Name: ______Date: ______Period: ______
Testing for Blood
The Kastle-Meyer Test
The Story:
The police were called to a house where they discovered a body near the back door. There was a fresh pool of blood by the victim’s head. Officers questioned the neighbors and learned that a plumber and an electrician made service calls to the house just hours before. They were able to locate each suspect. They recovered a shirt with dark stains from the service van of the plumber and rags with similar dark stains from the electrician’s truck.
Stains from each suspect were recovered, and each denies that the stains are blood. You must determine the truth. You will use the highly sensitive phenolphthalein (Kastle-Meyer) test. It can reveal blood spots practically invisible to the human eye.
The Chemistry:
The chemistry of the phenolphthalein test is essentially that of an organic molecule which can be oxidized by free hydroxyl ions liberated by peroxidase-like action. Peroxidases are enzymes that catalyze peroxide oxidation reactions. In this instance, heme or hemoglobin acts as the catalyst and colorless phenolphthalein in alkaline solution is oxidized by hydrogen peroxide to purple-violet phenolphthalein. None of the chemical screening tests for blood stains is absolutely specific for blood, because each can react to peroxidases found in most plant and some animal cells. Because peroxidases denature (change form and lose activity) rapidly with drying and aging, the screening tests are more reliable with older dry stains.
Development of purple-violet color at the end of the phenolphthalein test of an old dry stain is positive for blood. It is a non-destructive test, meaning the blood can still be used for DNA or other testing.
Procedure:
- Using a cotton swab, rub the stained area of the cloth.
- To the rubbed portion of the cotton swab add, in this order:
- one drop of ethanol (EtOH)
- one drop of Phenolphthalein solution,
- one drop of hydrogen peroxide (H2O2) solution
Development of a pink color before the addition of the hydrogen peroxide does not indicate blood, but may be a dye reaction. After the addition of the hydrogen peroxide, an almost immediate (one or two seconds maximum) purple-violet color reaction in the rubbed area of the cotton swab is a positive test for blood.
Questions/Conclusions:
- Based on the results of this test, which person (if either) is the more likely suspect? Stain A (the plumber) or Stain B (the electrician)?
- Provide circumstances under which a false positive may be obtained.
- Explain the significance of this technique to the operation of the field of forensics.