Illustrated Guide to Home Chemistry Experiments

Chapter 22 Laboratory: Forensic Chemistry 387

CUTIOA nS

The chemicals used in this experiment are reasonably safe.

Ethanol and the phenolphthalein solution are flammable,

so avoid open flame. Hydrogen peroxide is an oxidizer and

irritant, and may bleach clothing and other materials with

which it comes into contact. Wear splash goggles, gloves,

and protective clothing.

z

litre of water. You perceive that the resulting mixture has the

appearance of pure water. The proportion of blood cannot be

more than one in a million. I have no doubt, however, that

we shall be able to obtain the characteristic reaction.” As he

spoke, he threw into the vessel a few white crystals, and then

added some drops of a transparent fluid. In an instant the

contents assumed a dull mahogany colour, and a brownish

dust was precipitated to the bottom of the glass jar.

Sherlock Holmes, A Study in Scarlet

Although this description doesn’t correspond exactly to the test
we’re about to run, there’s little doubt that Sherlock Holmes was
the true inventor of this test. Arthur Conan Doyle, who was at
best an unreliable chronicler, probably didn’t take notes as he
watched Holmes perform the test, or perhaps Doyle obfuscated
the details for reasons of his own. (Holmes was very precise
in speech and action; the numbers given for the dilution are
an indication of Doyle’s sloppiness. One drop of blood—about
0.05 mL—in one litre of water is a dilution of about 1:20,000,
not 1:1,000,000. Coincidentally, or perhaps not, 1:20,000 is
about the threshold of detectability for this test, so Holmes was
speaking literally.)

The Sherlock Holmes test, usually called the Kastle-Meyer
test, is a presumptive test for blood, or, more precisely, for the
hemoglobin present in blood. The test is fast and inexpensive,
requiring only the sample and solutions of ethanol, hydrogen
peroxide, and phenolphthalein (C 20 H 14 O 4 ). The test is non-
destructive, so the sample can be preserved for later testing.

The ethanol functions as a solvent, but does not take part in the
reaction. The peroxidase enzyme present in the hemoglobin
component of human or animal blood catalyzes the reaction,
during which colorless phenolphthalein molecules are converted
to pink phenolphthalein ions. The peroxidase enzyme in
hemoglobin is extremely efficient catalyst. One peroxidase
molecule can catalyze the conversion of hundreds of thousands
of peroxide molecules per second, so the presence of even
a tiny amount of blood yields a distinct color change in the

POCEDURER

1.ou have not already done so, put on your splash If y

goggles, gloves, and protective clothing.

2. Moisten a cotton swab with distilled or deionized water,
   and touch it to the sample of dried blood. You needn’t rub
   vigorously, but try to get at least a small amount of the
   sample transferred to the swab.


3. Put a drop or two of ethanol on the sample. The goal is
   to moisten the swab with ethanol, not to drench it. The
   ethanol does not take part in the reaction, but merely
   serves to clean up the sample and expose the hemoglobin
   contained in the sample.


4. Allow the ethanol to work for a few seconds, and then
   put a drop or two of phenolphthalein solution on the
   sample. (The sample should remain colorless. If the
   phenolphthalein turns pink at this point, it is an indication
   that the phenolphthalein is old or oxidized. Rerun the test
   using fresh phenolphthalein.)


5. After a few seconds, put a drop or two of hydrogen
   peroxide on the sample. The sample should turn bright
   pink almost immediately, indicating the presence of
   blood.


6. Repeat steps 2 through 5, but using a sample of rust or
   another material that looks like dried blood but does not
   contain blood. When you apply the hydrogen peroxide,
   the sample should remain colorless. (Even in the absence
   of blood, a pink color will develop after 30 seconds or
   so as the hydrogen peroxide converts the colorless
   phenolphthalein molecules to pink phenolphthalein ions,
   but anything other than a nearly immediate color change
   does not indicate the presence of blood.) Figure 22-1
   shows swabs with negative and positive test results.

dISpoSAL: Used swabs can be discarded as ordinary

household waste.

kASTLE-mEyER SoLUTIoN

• To prepare the Kastle-Meyer phenolphthalein solution,
  dissolve 0.1 g of phenolphthalein powder in 10.0 mL
  of 25% w/v aqueous sodium hydroxide in a test tube
  and add 0.1 g of mossy zinc and a boiling chip. Boil
  the solution very gently, adding water as necessary
  to maintain the volume, until the bright pink solution
  turns colorless or slightly yellowish. Allow it to cool,
  decant off the liquid, and dilute it to 100 mL with 70%
  ethanol. Store in a tightly-capped brown bottle.

phenolphthalein. The high sensitivity of this test means that,

as Sherlock Holmes stated, one drop of blood can be detected

even if it is diluted with a liter of water. In addition to being very

sensitive, this test is also very specific for blood.