+<-->Fe2+ REDOX cycle that catalyzes the disociation of the H2O2 into HO' and HO-. (See Fenton reaction) Fe2+ is not strictly required as it is made in-situ.

Interestingly, luminol in DMSO (dimethylsulfoxide) will oxidize directly, in the presence of NaOH and air to perform the intersystem cross leading to a triplet phosphoresence (no iron required). Shaking the reaction to dissolve more oxygen is all that is required to make it glow again (so long as luminol is still present).

Take care though, DMSO facilitates the absorbance of many chemicals though the skin; wash carefully before handling other chemicals if you have made contact with it.

Luminol is great. Many years ago I synthesized it in organic lab from 3-nitrophthalic acid.



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