(For those unfamiliar with the Kastle-Meyer test,
a good overview is available
here on ScienceLine ).
The indication of a positive test is an intense
pink color, which signifies that the reduced form
phenolphthalein has been oxidized (i.e., has
lost electrons). The important reaction in the
test is actually that of hydrogen peroxide
(H2O2) with hemoglobin in
blood. As described
here, the phenolphthalein is not directly
involved in this reaction, but rather just
supplies electrons. The hemoglobin turns
the peroxide molecules into water by donating
electrons. The hemoglobin then wants more
electrons to be stable, and they are donated by
the phenolphthalein that is also in the solution.
I do not know quantum chromodynamics and so can't
describe it in the physical sense, but two
electrons come from phenolphthalein , which
join with two protons and one hydrogen peroxide
(H2O2) to create two
molecules of water. The protons have to come
from water, which means that the reaction
creates hydroxide ions.
Click Here to return to the search form.