Phenolphthalein in crime scene investigations
Phenolphthalein is a chemical used in the Kastle-Meyer test, a widely utilized presumptive test for detecting blood at crime scenes. When applied to a stain suspected of being blood, the presence of heme iron in red blood cells reacts with phenolphthalein and hydrogen peroxide, resulting in a bright pink color, indicating a positive test. This reaction is significant as it helps forensic investigators in the initial reconstruction of a crime scene. However, the test does not differentiate between human and animal blood, necessitating further analysis to confirm the source of the blood. Additionally, certain plant substances and other oxidizing agents can produce similar reactions, highlighting the need for caution and further testing.
The testing procedure involves applying a moist swab to the stain, adding phenolphthalein, and then hydrogen peroxide, while a control swab is used to validate results. Introduced in 1901, the phenolphthalein test is appreciated for its sensitivity and specificity, as well as its non-destructive nature, allowing samples to be preserved for additional examination. Furthermore, it is considered safer compared to other blood testing chemicals that may pose carcinogenic risks. Overall, phenolphthalein plays a crucial role in forensic investigations by providing preliminary evidence of blood presence, while also emphasizing the importance of follow-up analysis for definitive results.
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Phenolphthalein in crime scene investigations
DEFINITION: Chemical that is the basis for the Kastle-Meyer test, a commonly used presumptive test for blood at crime scenes.
SIGNIFICANCE: A positive reaction to phenolphthalein in a stain found at a crime scene suggests that the stain is probably blood; such information can facilitate an initial reconstruction of a crime and prompt follow-up.
In the presence of heme iron and hydrogen peroxide, phenolphthalein, which is clear in the reduced state, is converted to an oxidized state, which is pink. Because heme iron is present in hemoglobin, the protein that carries oxygen in red blood cells, a positive test can indicate the presence of blood. Such a test does not distinguish between human blood and animal blood, however; further testing is necessary to make that distinction and, if the blood is human, to determine whose blood it is. In addition, constituents of some plants, such as potatoes and horseradish, as well as oxidizing agents found in some cleansers, can catalyze the reaction. Accordingly, a phenolphthalein test is only presumptive for blood; a positive result must be confirmed by laboratory tests.
Typically, a forensic investigator performs the test by moistening a cotton swab with deionized water and rubbing the swab on the stain, adding a drop of phenolphthalein solution to the swab, waiting thirty seconds, and then adding a drop of 3 percent hydrogen peroxide to the swab. A positive reaction will turn the swab a bright pink color within fifteen seconds. Often, a swab taken near the stain of interest is used as a control. If the swab turns pinkish before the hydrogen peroxide is added, the test is invalid. To ensure that the reagents have not deteriorated before use, the investigator validates the test using a known blood standard.
First introduced in 1901, the phenolphthalein test is fairly sensitive and quite specific. In addition, phenolphthalein does not destroy the sample, which can be kept and used in further tests at the lab. The use of phenolphthalein does not interfere with subsequent tests. Furthermore, phenolphthalein is among the safest of the compounds used in presumptive tests for blood in the field; most others, including benzidine and orthotolidine, are known or probable carcinogens and thus pose a risk to investigators.
Bibliography
Geberth, Vernon J. Practical Homicide Investigation: Tactics, Procedures, and Forensic Techniques. 4th ed. Boca Raton, Fla.: CRC Press, 2006.
Lee, Henry C., Timothy Palmbach, and Marilyn T. Miller. Henry Lee’s Crime Scene Handbook. San Diego, Calif.: Academic Press, 2001.
Singh, Rajvinder. "Narration and Legacy of Important Chemical Spot Tests in Forensic Investigation." Critical Reviews in Analytical Chemistry, vol. 52, 2022, doi.org/10.1080/10408347.2020.1785837. Accessed 19 Aug. 2024.
Owen, David. Hidden Evidence: Forty True Crimes and How Forensic Science Helped Solve Them. Richmond Hill, Ont.: Firefly Books, 2000.
"Phenolphthalein Test for Blood." Federal Bureau of Investigations, 1 Aug. 2022, fbilabqsd.fbi.gov/file-repository/dna/casework/bio-410-00-phenolphthalein-test-for-blood.pdf. Accessed 19 Aug. 2024.