Investigating CSI – Background material Table of Contents I ...

VI. AN IMPORTANT NOTE
One should not “read” the result of the KM test after 30 seconds or so. This
is because the KM reagent may begin to turn pink after this time as the result of
oxygen in the ambient environment, and not because blood is present.
VII. HOW DOES THIS TEST WORK?
The diagram below helps illustrate what happens in the KM test. Follow along as
I describe each part.
1.) Hydrogen peroxide: Look at the chemical formula and compare it with the
chemical formulas for water and oxygen. Oxygen (elemental oxygen) typically
comes as a pair of molecules (as it has in the peroxide), but something in our test
breaks it up. What might that be?
2.) Heme: This is where we find peroxidase-like activity. This is what breaks up
peroxides. Peroxides are toxic to animal tissue, and are decomposed by the
heme in our blood. In this case, hydrogen peroxide is decomposed into two
parts: water, and a free oxygen radical.
3.) Follow the O (the free radical). It's going to combine with the KM, our
phenolphthalein color-indicator. Why?
4.) When the KM solution is initially prepared, it is boiled in a flask for several
hours to help remove oxygen. (A little bit of zinc metal dust added to the solution
28